OOOR79107
THE FUNCTIONAL ECOLOGY OF SUBMERGED AQUATIC VEGETATION
IN THE LOWER CHESAPEAKE BAY
by
R.L. Wetzel, K.L. Webb, P.A. Penhale,
R.J.-Orth , D. F. Boesch,
G.W. Boehlert and J.V. Merriner
Co-Principal Investigators
Virginia Institute of Marine Science and
School of Marine Science College of William and Mary
Gloucester Point, VA. 23062
Annual Data Report
for
EPA/CBP Grant No. R805974
to
Mr. Thomas W. Nugent
U.S. Environmental Protection Agency
Region III
6th & Walnut Street
Philadelphia, PA. 19106
Project Officer
H
November, 1979 -nO
^>;>^
CB 00292
-------
-Xii,
:.r" - \
COMMONWEALTH oj: VIRGINIA
Virginia institute of Marine Science
,/VUJ.iAMJ HARGIS, JR Gloi, CC^te r Poitn . \'] Y^'miU 2 30o2 Phone'(804) 642-2111
DIRECTOR
December 10, 1979
Mr. Bert Brun
U. S. Fish & Wildlife Service
Department of the Interior
Annapolis, Maryland 21401
Dear Bert:
Enclosed please find a bound copy of our first annual data report. The
original unbound copy was sent previously to Tom Nugent but as I under-
stand he will no longer serve as Project Officer. I am forwarding you
copies in the event receipt is delayed.
Please feel free to contact me concerning the report. Specific comments
should be addressed to the subproject principal investigators as indicated
in the summary.
Respectfully,
R. L. Wetzel
Program Manager
RLW/ck
Enclosure
-------
THE FUNCTIONAL ECOLOGY OF SUBMERGED AQUATIC VEGETATION
IN THE LOWER CHESAPEAKE BAY
by
R.L. Wetzel, K.L. Webb, P.A. Penhale,
R.J.-Orth , D. F. Boesch,
G.W. Boehlert and J.V. Merriner
Co-Principal Investigators
Virginia Institute of Marine Science and
School of Marine Science College of William and Mary
Gloucester Point, VA. 23062
Annual Data Report
for
EPA/CBP Grant No. R805974
to
Mr. Thomas W. Nugent
U.S. Environmental Protection Agency
Region III
6th & Walnut Street
Philadelphia, PA. 19106
Project Officer
November, 1979
-------
Executive Summary
The research program, "The Functional Ecology of Submerged Aquatic
Vegetation in the Lower Chesapeake Bay" (EPA/GBP Grant No. R805974), is
an integrative effort composed of seven principal investigators. The
research team has worked since July 1978 at one study site, the Vaucluse
Shores area, to develop and institute a coherent research program on
SAV ecological relationships.
The principal studies have focused on plant productivity, metabolism
and nutrient cycling, the role of resident consumers in SAV community
dynamics, the role of migratory species and efforts to develop a realistic,
ecosystem simulation model of SAV communities.
The preliminary results of the first years study in these research
areas are contained in the following report. Many interpretations
remain preliminary at this time. We welcome comments and criticisms and
in particular ideas concerning data interpretation.
Questions concerning specific aspects of the various sections
should be addressed to the following:
1. Productivity, Metabolism and Nutrient Cycling; R. L. Wetzel
2. Resident Consumers; R. J. Orth
3. Migratory Consumers; J. V. Merriner
4. Ecosystem Modelling; R. L. Wetzel
The above principal investigators are all members of faculty and staff of
v
the Virginia Institute of Marine Science and School of Marine Science,
College of William and Mary, Gloucester Point, Virginia 23062.
R. L. Wetzel, Ph.D.
Program Manager
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Productivity, Nutrient Cycling and Associated Microbial
Metabolic Activity in Eelgrass Communities in
the Lower Chesapeake Bay
R. L. Wetzel, K. L. Webb and P. A. Penhale
INTRODUCTION
The productivity of eelgrass communities in temperate zone estuaries
along the U.S. East coast is comparatively very high (Billion, 1971;
Thayer, et al., 1975; Penhale, 1977). Primary production in submerged
aquatic vegetation communities is partitioned among several components;
Zostera marina, Ruppia maritima,epiphytes, benthic macro- and micro-
scopic algae, and phytoplankton. Thayer et al., (1975) reports an annual
average production for eelgrass, Zostera marina, in North Carolina of
2 i
350 gC.m ,yr~. Associated macrophytes in this community produce an
7 1
approximately equivalent amount, 300 gC.m .yr (Thayer et al., 1975).
Partitioning of epiphytic and eelgrass production in this community was
investigated by Penhale (1977). Working in a recently established Zostera
community, she determined that approximately 25% of the total standing
stock (epiphytes & Zostera) was epiphytic biomass and concluded that the
attached epiphytes contributed 18% to productivity of the community.
Assuming the same relationships hold for mature communities, production
*
(exclusive of the phytoplankton and benthic microalgae) is on the order
of 600-700 gC.m .yr~l. Although productivity studies per se are lacking
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2
for the Chesapeake Bay, peak live biomass determinations,as a minimum
estimate of annual production in the lower bay,suggest production is
similar to the North Carolina area (Marsh, 1973; Orth, 1977).
The areal distribution of seagrass communities in mid-Atlantic
estuaries is limited and generally only a few percent of total area.
However, production on a unit area basis compares with salt marsh vascular
plant production (Keefe, 1972). In certain estuarine areas, production
by the submerged aquatic communities accounts for a significant fraction
of total estuarine primary production (Thayer et al., 1975;) and can in
large part be explained at the local level by basin morphology (Mann, 1975).
In the lower Chesapeake, seagrass communities occupy approximately
8400 hectares in mesohaline and polyhaline regions of the Bay (Orth,
Moore and fiorden, 1979).
Limitations on productivity in seagrass communities have been
ascribed to several environmental and nutrient related parameters. The
influence of light, temperature and salinity have received the major
research effort (Bachman and Barilotti, 1976; Biebl and McRoy, 1971;
Penhale, 1977; and references cited therein). It is generally accepted
that the local light regime limits the subtidal distribution of Zostera;
light, temperature and probably nutrient (nitrogen) regimes interact to
control specific rates of productivity during the annual cycle, and
geographically,temperature limits the distribution of the species.
Nutrient dynamics and specific aspects of mineral nutrient metabolism '
have received far less attention. For temperate zone seagrass communities
along the U.S. East Coast few data are available to suggest the major
*,
routes for nutrient flux and for one of the major nitrogen production
pathways, N-fixation, the reported data conflict (McRoy, et al., 1973;
Patriguin and Knowles, 1972).
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3
There is increasing evidence suggesting primary production in marine
and estuarine systems is generally nitrogen limited (Postgate, 1971;
Ryther and Dunstan, 1971, Valiela, et al., 1973; Gallagher, 1975;
Pomeroy, 1975; Orth, 1977; and others). If temperate sea grass communities
are limited by nitrogen, they may act as competitors with other estuarine
components for available nutrients. With the data available, we can
neither estimate the magnitude of the various nutrient interactions nor
guess their functional significance within the seagrass community or
the esturaine system as a whole.
Organic matter production, controls affecting the primary and annual
rates of energy fixation and the mechanisms inputing energy-matter and
nutrients to higher order trophic levels and supporting secondary production
are thus not well-known. In this report, we present our studies focused
on the above problems to investigate 1) plant distribution and relative
abundance, 2) substrate-plant relations, 3) total community metabolism
4) SAV component studies and 5) nutrient exchange studies.
Study Site
Selection of the principal study site was decided by consensus of a five
member research team associated with different aspects of seagrass
research. An approximately 260 hectare submerged aquatic vegetation bed
located in southeastern Chesapeake Bay in an area locally known as
Vaucluse Shores was chosen. Geographically the area is situated approximately
37° 25' N. latitude, 76° 51' W. longitude. Criteria for site selection
were;
1. The site has been previously studied and some background da£a
exists,
2. the bed is well established and historically stable,
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3. the area is relatively remote and unperturbed,
4. vegetationally, it contains the two dominant lower bay SAV
species, Zostera marina and Ruppia maritima, and,
5. it is large enough to simultaneously accomodate varied studies
and sampling regimes.
Also, the area was a site for intensive submerged vegetation mapping
(completed July-August, 1978). During this exercise, permanent transects
were established and were used for selection and identification of within-
site sampling stations. Figure 1 illustrates the seagrass bed showing
transects and the distribution of submerged vegetation. Figure 1 also
illustrates the spatial heterogeniety and distribution of the submerged
vegetation and indicates at least five distinct habitat types:
1. Ruppia maritima dominated community
2. Zostera marina dominated community
3. Miked vegetation areas
4. Within-bed bare bottom or sand areas
5. Sand Bar
Sampling sites were selected for each of the areas between transects
B and C and permanently marked (bouyed) to identify stations for routine
studies.
METHODS
Plant Distribution and Relative Abundance; Plant distribution and relative
abundance was determined along transects A, B and C in July, 1979 to
determine areal coverage by species and distribution with water depth.
A line-intersect method was employed using two divers and is discussed
in detail in Orth, Moore and Gordon, 1979. Briefly, the transects marked
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Figure 1: Vaucluse Shores Study Site.
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R = RUPPIA
Z = ZOSTERA
S=SAND
/ = MIXED
= TRANSECTS
CHESAPEAKE
BAY
(D)^.
/ R/Z .A ;
:
'?** \
H -
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in Figure 1 were followed from the sandbar beginning at low tide and
progressing toward the shore. A 100 meter line marked in 10 meter intervals
was employed along the transect line to locate point intersections for
determining species composition and estimating percent cover. At each
10 meter intersection, a 0.5m^ frame was randomly dropped and species
composition determined and percent coverage estimated by a diver. This
procedure was replicated twice at each sampling point. During each
transect study, time and water depth were recorded at each station for
comparison to a continuous relative tide height record kept near-shore
and for calculation of bottom depth relative to mean low water (see Orth,
Moore and Gordon, 1979). These data also provided direct comparisons
with the previous mapping effort (July, 1978) by Orth, Moore and Gordon,
(1979) for identifying any gross differences in distribution between
years.
Substrate-Plant Relation; Routine samples for sediment analyses in the
five habitat types were taken in July and October 1978, April, 1979 and
monthly for the period June through October, 1979. Analyses performed
in relation to community type were dissolved interstitial nutrients
(NH^"1", N03~, N02~ and P0^~^) , adenosine triphosphate (ATP), water content,
percent organic matter (POM), particulate organic carbon and nitrogen
(POC and PON). Sediment samples were taken by hand to a depth of approxima-
tely 30 cm using a 5 cm (diameter) acrylic core tube. The cores were
capped underwater and sealed with tape for transport to the laboratory.
Laboratory processing consisted of decanting the top water layer, filtered,
using glass fiber filters and analysed for dissolved nutrients. The
t,
cores were then extruded, split vertically and cut into 0-2, 2-5, 5-10,
10-15, 15-20 and >20 cm horizontal sections. For each core>processing
consisted of: duplicate 1 cc plugs extracted using boiling 0.1 M sodium
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7
bicarbonate for ATP analysis (Bancroft, Paul and Wiebe, 1974); the inter-
stitial water extracted by centrifugation and glass-fiber filtered as
above and the extracts analyzed either immediately or frozen (-20°C) until
analysis; the remaining sediment fraction was frozen for later water
content/organic matter and POC/PON analyses.
Determination of N03~, N02~, NH^+ and PC>4~3 utilized automated
analysis techniques (EPA 1974). Modifications to these techniques include
concentration of nitrate/nitrite reagents with a corresponding reduction
in sampling rate to reduce volumes of reagent needed for analysis, a
two reagent chemistry for phosphate determination resulting in better
reagent stability and a two reagent chemistry for ammonia (Koroleff,
1970; Solorzano, 1969; Liddicoat, Tibbits and Butler, 1975; Gronuty
and Gleye, 1975).
POC and PON analyses will be performed using a Perkin-Elmer Model
240B Elemental Analyzer.
Water content and organic matter content was determined on freshly
frozen sediments by drying at 60°C to constant weight for water content
and ashing @ 550°C for organic matter determination. All weights were
determined to the nearest 0.01 mg.
Rooting depth was determined during July 1979 in the three major
vegetation areas (i.e., Ruppia, Zostera and mixed areas) by hand coring
using a 33 cm (diameter) acrylic corer. The cores (4 replicates per area)
were taken and sectioned into 0-2, 2-5, 5-10, and 10-15 cm horizontal
sections. Each section was washed free of sediment through a 1.0 mm screen
and roots and rhizomes sorted by hand. The replicate samples were dried
«.
at 60°C to constant weight (48 hours) and percent contribution to total
weight determined for each section.
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8
Total Community Metabolism; Total community metabolism (net community
production) within the various habitats was determined using dome enclosures.
The domes are hemispherical and measure approximately 1 m inside diameter
by 0.5 height. Volume of the domes is ca. 260 liters and enclose a bottom
area of 0.78 m 2. Areas within each SAV habitat type (Zostera, Ruppia,
mixed vegetation, or bare bottom) were randomly selected between transects
B and C for study. The domes are submerged by diver and mixed using a
modified 12 VDC bilge pump in a closed loop. The domes were sampled
through septums at regularly spaced time intervals for dissolved nutrients,
temperature and dissolved oxygen. The dissolved inorganic nutrients
(NH4+, N02~, N03~ and PQ^~^) were determined on filtered 50 ml samples
and analyzed as discussed previously. Dissolved oxygen was continuously
monitored using temperature compensated polarographic (Clark-type)
electrodes and calibrated using both water saturated-air nomographs and
the Winkler technique (Strickland and Parsons, 1972). Experiments in
each area lasted a minimum of 24 hours to bracket a complete diel cycle.
The domes within each habitat and for each study were run in duplicate
always and for some studies, four experimental domes were deployed.
Figure 2 illustrates the general experimental design.
In addition to the parameters measured within the domes, ambient
(outside water) samples were routinely taken for the same analyses.
Light, as photosynthetically active radiation (PAR),W$B determined at the
surface and routinely light profiles taken through the water column using
a LI-COR 185A Quantum Meter.
At the termination of experiments conducted earlier in our program^
2 or 3 randomly placed 0.085 m^ cores were taken from within the dome
enclosed area to determine SAV biomass. Because of the high degree of
variability in these samples, we have since harvested the entire
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Figure 2: Dome Enclosure Experimental Design.
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10
dome enclosed area for biomass determination. Although very time consuming,
we feel this is the only adequate way, at the present, to evaluate replicate
dome variablity.
SAV Component Studies; As discussed previously, the SAV community is
composed of both autotrophic and heterotrophic components in addition to
the dominant vascular plant species. Production and metabolism (respira-
tion) by these components can significantly augment production and meta-
bolism in the seagrass community. Studies of their production and metabolism
have been recently initiated. We report here some preliminary findings
relative to partitioning production and metabolism between components.
To date, component studies have focused on 1) the effects of light
on total community metabolism and specific rates of CO 2 fixation,2)
partitioning 02 and dissolved nutrient exchange between the above-ground
plant community and the sediment substrate, 3) partitioning Q£ and dissolved
nutrient exchange between the plankton, epiphytic, benthic microalgae
and the SAV community.
The effect of light on total community metabolism was studied
intensively during July, 1979 using the dome enclosures. Light was
reduced by 53.3% and 85.7% in adjacent domes using netting and 62
and nutrient exchange followed for 24 hours. The effects of light level
on the specific rates of CC>2 fixation by intact plants has been evaluated
routinely (monthly) since July, 1979 using a 14CC>2 incubation technique
(Penhale, 1977) and light level modified by neutral density screening.
Partitioning 02 and nutrient exchange between the above-ground SAV
community and the sediment substrate was investigated in July, 1979
using dome enclosures. Areas in the Ruppia and Zostera dominated communities
were clipped and all above-ground plant material removed in the area of
dome placement. Adjacent non-clipped areas were also enclosed and
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11
02 and dissolved nutrients determined over a 24 hour period.
Partitioning the major primary producer components has just begun.
Production by the attached epiphytic community is being investigated using
a 4C02 incubation technique (Penhale, 1977). Plankton community meta-
bolism and primary production are being investigated using the standard
light-dark bottle technique and ^C02 incubation technique respectively
(Strickland and Parsons, 1972). Benthic microalgae metabolism is being
studied using 1 liter in situ chamber incubations. Oxygen determinations
from light and dark chambers are determined at routine sampling intervals
on 10 ml fixed samples and titrated using a microliter buret (resolution
=0.1 ul). We present in this report some preliminary data to illustrate
the experimental designs.
Nutrient Exchange Studies; Kinetic studies of dissolved inorganic nutrient
exchange employs the same experimental design as the production/metabolism
studies of plankton and SAV communities. The incubations (glass 300 ml
bottles for plankton and domes for SAV habitats) are run at ambient levels
and spiked (nutrient enriched) to approximately lOx ambient levels. Decay
of the spiked samples is followed over time with the sampling intervals
adjusted to the decay rate and continued sampling until ambient levels
are reached. These studies are conducted simultaneously with the production/
metabolism experiments. Analyses for Ntfy"*", N03~, N02~ and P04~^ are
as previously discussed.
RESULTS
Plant Distribution and Relative Abundance; Plant distribution and percent
cover, as a estimate of relative abundance, was determined along transects
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12
A, B, and C in July, 1979. Figure 3 illustrates the distribution by
species relative to water depth at mean low water and percent cover
along the transects. Ruppia predominated in the shallower depths and
Zostera predominated in the deeper areas (adjacent to the sand bar).
In the shallower subtidal areas along transects B and C an unidentified
algae contributed significantly to percent cover. These data support
the general conclusions reported by Orth, Moore and Gordon (1979) of the
depth dependent distribution of Ruppia - Zostera plant association
but also indicate that the relationship is not a simple function of water
depth alone. Ruppia occurs in the deeper areas generally dominated
by Zostera (Transects A, B and C) and Zostera occurs in some shallower
depths normally occupied only by Ruppia (Transect A).
Substrate - Plant Relationships; The results of the analyses for
rooting depth of the dominant plant species in the three vegetated zones
are presented in Table 1. The data suggest that in the Ruppia community
a greater percentage of the below ground root and rhizome biomass is
located in the top 5 cm and Zostera appears rooted deeper in the substrate.
However, for all vegetated areas, greater than 98% of the root-rhizome
system is located in the upper 10 cm of sediment.
Sediment profiles of dissolved interstitial NH^+ and N03~ and
sediment ATP concentration for various times of year are presented
in Figure 4 and Figure 5, respectively,for the four habitats within the
seagrass bed. Table 2 summarizes the percent of total ATP (to a depth
of 30 cm), contained in the top 5 and 10 cm core fractions. The data
indicate both between habitat variability and suggest strong seasonal
v
changes in all areas. The major distributional changes with depth for
both NH, and ATP in the vegetated areas appears to occur at or near the
maximum depth of rooting (see Table 1).
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13
Figure 3: SAV Distribution and Relative Abundance along Transects A, B,
and C, Vaucluse Shores, July 1979.
