Chapter 2
Section A
PLANT COMMUNITY STRUCTURE AND PHYSICAL-CHEMICAL
REGIMES AT THE VAUCLUSE SHORES STUDY SITE
Richard L. Wetzel, Polly A. Penhale and
Kenneth L. Webb
Virginia Institute of Marine Science
College of William and Mary
Gloucester Point, Virginia 23062
Draft Final Report
To
Mr. William A. Cook, Project Officer
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street
Annapolis, Maryland 21401
June 1981
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600R811Q6
CHAPTER 2
SECTION A
PLANT COMMUNITY STRUCTURE AND PHYSICAL-CHEMICAL
REGIMES AT THE VAUCLUSE SHORES STUDY SITE
Richard L. Wetzel, Polly A. Penhale, arid
Kenneth L. Webb
June, 1981
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INTRODUCTION
Various autotrophic components contribute to the total primary production
in estuarine and coastal marine environments. The overall distribution and
relative contribution of each is determined in large part by geomorphology
and local physical and chemical regimes of a particular area (Mann, 1975).
In open water, phytoplankton dominate the autotrophic input of fixed energy
while in shoal-benthic habitats,algae, seagrasses and marshes contribute to
primary production as well. Unit area comparisons generally result in the
ordering: salt marshes > seagrasses > macro-algae > phytoplankton for many
Gulf of Mexico and U.S. Atlantic Coast estuarines. However, areal comparisons
and ranking vary considerably from system to system. Coastal, temperate
estuaries along the U.S. Atlantic Coast are generally characterized by inputs
from all. The Chesapeake Bay is a good example.
Partitioning primary production among these various components in the
Chesapeake Bay for comparative purposes is difficult due in large part to
the extreme complexity and physical characteristics of the Bay itself. At
least three major subdivisions can be identified based on hydrological regimes.
These consist of; 1. Bay stem, the major open water area of the bay, 2.
>
Shoal-benthic areas within the major water body delineated grossly by the
shoreline and extending to a mean subtidal depth of 2 to 3 meters, and 3.
Sub-estuarine water bodies made up of the principal tributaries entering
the Bay proper. The Bay stem is a major open water area and is dominated
by phytoplankton production. Shoal, benthic areas and the tributaries however,
make up a significant part of the Bay environment and are dominated by benthic
microalgae and macroalgae, submerged aquatic vegetation, and the wetland,
tidal marshes. Based on a gross areal approximation of the Virginia portion
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of Chesapeake Bay and estimates of annual net primary production for each
component, the shoal benthic habitat and the tidal marshes contribute
30-40% of the annual net primary production for the lower Bay.
Within the shoal-benthic areas, the relative contribution of submerged
aquatic vegetation (SAV) to primary production varies considerably. Orth
et al. (1979) estimated that Virginia SAV occupied a bottom area of approxi-
mately 85.4 km2 in 1978. Using their figure of 5300 km shoreline length
and assuming an average lateral extent of 300m, the shoal-benthic habitat
o
area would approximate 1600 km ; thus, SAV would occupy approximately 5%
of the area. However, these figures include the shoreline areas of the
tributaries where the major portion of the habitat is oligohaline or fresh-
water tidal where the two dominant species of SAV for the lower Bay, Zostera
marina and Ruppia marititna do not exist. By utilizing the present relative
abundance and distribution data for these two species (Orth et al., draft
report to the EPA, April 1981) and limiting the estimated shoal-benthic
habitats to brackish water and mesohaline areas, the percentage of shoal-
benthic habitat occupied by Zostera-Ruppia communities would significantly
increase.
The productivity of SAV communities in U.S. Atlantic temperate estuaries
is very high based on studies of similar Zostera-dominated systems (Billion,
1971; Thayer, et al., 1975; Penhale, 1977). Within SAV communities typical
of the lower Chesapeake Bay, primary production is partitioned among several
components: Zostera marina, Ruppia maritime, epiphytic algae (that attached
to seagrass leaves), benthic microscopic and macroscopic algae, and phyto-
plankton* A comparison of the productivity estimates of four major primary
producers in an estuarine system near Beaufort, North Carolina, points to
the importance of seagrasses in shallow coastal systems. For this system
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annual net production values as g C m yr~l were : 66 for phytoplankton
(Thayer, 1971), 249 for Spartina (cordgrass) (Williams, 1973), 330 for
Zostera and 73 for epiphytes attached to Zostera (Penhale, 1977). Therefore,
at least at the local level, SAV production is comparatively high and for
some systems may dominate total estuarine autotrophic production (Thayer et al.,
1975; Mann, 1975). For certain areas of the Lower Chesapeake Bay, e.g.
Bayside Eastern Shore, Mobjack Bay and shoal benthic areas in and around the
York River mouth, (Orth et al., 1979; Figure 4), SAV primary production is
a significant source of fixed energy and organic matter production available
to support either directly or indirectly heterotrophic, secondary production.