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E
o
cc
UJ
o
o
JOO
200
300 400 5OO
600
TOO
MO
601
ioo
TRANSECT C
O
o
KEY
P Z. marine
* ft. moritima
" Algae
300 400
50O
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14
TABLE 1: ROOTING DEPTH ANALYSES
SPECIES
RUPPIA MARITIMA
MIXED BED
ZOSTERA MARINA
DEPTH
TCM)
0-2
2-5
5-10
1015
0-2
2-5
5-10
10-15
02
2-5
5-10
10-15
x + S.D,
% TOTAL WT, % 0-5 CM
67 + 9,7 89, +3,0
22 + 10,0
9 + 3,3
2 + 0,96
55 ±4,6 83 ±11,
28 + 15,2
15 + 10,1
2 + 2
51, + 14, 93, + 3,2
42, + 15,9
6, +2,6
0,7 + 0,58
%0-10 CM
98, +1,0
98 + 0,6
99 + 0,6
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15
Figure 4: Sediment Profiles of NH* and N0~; October 1978 and July 1979.
-------
jil M N-MH4
125 250 379 5OO
N-N03
10 15 20
Z
J I I 1
I I I I
20-20
= OCTOBER, 1978
= J ULY, 1979
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16
Figure 5: Sediment Profiles of ATP by habitat for various times of year.
-------
IL.L u.
O I
o O
" c
tl
< t-
09
viddna
HDiVd QNVS
VH31SOZ
«va QNVS
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17
TABLE 2: PERCENT TOTAL ATP IN 0-5 AND 0-10 VERITAL CORE SECTIONS
78 78 . 79 79
SPECIES
RUPPIA
SAND PATCH
MIXED
ZOSTERA
SAND BAR
DEPTH (CM)
0-5
0-10
0-5
0-10
0-5
0-10
0-5
0-10
0-5
0-10
JULY
61
90
49
79
N,D,
N,D.
55
80
37
65
OCT
32
68
43
70
46
82
54
80
64
83
APRIL
65
86
55
75
48
70
56
81
35
57
JUNE
58
91
37
66
35
55
65
86
23
38
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18
Total Community Metabolism; Tables 3 and 4 summarize the results of the
dome 0- exchange studies. The 24 hour rate estimates net community daily
production or consumption of oxygen and is the simple difference between
starting and ending concentrations. The mean apparent 02 production
rates during the daylight period were determined by averaging all daytime
rates from the domes. The mean apparent 0- consumption (community
respiration) rates were determined by averaging all nightime rates from
the domes.
Net Community Production (NCP) as estimated by the 24 hour difference
was highly variable in both Ruppia and Zostera areas for the periods
reported. In Ruppia, the highest negative values (6/79 and 7/79) for
community consumption over a 24 hour period occurred when low tide
coincided with the period of maximum insolation and may suggest inhibition
of photosynthesis under high light regimes. (PAR = 1134 to 1323
-2 -1
uE-m -sec at bottom). At other times of the year, NCP was generally
positive except during the September, 1979 study. During this period
high tide occurred during the middle of the day and light was reduced
-2 -1
(PAR = 30 - 300 uE-m -sec ) during the period of maximum potential
photosynthetic activity. In the Zostera area, NCP was variable and for
the majority of dome studies (4 of 7) was negative. Net negative values
for 0~ exchange in Zostera were associated with high tides during the
2
day coinciding with periods of maximum insolation (PAR = 350 uE'm -sec-1,
1330 hours at the bottom in September, 1979). However in July 1979, NCP
was highly positive with high tide occurring during the middle of the
day. During this period water turbidity was low, no cloud cover and
t
maximum insolation for the dates were reported (.100% maximum sunshine,
Norfolk Weather Station). Light reaching the bottom during the study
-2 -1
period ranged from 500 to 760 uE'm -sec
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19
(MO/DY/YR)
10/11/78
10/12/78
4/27/79
4/28/79
5/2/79
5/2/79
6/23/79
6/23/79
7/17/79
7/18/79
7/20/79
8/27/79
8/27/79
9/25/79
9/25/79
10/24/79
10/24/79
TIDE
(TIME)
0947 HT
1030 HT
1343 HT
1435 HT
1416 LT
1416 LT
0926 LT
1027 LT
1126 LT
1156 HT
1122 HT
1056 HT
L.
, 2S HR-?N
(MG02-M O
+863,
- 99,6*
«_
+127,
- 78,5
+181,*
...
-1760,
-1089 ,
790,*
+332,
+498,
+996,*
-108,
+266,*
+266,
+448,*
, X DAY9 v
(MG02'MZ'HR -1)
339,
392,*
mm
86,4
108,
165,*
398,
418,
286,
365,*
129,
+335,
+339 , *
+116,
+359,*
304,
335,*
X NIGHT i
' (MG02'M-2.HR -1)
-325,
-246,*
- 84,8
-144,
-122,
-154,*
«_
-491,
-309,
-236,*
-282,
-136,
-123,*
- 66,
-116,*
- 37,2
- 25,2*
= NUTRIENT SPIKE EXPERIMENT
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20
DATE TIDE
(TIME) (
10/16/78
10/17/78
4/30/79 1202 HT
6/25/79 0947 HT
6/26/79 1029 HT
7/26/79 1040 HT
7/27/79- 1117 HT
7/27/79
8/29/79 1321 HT
9/27/79 1250 HT
10/29/79 1535 HT
t.
24 HRo
^MGuo'M /
+239,
-322,
-883,
- 39,8*
+551,*
+770,*
-2025,
-1643,*
-926,
-1368,*
+598,*
+531,*
, X DAY_o n
(MG02-M ^'HR -1)
196,
193,*
276,
212,
335,*
+333,
+307,*
+385,
116,
178,*
252,
219,*
316,
261,*
X NIGHIo i
I Wl^**f \f\ M LJ D ^^ 1
-86,3
-226,*
-177,
-99,6
-26,6*
-86,2
-133,
-133,
-199,
-206,*
-166,
-173,*
-81,*
* _
= NUTRIENT SPIKE EXPERIMENT
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21
Mean hourly 0- production rates during the day and mean hourly
0- consumption during the night were less variable both among and between
vegetated communities. 0- production in both communities peaked in early
summer followed by a late summer depression and again increased in early
fall. 0- consumption in the Zostera community remained relatively constant
during the period April to October, 1979 with values ranging from 100-200
-2 -1
mgO^-m -hr . 0- consumption in the Ruppia community varied over the
-2 -1 -2 -1
range of -50 mgO_-m -hr to a late June maximum of -570 mgO?-m -hr
During this later period of maximum values (June to July) it was observed
that Ruppia was highly colonized by hydrozoans.
Nutrient Exchange Studies; The experimental approach we have used to
produce the kinetic parameters for relationships between nutrient uptake
and rate of utilization have been in part to enrich the dome incubation
chambers to elevated concentrations and follow the concentration decay
with time. Uptake kinetics for inorganic nutrients and other reactive
substrates are most often described by the Michaelis-Menten enzyme-
substrate analogy: i.e., a retangular hyperbola of the rate vs substrate
concentration or by the hyperbola plus a linear diffusion component.
The relationship is described mathematically by the equation:
Vm. Cs3
V - CS3 -k
s
where: Vm = maximum rate
fe] = substrate concentration
k = substrate concentration at which V = 0.5 Vm
s
The equation is modified by:
v-PCJ- \ TU
where d = diffusion constant
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22
to include diffusion. The mathematical description is both useful for
data reduction, understanding the system and for modelling purposes.
The decay curves resemble the solid line of Fig. 6C for the hyperbola
represented in Fig. 6A. The linear portion of the decay curve is
representative of the maximum rate. In some experimental situations the
concentration never decays to zero but comes to equilibrum at some positive
concentration (dotted lines of Fig. 6C & 6D). This is interpreted as
either (a) there exists a threshold concentration below which there is no
uptake or (b) there are components of the system which are producing the
material in question and this is the concentration at which the processes
balance. If there is a diffusion component (as there often is for ammonia)
the decay curve will resemble that of Fig. 6B and there will be no linear
portion of the uptake curve, i.e., the rate cannot be saturated. At
this point in time our data analysis has progressed only to the point of
calculating V , i.e., the saturated rate.
6 max' '
Figures 7A and B illustrate the decay (uptake) of NH, in replicate
experiments in the Ruppia community. Uptake was extremely rapid during
the photoperiod (1200 to 1600 hours) and decayed asympotically, to
ambient levels in less than 24 hours. The data suggest at least a two
component system perhaps related to photoperiod. Figures 7C and D
-3 -
illustrate the decay (uptake) of PO, and N0_ during the same period
in the Zostera area. N0~ uptake followed a pattern similar to NH,
in the Ruppia area but appears slower from inspection of the slopes of
the decay curves. Figures 8A and B illustrates the uptake of NflT and
NO, in a mixed spiking experiment (i.e. both NH, and NO, added to the
domes in combination). The initial decay rates indicate NH, is taken up
faster than N0_, and suggest a preference for NH,. Figure 9 illustrates
the uptake of NH, in a double spiking experiment in the Zostera community
-------
23
Figure 6: Mlchaelis-Menten Uptake vs Substrate Concentrate Curves with
and without a Diffusion Component (A & B) and Typical Decay
Curves for Substrate Utilization (C & D).
-------
UJ
E
>
m
-------
24
T -'
Figure 7: Decay Curves for NH, Uptake in the Ruppia Community and PO ~
and NO" Uptake in the Zostera Community (October 1978) .
-------
8 8 8 S § 8 °
o
ID
UJ
2
o
o
o
UJ
- Q.
I 10
O
z
CO
UJ
z
o
Q
C
5
00
co
o o
Nj O
-oooi
- 0083
- 0093
-00t>3
-0033 9j
O
r 0003 3
UJ
- 0081 2
H
-0091
- OOtH
-0031
-0001
-0080
-OOOZ
-0083
-0093
-00*3 _
H0033 ^
O
- 0003 X
UJ
-0081 £
I-
-O09I
-OOfrl
-0031
- 0001
0080
9
9 o
£-
Od
-------
25
Figure 8: Decay Curves for NH, and NOl Uptake in Ruppia Community (July, 1979)
-------
I
z
10-
9-
8-
7-
6-
5-
4-
3-
2-
I-
SPIKE
Ruppia maritime
JULY 1979
o
z
0.5
(1100) (1130)
i.O 1.5 2.0 25 3.0 5.5 9.5 13.5 17.5
(1200) (1230) (1300) (1330) (1400) (1630) (2030) (0145) (0545)
T(hr)
-------
26
Figure 9: Decay Curves for Double NH, Spike in Zostera Community (July, 1979)
-------
~
m
r
in
~ o
in
in
CM
-SI9I
-0191
-S09I
-0091
-QQSI
-OSSI
-SfrQI
-SCSI
-S3SI
-OZSI
-SIS)
-01 SI
-SOS I
-OOSI
-SSfrl
-OCfrl
-SEfrl
- O
-------
27
during the day. The calculated uptake rates from the two spikes are not
significantly different and indicate the uptake system is saturated and
operating at Vmax.
Table 5 summarizes the summer experiments from the Ruppia and
Zostera areas in terms of nitrogen uptake rates assuming saturated systems
kinetics. These few data suggest that Vm is similar for NH, and NO,
and the rates appear higher in the Zostera community. During this sampling
period, the ambient or control domes were variable in relation to NH, and
N0_ behavior and indicated no net uptake or release of either nitrogen
species except for the August, 1979 study in Zostera. In the ambient dome,
-2 -1 +
a net release of +110 ug-at-m -hr and NH.-N was realized.
These types of experiments have been done for each study period
indicated on the table's summarizing the 0~ exchange data. However, the
data on nitrogen exchange has not been reduced to the point where kinetic
parameters can be calculated. The above data is presented to illustrate
the potential information contained in the experimental designs and data
analysis technique employed with such information.
SAV Component Studies; Studies were initiated during 1978 and 1979 to
partition total community metabolism and nutrient exchange into water
column processes, above-ground plant community processes, sediment
processes and to begin studies on the relations between light, nutrient
exchange and community metabolism. These efforts are just beginning and
the experiments reported herein were designed primarily to illustrate our
approach and as screening experiments for designing and allocating future
research effort (1980).
V
Photosynthesis and respiration of the plankton community was
investigated using light and dark bottle incubations (300 ml BOD bottles)
and the CO- incubation technique using various light regimes. Table 6
------- OCR error (C:\Conversion\JobRoot\00000ANE\tiff\2000W6CW.tif): Unspecified error
-------
29
TABLE 6: SUMMARY OF LIGHT-DARK BOTTLE EXPERIMENTS,
JULY, 1978
OCTOBER, 1978
X MG(
GP1
39,4
(6,0)
M J LJD *
s ' I I nr\
NP2
19,9
(5,2)
63,8
(±1,9)
R5
23,4
(4,4)
APRIL, 1979 63,6 46,4 21,6
(40,8) (39,8) (9,0)
1, GROSS PRODUCTION
2, NET PRODUCTION
3, RESPIRATION
-------
30
TABLE 7: PHYTOPLANKTON PHOTOSYNTHESIS
DATE PMAX
(MG C M~* HR~1) (u E
JULY, 1978 180, 75,
OCT., 1978 93, WO,
JULY, 1979 160, 83,
-------
31
TABLE 8A:
PHOTOSYNTHESIS:
PERCENT OF TOTAL
IN 15 UM & LESS
FRACTION,
CHLOROPHYLL A:
PERCENT OF TOTAL
IN 15 UM & LESS
FRACTION,
TIME
(H)
1000
1400
1800
2200
0600
MGC-M ,HR~-"-
67,
82,
71,
75,
83,
DRIFT STUDY
43,
103,
99,
80,
102,
BAG ENCLOSURE
43,
64,
86,
78,
78,
TABLE
(±S,D,)
TIME
1000
1400
1800
2200
0600
DRIFT
T
10
2
2
4
1
, +
,81
,5
,11
,99
,66
,064
,36
,179
,192
STUDY
F
4,28 +
2,88
2,48
3,29
2,03
BAG STUDY
,128
,192
,256
,128
,256
10,
5,
4.
4,
4,
T
+
61
37
5
54
,66
,279
,144
,38
,111
4,
3,
3,
3,
3,
F
28 +
58
73
55
55
,128
,064
,064
,111
,000
-------
32
summarizes light-dark bottle studies for various seasons as measures of
community metabolism. Characterization of phytoplankton photosynthesis
for various seasons and light saturated rates are summarized in Table 7.
The light intensity photosynthesis relationship can be described by
a hyperbola where P is the maximum rate of photosynthesis and I, is
max k
the light intensity at which the initial slope would intersect the P
max
value if it were extended. These values will be used to model phytoplankton
production as well as indicate the physiological state of the phytoplankton.
The phytoplankton responsible for most of the primary production in
Chesapeake Bay as well as other estuaries are usually the very small forms,
i.e. less than 15 micrometers in diameter. During July 1979, we monitored
both the total (T) chlorophyll ji and the chlorophyll a_ in the size fraction
less than 15 urn (F). Samples were taken both from a boat drifting with
the water mass across the grass bed and from a large bag enclosure filled
at the start of the "drift study" and kept suspended at the surface of
the water column. Photosynthesis was measured on water samples taken
from the middle of the grass bed. Table 8A and B summarize the results
of these analyses. Table 8A summarizes chlorophyll ji and photosynthesis
distribution by phytoplankton size class. Table 8B summarizes chlorophyll
a^ concentrations for the total (T) and less than 15 urn size fractions(F).
Table 9 summarizes the values for P and I, of the light photosynthesis
max K.
relationship for the total (T) and less than 15 urn size fraction (F)
during the study. All data in the above studies is for July 28 to 29,
1979.
The effect of light reduction (shading) on total community metabolism
*,
in the Zostera area was studied in July, 1979. Shades were constructed
of seine netting to reduce ambient light in the dome enclosures by
approximately 50 and 85 percent. Table 10 summarizes the data for 0_
-------
TABLE 9: 33
RELATIONSHIP BETWEEN PMAX AND IK FOR TOTAL (T) AND LESS THAN
15 JJM FRACTION (F); vIllLY, 1979,
PMAX
(MG C M JH ±
TIME
1030
1414
1815
2400
0600
T
157,
87,8
50,2
63,6
56,9
F
105
72,1
35,6
47,4
47,3
F
(MG C
T
15,8
31,3
20,0
15,5
28,8
DMAX 11 S 1
MG CHLA H ) (U E M '-S -1)
F
24,6
25,1
14,4
14,4
23,3
T F
83,8 94,2
112, 80,5
79,5 84,2
78,5 78,5
-------
34
TABLE 10: EFFECTS OF SHADING ON ZOSTERA COMMUNITY METABOLISM,
TIME 0
PRE-SHADE 554,
SHADED
(LATE AFTERNOON) -
SHADED
(MORNING)
POST- SHADE
53-32
498,
50,
153,
299,
85,7%
774,
7,
86,
531,
-------
35
exchange. The reduction in apparent 0_ production rates, as a percentage
of pre-shading rates, was 69% and 89% in the 53% and 85% treatments
respectively. Ambient light reaching the bottom was high (200-750
-2 -1
uE'tn -sec ) during the photoperiod and suggests that if the community
-2 -1
does light saturate, saturation occurs above 300 uE-m -sec . Specific
14
experiments using C tracer techniques have been designed to address
this relationship but the data have not been analyzed due to radioisotope
counting equipment failure. However, experiments have been conducted
since July, 1979. Nightly respiration values (rates) were not significantly
different between shaded and non-shaded domes and are not included in the
summary table.
Partitioning 0~ exchange and nitrogen exchange between the above-
ground community and the substrate was investigated in August, 1979.
Areas in both Ruppia and Zostera communities were clipped (i.e. all above-
ground plant material harvested) to allow placement of the domes. Table
HA summarized 0~ exchange in clipped and non-clipped areas. Table 11B
summarizes the nitrogen exchange information for the Ruppia area. For
0« exchange, mean 0 production was reducedc during the day by 50% and
the 24 hour estimate of net community production was reduced by 89% in
the Ruppia dominated community. Under the conditions prevailing during
the Zostera study, mean 0_ production during the day was reduced by 78%
and the net negative community production rate for the 24 hour reduced
by 93%. The maximum velocity (Vm) for uptake of both NH,- and N0~ was
reduced by 55% and 53% respectively in the Ruppia community by eliminating
the above-ground material. Assuming that the plankton community is
t
contributing little to the calculated rates, it would appear that in the
shallower Ruppia area, photosynthesis by autotrophs associated with the
sediments is high and in both areas, the major component influencing
respiration is sediment related.
-------
36
TABLE 11; ABOVE-GROUND AND SUBSTRATE PARTITIONING EXPERIMENT:
A, 0 EXCHANGE
DATE
8-28-79
8-29-79
AREA
RUPPIA*
RUPPIA**
ZOSTERA*
ZOSTERA**
A24 HR,
+854,
+ 93,5
- 20,4
-308,0
°2 ? .
X MGQ2'M~ -HR"1
(DAY)
+195,
+ 98,
+ 67,5
+ 15,7
B, NITROGEN EXCHANGE
9 1
DATE AREA UG-ATN-M -HR
NHj N03
8-28-79RUPPIA*61CK475?
RUPPIA** 250, 224,
*= NON-CLIPPED
**=CLIPPED
-------
37
DISCUSSION
Plant distribution and relative abundance along transects A, B,
and C at the Vaucluse study site closely follows the results reported
by Orth, Moore and Gordon (1979). This distribution relative to water
depth appears characteristic of the lower Bay mesohaline and polyhaline
environments. The lower limit for Zostera marina is probably controlled
by available light. However, there is some indication in the distribution
data (Figure 3) that the interaction between Ruppia and Zostera is not
simply explained by water depth. For both species, occurrance has been
recorded at depths outside their typical ranges. At Vaucluse, in the area
adjacent to the sand bar occupied predominately by monospecific stands of
Zostera marina, Ruppia was observed inhabiting the fringes of what appeared
to be recently established bare sand areas. In addition to light, it
appears that the relative distribution and abundance of Ruppia-Zostera
seagrass communities is influences by other factors; current is suggested
as one possible contributing factor to the observed patterns.
Plant-substrate relations in terms of nutrient relations are difficult
to interpret due to the limited amount of data reduced at this time.
The depth of the rooting zone in the vegetated areas appears to correlate
with the distribution of ATP and NH,; the major inflections occurring
at 5-10 cm deep. In the summer (July), NH, pore water concentrations are
high in the deeper sediments (>10 cm) and N0_ low. In the fall (October)
during the second growth period, NH, is generally reduced throughout the
sediment with depth and N0_ increased. ATP concentrations, as an estimator
of heterotrophic biomass, is highest in the warmer months and corresponds
k
to the periods of highest community respiration measurements.
-------
38
Nixon and Oviatt (1972) repotted apparent 0« production and respiration
-2 -1 -2 -1
rates of 2.9 gO^-m -day and 3.6 gO-'m -day respectively for a pond
_2
eel grass community and 0,, production values of 3.6 gO~;m 'day for a
riverine eelgrass community. These values are for the midsummer. Assuming
the mean rates presented in Table 4 for June, July and August represent
the mean hourly rate over the photoperiod and the respiration values are
typical of the hourly rate over the diel cycle, mean hourly rate of
-2 -1
respiration for the three month period is -143 mgO -m -hrr . This
compares favorably with the value reported for the pond eelgrass community.
-2 -1
For the same period, the mean hourly production rate equals +267 mgO -m -hr
The mean daylight period at this latitude during the period June-August
is 14.1 hours. If the photoperiod during which this mean production rate
is realized is 80% of the daylight period, or approximately 11.5 hours,
mean daily CL producting during the period for the Zostera area would
-2 -1
equal +3.07 gO -m 'day . This compares favorably with the results of
Nixon and Oviatt (1972) and suggests the community is net negative with
respect to daily production. Mean daylight 0« production and night
respiration values in the Ruppia community compare with the Zostera area
(Table 3).
The data from the dome 0_ exchange studies suggest two potentially
valuable lines of research with respect to factors controlling community
metabolism, productivity and nutrient cycling. The effect of light on
distribution and production has been alluded to previously. It appears
from the data that available light governs the specific rate of net
community production or consumption of oxygen and that the community,
\
at least the Zostera area, is not operating in a light-saturated environment.
-------
39
Small changes in available light due to either weather conditions or
possibly turbidity levels have pronounced effects. The results of one study
in the Ruppia dominated community also suggest that high light may inhibit
apparent production by this species. The reasons for this are not clear
but do suggest that available light is singularily a primary control in
both communities. Tables 3 and 4 also summarize, in terms of mean hourly
rates, 0_ production and respiration and the effect of nutrient (NH, and
N0_) additions to plant communities. In the Ruppia area, the majority
of spike experiments resulted in increased apparent CL production during
the day. The percent increase over ambient rates ranged from 1% to
210% with maximum increases occurring in early summar and fall periods.
In the Zostera community the effect was not as pronounced. Increases
were observed in the summer months (June through August) and averaged
30% increase in apparent hourly CL production rates during the day. At
other times of the year either no effect was observed or the rate slightly
reduced (Table 4). Whether these increases can be attributed to the
vascular plants per se is not known but the community as a whole responded
to the increased nitrogen supply. Based on this information more refined
experiments are planned for light, nutrient and productivity studies for
the following year.
Partitioning the processes of production, respiration and nutrient
exchange into components of the SAV community has just began and the data are
preliminary. It appears from the light-dark bottle studies that plankton
community production compared to the SAV communities is small; i.e. on
the order of 10% net daytime community production. The clipping experiments
%
(Table 11) indicate that in certain areas, benthic algal production may be
significant and the major respiratory demand of the community is associated
with the sediments. Our future studies will evaluate these processes in
more detail with regard to energy flow and nutrient exchange.
-------
40
These studies and the data gathered from the various experiments
will provide the modelling effort the necessary information to realistically
simulate energy flow and the effects of light and nutrients in the plant
dominated communities. Unfortunately for this report much of the data
has not yet been reduced and interpreted for inclusion. The data presented
however is representative of our overall effort and indicative of our
findings to date.
-------
41
Literature Cited
Blaclaiian, T. W. and D. C. Barilotti. 1976. Irradiance Reduction: effects
on standing crops of the eelgrass Zostera marina in a coastal lagoon.
Mar. Biol. 34:33-40.
Bancroft, K., E. A. Paul and W. J. Wiebe. 1976. The extraction and
measurement of adenosine triphosphate from marine sediments. Limnol.
Oceanogr. 21:473-480.
Biebl, R. and C. P. McRoy. 1971. Plasmatic resistance and rate of
respiration and photosynthesis of Zostera marina at different salinities
and temperatures. Mar. Biol. 8(l):48-56.
Billion, C. R. 1971. A comparative study of the primary productivity of
estuarine phytoplankton and macrobenthic plants. Ph.D. Dissertation,
Univ. North Carolina, Chapel Hill.
Gallagher, J. L. 1975. Effect of an ammonium nitrate pulse on the growth
and elemental composition of natural stands of Spartina alterniflora
and Juncus roemerianus in a Georgia salt marsh. Amer. J. Bot. 62:644-648.
Gravitz, N. and L. Gleye. 1975. A photochemical side reaction that interferes
with the phenolhypochlopite assay for ammonia. Limnol. Oceanogr.
20:1015-1017.
Keefe, C. W. 1972. Marsh production: a summary of the literature. Contri.
Mar. Sci. 16:163-181.
Koroleff, F. 1970. Direct determination of ammonia in natural waters as
indophenol blue. pp. 19-22. IN; Information of Techniques and
Methods for Seawater Analysis. ICES, Service Hydrographique, Interlab
Report 3.
Liddicoat, M. I., S. Tibbitts and E. I. Bulter. 1975. The determination of
ammonia in seawater. Limnol. Oceanogr. 20:131-132.
Mann, K. H. 1975. Relationship between morphometry and biological functioning
in three coastal inlets of Nova Scota. pp. 634-644. IN; Cronin, L. E.
(ed.). Estuarine Research, Vol. I., Academic Press, N.Y. 738 pp.
Marsh, G. A. 1973. The Zostera epifaunal community in the York River,
Virginia. Chesapeake Sci., 14:87-97.
McRoy, C. P., J. J. Goering and B. Chaney. 1973. Nitrogen fixation associated
with sea grasses. Limnol. Oceanogr. 18:998-1002.
Nixon, S. W. and C. A. Oviatt. 1972. Preliminary measurements of mid-
summer metabolism in beds of eelgrass, Zostera marina. Ecology 53:150-153.
\
Orth, R. J. 1977. Effect of nutrient enrichment on growth of the eelgrass
Zostera marina in the Chesapeake Bay, Virginia, U.S.A. Mar. Biol.
44:187-194.
-------
42
Orth, R. J., K. A. Moore and H. Gordon. 1979. Distribution and Abundance
of Submerged Aquatic Vegetation in the Lower Chesapeake Bay. Draft
Report to EPA; Contract No. R805951010.
Patriquin, D. and R. Knowles. 1972. Nitrogen fixation in the rhizosphere
of marine angiosperms. Mar. Biol. 16:49-58.
Penhale, Polly A. 1977. Macrophyte-epiphyte biomass and productivity in
an eelgrass (Zostera marina L.) community. J. exp. mar. Biol. Ecol.,
26:211-224.
Pomeroy, L. R. 1975. Mineral cycling in marine ecosystems, pp. 209-223.
IN; F. G. Howell, J. B. Gentry and M. H. Smith (eds.). Mineral
Cycling in Southeastern Ecosystems, NTIS, CONF-740513, Springfield,
Va.
Postgate, J. R. 1971. Relevant aspects of the physiological chemistry
of nitrogen fixation, pp. 287-307. IN; D. E. Hughes and A. H. Rose
(eds.)., Microbes and Biology Productivity. Symp. Soc. Gen. Microbial.,
Vol. 21, Cambridge University Press, London.
\
Ryther, J. H. and W. M. Dunstan. 1971. Nitrogen, phosphorous and eutro-
phication in the coastal marine environment. Science, 171:1008-1012.
Solorzano, L. 1969. Determination of ammonia in natural waters by the
phenolhypochlorite method. Limnol. Oceanogr. 14:799-801.
Strickland, J. D. H. and T. R. Parsons. 1972. A practical Handbook of
Seawater Analysis. 2nd ed. Bull. Fish. Res. Bd. Canada 167.
Thayer, G. W., S. M. Adams, and M. W. LaCroix. 1975. Structural and
functional aspects of a recently established Zostera marina community.
pp. 518-540. IN; Cronin, L. E. (ed.). Estuarine Research, Vol. I.,
Academic Press, N.Y. 738 pp.
Valiela, I., J. M. Teal, and W. Sass. 1973. Nutrient retention in salt
marsh plots experimentally fertilized with sewage sludge. Estuarine
Coastal Mar. Sci. 1:261-269.
-------
INTERACTIONS INVOLVING RESIDENT CONSUMERS
R. J. Orth and D. F. Boesch
Introduction
One of the most notable features of SAV habitats is the
characteristically high density of animals residing in the grass bed.
This large standing stock of animals is thought to be fundamental to
the resource value of SAV beds. The grass bed provides substrate,
protection and food resources which allow maintenance of high faunal
densities. The protection provided by the bed and the high prey
density serve migratory utilizers of the SAV habitat, i.e. crabs,
fishes and waterfowl.
The epifauna and infauna represent a diverse and complex
assemblage which includes micro- and macroalgae, protozoans,
polychaetes, oligochaetes, bivalves, decapods and barnacles.
Many of these groups exhibit distinct seasonal pulses of abundance
depending on their individual spawning periods (Stevenson and Confer,
1978).
The biotic community within grass beds is quite distinct from
the communities in adjacent unvegetated areas. Because of the lack of
suitable substratum, there is usually very little epifauna in bare
sand or mud areas. These animals are primarily using the blades as a
substratum or in the case of herbivorous gastropods, grazing on the
microalgae that grow on the blades.
The fact that the epifauna may not be totally dependent on the
V
presence of grass but rather any substrate, biotic or abiotic, for its
survival, does not take away from the importance of the presence of the
-------
45
grass itself. The grass is a renewable resource, unlike many other
substrata, and provides a suitable substrate for growth every year.
In addition, the grasses serve other functional roles that are equally
important (e.g. erosion buffer, detrital source, nutrient pump), which
could not be achieved with an abiotic substance.
The infaunal community is also quite distinct from that in
adjacent unvegetated areas. There is a tremendous increase in numbers
of species and individuals in grass areas and this may be related to
increased sediment stability and/or microhabitat complexity of food
supply (Orth, 1977; Thayer, Adams and La Croix, 1975). Orth (1977),
working with infauna of Chesapeake Bay eelgrass, found infauna to
increase in density and diversity from the edge of an eelgrass bed
to the center of the bed and also with increasing size of the bed.
He related this increase to the sediment stabilizing function of
eelgrass and showed that decreasing the stability of sediments
experimentally (removing blades of grass by clipping, simulating wave
action) and naturally [cownose ray activity (Orth 1975)] decreased
the density and diversity of the infauna.
The motile community is also diverse and quite distinct from
surrounding unvegetated areas (Orth and Heck, unpublished; Kikuchi,
1974). Hardwick (1973) found that on the West Coast, the herring,
Clupea harengus pallasi, used eelgrass leaves to lay most of their
eggs on.Though Clupea spp. on the east coast do not use grass beds like
the west coast species, the toadfish Opsanus tau uses the rhizomes as
attachment sites for its eggs. Juveniles and adults of many species
may utilize eelgrass for protection. The blue crab, Callinectes sapidus,
-------
46
is found in greater abundance in eelgrass beds both as juveniles and
adults (Lippson, 1970, Orth and Heck, unpublished). Changes in eelgrass
abundance are thought to be a factor in variations in the commercial
catch (W. A. Van Engel, personal communication). Many species use SAV
habitats primarily because of the abundance of food.
One of the more complete studies of fish communities of eelgrass
was done by Adams (1976a,b,c) in North Carolina. He found the highest
fish biomass when temperature and eelgrass biomass were maximal.
Further, food produced within the grass bed could have accounted for
approximately 56% by weight of the diet of the fish community. The
high fish production was due to juveniles which had higher growth
efficiences than older fishes. They accounted for 79-84% of the
total annual fish production.
By simply looking at the structural complexity of SAV habitats
one does not get an appreciation for the flow of energy needed to
support the complex trophic structure. From analysis of feeding habits
of the higher level consumers (fish and crabs) it is obvious that
benthic invertebrates play a major role in the flux of energy through
the seagrass system. The benthos represents the major link between
primary production, detritus, and higher trophic levels.
The amount of energy or biomass produced within the system can
only be estimated by a detailed look at the secondary production of the
individual species inhabiting the grass bed. Secondary production
estimates will provide the basis for determining the amount of energy
available and the rate at which it is transferred to the higher *
consumers.
-------
47
Although few marine organisms consume SAV directly, the SAV
resource is recognized as a preferred food for many waterfowl species
(Bent 1925; Cottam 1939; Cottam and Munro 1954). The availability of
SAV fluctuates widely and is currently declining in the Chesapeake
Bay (Stevenson & Confer 1978, Bayley et al. 1978). The impact of
such a decline on waterfowl depends on the degree of dietary
specificity. While the abundance of SAV is presently inadequate
to totally support wintering waterfowl populations, it may be very
important early in the season. The degree to which waterfowl use
this resource and possibly affect the ecology of SAV is poorly
understood. Birds may utilize the limited resource for conditioning
and building of fat reserves to survive winter stress, when feeding
is more difficult.
The general importance of seagrass beds in the marine and
estuarine environment has been well documented. Although much work
has been done on the structural components of eelgrass beds in the
Chesapeake Bay, little information is available on the functional
ecology of these beds.
Our efforts from July 1978 to the present have been directed
at determining the relative importance of SAV beds and understanding
the trophic role of resident consumers in such systems by: a.)
determining the bases of secondary production; b.) quantifying secondary
production of important consumers; c.) determining which resident
consumers are trophically important to migratory consumers and d.)
determining the degree to which migratory consumers control populations
of resident consumers.
-------
48
Methods and Materials
A. Habitat Differences;
Routine sampling was scheduledto coincide with major biological
events in the grassbed and adjacent areas. These events included the
arrival of major predators in the system (early spring), the partial
defoliation of Zostera (mid-summer), and the major larval settling
periods (spring and fall). Such timing,rather than quarterly sampling,
would yield the best data on the dynamics of the grass bed and adjacent
habitats.
Three habitats were sampled five times (July and October, 1978
and April, June and August, 1979) to determine quantitative and
qualitative differences in their associated fauna. The habitats
included an offshore sandbar system (outside sand), sandy patches
within the grass bed (inside sand) and the grass bed proper (Fig. 1).
Initially, 10 stations were established in each habitat. Analyses of
data from the initial sampling indicated that 5 rather than 10 stations
adequately represented the infauna in the two sand habitats. One
9 9
sediment (3.8 cnr) and three macroinfaunal (0.007 nr each) cores were
taken at each station. Prior to taking infaunal and sediment cores
in the grass bed an epifaunal sample was taken at each station by
clipping and collecting grass from the area to be sampled. Coring
was then conducted within the clipped area.
Vertical distribution of infauna was examined in July 1978.
2
A 35 cm long plexiglass core 9.4 cm in diameter (0.007 m ) was used to
collect infaunal samples. One such core sample was taken at each
station. The top 10 cm of each sample was sectioned vertically into 2
-------
49
cm intervals and the remaining material was divided into 5 cm intervals.
Based on these data, it was determined that a sample depth of 15 cm
adequately collected the infauna.
Before sieving and preservation samples were held for at least
30 min. in labelled plastic bags containing isotonic MgCl2 as a relaxant.
This kept many of the smaller polychaetes and oligochaetes from crawling
through the sieve. All infaunal samples were washed through 0.5 mm mesh
sieves and the retained material was preserved in 10% buffered seawater
formalin. A vital stain (Rose Bengal) was added to facilitate laboratory
sorting.
Epifaunal samples were collected by clipping plants to within
2-3 cm of the sediment surface and easing the blades into a collecting
bags with a 0.5 mm mesh bottom (Marsh 1973). Samples were kept in
water and processed live by stripping all epifauna from the blades
and preserving them in 10% buffered seawater formalin containing the
vital stain Rose Bengal. The remaining plant material was sorted
according to species (Ruppia, Zostera and algae), oven-dried at 80°C
for at least 48 h and then weighed to the nearest 0.1 g.
B. Predator Exclusion Experiments;
Exclosures consisting of large circular topless pens 5 m in
diameter (20 m^) and smaller, square cages (0.25 m2) were used in the
manipulative predator exclusion experiments. One pen was constructed
in a mixed Ruppia-Zostera bed and another in an adjacent inshore sandy
area. The pens were made of 4.3 m long salt treated wooden pilings
placed 1.5 m into the bottom. Initially, thick-wall galvanized pipes
(240 cm x 2 cm) were placed between the equally spaced wooden pilings
-------
50
to provide shape ( Fig. 1). The pipes inadequately supported the
weight of the netting during storms and were later replaced with 10 cm
x 10 cm x 360 cm wooden posts. Pens were encircled by a piece of
black plastic 0.63 cm mesh netting with a uv retardant (Conwed Corp.
Plastic Netting #OV3010). The netting,which was 324 cm wide, was
attached to the posts at a height of 240 cm above the bottom. Excess
netting was stapled along the bottom with 18 cm long wire staples to
form an 84 cm wide skirt which extended outward from each pen. The
skirt prevented predators from burrowing into the pen. An entrance
into each pen was constructed by sewing a 5 cm wide x 324 cm long
strip of VELCRO to one end of the netting with the opposing piece
attached to a piling.
Smaller square cages measuring 50 cm on a side and 50 cm high
were constructed of reinforcing rod frames covered with the same
plastic netting as used on the pens. Each cage had 30 cm long legs
which were pushed into the bottom, anchoring the cage. A top attached
with VELCRO strips on three sides allowed easy access into each cage.
Panels simulating only the sides of cages were similarly constructed.
Triads of experimental treatments were randomly arranged in
triplicate both within and outside of the pens in each of the two
habitats. A triad consisted of three experimental treatments: a
complete cage enclosing 0.25 rar of bottom area, and open cage with no
top and parallel sides of 0.25 nr, and an uncaged control area ( Fig.
1). One of the three triads per experimental condition (sand; sand
plus pen; grass, grass plus pen) was designed to be destructively
sampled after an appropriate time interval.
-------
51
Fig. 1. Design of predator exclusion experiments showing the
construction of the large pen and the placement of
experimental triads. Closed circles around the perimeter
of the pen indicate the placement of pilings and open
circles show the position of galvanized pipes which
were later replaced by wooden poles.
-------
52
Sampling for predator exclusion work was scheduled to take
spring and fall larval sets into account. Four sample times were
designated for 1979: TQ - April; T^ - June; T2 - September; T3 -
November. Infauna in both the unvegetated habitat and the grass bed,
and epifauna in the grass bed were collected and preserved using the
same methods described for routine sampling. Ten core samples for
infauna and one grass clipping (vegetated area only) were taken when
each experimental area and treatment were sampled.
Pen and cage effects were examined by placing larval and
sediment traps inside additional cages (sediment traps in sand area
only) to assess variations in larval recruitment and sedimentation
rates.
Cages and pens were cleared of fouling organisms when necessary.
Crab pots and minnow traps were placed within pens to remove predators
which entered as larvae on juveniles. Several blue crabs were also
removed by spearing.
Two days prior to the first sampling period of 12 June, pens
were breached during a severe storm. After sampling, pens were rebuilt
and a backing of heavier larger mesh (13 mm) netting (Conwed Corp.
Plastic Netting //OV1580) was added to the smaller mesh. After the
first sampling period but prior to reconstruction of the pen, blue
crabs had burrowed into all sand cages. Because of this disturbance
cages were removed and replaced by new ones positioned over bottom
which had been uncaged. In addition, a 24 cm wide skirt was placed
around each sand area cage.
-------
53
C. Secondary Production;
Eight consecutive monthly samples were taken for secondary
production using a suction dredge (Fig.2 ). Quantitative samples
were collected from within a weighted plexiglass cylinder with a
diameter of 28.6 cm (0.065 m2) and a height of 65 cm. The cylinder
was carefully placed over the grass blades and the sample was taken
from within by filtering water through a clear plastic bag with a
removable 0.5 mm mesh sieve bottom. Samples of larger,more motile, or
widely spaced species were collected from within a weighted fiberglass
r\
cylinder 110 cm in daimeter (0.95 nr1) and 30 cm high equipped with a
0.5 mm mesh screened top (Fig. 2 ). All samples from the larger
fiberglass frame were filtered through a 1 mm x 1.5 mm mesh bag.
The sampling frame was dropped from a boat over dense vegetation.
Only drops over 100% vegetation cover were sampled. The majority
of samples were taken from mixed Zostera-Ruppia areas where abundances
of vagile epifauna appeared to be the greatest. Attached epifauna,
Crepidula plana, was sampled by clipping the grass from within a
2
0.1 m ring as close to the sediment surface as possible. Grass
blades were then cleaned of all attached epifauna and saved for
future processing. All samples for production estimates were
preserved in 10% buffered formalin. Samples were sorted in the
laboratory and up to 200 complete individuals for each species were
measured, dried and weighed. Based on their trophic importance to
higher level consumers 9 species were selected for production estimates:
-------
54
Fig. 2 . Schematic diagram of suction dredge sampler. A
venturi effect in the suction head draws the sample
from within the sampling frame through the collecting
bag.
-------
55
Decapods Amphipods Isopods
Callinectes sapidus Gammarus mucronatus Erichsonella attenuata
Crangon septemsplnosa Microprotopus raneyi Edotea triloba
Palaemonetes vulgaris Caprella penantis Mysid
Neornysis americana
Presently 4 months of data have been worked up. This report contains
information on growth rate and preliminary details of life histories.
Final estimates of production must wait until all the data are available.
Species need to be separated into cohorts or recruit groups if possible
for use of the instantaneous growth rate or removal summation methods
(Water and Crawford 1973), or combined into average cohorts for
production estimation by the Hynes method (Hamilton 1969).
D. Tissue Samples;
Tissue samples for the Carbon-Hydrogen-Nitrogen ratio, C /C
13
carbon ratio (6 C) and calorimetry were collected throughout the summer
of 1978. Plant material was carefully checked for epiphytes or epifauna
which were removed by scraping or brushing prior to drying. Benthos and
fish had their guts removed or were held in screened containers in
aquaria for 24 hr. to permit the voiding of gut contents. Specimens
of resident consumers and predators were grouped by size. A special
effort was made to examine changes in tissue chemistry with growth,
especially with regard to 6^-^C values. Shelled animals were treated
with 10% HCl prior to analysis to remove shell fragments. All tissues
collected were then dried, ground to a fine powder and distributed to
x
subproject principal investigators or consultants for further analyses.
-------
56
E. Stomach Analyses (Callinectes sapidus):
Eighty-three blue crab stomachs were analyzed in 1978.
Individuals were collected with a 4.87 m (16 ft) otter trawl with
19 mm (3/4 in) wings and a 6.3 mm (1/4 in) cod end liner. The trawl
was pulled for a period of 2 min. at a speed of 2-3 knots. Crabs
collected were subsampled and those selected were immediately weighed,
measured, sexed, and the molt stage noted. Stomachs were removed
in the field and preserved in 10% buffered seawater formalin with the
vital stain, Rose Bengal. Each stomach was carefully dissected in
the laboratory and the contents enumerated and identified when possible.
F. Waterfowl Interactions:
A preliminary field effort was undertaken in 1978-1979,
consisting of 34 censuses and feeding observations between 16 December
and 22 March. Birds were censused during daylight hours at approximately
3-hourly intervals on 6 days. The remaining censuses were limited by
poor weather or other activities at the site. Waterfowl were counted,
located by transect interval, and behavior was recorded as feeding or
non-feeding.
Results and Discussion
A. Habitat Differences:
Cumulative species curves for vertically sectioned cores from
each habitat (Fig. 3 ) flattened at a depth of approximately 15 cm.
Most species in each habitat were found in the top 15 cm of sediment
%
but the composition and numbers of individuals of the dominant taxa
differed from one area to the next (Table 1 ). A greater number of
-------
57
Fig. 3. Cumulative species curves of vertically sectioned cores
from three habitats.
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-------
59
individuals per section were found in the grass bed below the top 2 cm
than in the other two habitats (Table 3 ). It is possible the grass
bed afforded some degree of protection from predators or the root and
rhizome mat provided a food source supporting a greater number of
infauna than unvegetated habitats.
2
Generally twice as many infaunal species per core (0.007 m )
were associated with the grass bed than were found in the adjacent sandy
habitats ( Fig. 4 ). An average of 21 infaunal species per core were
found in SAV beds 'where only 9 were associated with sand patches within
the grass bed:and 7 were found in the offshore sand bar area.
With the exception of the July (1978) sample date, there were
also a greater number of individuals per nr in the grass bed than in
either of the sand areas due in part to the relative stability of each
habitat ( Fig.5 ). The reversal in this trend during July was due
2
to a large set of the bivalve, Gemma gemma (32,648/m ). By the next
sample date they had greatly declined in abundance. In addition to
sediment stability, the refuge factor and structural complexity of
seagrasses may be a cause for the greater abundance of grass bed fauna.
In October, 1978 and June, 1979 Zostera had a greater number
of epifaunal individuals per gram of grass than either mixed Zostera-
Ruppia (where both species contribute at least 15% to the sample
biomass) or Ruppia ( Fig.6 ). However, in April, 1979, Ruppia
contained almost two orders of magnitude more individuals than Zostera.
During July, 1978 individuals per gram of grass were more equitably
distributed between the three areas of the grass bed. There is little
doubt that in vegetated areas the increased habitat complexity and
-------
60
Fig. 4. Mean number of infaunal species/core (0.007 m^) from the
three habitats found during routine sampling.
-------
61
Fig. 5. Mean number of infaunal individuals/m^ from the three
habitats found during routine sampling.
-------
62
Fig. 6. Mean number of epifaunal individuals per gram of grass found
during routine sampling.
-------
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-------
63
surface area provided by the grass blades significantly enhances the
number of species inhabiting the area.
From an analysis of fish gut contents, it appears that the
epifauna are an important link in transferring energy from primary
producers and decomposers: to higher trophic levels. For example, spot
(Leiostomus xanthurus) fed primarily on epifaunal organisms (amphipods,
polychaetes) or those species associated with plant detritus at the
sediment surface (copepods, nematodes, ostracods) (see Merriner and
Boehlert, this report). Few truly infaunal species were consumed. Spot
have a "vacuum cleaning" mode of feeding where an individual moves along
a grass blade or the sediment surface ingesting food items and detritus
and may significantly influence structure of the epifaunal community.
Spot appear to have difficult foraging on the bottom in dense vegetation.
Late juvenile and early adult silver perch (Bairdiella chrysoura)
feed exclusively on motile and vagile epifauna (see Merriner and Boehlert,
this report) by visual cues and may also effect epifaunal community
structure. Bairdiella first appears in the bed in August as juveniles and
remains until water temperatures drop in the fall. The most frequent preyed
upon food item, mysid shrimp, also make their appearance in the early
fall. Other food sources, the majority of which are amphipods and
isopods, are permanent residents in the grass bed.
Pipefish (Syngnathus fuscus) are also trophically important in
grass bed ecosystems. This species forages by sight among grass blades
and feeds primarily on epifauna (see Merriner and Boehlert, this report).
Principal food items included calanoid copepods, mysids, and amphipods.
*
Mysids (Neomysis americana) were more frequently consumed in October 1978,
when they were more abundant in the grass bed. Capiellid amphipods were
also a common food item.
-------
64
B. Predator Exclusion;
Initial results of predator exclusion experiments suggested
that 1) predation plays an important role in structuring the benthic
communities in grass bed areas and 2) that predation may have a greater
impact on community associated with unvegetated habitats than grass
beds.
There were more species and individuals in the sand area cage
treatments for both the penned and unpenned conditions after 2 months
of caging with no consistent patterns in the vegetated area for similar
treatments ( Figs. 7 and 8 ) However, the abundance of infauna in
the vegetated area was initially much higher than in unvegetated areas.
In June 1979 vegetated areas contained 5 to 10 times the number of
individuals found in adjacent sand patches. The abundance of indi-
viduals and numbers of species in the caged sand area was similar to
the untreated vegetated areas although differences in the species
composition did exist. Mya arenaria, the soft shell clam, was
extremely abundant in the caged sand area (. Fig. 9 ) but its
abundances were much lower in the grass area. The caged sand
community developed only after two months, primarily by recruitment
from planktonic larvae>whereas the grass infaunal community represented
an older,more established community which had developed from the
start of the growing season.
v
Based on this preliminary data, we suggest that the infauna in
sandy areas was more susceptible to predation. Their response to
-------
65
Fig. 7. Total number of individuals per core for each treatment
(S = sand area; M = mixed grass area; OC = cage with two
sides only; C = complete cage; P = pen. e.g. I'B-P+C =
complete cage treatment located inside the pen in the
grass bed).
-------
66
Fig. 8. Total number of species found in five cores for each
treatment (see Fig. 7 for treatment designation).
-------
67
Fig. 9. Total number of Individuals of the soft shell clam, Mya
arenaria, in five cores, for each treatment.
-------
68
protection was most pronounced. The infauna of the more spatially
diverse grass area are protected in part by the roots and rhizomes
and respond less dramatically . This community may already be near
maximum densities.
Predator exclusion studies conducted both in the Chesapeake
Bay and Europe have shown similar patterns of community response
(Virnstein, 1977; Reise, 1976; Orth, 1977). However, our data only
represent one sampling period and definitive habitat comparisons
and species response patterns will be discussed in detail following
the end of the experiment.
Data for the grass bed epifauna (Table 2) are also preliminary.
Uncaged treatments contained a greater abundance of individuals than
caged treatments due to fewer barnacles, Balanus improvisus, in the
caged areas, both within and outside the pen. Observations of barnacle
set on the sediment traps placed in the sand area suggested fewer
barnacles set inside than outside the cages.
C. Secondary Production;
The various methods of production estimation are sensitive to
growth type (Water 1977). To accurately estimate production it is
therefore necessary to know the type of growth exhibited by a species.
Growth is basically the process of increasing mass in developing
organisms and involves following changes in body weight or some
measure proportional to weight. We have chosen to measure the
lengths of various parts for the 9 species and have calculated the
v
relationship between length and weight (Table 3). Gammarus was the
only species to exhibit isometric growth (weight increased as the cube
-------
69
Table 2. Numbers of epifaunal species and individuals per gram of
SAV for each treatment in the predator exclusion experiment
taken in June, 1979.
Balanus improvisus
Bittium varium
Polydora ligni
Erichsonella attenuata
Gammarus mucronatus
Crepidula convexa
Caprella penantis
Astyris lunata
Cymadusa compta
Nereis succinea
Styllochus ellipticus
Anadara trans versa
Ilyanassa obsoleta
Urosalpinx cinerea
Mya arenaria
Idotea baltica
Mytilus edulis
Triphora nigrocincta
Heteromastus filiformis
Ampithoe longimana
Doridella obscura
Ampithoe valida
Palaemonetes sp.
Sabellaria vulgaris
Brania clavata
Nemertean
Nudibranch
Hit re 11 a lunata
Microprotopus raneyi
Edotea triloba
Paracaprella tenuis
Odostomia bisuturalis
Number of Species
Number of Ind./g. SAV
M
180.0
2.8
3.7
4.2
4.6
9.7
0.9
1.6
0.2
0.7
0.2
0.5
0.2
0.9
0.2
15
210.4
MfOC
135.6
4.3
3.9
2.2
2.4
15.9
0.6
0.2
0.7
4.3
1.2
10
170.1
W-C
118.5
6.4
9.4
1.2
0.6
9.9
0.1
0.1
0.4
1.0
0.6
0.1
0.1
13
148.5
MfP
324.3
3.0
2.2
10.1
1.8
9.1
1.0
0.8
1.4
2.7
0.2
0.2
0.6
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
21
359.1
MfP+OC
166.4
3.6
7.4
11.0
5.0
10.0
1.1
1.5
2.0
1.1
1.2
0.1
0.1
0.1
- 0.4
2.1
0.1
0.1
0.1
19
213.4
MfP+C
104.7
0.7
6.4
1.4
1.0
9.6
0.2
0.4
0.1
1.1
1.7
0.1
0.4
0.2
0.2
0.2
0.1
0.1
0.1
19
128.7
-------
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-------
71
of length). Allometric growth (weight did not increase as the cube of
length) was exhibited by all other species. The weight of Neomysis
increased faster than the cube of length whereas the opposite was true
for the remaining species.
While working up the samples we noticed that species might be
growing at different rates in pure Zostera, pure Ruppia or mixed
Zostera and Ruppia stands. Analysis of covariance was used for
determining significant differences in growth rates between these
three habitats (Table 4). Significant differences were found between
habitats for Erichsonella and Neomysis. Erichsonella, an isopod of
limited mobility, grows larger in Zostera. Neomysis, a motile mysid
shrimp, grows larger in mixed stands. Presently we are not certain
what causes these growth differences but they may be related to: 1)
the differential occurrence of a preferred food source in one habitat
or 2) predatory cropping of larger older individuals. For Neomysis
it may also be related to a refuge function since the species is very
motile and may seek mixed stands to hide in since they tend to be
denser than pure stands.
Life History
The life history of a species greatly influences its annual
production. For marine invertebrates the number of generations/year,
maximum size, life span, and time spent in the plankton as larvae are
most influential. We will not know what many of these values are for
our selected species until a year's data are worked up, but fortthe
first 4 months from April to July we have some approximations (Table 5).
-------
72
Table 4. Analysis of covariance of growth rates between habitats.
Probability Habitats*
Decapods
Callinectes sapidus
Crangon septemspinosa
Palaemonetes vulgaris
df
2
2
2
F
2.15
0.61
2.26
of >F
0.12
0.55
0.12
compared
Z,M,R
Z,M,R
Z,M,R
Amphipods
Caprella penantis
Gammarus mucronatus
Microprotopus raneyi
Isopods
Erichsonella attenuata
Edotea triloba
2
1
1
1
1.29
0.72
5.90
1.26
0.28
0.40
0.02
0.27
Z,M,R
Z,M
Z,M**
M,R
My s ids
Neomysis americana
19.82
0.0001
Z,M,R**
* Z = Zostera. R = Ruppia. M = mixed Z and R
** significant difference
-------
73
Table 5. Some life history parameters for secondary production species
from April to July.
Decapods
Callinectes sapidus
Crangon septemspinosa
Palaemonetes vulgaris
Amphipods
Caprella penantis
Gammarus mucronatus
Microprotopus raneyi
Isopods
Erichsonella attenuata
Edotea triloba
Maximum
size
(mg)
6574.0
130.00
142.00
3.10
6.20
0.11
6.90
3.00
e QJ <"*
H N 00
C -H 6
g cn^
4.00
1.00
11.00
0.01
0.01
0.04
0.10
0.10
Cohort or
generations
to date
1
1
2
2
2
2
2
2
mobility
good
good
good
limited
?
7
limited
limited
feeding
type
predator
omnivore
omnivore
predator
herbivore
herbivore
herbivore
herbivore
Mysids
Neomysis americanus
5.00
0.01
good
omnivore
-------
74
While it is too soon to determine life span it appears that the
amphipods and isopods live about 4 months. No determination for
the life span of the other species can be made at this time since a
complete generation or cohort has not appeared or disappeared within
the 4 months of data analyzed. At least 2 cohorts have appeared for
all amphipods and isopods. Only 1 cohort is present for Callinectes
and Crangon. Palaemonetes is the only decapod with 2 cohorts present
(Table 6) . Neomysis was found only in April. Through July it had
not reappeared in the secondary production samples but was taken in
June from the sand bar habitat.
Production
Both the instantaneous growth method, which basically sums
growth increments, and removal summation method, which sums increments
of mortality, will be applied for production estimates for species with
definable cohorts (Waters 1977, Crisp 1971). The Hynes method will be
applied to species which cannot be separated into cohorts after analysis
of 12 months of data (Hamilton 1969). To date cohorts can be recognized
only for Callinectes, Palaemonetes, Microprotopus, Erichsonella and
Edotea.
The most complete cohort recognized to date is cohort 1 for
Erichsonella which will be used as an example to calculate production
(Table 7). These production values are for one generation or cohort
of Erichsonella and represent only a fraction of the annual production.
2
The difference between the removal summation (0.63 g dry wt/m ) and
0 *
instantaneous growth (0.56 g dry wt/nr) is due primarily to not having
sampled the early part of the cohort in March and possibly February.
-------
75
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-------
76
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-------
77
Table 6 (continued)
P. vulgaris
0-1 nun
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
Cohort 1
Cohort 2
April
.22
.38
.11
.11
.82
May
1.14
1.86
1.24
.72
.31
.21
5.48
June
6.08
.82
5.96
7.95
.82
21.63
July
.13
.13
.13
5.35
2.22
2.35
2.35
.65
10.81
.39
* cohorts not distinguishable.
-------
78
Table 7. Production calculation for one cohort of Erichsonella from
April to July.
Removal Summation Method
April
May
June
July
No./m2
112.2
443.5
46.1
3.8
w1
(mg)
1.13
2.18
2.99
3.98
No. Iost/m2 x2
-331.3
397.4
42.3
wt . at =
loss
(ing)
1.66
2.57
3.49
= Production
(mg/m2)
-549.96
1029.27
147.42
0.63 g dry wt/zn2
626.73
Instantaneous Growth Rate
April
May
June
July
Standing
Crop
(mg)
126.50
964.67
137.63
15.12
-1 L
w G4 x
(mg)
1.13 0.66
2.18 0.32
2.99 0.29
3.98
B
(mg/m2)
545.59
551.15
76.38
P
(mg/m2)
360.09
176.37
22.15
0.56 g dry wt/m
558.61
w"
= mean individual weight - calculated from length-weight
regression and size frequency distribution
number of individuals lost between sample dates
weight at loss as average of v7 for two consecutive sample dates
G = instantaneous growth rate as In
-------
79
Thus for the one cohort of Erichsonella approximately 0.56 to 0.63 g
/\
dry wt/m^ of tissue were produced and available for consumption by
other trophic levels, which could be either higher level consumers
or lower level decomposers . Assuming this cohort of Erichsonella
occurred and produced evenly over the entire grass bed (140 hectares)
then 782 to 877 kg of dry tissue were available to other trophic levels.
When more data become available from feeding habits studies of higher
level consumers it will be possible to determine what fraction of the
secondary production is utilized.
D. Tissue Samples
A preliminary analysis of 6-^C ratios in some floral and faunal
components of the SAV habitat ( Table 8) revealed similar values to
those found by Thayer et al. (1978). Spyridia filamentosa, a macro-
epiphyte on Zostera and Ruppia had 6^C values of -17.7 which were
similar to Zostera epiphytes (-16.3) in North Carolina (Thayer et al.,
1979) . Some of the dominant faunal components had values ranging from
-13.3 (Penaeus aztecus) to -15.4 (Syngnathus fuscus) . Although
additional components of the grass bed await examination it appears
those analyzed to date may be linked more directly to a plankton-carbon
food chain than to a seagrass-carbon system. These findings are in
agreement with those of Thayer et al. (1978) who examined trophic
relationships in a relatively young eelgrass bed.
E. Stomach-Analyses (Callinectes sapidus) :
The masticatory mode of feeding made the identification of gut
contents to the species level difficult. Percent frequency of occurrence
-------
80
Table 8. 513C Values in SAV Systems
Values
Thayer. G. W. et al. , 1978 VIMS
Zostora marina (live) -10.2
.£ n'a r. .* "-** (dead) -10.6
7.. marjnrt cplphytcw -16.0
r Idl/i f I I .niicnt tmi - -37.7
Suspended particulates -20.0
Palaeroonetes vulgaris -16.3 -14.9
Cran^'Mi scptemspinosa - -14.5
Callinectes sapidus - -13.8-
Penaeus aztecas - -13.3
B.I! nlh-l la cliryiuira -]6.8 -Jr>.2
Lc-i(i:.t()Mius xaiiihurus - -15.2
Syn^nathus fuse us -17.0 -15.4
f T oridae -15.3
Illy anassa oh.solct_a_ - -14.0
Nassarias vibex -15.4
-------
81
of food item indicated that blue crabs feed on both epifaunal and
infaunal species ( Fig.10 ). Zostera was found in 70% of the
stomachs analyzed. Generally, live,intact,and very uniformly cut
sections of leaf material were present,indicating that crabs may
ingest the blades but digest only the encrusting organisms. Epifaunal
molluscs, isopods and Balanus improvisus were among the major food
items in crab stomachs. Callinectes also foraged among the root and
rhizome mat on infaunal molluscs. Feeding burrows and infaunal
feeding were frequently observed in the field. Callinectes may be
an important predator on the infauna in vegetated habitats. In
addition to nutritional needs derived from the grass beds, crabs
also utilize these habitats for protection from predators during
the critical soft shell phase of the molt cycle.
F. Waterfowl Interactions:
The Canada goose was the most abundant waterfowl species,
averaging 556 individuals per census date and exceedtig 2000 individuals
in one census (Table 10). Second in abundance were redheads.
present primarily at dawn and dusk. This species probably foraged
in the grass beds nocturnally and were inadequately censused.
Buffleheads (Bicephala albeola) consistently utilized the area,
and averaged 44 birds per day. Brant were abundant only on one census
date. Whistling swans, red-breasted mergansers (Mergus serrator), and
widgeons (Anas americana) were regularly encountered in low numbers.
Canada geese and redheads showed differential habitat use
i,
( Fig. ll). Canada geese, which forage by tipping up rather than
diving, avoided the deeper areas (>60-80 cm). At tide levels above
-------
82
Fig. 10. Percent frequency of occurrence of food items in Callinectes
sapidus stomachs.
-------
83
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-------
84
Fig. 111. Distribution of dominant waterfowl species at the study
site.
-------
85
MLW, goose foraging is restricted to an average of 40-50% of the
vegetated area throughout the grass bed. The relationship between
tide level and numbers of foraging geese further emphasizes the
influence of water depth ( Fig.12 ). Redheads, which are diving
ducks,were censused in deeper water, and may have utilized the same
areas for nocturnal foraging. Diving buffleheads were not restricted
to shallow water, and were more evenly distributed although they
showed a slight preference for deeper water ( Fig.12 ). Reduced
foraging in the shallowest areas (E-F) may relate to timing of field
observations. Grass in the shallows may have been depleted early in
the season, when observations were not made.
Preliminary work indicated consistent utilization of the grass
bed by Canada geese, redheads, buffleheads and red-breasted mergansers.
However, degree of trophic support can be assessed only by comparing
estimates of consumption with known dietary requirements, and the
work proposed for the 1979-80 season will emphasize this approach.
Several techniques including the use of exclosures, intensive censusing,
gizzard content analyses, and changes in S-^C values of liver tissue
will be used to quantify the utilization and importance of SAV to
waterfowl.
-------
86
Fig. 12. Numbers of feeding vs. non-feeding Canada geese in
relation to tide level.
-------
References
Adams, S. M. 1976a. The ecology of eelgrass Zostera marina (L.)>
fish communities. I. Structural analysis. J. Exp. Mar.
Biol. Ecol. 22:269-291.
Adams, S. M. 1976b. The ecology of eelgrass, Zostera marina (L.),
fish communities. II. Functional analysis. J. Exp. Mar.
Biol. Ecol. 22:293-311.
Adams, S. M. 1976c. Feeding ecology of eelgrass fish communities.
Trans. Amer. Fish. Soc. 105:514-519.
Bayley, S., V. D. Stotts, D. F. Springer, and L. Steenis. 1978.
Changes in submerged aquatic macrophyte populations at the
head of Chesapeake Bay, 1958-1975. Estuaries l(3):73-84.
Bent, A. C. 1925. Life histories of North American wildfowl. U. S.
Nat. Mus. Bull. 130, part II., pp. 281-293.
Cottam, C. 1939. Food habits of North America diving ducks. U.S. D.A.
Tech. Bull. No. 643.
Cottam, C. and D. A. Munro. 1954. Eelgrass status and environmental
relations. J. Wildl. Mgt. 18(4):449-460.
Hamilton, A. L. 1969. On estimating annual production. Limnol. and
Oceanogr. 14:771-782.
Hardwick, J. E. 1973. Biomass estimates of spawning herring Clupea
harengus pallasi, herring eggs, and associated vegetation in
Tomales Bay. Calif. Fish & Game 59:36-61.
Kikuchi, T. 1974. Japanese contributions on consumer ecology in
eelgrass (Zostera marina L.) beds, with special reference
to trophic relationships and resources in inshore" fisheries.
Aquaculture, 4:145-160.
Lippson, R. L. 1970. Blue crab study in Chesapeake Bay, Maryland.
Final Progress Report, NRI Ref. No. 70-46. Univ. Md.
Marsh, G. A. 1973. The Zostera epifaunal community in the York River,
Virginia. Chesapeake Sci. 14:87-97.
Orth, R. J. 1975. Destruction of eelgrass, Zostera marina, by the
cownose ray, Rhinoptera bonasus, in the Chesapeake Bay>
Virginia. Chesapeake Sci. 10:205-208.
-------
88
Orth, R. J. 1977. The importance of sediment stability in seagrass
communities. In; Ecology of Marine Benthos, B. C. Coull
eds., Univ. of South Carolina Press, Columbia, South
Carolina, pp. 281-300.
Reise, K. 1978. Experiments on epibenthic predation in the Wadden
Sea. Helgolander wiss. Meeresunters. 31:55-101.
Stevenson, J. C. and N. M. Confer. 1978. Summary of available
information on Chesapeake Bay Submerged Vegetation. U.S.
Dept. of Interior, Fish and Wildlife Service. FWS/OBS-78/
66, 335 pp.
Thayer, G. W., S. M. Adams & M. W. LaCroix. 1975. Structural and
functional aspects of a recently established Zostera marina
community. Estuarine Res. 1:518-540.
Thayer, G. W., P. L. Parker, M. W. LaCroix and B. Fry. 1978. The
Stable Carbon Isotope Ratio of Some Components of an Eelgrass,
Zostera marina, Bed. Oecologia (Berl.) 35, 1-12.
Virnstein, R. W. 1977. The importance of predation by crabs and
fishes on benthic infauna in Chesapeake Bay. Ecol. 58:1199-1217.
Waters, T. F. 1977. Secondary production in inland waters. Adv.
Ecol. Res. 14:91-164.
Waters, T. F. and G. W. Crawford. 1973. Annual production of a stream
mayfly population: a comparison of methods. Limnol. and
Oceanogr. 18:286-296.
Wood, E. J. F., W. E. Odum, and J. Zieman. 1969. Influence of sea-
grasses on the productivity of coastal lagoons. Lagunas
Costeras, un Simposio. Mem. Simp. Intern. Lagunas Costeras.
UNAMVNESCO., pp. 495-502.
-------
HIGHER LEVEL CONSUMER INTERACTIONS
J.V. Merriner and G. W. Boehlert
INTRODUCTION
The basic objectives within this subtask of the grant are to analyze
the structural and functional ecology of fish communities in submerged
aquatic vegetation (SAV) and to assess the importance of SAV to the
production and maintenance of important commercial fish populations. Our
approach has been to combine a program of field sampling with laboratory
study. Areas to be addressed include the processes of recruitment and
emigration from the SAV areas, the relative benefit of SAV from trophic
and refuge standpoints, the effects of major predators which may frequent
the SAV areas, biomass estimates of the components of the fish community,
the sources of production consumed by the fish populations, and ultimately,
the levels of secondary production by the fishes.
The fish community in the present study divides to three components;
these are i) fish eggs, larvae, postlarvae, and pelagic juveniles, ii)
resident fishes, and iii) migratory predators. Ecology of resident fish
communities in eelgrass (Zostera marina) beds has been studied in the
Beaufort, North Carolina area (Adams 1976a, b); species composition of
the benthic fish community in the study site used in the current work
has been qualitatively described (Orth and Heck, in press). The dominant
resident species in the lower Chesapeake Bay eelgrass bed was spot
(Leiostomus xanthurus), contrasting with the North Carolina eelgrass fish
community, where pinfish (Lagodon rhomboides) and pigfish (Orthopr^stis
chrysoptera) were the dominant species (Adams 1976a). Analysis of
feeding behavior in the current study will allow determination of the
trophic interrelationships and the effects on secondary producers within
-------
90
the system. Mld-and late-summer gill netting also revealed certain of the
migratory predators (Orth and Heck, in press), including the sandbar
shark (Carcharinus milberti) and bluefish (Pomatomus saltatrix). Preliminary
evidence suggested that these predators were feeding in the eelgrass
area. In other parts of the lower Chesapeake Bay, the cownose ray
(Rhinoptera bonasus) has been shown to feed and have dramatic effects in
eelgrass beds (Orth 1975).
Previous characterizations of Chesapeake Bay ichthyoplankton assem-
blages (Pearson 1941; Dovel 1971; Olney 1978) have concentrated on mid-
channel portions of the estuary and have neglected the generally
inaccessible nearshore, shallow environments. As a result, the extent
to which Chesapeake Bay fish stocks utilize these nearshore zones as
spawning and/or nursery sites is unknown. This lack of data takes on
added significance as a result of the recent emphasis on the importance
of shallow seagrass beds as refuge and feeding grounds for many species
of marine and estuarine fishes (Ried 1954; Adams 1976a, c). The
greater utilization of vegetated over unvegetated habitats by juveniles
and adults of many species of fishes is well documented (Briggs and
O'Connor 1971; Orth and Heck, in press); the current study will document
the importance of these areas to the early life history stages of
fishes and will determine the time of immigration and residence for the
important species. The contribution of zooplankton derived from sources
outside the vegetated areas will also be analyzed in the present study.
-------
91
MATERIALS AND METHODS
Field Sampling
The field sampling is conducted at the Vaucluse Shores study site,
north of the channel of Hunger's Creek (Figure 1). Sampling of relatively
large areas is required for adequate estimations of fish densities; for
this reason our sampling areas are not distinctly defined with respect to
vegetation type. Sampling is divided to three areas, designated as
representative of Zostera marina, Ruppia maritima, and an adjacent un-
vegetated area. The nominal Zostera area is located between the sandbar
and land, along transect A. The nominal Ruppia area is located on and
northeast of transect C. The unvegetated sampling area is on the sandbar
west of transect markers B and A in depths appropriate for the particular
sampling gear. As is apparent in vegetation maps of the bed, the nominal
sampling areas for Ruppia and Zostera contain mixed stands as well as pure
stands of the respective vegetation types (Figure 1). Differences noted
between the two sampling areas may therefore represent faunal changes
due to isolation from deeper water rather than differences attributable
to vegetation type.
Sampling gears generally break down to those for 1) ichthyoplankton
and zooplankton, 2) resident fishes, and 3) migratory predators. A
variety of gears were tested for sampling these components of the fauna
during the first six months of the project. Ichthyoplankton and zooplankton
were initially sampled with towed, bridled nets; these were abandoned due
to excessive disturbance ahead of the net from the outboard motor which
v
resulted in avoidance by fishes and samples with excessive silt, detritus,
and dislodged vegetation. Resulting samples were often impossible to
-------
92
Figure 1: Vaucluse Shores study site. Location of the vegetation types are
shown.
-------
R = RUPPIA
Z = ZOSTERA
S=SAND
/ = MIXED
= TRANSECTS
CHESAPEAKE
BAY
M
-------
93
preserve and sort (especially zooplankton samples with large amounts of
sand). Routine sampling for ich- and zooplankton currently consists
of two replicate collections in each habitat (Zostera, Ruppia, and sand)
utilizing a pushnet (Figure 2) constructed of h" diameter galvanized
pipe and deployed over the bow of a 19 foot outboard craft. The frame
is equipped with a 1 meter ichthyoplankton net (505 Mm mesh) and two
18.5 cm zooplankton (202 ym mesh); the ichthyoplankton net and one zoo-
plankton net are fitted with calibrated General Oceanics flowmeters to
assess the volumes of water filtered. Nets are fished at high tide for
2-3 minutes depending on abundance of plankton. The sampling duration
and boat speed allows the ichthyoplankton net to cover 74-174 m2 of sea
surface and filter from 68-117 m3 of water. All samples are taken in
the bed at high tide; routine monthly sampling is conducted at night;
daylight samples are taken at high tide in selected months.
Each time the net is deployed, one ichthyoplankton and two zoo-
plankton samples result. One of the zooplankton samples is preserved
in 10% formalin for later taxonomic analysis and estimation of abundance;
the other is washed with distilled water, frozen in the field on dry ice,
lyophilized, weighed, and ashed in a muffle furnace (6 hours at 500°C)
to determine organic biomass per unit volume. Ichthyoplankton samples
are preserved in 5-10% buffered formalin. In the laboratory they are
whole sorted for all fish eggs, larvae, postlarvae, juvenile, and adult
stages. Specimens are later identified to the lowest taxon possible,
measured, and curated.
v
For sampling resident fishes, a portable dropnet similar to those
described in Moseley and Copeland (1969) and Adams (1976a) was built;
-------
94
Figure 2: Zoo- and ichthyoplankton sampling pushnet. A. Gear array. As
presently designed, the net is fished with three nets. The central
net (505 urn mesh) is designed to sample ichthyoplankton and larger
components of the demersal plankton. The smaller two nets, fished
at a depth of approximately one meter, sample zooplankton (202
urn mesh). B. Design of the net frame. The gear is designed to
fish off the bow of the boat prior to any bow wake; the nets
trail under the hull of the boat. The frame pivots onto the boat
to allow access to the cod ends and ease in sample processing.
-------
k
)
>
1
202 M
NET
< 70cm >
505pm
METER
NET
23cm
202 M
NET
B
Figure 2
-------
95
it covered an area of 9.3 m . Our initial experiences with this gear
proved it to be unsatisfactory due to the small area covered, long deployment
times, and instability in rough weather. We therefore abandoned the dropnet
in favor of a 40 m long, 2.4 m deep seine (Figure 3) fished in the manner
described for long haul seines by Kjelson and Johnson (1974). Briefly,
the seine is deployed bag end first from the bow of an outboard craft
travelling in reverse. The net is set in a circle and the long wing
pulled past the bag end to decrease the circumference of the circle to
approximately 7.3 m, after which the bottom of the net is closed off
by tightening a purse line. The catch remains in the pursed section of net
and is brought on board the boat for processing. When set in an ideal
circle, this gear encompasses an area of 127 m2. Duplicate or triplicate
samples are taken monthly (from March through November) in each of the
three habitats. Daylight samples are also taken in selected months for
diel comparisons. Large specimens are identified, measured, and noted
on the field sheets; the remainder of the catch is preserved in 10%
buffered formalin for later identification in the laboratory.
Migratory predators are sampled in gill nets. Monthly sampling
consists of deploying 30.5 meters each of 12.7 and 17.8 cm stretch mesh
gill net perpendicular from shore in each of the three sampling habitats.
These nets are fished every four hours over a 24 hour period. At each
sampling time, the catch is removed, identified, measured, and weighed,
and the net is reset. Observations are made on relative fullness of
stomach contents and selected stomachs are removed and preserved for
t
analysis of contents. As with other collections, additional information taken
at the time of collection include date, time, habitat, tide stage, depth,
water temperature, salinity, dissolved oxygen, and comments on weather.
-------
96
Figure 3: Haul seine used in the collection of resident fishes; mesh size
is 3.2 mm (square) throughout. The net is set from the bow of
an outboard craft travelling in reverse. The short wing and bag
end (a) is set first and the net paid out in a circle. After the
circle is closed, the long wing end; (b) is pulled past point "a"
until reaching the last 7.3 meters of net. The remaining circle
is then pursed with the purse rings in this section of net and
the catch brought on board the boat.
-------
Figure 3
-------
97
Laboratory Procedures
To determine the feeding behavior of the fishes and their impact upon
the resident secondary producers, stomach contents and feeding periodicity
studies are being conducted. The resident fishes are collected by trawl
during the times of day when feeding is actively occurring for taxonomic
analysis of stomach contents. For determination of feeding periodicity,
trawling was conducted over 24 hour periods in May and August 1979. Stomachs
from the larger, migratory predators are sampled during the monthly gill
net collections.
The method of stomach collection depends upon the size of the fish.
For resident fishes larger than 150 mm and for all migratory predators,
stomachs are removed in the field and preserved in 10% buffered formalin
immediately after capture. Tags are placed with the stomach describing
fish length, species, and collection number to associate the stomach with
further information available on the field data sheets. For resident
fishes smaller than 150 mm, specimens are preserved whole in 20% buffered
formalin; the body cavity is slit to facilitate penetration of the formalin.
When stomachs are removed a qualitative index of fullness based upon the
size of the specimen is assigned. Contents are transferred to 40%
isopropyl alcohol prior to analysis.
Analysis of stomach contents of planktivorous and piscivorous fishes
is conducted by the Higher Level Consumer Interactions group; identification
of stomach contents of fishes feeding on invertebrate secondary consumers
is conducted by the Resident Consumer Interactions group. When contents
\
are removed, a second qualitative index of the state of digestion of the
food items is determined. The combination of these two indices allows
-------
98
preliminary analysis of feeding periodicity. After contents are identified
to the lowest taxon possible, individual food items are dried to constant
weight at 56°C and weighed. Certain items, such as nematodes and
harpacticoid copepods are assigned weights from literature values for
dry weight. Feeding of zooplanktivorous fishes will be conducted in the
coming year using the technique of Carr and Adams (1972).
Feeding periodicity is being determined for spot (Leiostomus xanthurus),
pipefish (Syngnathus fuscus), bay anchovy (Anchoa mitchilli), and silver
perch (Bairdiella chrysoura). Collections are made by otter trawl over a
24 hour period. From each sampling period, total gut contents of up to
six specimens are removed. The contents and the fish are then dried and
weighed separately; the ratio of dry gut content weight to dry body weight
gives an analysis of feeding periodicity which, when combined with estimates
of evacuation rate at the temperature of collection, will allow analysis
of daily ration. Analyses of samples taken in May and August are currently
underway.
Preliminary experiments are being run in the laboratory to examine
the effect of artificial Zostera marina on predator-prey relationships of
migratory predators and resident fishes. The experimental setup (Figure 4)
consists of two circular wading pools, (3.66 m in diameter, 0.9 meters
deep) with a volume of approximately 9500 liters each. The present
design utilizes a closed system with a biological filter comprised of
0.24 m3 of coarse sand, oyster shell, and gravel; circulation is pro-
vided by two 38 liter per minute pumps. Predators are captured by hook
x
and line, prey fishes by cast net, otter trawl, and dipnet. Predators are
maintained as residents in the tanks; holding tanks provide a supply of
-------
99
Figure 4: Present laboratory ta,nk setup for the preda,tor-
-------
Figure 4
-------
100
both predator and prey fishes. Artificial eelgrass (3/16" wide green
polypropylene ribbon, 0.6 density) mats have been woven to observed field
densities (dense- 1750 blades/m2; average 875 blades/m2). Mats (1 m2)
will be placed in the center of the experimental pools to mimic an
eelgrass habitat; prey will be released into the center of the tanks
in both eelgrass densities and in bare bottom controls. Preliminary work
has involved setting up the experimental system, determining the proper
size of predators for the tank, determining appropriately sized prey for
the predators, and analyzing methodological problems as necessary for
determining final experimental design.
Temperature acclimation tanks have been set up in the laboratory
with optional flow-through or closed system capabilities. Current
acclimation temperatures are 12°, 17°, 22°, and 27°C. This will allow
temperature related analysis of respiration rates and evacuation rates
of Bairdiella chrysoura, the silver perch, as part of a study on the
bioenergetics and physiology of this species. Evacuation rate analysis
is also planned for the pipefish Syngnathus fuscus. Respiration chambers
(Figure 5) have been constructed with flow-through characteristics to
allow analysis of metabolic rate at different temperatures. Experiments
are currently being run with JB. chrysoura.
-------
101
Figure 5: Flow-through respirometer. Although only two are shown, the system
currently in use has five fish chambers (A), four of which contain
fish and one of which is a control blank to monitor bacterial
respiration. Water is pumped through chambers and tubing by a.
peristaltic pump (D) past oxygen probes (C) contained in special
probe holders (B). Flow rates are varied between 3 and 18 m/min
depending on fish size and experimental temperature and are measured
(E) during each experiment.
-------
Figure 5
-------
102
RESULTS
Field Program
Migratory predators sampled with the gill nets are represented by
264 specimens of twelve species in nine families. Data on catch for March
through August 1979 are presented in Table 1. Catches (representing
number caught over the 24 hour period with nets fished every four hours)
were very low in both March and April. The April catch, represented by
a single bluefish (Pomatomus saltatrix) points up the variability in
catch of migratory predators. One net in the sand area fished overnight
3 days prior to sampling (a set aborted by weather) caught 45 bluefish
as compared to 8 captured in the vegetated areas. In May, catch increased
with movement into the bay of the teleosts Pomatomus saltatrix, Cvnoscion
nebulosus, JS. regalis, and the elasmobranchs Rhlnoptera bonasus and
DasyatIs say 1. In June the sandbar shark, Ca_r_charhlnus milberti dominated
the catch and has continued as the dominant through July and August
(Table 1).
The greatest catch of migratory predators was made in the Zostera
area (48%) followed by Ruppia and sand areas (26% each). The combined
catch In the vegetated areas (representing twice the fishing effort in
the sand area) provides preliminary evidence for the distribution of
the species relative to vegetation type. The bluntnose stingray (Dasyatis
sayi) and the cownose ray (Rhinoptera bonasus) are equally abundant in
sand and vegetated areas during months with low abundance; in May,
however, when the catch of the cownose ray was highest it occurred most
v
frequently in the sand area. The sandbar shark (C. milberti) was clearly
more abundant in vegetated areas. Spotted seatrout (Cynoscion nebulosus)
-------
103
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-------
104
and weakfish (C. regalIs) were captured almost exclusively in vegetated
areas whereas bluefish (Pomatomus saltatrix) was dominant in the sand area
in two out of three months of collection.
Two mesh sizes (12.7 and 17.8 cm square mesh) were used for migratory
predators; the larger mesh size was chosen to catch the sandbar shark
(Carcharinus milberti); only 12% of the catch of this species, however,
was made in the 17.8 cm mesh. With the exception of Dasyatis sayi and
Rhinoptera bonasus, all species were captured to a much greater extent
in the 12.7 cin mesh. These two species are probably sampled poorly in
gill nets; most catches occur through entanglement rather than via
"gilling" due to body shape.
For most species there are insufficient captures to provide an adequate
estimate of diel temporal abundance patterns. Diel pattern of catch for the
most abundant species (C. milberti) is presented in Figure 6 for June,
July, and August. A cursory examination of the data suggests that the
highest rate of catch is in the late afternoon and dark hours. The low
catch in the late afternoon of the second day in the June and August
collections, however, suggests this may not be the case; in all three months,
the first collection was made between 1200 and 1600. In June, an additional
collection was made 24 hours after the first collection; the first collection
at 1500 resulted in a catch of 20 fish while none were caught at 1500 the
following day. Similarly, the catch at 1630 was high the first day in the
August collection whereas only one individual was captured at 1500 the
following day. This suggests that the population of £. milberti may be
W
limited in this system and that 12 to 14 hours of fishing effectively removes
them; by comparison the catches of bluefish and Cynoscion do not appear
to show the same phenomenon.
-------
105
Figure 6: Temporal gill net catch of Carcharhinus milberti in all three
sampling areas combined. Nets are fished approximately each four
hours over a 24 hour period each month; the points for the time
of day represent the midpoint between setting and fishing the net.
In all three months the first set was at approximately 1200 EDT.
-------
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OO JUNE N=34
JULY N = 58
AUGUST N=48
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1200
1600
2000
0000
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0800
1200
1600
TIME OF DAY
Figure 6
-------
106
Resident fishes were sampled with the haul seine. Data are considered
in this report from March through August 1979, which include 47 night and
9 day sets of the seine. The resulting 4856 specimens represent 30 species
in 20 families. Densities of the resident species taken in the monthly
night collections are presented Table 2. Generally, numbers and diversity
of species were greatest in the Zostera area followed by the Ruppia and
sand areas. The number of species captured and total fish density increase
with temperature through April and May. Anchoa mitchilli was the most
frequently and consistently captured species; it was the numerical
dominant in the sand area in March and May and in all habitats during
the months of June through August. The Atlantic silverside, Menidia menidia,
was the dominant species in both vegetated areas in March, but decreased
in abundance in April and has been largely absent from night collections
since that time. Spot, Leiostomus xanthurus, recruited to the Chesapeake
Bay in April and was clearly the numerically dominant species of resident
fish in all habitats. Atlantic menhaden, Brevoortia tyrannus. has been
present in all months since April and was the dominant species in the
vegetated areas during May.
Most species captured in the haul seine are relatively uncommon and
appear only sporadically. Biomass (dry weight) of seven species is
presented in Table 3. The dominant species in terms of biomass differs
from the numerical dominant in certain months; with few exceptions, however,
Anchoa mitchilli remains the dominant species. In March, M. menidia is
dominant in all habitats; L,. xanthurus is the dominant species only in May
t
in the Zostera sampling area. Although clearly the numerical dominant in
April (Table 2), all specimens are newly recruited postlarvae (mean length
18.1 mm) which individually contribute little to the fish biomass.
-------
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109
The day haul seine catches made in June included four species not
taken at night in any month; these were Gobiesox s trumosus, (1),
Fundulus heteroclitus (1), Apeltes quadracus (3), and Pomatomus saltatrix
(1). Biomass of the dominant species for day and night collections from
June is presented in Tahle A. Brevoortia tyrannus is dominant in the
Zostera area during the day; all specimens, however, were taken in a
single collection and none were taken in the other two made in daylight
in Zostera. By comparison this species was common at night only in the sand
area, where it was taken in all three collections. It is possible that
these juveniles school in daylight and disperse at night. Anchoa mitchilli
and Membras martinica occur in low densities (except for the latter species
in sand) during the day and are abundant during the night; for these two
species it is unlikely that the difference is an effect of enhanced
avoidance during the daylight samples, since Menidia menidia is captured
during the day. The increased abundance of Syngnathus fuscus during the
day is probably due to increased activity during the day and better
availability to the sampling gear. The lower daytime catches of Leiostomus
xanthurus in vegetated areas, however, probably represents increased
avoidance of the sampling gear; the ratios of night to day catch are
much greater in sand, however, suggesting that some movement from the
vegetated areas may occur at night for this species.
Push net sampling has been conducted monthly at night with day
samples taken in May and August. Sorting and identification of catch
for the ichthyoplankton samples (505 ym mesh) have been completed for
samples taken in March through July; this represents 38 total collections,
including 12 collections in the Ruppia area and 13 each in Zostera and
-------
110
TABLE 4
Resident Fishes
Day-Night Comparison (June)
Biomass (mg dry wt/m2)
Zostera Ruppia Sand
D N D N D N
Brevoortia tyrannus 573.30 0 0 7.20 0 79.98
Anchoa mitchilli 3.64 176.95 12.70 95.23 0 80.73
Membras martinica 6.91 68.83 5.90 14.07 27.53 4.03
Menidia menidia 37.39 0 9.64 0 6.01 0
Syngnathus fuscus 15.82 8.19 1.08 0.58 0 0
Leiostomus xanthurus 6.33 54.59 16.82 43.87 0.87 17.18
-------
Ill
sand habitats. Volumetric and areal estimates of sampling effort
(Table 5) reveal moderate monthly variability but almost equal effort
(expressed as percent of total) between habitats. Push net collections
yielded 2669 juvenile/adult fishes, 3243 larval/postlarval specimens
and 8235 fish eggs.
Eggs of the windowpane flounder, Scopthalmus aquosus, the bay anchovy,
Anchoa mitchilli, and unidentified species of the family Sciaenidae
dominated push net collections (Tables 6 and 9). Additional species
represented were Tautoga onitis, Trinectes maculatus, Membras martinica,
Hyporhamphus sp. and an unidentified goby species. Eggs of the latter
three species are demersal, being attached to vegetation by chorionic
filaments (Atheriniformes) or laid in open shell nest sites (Gobiidae).
As a consequence, density estimates of eggs of these species cannot be
considered quantitative.
Eggs of _A. mitchilli slightly outnumbered those of sciaenids
(1'63:1) and density estimates (May - July) were roughly comparable in
all habitats (Table 6). During each month of occurrence, peak densities
of anchovy and sciaenid eggs were observed over the sand habitat and
lowest densities over Ruppia beds. A^. mitchilli and sciaenid egg
abundance estimates ranged from 0.8 - 2018 eggs/100 m3 and 0.9 - 1159
eggs/100 m3 respectively.
Larval and postlarval stages of 14 species representing 11 families
were taken in push net samples (Table 7). In addition larval atherinids
(probably both Menidia menidia and Membras martinica) and Gobiosoma
V
(probably both bosci and ginsburgi) were collected but reliable species
separation was not possible.
-------
112
Table 5
Volumetric and areal estimates of sampling effort by pushnet
at Vaucluse Shore study site.
Zostera
m3 m2
March
April
May (N)
(D)
June
July
Totals
% of Total
194.37
269.09
152.90
258.82
275.14
206.18
1,356.50
31.9
198.50
274.81
164.69
278.77
296.35
257.27
1,470.39
35.4
Ruppia
m3 m2
180.66
246.71
219.59
212.93
221.49
192.81
1,274.19
33.9
194.60
265.73
274.01
293.64
328.41
240.60
1,596.99
32.6
Sand
m3 m2
272.19
271.56
180.67
217.42
258.25
164.00
1,364.09
34.2
277.96
277.33
194.60
234.19
278.17
176.65
1,438.90
31.9
-------
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115
Ichthyoplankton collections were seasonally distinct. The winter-spring
assemblage was dominated by postlarvae of Ammodytes hexapterus and
Brevoortia tyrannus but also included postlarval Paralichthys dentatus
and larvae of JS. aquosus and Atherinidae species. Premetamorphic (< 30 mm SL)
Ji. tyrannus peaked in density during April and appeared more abundant
over vegetated habitats. In May, metamorphosed (juvenile) specimens
(Table 8) exhibited a similar distributional patterns were apparent in
other winter-spring assemblage species.
The summer was characterized by greater diversity and
abundance and was dominated by larval anchovies, gobies, and pipefishes
(Table 7). Young Syngnathus fuscus and larval A_. mitchilli peaked in
abundance in July while the largest concentrations of Gobiosoma sp.
larvae appeared in June. Larval anchovies and gobies were taken in
greatest densities over non-vegetated habitat. Collections of young
pipefish as well as the additional 7 species making up this summer
ichthyoplankton assemblage revealed no distributional patterns.
Data on juvenile and adult fishes occurring in evening push net
samples are summarized in Table 8. Four species appear to be consistently
available to the push net. These include juvenile/adult A^ mitchilli
and M. martinica and early juvenile stages of L_. xanthurus and 15. tyrannus.
The remaining species (as well as larger size classes of B_. tyrannus and
_L. xanthurus) are either effective avoiders of the gear, occur below
the sampling depth of the push net, or are infrequent in the habitats
sampled.
V.
Day/night catch data (Table 9) for eggs and larvae was highly variable,
with no trends apparent. As expected, however, catches of juvenile/adult
fishes were consistently highest during evening collections.
-------
117
Table 91
Day versus night push net catch comparisons at Vaucluse Shores.
31 May 1979
Species
Eggs (#/100 m3)
A. mitchilli
Sciaenidae
M. martinica
Unknown
Goniidae
Larvae (#/100 m2)
S. fuscus
C. re^alis
Gobiesox strumosus
Atherinidae
Gobiosoma sp.
A. mitchilli
H. hentizi
Juvenile (#/100 m2)
M. martinica
J5. tyrannus
A. mitchilli
L . xanthurus
S. fuscus
Hyporhamphus sp.
Zostera
Day Night
0.8
1.2
0.4
3.9
4.7
0.7
0.7
5.7
0.7
139.3
12.4
1.9
1.8
20.0
18.9
38.9
2.4
0.6
Ruppia Sand
Day Night Day Night
10.5 55.7 491.5
11.4 - 29.3
0.9 - 1.8
0.7 0.7 14.5 0.5
0.5
1.3
0.3 - 1.3
0.4
0.3
12.0 - 1.5
392.3 - 2.6
50.0 - 15.4
3.1
1.1 0.9
0.4
-------
116
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Zooplankton samples have been taken concurrently with ichthyoplankton
samples. Most samples have been curated and analysis will begin in October
1979 in conjunction with the addition of personnel as requested in the amend-
ment to EPA. In general the number of samples is double that for the
ichthyoplankton sampling with half used for biomass determination and
half for taxonomic analysis. Zooplankton biomass (mg ash-free dry weight/m3)
has been determined for March through July 1979 and is presented in Table
10 and Figure 7. Biomass is determined as ash-free dry weight ("organic
weight") due to the occurrence of sand and particulate matter in these
shallow-water samples which are variable and often render dry weights
alone useless. The trend shows that biomass is consistently greatest
in the Ruppia area, the shallowest area sampled; Zostera and sand areas
are approximately equal with fluctuations in certain months. All values
are low in April and June. April samples were taken after a storm which had
an effect on the fish populations; it may also have had an effect on the
zooplankton biomass. It is possible, however, that the large populations
of postlarval spot (Leiostomus xanthurus, see Table 2) may cause a reduction
through predation. No explanation is apparent for the low values observed
in June. Analysis of the taxonomic composition should help determine
certain of the causative mechanisms in the low values. Day samples were
taken in May; the diurnal differences in biomass are apparent (Table 10).
The day samples, however, were taken after a storm had dropped approximately
two inches of rain on the area. It is possible that the low biomass may be-
at least partially due to lowered salinity from the storm as well as
t
vertical migration of components of the zooplankton.
-------
119
TABLE .10
Zooplankton Biomass
(rag AFDW/m3)
Nig
Day
ht Samples
March
April
May
June
July
Samples
May
Zostera
129
35
132
61
151
21
.3
.1
.4
.4
.8
.5
Ruppia
225.
69.
397.
91.
380.
55.
3
3
5
7
6
7
Sand
234.2
39.8
118.6
32.1
164.5
51.2
-------
120
Figure 7: Organic biomass (Ash-free dry weight per m ) of zooplankton
by habitat and month. Each value represents the mean of two
determinations. R: Ruppia area, Z: Zostera area, S: sand area.
-------
a
u.
400-1
300 -
200 -
100
MARCH
APRIL
MAY
JUNE
JULY
Figure 7
-------
121
Non-empty stomachs from 343 resident fishes representing 10 species
and 117 migratory predators representing 9 species have been collected,
sorted, and contents identified. We are in the process of determining
dry weights of all contents and coding the data for computer analysis.
Additional specimens of these and other species have been taken to
complete size ranges and seasonal sampling of the resident fishes.
Stomachs from migratory predators continue to be taken as collected.
The species sampled, number of stomachs, and length range are presented
in Table 11. Summer flounder (Paralichthys dentatus) has been treated
both as a resident fish and as a migratory predator. Specimens under
200 mm SL are considered resident fishes; prey items are almost ex-
clusively invertebrates from within the eelgrass habitat, principally
mysids and Crangon septemspinosa. Those specimens larger than 200 mm
prey almost exclusively on fishes, including Syngnathus fuscus and
Leiostomus xanthurus.
Gravimetric analysis of the stomach contents and coding of the data
are currently underway. Preliminary analysis of the feeding behavior of
three important members of the "true resident" group (based upon percent
frequency of occurrence of prey items) is presented in Figures 8-11.
Stomach contents of spot OL. xanthurus) collected in July 1978 and October
1978 are presented in Figures 8 and 9, respectively. Fish collected in
July, when mean length was approximately 65 mm SL, fed predominantly upon
benthos as is apparent from the abundance of copepods (primarily harpacticoids),
nematodes, ostracods, polychaetes, and detritus (Figure 8). Epibenthic
u
and possibly planktonic feeding also occurred as shown by the presence
of amphipods, mysids, and fish eggs. In October, when mean length of the
-------
122
TABLE ' 11
Fish Stomachs
Species Number Length Range
Resident fishes
Urophysis regius
Syngnathus fuscus
Centropristis striata
Orthopristis chrysoptera
Bairdiella chrysoura
Leiostomus xanthurus
Prionotus carolinus
Paralichthys den tat us
Pseudopleuronectes americanus
Trinectes maculatus
8
84
3
16
111
87
11
14
8
1
41-120
61-160
61-160
21-120
41-140
61-160
21-120
121-200
41-100
121
Migratory predators
Carcharhinus milberti 56 461-860
Rhinoptera bonasus 2 741-946
Pomatomus saltatrix 32 281-880
Rachycentron canadum 1 410
jjynps cion regal is 6 321-570
_C. nebulosus 11 381-590
Micropogpn undulatus 1 350
S_ciaenops ocellata 2 381-780
Paralichthys dentatus 6 241-440
-------
123
Figure 8: Percent frequency of occurrence of specific prey items in
stomachs of resident fishes from the Vaucluse Shores study site.
Numbers in the figures may not equal those in Table 11 due either
to inclusion of empty stomachs (in the figures) or due to sub-
sampling of available stomachs; Spot, Leiostomus xanthurus collected
in July, 1978.
-------
lOOr
75 -
65.6
PERCENT FREQUENCY OF OCCURRENCE
? K 8
59,4
50.0
48-4
40-6
39-1
35,9
N=64
297
20.3
15.6 15.6 . , .
' 14.1
9.4
3.1
Figure 8
-------
124
spot collected was 101 mm SL, mysid shrimp were more conspicuous in the
diet, but benthic feeding remained important (Figure 9).
Feeding by the silver perch OB. chrysoura), all collected in the
month of October, is notably different from that of spot (Figure 10).
Harpacticoid copepods, nematodes, and other evidence of benthic feeding
is lacking. Mysids are clearly the dominant food item, as both numerical
and preliminary gravimetric analyses confirm. Planktivorous feeding
is taking place as demonstrated by presence of mysids and calanoid
copepods. Feeding within the vegetated areas is suggested by the
abundance of amphipods and other epifauna. Pipefish (Syngnathus fuscus),
captured almost exclusively in vegetated areas (Table 2), feeds on a combination
of prey items from within and outside of the vegetated areas (Figure 11).
Calanoid copepods occur most frequently in the fish collected in July,
along with Caprella penantis and other amphipods. In the October
collections, however, mysids (Neomysis americana) clearly dominate the
diet; calanoids remain relatively important in the diet, but amphipods
are consumed less frequently (Figure 11).
The migratory predators feed primarily on fishes and blue crab,
Callinectes sapidus. Spotted seatrout (Cynoscion nebulosus) and weakfish
(C. regalis) show similar feeding habits. Both species have been caught
mostly in the vegetated areas (Table 1). Diet is comprised primarily of fishes
(Including small Brevoortia tyrannus and Leiostomus xanthurus) with lower
frequencies of invertebrates (Crangon septemspinosa, Palaemonetes vulgar!s
and a single small Callinectes japidus). The sizes of fish preyed upon
\,
suggest that they are captured within the vegetated areas where the fish
-------
125
Figure 9: Percent frequency of occurrence of specific prey items in stomachs
of resident fishes from the Vaucluse Shores study site. Numbers
in the figures may not equal those in Table 11 due either to
inclusion of empty stomachs (in the figures) or due to sub-sampling
of available stomachs; Spot, Leiostomus xanthurus, collected in
October 1978.
-------
100 T
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-------
126
Figure 10: Percent frequency of occurrence of specific prey items in stomachs
of resident fishes from the Vaucluse Shores study site. Numbers
in the figures may not equal those in Table 11 due either to
inclusion of empty stomachs (in the figures) or due to sub-
sampling of available stomachs; Silver perch, Bairdiella
chrysoura, collected in October 1978.
-------
100-r
75f
u
U
ff
K
D
u
8
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z
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oc
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N=I3O
274
Figure 10
-------
127
Figure 11: Percent frequency of occurrenct of specific prey items in stomachs
of resident fishes from the Vaucluse Shores study site. Numbers
in the figures may not equal those in Table 11 due either to
inclusion of empty stomachs (in the figures) or due to sub-
sampling of available stomachs; Pipefish, Syngnathus fuscus
collected in July 1978(left) and October 1978 (right).
-------
oo
r-l
30N38UnOOO dO AON3riO3Hd iN3DH3d
-------
128
were caught. The dominant migratory predator after the May collections
was the sandbar shark (Carcharhinus milberti) for which the dominant food
items have clearly been fish and blue crab. Based upon frequency of
occurrence, 54% have contained both fish and crab, 15% exclusively crab,
and 31% exclusively fish. Of those fish which were identifiable,
Brevoortia tyrannus, Leiostomus xanthurus, and Hypsoblennius hentzi were
represented; all of these species are probably taken in the vegetated
areas. For those stomachs where dry weight of contents has been
determined, fish represents, on the average, 2.1 times the weight of
crab consumed. The final species of migratory predator, Pomatomus saltatrix,
feeds almost exclusively on fish with a single occurrence of blue crab.
The dominant prey species have been spot (25% of identifiable fish
occurrences) and menhaden (58%); in contrast to the sandbar shark and
Cynoscion spp., however, the menhaden consumed by bluefish are generally
larger and probably are captured outside the vegetated area.
One species included in the resident fishes is the spotted hake,
Urophysis regius. This species was captured in a single trawl taken in
May during a sampling effort aborted due to weather; it has not been taken
prior to this time or in a series of trawls made the following week. Eight
specimens have been examined for stomach contents, all of which had
consumed fish. The only identifiable fish species was spot, Leiostomus
xanthurus. From this consideration this species, although small (41-120 mm SL)
might be defined as a migratory predator.
Adequate predators for the predator-prey laboratory experiments have
v
been determined. Summer flounder (Paralichthys dentatus) bluefish
(Pomatomus saltatrix), and weakfish (Cynoscion regalis) have been success-
fully maintained in both holding and experimental tanks. The size range
-------
129
best suited to the size of experimental tanks is from 250 to 350 mm
standard length. Summer flounder and bluefish generally commence
feeding on live food after one week or less in captivity, whereas weakfish
would not feed for a minimum of three weeks. All three of these species
are taken in the vegetated areas at the study site; feeding behavior indicated
that fish is the primary food.
Five species have been assessed as potential prey for the experiments.
The mummichog (Fundulus heteroclitus) showed excessive orientation to the
sides of the tanks; its availability, however, is such that it may be used
for feeding predators between experiments. The pipefish (Syngnathus fuscus),
although important in vegetated areas and showing proper behavior to
artificial eelgrass, orients to the bottom and walls of the experimental tank
such that predation is nil or low even in the unvegetated controls. The
three species chosen for the experiments are spot (Leiostomus xanthurus),
silver perch (Bairdiella chrysoura), and menhaden (Brevoortia tyrannus).
All are important species in the vegetated areas of the study site in
the diets of the three predator species. Spot remain motionless in the
bottom of the tank in unvegetated controls, showing movement when con-
fronted by a predator species. Bairdiella, by contrast, generally remains
in midwater. Menhaden show schooling behavior, especially when pursued
by a predator. The size ranges of all prey species are from 40 to 80 mm SL.
The predators behave differently with respect of prey pursuit. Bluefish
approach the prey directly and usually slash or bite the prey to pieces, as
described in Olla et al (1970). Weakfish show a more cautious pursuit
and attack usually from below. Flounder stalk prey and usually approach
from behind. The behavior of predators and preliminary experiments suggest
that the predators will have varying success in the presence of artificial
eelgrass.
-------
130
DISCUSSION
Although an entire sampling season has not been completed, the trends
in distribution and abundance of the migratory predators and resident
fishes recorded in the present study generally show agreement with other
studies in shallow-water habitats in the lower Chesapeake Bay (Orth and
Heck, in press). The migratory predators (Table 1) show sporadic occurrences
with the exception of the sandbar shark, C. milberti, which was consistently
abundant during the last three months addressed in the present report.
Although gill nets are selective (Hamley 1975), the catch in this study
appears to give an estimate of relative abundance of most species with
the probable exception of the rays Rhinoptera bonasus and Dasyatis sayi
and probably the summer flounder Paralichthys dentatus. The rays are
dorso-ventrally flattened and are captured largely due to entanglement.
Although the nets foul visibly with jellyfish, large ctenophores, and
drifting aquatic vegetation due to current flow, the catch is not
markedly greater at night when visual detection would be less effective;
this may be due to the low water clarity during most months. The temporal
catch of Q. milberti shows increases at night but temporal catch is
difficult to analyze due to removal of an apparently resident population
without replacement (Figure 6). Combining catch of both species of
Cynoscion from May through September, 43% are captured during daylight
hours, whereas 63% of bluefish (jP. saltatrix) were caotured in daylight
hours. This contrasts with the data of Pristas and Trent (1977), who found
93% of the twelve most abundant species taken at night in gill nets. Thus
v
movement to the shallow water areas is probably greatest during daylight
hours when active feeding takes place.
-------
131
Availability of most species arises from populations moving through
the area or coming from adjacent deeper water areas. This is apparently
not true for the sandbar shark (C. milberti), which appears to exist in
essentially resident populations which are removed within 12-14 hours after
initial setting of the gill nets (Figure 6); status as a "resident"
predator is consistent with the fact that it was captured primarily in
the vegetated areas with slightly greater abundance in the Zostera area
(Table 1). Although captured most frequently in the vegetated areas as
well, the spotted seatrout and weakfish (Cynpscion nebulosus and £. regalis)
do not show a pattern of catch indicative of residence in the bed; bluefish
(£ saltatrix), on the other hand, exhibits a greater variability in catch
from month to month but in general is most frequently captured in the
sand area. The feeding habits of these migratory predators reflect the
habitat of occurrence.
Resident fishes sampled by the haul seine show the seasonal trends
observed for collections made two years earlier using a trawl at the same
study site (Orth and Heck, in press). In the current study, the
immigration of spot (_L. xanthurus) to the seagrass bed did not occur
until mid to late April (Table 2), later than observed in 1977 (Orth and
Heck, in press) in lower Chesapeake Bay or for most years in vegetated
areas south of Cape Hatteras (Adams 1976a; Thayer et al 1974). The
numerically dominant species observed by Orth and Heck was spot, whereas
the numerical dominant in the current study is the bay anchovy (A. mitchilli)
The increased importance of the pelagic species (B. tyrannus, A^. mitchilli,
t.
M- nienidia, and M. martinica) in the current study is probably due to the
difference in gear type. Orth and Heck used a 16 foot otter trawl towed
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132
behind an outboard vessel which fishes effectively only one meter off the
bottom; thus both avoidance and fishing of the net below the depth of
occurrence of these species suggests that their relative abundance was
underestimated. Adams (1976a) showed dominance of pinfish (Lagodon
rhomboides) and pigfish (Orthopristis chrysoptera) in North Carolina
eelgrass beds; the former, although present in low abundance in the study
by Orth and Heck, was not captured in the current study, and the latter
was present only in low numbers (Table 2). Spot, however, occurred
later in the year in densities similar to those observed by Adams (1976a)
using a dropnet.
Within the resident fishes, two subgroups are apparent. The first
is comprised of the pelagic and/or schooling group ("pelagic residents")
including Ji. tyrannus, _A. mitchilli, M. martinica, and M. menidia. Adams
(1976a) did not consider these species as true residents of the bed.
Although the same is probably true in the present study for all four
of the above species, they are considered with the residents in terms
of ecological impact upon the ecosystem due to the relatively high
biomass in the vegetated areas. In the night collections these species
were taken in all three habitats (Table 2) without clear trends in
abundance. Comparing day and night collections for June (Table 4),
no trend is apparent for JJ. tyrannus due to its highly contagious
distribution; A. mitchilli is present in vegetated areas in the day,
increasing greatly in these areas and in the sand area at night. M.
martinica is taken in low abundance during the day except in the sand
V
area; the situation reversed during the night. The other atherinid,
M. menidia, however, shows an opposite pattern; none were captured at
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133
night except in March and April while Membras densities were low. In
the day collections, however, Menidia was common, especially in Zostera.
The second group of resident fishes ("true residents") is dominated
by spot (L. xanthurus) and pipefish (S^. fuscus) and includes the
majority of other species included in Table 2. In August, silver perch
(_B. chrysoura) appeared and has remained important in subsequent months
as observed by Orth and Heck (in press); the other sciaenids (C. nebulosus,
JC. regal is, M. ajnericanus) captured in August did not remain as important
components of the community. Members of this component of the resident
fish group are captured most frequently in the vegetated areas with
greater catches in the Zostera sampling area (Table 2). Day-night
sampling conducted in June suggests that S_. fuscus is more abundant in the
day, but this probably reflects greater availability to the haul seine.
Spot, on the other hand, appear more abundant in the night collections.
Orth and Heck (in press) observed increased catch of spot in all habitats
at night as observed in the present study. It remains to be determined, however,
whether the increases at night are due to increased daytime avoidance or
to actual movements to the bed from other areas. The relative increases
are greater in unvegetated areas, however, suggesting that some movement
may occur between vegetated and unvegetated areas, as suggested by Orth
and Heck. In general, the biotnass reported in the present study
for the seven major species falls within the range of total fish biomass
for Zostera marina beds in New England by Nixon and Oviatt (1972) but
is less than that reported in studies to the south (North Carolina, Adams 1976a;
Texas, Hoese and Jones 1963).
Ichthyoplankton collections from shallow-water vegetated habitats
have not been analyzed in the lower Chesapeake Bay; qualitative comparisons,
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134
however, of the present Ichthyoplankton data with those of previous
studies on Chesapeake Bay fish eggs and larvae (Pearson 1941, Dovel
1971, Olney 1978) indicate that the push net presently employed as
primary ichthyoplankton gear in this study adequately samples nearshore
ichthyoplankton assemblages. Species composition and seasonality of
Vaucluse Shores ichthyoplankton (March-July) are in general agreement
with all previous Chesapeake Bay studies. Without exception, all
species encountered in the present collections have been previously
recorded as eggs, larvae or juveniles in similar seasonal patterns.
Although quantitative comparisons are limited by natural variability,
difference in methodology, and lack of comparative gear efficiency data,
relative ichthyoplankton abundance as measured in the present study
compares favorably with the most recent data on lower Bay fish eggs and
larvae (Olney 1978). Differences noted may be instructive in pointing
out the importance of nearshore spawning nursery habitats. Present ranges
of density estimates (///100 m ) for eggs of A. mitchilli and sciaenid
fishes (0.5 - 2018.3 and 0.9 - 1159.2 respectively) are comparable to
those reported by Olney (3200-14000 A. mitchilli eggs; 6.0 - 819
sciaenid eggs). Both studies found eggs of Anchoa and sciaenid fishes
to dominate fish egg collections, but differences in absolute ratios of
Anchoa to sciaenid eggs (1.63:1, present study; 15:1 Olney's data)
suggest reduced Anchoa nearshore spawning activity and greater utilization
of nearshore spawning habitat by sciaenids. Similarity, differences in
abundances of goby and pipefish larvae relative to larval Anchoa point
v
out increased utilization of shoal, vegetated habitats as spawning grounds
for these species. In the present data, pipefish and goby larvae occurred
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135
in equal and sometimes greater concentrations than anchovies. In contrast,
larval concentrations of these species never surpass those of Anchoa in
deeper waters (Olney 1978).
Continued examination of nearshore ichthyoplankton assemblages
utilizing this gear will be instructive. In future reports, we will include
length-frequency analysis, additional day-night comparisons, data on the
relationship of hydrographic parameters to species occurrence, and comparative
gear evaluation.
Variation between habitats for components of the ichthyoplankton
differ between stages (Tables 6-8). Eggs of Scopthalmus aquosus are about
equally distributed between the three habitats during the months of March
and April. The more abundant eggs of Anchoa mitchilli and of sciaenids
show virtually the same pattern (lowest densities in the Ruppia area,
increasing in the Zostera area, and highest in the sand area). Since
much of the water flow to the bed occurs up the main channel during
flood tide (Figure .1), this is consistent with spawning activity either
in deeper water, which communicate directly with the sand area, or
possibly in the main channel or upstream in Hungar's Creek. The
great density of both egg types in the sand area in July, however, favors
the former explanation (Table 6). Very small larvae (Table 7) show
either no difference between habitats or show a pattern similar to
that of the eggs of Anchoa. With growth, however, postlarvae of some
species show increased densities in the vegetated areas (Table 7). 15.
tyrannus and 1^. xanthurus, for example, are spawned off the Atlantic
t
shelf and move into the Chesapeake Bay, arriving as postlarvae in the
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136
shallow water habitats. Recruits of both IJ. tyrannus (Table 7)
and L. xanthurus (Tables 2 and 8) appear to prefer vegetated habitats.
The temporal pattern of variation in zooplankton biomass is similar
to that observed in a study of the plankton of the lower Chesapeake Bay
(Jacobs 1978). Values observed in the sand and Zostera areas fall within
the ranges observed by Jacobs, but the values for the Ruppia area greatly
exceed those observed in open bay waters. The low values observed in
the month of April coincide with the recruitment of large numbers of
postlarval spot (L. xanthurus) and menhaden (B. tyrannus) to the lower
Bay (Tables 2 and 7). These species are planktivorous in the postlarval
stage; it has been suggested that immigration of large numbers of
postlarval fishes may significantly reduce the standing crop of zooplankton
in estuarine systems (Thayer et al 1974). In sampling deeper water,
however, Jacobs (1978), although noting a decrease in zooplankton biomass
in April, did not demonstrate a significant reduction in copepod density;
a reduction would be expected if postlarval feeding was the causative
factor since copepods make up 76-99^ of the food of these fishes in this
stage of the life history (Kjelson et al 1974). The diurnal differences
in zooplankton biomass (Table 10) for May demonstrate a dramatic reduction
in the day samples. Although Ruppia remains the highest value, the day
value in vegetated areas is 15% that of the night value as compared to
43% in the sand area. Further analysis of the curated samples will
elucidate the meaning of the temporal changes in zooplankton biomass.
Organic biomass of zooplankton collections shows clear differences
between habitats (Table 10, Figure 7). Generally, biomass is greatest in
Ruppia followed by sand and Zostera with the latter two showing similar
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137
values. Although the Importance of these differences must await analysis
of the taxonomic composition of the plankton, three hypotheses are
consistent with this observation, as follows: 1) obligate planktonic
organisms collect in high densities in the upper end of the channel
through hydrodynamic winnowing, active orientation, or swarming behavior;
2) facultative or demersal plankton are more abundant in the shallower
water of the Ruppia zone; or 3) organic detritus and particulate matter
retained by the 202 ym mesh is more common in the Ruppia area. Evidence
against hydrodynamic winnowing is provided by the abundance pattern of
fish eggs (Table 6) as discussed above, which are in lowest densities in
the Ruppia area (unless high planktonic predation rates on egg stages
lowers density). Swarming behavior has been observed in several shallow water
habitats, including coral reefs, marine lakes, seagrass beds, and rock
and sand bottoms by a variety of obligate planktonic taxa including copepods,
euphausids, and mysids (Emery 1968; Fenwick 1978; Hamner and Carleton
1979). The samples in the Ruppia area are generally taken at peak high
tide (before ebb) to provide the water depth necessary for the push net.
Swarming would probably be facilitated during times of slack water. The
contribution of obligate or facultative plankton or of organic detritus
to the high values of zooplankton biomass in the Ruppia area must await
analysis of the taxonomic composition of the curated samples.
Feeding relationships of fishes within the Vaucluse Shores study site
are generally similar to those of the dominant speices observed in other
studies in vegetated habitats (Carr and Adams 1973; Adams 1976c). The
*.
lack of the dominant species from North Carolina (Lagodon rhomboides and
Orthopristis chrysoptera) , however, may alter the feeding behavior of
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138
L. xanthurus in the current study through availability of other food sources.
Although plant material and detritus occur frequently (Figures 9 and 10),
preliminary gravimetric data suggest that they are less important in the
diet than in North Carolina Zostera beds (Adams 1976c) or in Florida
(Sheridan 1978). The frequency of occurrence reflects the frequency of
benthic feeding rather than dietary importance. Spot are initially
planktivorous, after which they switch to predominantly benthic feeding
(Kjelson et al. 1974; Sheridan 1978); this will be confirmed in the current
study in conduction with zooplankton sampling in the spring of 1980.
Smaller (average 65 mm SL, July data) spot exhibit predominantly benthic
feeding (Figure 8). Although benthic feeding remains important in larger
specimens collected in October (Figure 10), planktonic feeding represents
the major food intake; analysis shows that numbers of harpacticoids and
nematodes decline rapidly with size whereas the numbers of raysids per
stomach increases. The data is confounded, however, by the sampling
of smaller fish in July, when mysids were rare in the eelgrass bed, and
larger fish in October, when mysids were abundant (see part II of this
report). The increases in importance of mysids in the diet are likely
evidence of high availability, since I,, xanthurus has a subterminal mouth
adapted primarily to feeding on infauin and benthic organisms (Chao and
Musick 1976).
Bairdiella chrysoura immigrates to the vegetated areas in August
(Table 2). Stomachs have been analyzed from collections in October 1978,
when the fish were relatively large (mean length approximately 92 mm SL;
t
see Table 11). This species has a terminal mouth and is adapted for pelagic
feeding, although some epibenthic feeding takes place (Figure 10). Most
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139
studies on the feeding by this species (summarized in Chao and Musick
1976) show fish, mysids, and decapod shrimp to be the predominant dietary
items. Adams (1976c), by contrast, observed no mysids in the diet of
this species in North Carolina eelgrass beds. The abundance of mysids
in the diet is undoubtedly related to the high levels of abundance in
the eelgrass habitat. The importance of availability upon feeding on
mysids within the bed is stressed by the change in frequency of mysids
in stomachs of pipefish (S_. fuscus). Mysids were not present in stomachs
of specimens taken in July but represented the major food item in October
(Figure 11). The ongoing gravimetric analysis of prey items will provide
more precise definition of ontogenetic and seasonal trends of feeding
behavior of the fish community, their relationship to prey availability,
and the impact on other components of the ecosystem.
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140
LITERATURE CITED
Adams, S. M. 1976a. The ecology of eelgrass, Zostera marina (L.)> fish
communities. I. Structural analysis. J. exp. mar. Biol. Ecol.
22:269-291.
Adams, S. M. 1976b. The ecology of eelgrass, Zostera marina (L.), fish
communities. II. Functional analysis. J. exp. mar. Biol. Ecol.
22:293-311.
Adams, S. M. 1976c. Feeding ecology of eelgrass fish communities.
Trans. Amer. Fish. Soc. 105:514-519.
Briggs, P. T., and J. S. O'Connor. 1971. Comparison of shore-zone fishes
over naturally vegetated and sand-filled bottoms in Great South
Bay. N.Y. Fish Game J. 18:15-41.
Carr, W. E. S., and C. A. Adams. 1972. Food habits of juvenile marine
fishes: Evidence of the cleaning habit in the leather jacket
Oligoplites jjaurus, and the spottail pinfish Diplodus holbrpoki.
Fish. Bull. 70:1111-1120.
Carr, W. E. S., and C. A. Adams. 1973. Food habits of juvenile marine
fishes occupying seagrass beds in the estuarine zone near Crystal
* River, Florida. Trans. Am. Fish. Soc. 102:511-540.
Dovel, W. 1971. Fish eggs and larvae of the upper Chesapeake Bay.
Nat. Res. Inst., Univ. of Md. Contrib. #460, 71 pp.
Emery, A. R. 1968. Preliminary observations on coral reef plankton.
Limn. Oceanog. 13(2):293-303.
Fenwick, G. 1978. Plankton swarms and their predators at the Snarves
Islands. N.Z. Journal Mar. Freshwater Res. 12:223-224.
*Chao, L. N., and J. A. Musick. 1976. Life history, feeding habits, and
functional morphology of juvenile sciaenid fishes in the York River
estuary, Virginia, Fish. Bull. U.S. 75:657-702.
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141
Hamley, J. M. 1975. Review of gill net selectivity. J. Fish. Res.
Bd. Can. 32(11):1943-1969.
Hamner, W. H., and J. H. Carleton. 1979. Copepod swarms: attributes
and role in coral reef ecosystems. Limnol. Oceanogr. 24:1-14.
Hoese, H. D., and R. S. Jones. 1963. Seasonality of larger animals in
a Texas grass community. Publs. Inst. mar. Sci. Univ. of Texas.
9:37-46.
Jacobs, F. 1978. Zooplankton distribution, biomass, biochemical composition,
and seasonal community structure in lower Chesapeake Bay. Ph.D.
Dissertation, University of Virginia. Charlottesville, Va. 105 pp.
Kjelson, M. A., D. S. Peters, G. W. Thayer, and G. N. Johnson. 1974.
The general feeding ecology of postlarval fishes in the Newport
River Estuary. Fish. Bull. 73:137-144.
Kjelson, M. A. and G. N. Johnson. 1974. Description and evaluation of a
long haul seine for sampling fish populations in offshore estuarine
habitats. Proc. 28th Ann. Conf. S. E. Assoc. Game Fish Conm.
pp. 171-175.
Moseley, F. N. and B. J. Copeland. 1969. A portable drop-net for
representative sampling of nekton. Contr. Mar. Sci. Tex. 14:37-45.
Nixon, S. W. and C. A. Oviatt. 1972. Preliminary measurements of
mid-summer metabolism in beds of eelgrass, Zostera marina. Ecology.
53:150-153.
Olla, B. L., H. M. Katz, and A. L. Studholme. 1970. Prey capture and
feeding motivation in the bluefish, Pomatomus saltatrix. Copeia.
14:360-362.
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142
Olney, J. E. 1978. Planktonic fish eggs and larvae of the lower Chesapeake
Bay. MS Thesis, College of William and Mary, Williamsburg,
Virginia. 123 pp.
Orth, R. J. 1975. Destruction of eelgrass, Zostera marina, by the
cownose ray, Rhinoptera bonasus, in the Chesapeake Bay. Ches.
Sci. 16:206-208.
Orth, R. J., and K. L. Heck, Jr. In Press. Structural components of
eelgrass (Zostera marina) meadows in the lower Chesapeake Bay:
fishes. Estuaries.
Pearson, J. C. 1941. The young of some marine fishes taken in lower
Chesapeake Bay, Virginia, with special reference to the grey sea
trout, Cynoscion regalis (Bloch). USFWS Fish. Bull. 50:79-102.
Pristas, P. J., and L. Trent. 1977. Comparisons of catches of fishes
in gill nets in relation to webbing material, time of day, and
water depth in St. Andrew Bay, Florida. Fish. Bull. 75:103-108.
Ried, G. K., Jr. 1954. An ecological study of the Gulf of Mexico
fishes, in the vicinity of Cedar Key, Florida. Bull. mar. Sci.
Gulf. Caribb. 4:1-94.
Sheridan, P. F. 1978. Trophic relationships of dominant fishes in the
Apalachicola Bay system (Florida). Ph.D. Thesis, Florida State
Univ. 232 pp.
Thayer, G. W., M. A. Kjelson, D. E. Hoss, W. F. Hettler, Jr., and M. W.
LaCroix. 1974. Influence of postlarval fishes on the distribution
of zooplankton in the Newport River estuary. Ches. Sci. 15:9-16.
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ECOSYSTEM MODELING
R. L. Wetzel
INTRODUCTION
The ecosystem modeling project is designed and being implemented as
an integrative tool for ecosystem analysis. The modeling effort will
describe the principal pathways for energy flow and evaluate those parameters
associated with specific processes that control behavior. The tasks to
date have centered on model conceptualization, parameterization and
mathematical formulation for digital computer simulation.
METHODS
Conceptualization; Model conceptualization or compartmentalization is based
on trophic interactions. The information used to decide the compartmental
structure for the model came primarily from the literature and relied
heavily on the expertise and experience of the several principal investiga-
tors participating in the overall research program. Following two prelimi-
nary versions, the model proposed for simulation was decided. Table 1
lists and describes the 1? compartments of the model and Figure 1 gives
the interaction matrix describing compartmental exchanges. Emphasis in
this model version is placed on biological organization and trophic function
in response to the overall program objective to evaluate predator-prey
interactions and secondary production. Evaluation of controls such as
temperature, light and nutrients on primary production within the context
of the model will be studied in a second model version following preliminary
analysis using the current model structure.
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145
Table 1: Model compartmentalization
Symbol Name
XI C02-H20
X2 POC-H20
X3 DOC-H20
X4 POC'Sed
X5 DOC'Sed
X6 Phytoplkankton
X7 Vascular Plants
X8 Epiphytes
X9 Benthic Algae
X10 Zooplk. & Meroplankton
XI1 Microheterotrophs'H 0
XI2 Attached Epifauna
XI3 Motile Epifauna
X14 Heterotrophs-Sed
X15 Microheterotrophs-Sed
X16 Natant residents
XI7 Megapredators &
Waterfowl
Description
Carbon dioxide J.n water
Particulate organic carbon in water
Dissolved, organic carbon in water
Particulate organic carbon in, sediments
Dissolved organic carbon in sediments
All autotrophic water column components
Zostera marina, Ruppia maritima
Autotrophes associated with emergent
vascular plant leaves
Both macrophytic and microautotrophs assoc.
with sediments.
Heterotrophs in water column. Includes both
resident and seasonally abundant larval and
juvenile forms.
Primarily bacteria in water
Sessile heterotrophs associated with emergent
vascular plant parts
Heterotrophs that are capable of free movement
within the vascular plant community.
Primarily infauna
Primarily bacteria in sediments
Large predatory, motile species
Self-explanatory
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146
Parameterization; The parameters necessary as input data for model simula-
tion can be grouped according to equation structure. The equational structure
is based on the type of exchange in question. Generally these fall in one
of four categories: 1. abiotic-*-abiotic, 2. abiotic Abiotic, 3. biotic ->
biotic and 4. biotic-> abiotic exchanges. Because of the high degree of
biological interaction represented in the current model version, parameters
associated with the last two categories of exchange dominate the data
input requirements. The following summary defines these parameters and
together with the next section on mathematical structure forms the basis
for the computer simulation version of the model. Briefly the parameters
currently being evaluated;
Given two generalized compartments and represented as:
Fij
where; Xi = donor compartment "i"
Xj = recipient compartment "j"
Fij = flux of matter-energy from "i" to "j"
the parameters necessary to describe the various flows in the model are:
1. Pi j ; a dimensionless number where
that gives the preference assigned an ingestion or uptake pathway, Fij.
For any biotic compartment having multiple resources (2 or greater) , a
preference value in the above range must be decided such that,
n
£pij=1.0, n= number of inputs to Xj
i=l
This applies, in its present form, to only biological uptake pathways.
V
2. Ti j : The maximum specific rate of uptake or ingestion as;
gC(Xi) . gC(Xj)-1 . At'1
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Figure 1: Model Interaction Matrix
144
From; 1
I. C02.H20
2. POC.H20
3. DOC.H20
4. POC.Sed.
5. DOC.Sed.
6. Phytoplk. D
7. Vas. Plants D
8. Epiphytes D
9. Benthic D
Algae
0. Zooplk & D
Meroplk
1. Microhetero D
2 . Attached D
Epi fauna
3. Motile D
Epi fauna
4. Hetero.Sed. D
5. Microhetero
Sed.
6 . Natant
Residents
7. Megapred.
Waterfowl
2
RS
M
M
M
M/Eg
M/Eg
M/Eg
M£g
3
Ex
EC
EC
EC
EC
EC
EC
EC
EC
4
S
M
M
M/Eg
M/Eg
M/Eg
5
Ex
EC
EC
EC
EC
EC
EC
6
R
7
R
8
R
9
R
10
DR
DR
DR
11
DR
DR
12
DR
DR
DR
13
DR
DR
DR
DR
DR
DR
14
DR
DR
DR
15
DR
DR
16
DR
DR
DR
DR
DR
DR
DR
17
DR
DR
DR
DR
4
2
2
2
2
5
8
5
6
6
5
_3
76
D = Linear Donor controlled (LDC)
RS = Resuspension - LDC
M = Mortality - LDC
Eg = Egestion - LDC
EC * Excretion - LDC
S = Sedimentation
DR = Donor-Recipient Controlled
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147
where,
Xi and Xj as above
gC = grams of carbon
At = time interval
For the current model version carbon is the unit of exchange and the unit
of time equals one day. For our preliminary simulation work with the model,
four (4) seasonal values are assigned each ingestion or uptake pathway, i.e.
winter, spring, summer and fall values.
3. AEi j : The assimilation efficiency for the corresponding ingestion or
uptake pathway, Fij where,
0
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148
maximum maintainable density for the recipient population. This is equivalent
to the "carrying capacity" or K-value from other works. Units as above.
n
At, Xj =y jj, EFij = maintenance costs of Xj
i-1
n = no. input pathways
and A xj.At"1=0. no growth
The above seven parameters in equational form define the principal biological
interactions for uptake or ingestion of matter-energy. The following describe
the losses of matter-energy from the various populations.
8. Pj ; Specific rate of respiration as gC.gC~l. t~l.
9. Cj : Specific rate of excretion as gC.gC'l. t~*.
10. yj ; Specific rate of natural mortality exclusive of predatory mortality
as gC.gC"1. t"1.
Mathematical Structure
The mathematical structure of the simulation model and formulation of
interaction equations follows the general guides presented by Wiegert
(1975, 1978). The interaction equations coupling the compartments (Table 1)
are based on testable assumptions and incorporate measurable parameters;
i.e. the interaction coefficients are dimensioned and have ecological meaning.
The technique used for digital computer solution of the simulation model
follows Wiegert and Wetzel (1974).
The current model version has 76 pathways for exchange. The equations
used to describe these can be mathematically classed into one of three
general categories:
v
1. linear, donor controlled pathways
2. linear, recipient controlled pathways
3. non-linear, recipient controlled, donor-recipient determined pathways
(feedback controlled)
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149
Of the 76 pathways the majority (34) are classed as category three, i.e.
feedback controlled ingestion or uptake fluxes. Thirty (30) pathways represent
metabolic loss or natural mortality for the biotic compartments and are
classed as category 1, i.e. linear, donor controlled fluxes. The remainder,
12, are abiotic-^abiotic exchanges and are either linear donor or recipient
controlled fluxes.
The three mathematical categories for classifying the various exchanges
can be generally represented as follows;
1. Linear, donor controlled;
Given the conceptual interaction;
where Xi = Donor compartment
Xj = Recipient compartment
Fij = Flux of carbon from "i" to "j"
then Fij = Cij.Xi
Cij = specific rate of transfer; gC.gC" .At""!
For the model, respiration, excretion, natural mortality, sedimentation
and resuspension are represented in this manner with "Cij" replaced with the
appropriate parameter.
2. Linear, recipient controlled:
As above, except
Fij = cij.Xj,
the recipient compartment controlling the realized amount of transfer. At
present, no flux in the model is represented with this function.
3. Non-linear, recipient controlled, donor-recipient determined;
The general form of the equation can be given as:
Fij =Tij.Xj (l.-(fij.fij)} +
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150
where; Fij, Xj as defined previously
fij = maximum specific rate of ingestion or
uptake; gC.gC~l.day~l
fij = resource controlled negative feedback term
fjj = self or recipient controlled negative
feedback term
The form of the feedback can vary depending on how a population responds
to intense predation or, oppositely, to crowding or some space related
limitation. Without specific information relative to the form of these
feedbacks, the following general forms have been adopted;
i
- Xi
a i j - Y i Jj ,and,
Yjj - Xj +
Wiegert (1975 ) discusses the various forms used in the past and Christian
and Wetzel (1978) give specific examples of changes in feedback functions
for microbial interactions using this modeling approach.
RESULTS AND DISCUSSION
The ecosystem modeling project during the first year of effort has;
a. determined the compartmental model structure,
b. formulated the mathematical structure for the interaction equations
describing compartmental coupling,
c. began data summaries for parameter estimation, and,
d. established computer software and remote terminal communications
for model simulation and analysis.
t
The modeling study is designed to reflect the trophic structure of
submerged aquatic communities in Chesapeake Bay that are dominated by Zostera
marina and Ruppia maritima. The model incorporates biologically and
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151
ecologically realistic mathematical representations of flux pathways that
are based on testable assumptions and measureable parameters. Hierarchical
analysis is possible at the level of processes influencing or controlling
specific fluxes as well as analysis of overall community behavior and
interaction with other bay system components. The simulation model provides
an organizational structure and vehicle for incorporating and evaluating
the results of individual research efforts.
The overall research incorporates three other research efforts;
1. Productivity and nutrient dynamics associated with micro auto-
trophic and heterotrophic components of the eelgrass community
including the environmental controls of light, salinity and
temperature and measures of total community metabolish (KLW and RLW).
2. Within community macro-consumer dynamics (DFB and RJO).
3. SAV community interaction with bay consumer components; e.g.
migratory shellfish, finfish and waterfowl (JVM).
The first model version (Fig. 1) explicitly represents these major
research efforts both compartmentally and through the mathematical structure
proposed, the postulated mechanisms controlling interactions and community
dynamics.
The overall design is thus complimentary and highly interactive.
Specific aspects of SAV community structure and function addressed by other
CBP research efforts (e.g. R. J. Orth; M. Kemp, et al., ) will be incorporated
to the extent that the results suggest fundamental changes in design
or systems conceptualization.
The effort is approximately 30% complete to date (October, 1979).
The effort for the next six months will be to have a running computer version
i,
and to have preformed a sensitivity analysis with the current model. The
effort will identify parameters particularly sensitive to change and aid
in designing future work (see Wiegert and Wetzel, 1979).
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152
Literature Cited
Christian, R. R. and R. L. Wetzel. 1978. Interaction between substrate,
microbes and consumers of Spartina detritus in estuaries, p. 93-113.
IN; M. L. Wiley (ed.) Estuarine Interactions, Academic Press, New
York.
Wiegert, R. G. 1975. Mathematical representation of ecological interactions.
p. 43-54 IN; S. A. Levin (ed.) Ecosystem Analysis and Prediction,
SIAM, Philadelphia.
Wiegert, R. G. 1978. Population models: experimental tools for analysis
of ecosystems. IN; Horn, D. J. et al. (eds.) Proceedings of Colloquim
on Analysis of Ecosystems. Ohio State University Press, Columbus.
Wiegert, R. G. and R. L. Wetzel. 1974. The effect of numerical integration
technique on the simulation of carbon flow in a Georgia salt marsh.
Proc. Summer Computer Simulation Conf., Houston. Vol. 2: 575-577.
Wiegert, R. G. and R. L. Wetzel. 1979. Simulation experiments with a
fourteen-compartment model of a Spartina salt marsh, p. 7-39. IN;
R. F. Dame (ed.). Marsh-Estuarine Systems Simulation, University of
South Carolina Press, Columbia.
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