Simply comparing the autotrophic production of the various estuarine
primary producers, oversimplifies arid perhaps underestimates the role and
value of SAV communities in the Lower Chesapeake Bay.system as a whole. The
SAV communities add structural complexity and diversity to the shoal-benthic
habitat, thus creating habitats for epifaunal, infaunal and motile species
and refuge areas for prey species. In addition to these non-trophic character-
istics, SAV also function in the stabilization of sub-tidal sediments and
probably influence shoreline erosion processes by dissipating both tidal
and wind generated wave energy. SAV communities in the Lower Chesapeake Bay
therefore have both trophic and non-trophic significance in the ecology of
the overall system.
Historical studies of seagrass distribution and relative abundance of
SAV communities in the Lower Chesapeake Bay show major periods of decline
followed by periods of recovery. Eelgrass (Zostera) has reportedly undergone
major changes in abundance in 1854 and during the period 1889-1894 (Cottam,
1935 a,b). More recently, major declines in Chesapeake Bay SAV were observed
during the wasting disease of the 1930Vs (Cottam and Munro, 1954) and during
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a decline which began in 1973 and continued until 1978 which resulted in the
lowest population levels in 40 years (Orth et al., 1979). Despite the
documentation of these events, these fluctuations in abundance remain largely
unexplained.
Geographically, the dominant lower Bay species, Zostera marina, has a
world wide distribution in the temperate and subarctic regions in the Northern
Hemisphere (den Hartog, 1970). The southern range limit along the U.S.
East Coast is North Carolina. This limit is ascribed to high temperature
effects; thus Zostera in the lower Chesapeake Bay is existing very near its
geographical limit of distribution.
Apparently Ruppia maritima has broader temperature and salinity tolerances
than Zostera (Richardson 1980 and references therein); this is reflected in
the biogeographical distribution of Ruppia maritima. Within Chesapeake Bay,
Zostera marina and Ruppia maritima are limited to shoal benthic habitats
less than two meters mean depth (Orth et al., 1979).
Limitation of primary productivity of seagrass communities have been
ascribed to various environmental parameters. The influence of light, tempera-
ture and salinity have received the major research effort (Biebl and McRoy,
1971; Bachman and Barilotti, 1976; Penhale, 1977; Congdon and McComb 1979; and
references therein). It is generally accepted that the local light regime
limits the subtidal distribution of Zostera, while light, temperature and
nutrient regimes (primarily nitrogen) interact to control specific rates of
productivity over an annual cycle.
There is increasing evidence that suggests primary production in both
marine and estuarine systems is generally nitrogen limited (Postgate, 1971;
Ryther and Dunstan, 1971; Valiela, et al., 1973; Gallagher, 1975; Pomeroy,
1975; Orth, 1977; Nixon, 1980). Orth (1977) recently demonstrated a positive
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growth response in Zostera communities in the Bay to added commercial fertilizer
treatments. These data together with the consistent and predictable pattern
for the depth distribution of the seagrasses in the Lower Chesapeake Bay
suggests that light (and/or factors influencing both the quality and quantity
of the light regime) and nutrients are primary factors governing the distribu-
tion and metabolism of the submerged aquatic plant communities.
The overall objectives of the studies reported in the following sections
have been, 1. to describe the physical-chemical regime and plant community
structure of a naturally, unperturbed SAV community in the Lower Chesapeake Bay,
2. to derive estimates of community productivity and metabolism for diel,
seasonal and annual periods, and 3. to evaluate the principal mechanisms
controlling productivity and community dynamics. Various studies and experi-
mental designs have been initiated and completed during the period July,
1978 through November, 1980 to accomplish these overall objectives.
Study Site
Selection of the principal study site was decided by consensus of the
five original principal investigators and was composed of members associated
with different aspects of SAV research within the overall program. A seagrass
bed approximately 140 hectares in size located on the southeastern shore of
Chesapeake Bay was chosen as the principal study area. This area is known
locally as Vaucluse Shores and is situated north of Hungar's Creek at approxi-
mately 37°25'N. latitude, 75°59'W. longitude. Critera used for site selection
were:
1. the site had been previously studied and some background information
was available.
2. the bed is well established and historically stable,
3. the area is relatively remote and unperturbed,
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4. vegetationally, the bed contained the two dominant lower Bay species,
Zpstera marina and Ruppia maritima, and,
5. the bed was large enough to simultaneously accomodate our varied
studies and sampling regimes.
The bed is roughly triangular in shape with the apex to the north and
bordered on the east by the Vaucluse Shoreline, the south by the Hungar's
Creek channel and to the west by an off-shore sandbar (Figure 1). The area
was also a site for intensive vegetational mapping program and was completed
by the initiation of our studies in July 1978. During this studjj permanent
transects were established and are represented in Figure 1 as transect lines
labeled A through F, proceeding south to north. These were used in our
current studies for selection and identification of within-site sampling
stations. The figure also illustrates the submerged plant distribution and
zonal dominance by species. From these data, five habitat types were indicated
and have subsequently been used for within habitat sample site selection.
These areas are;
1. Ruppia maritima dominated community
2. Mixed vegetation areas
3. Sand Patches or Unvegetated bottom within the bed
4. Zostera marina dominated community, and
5. Sand Bar.
Sampling sites were selected for each of the areas between transects B and C
and permanently marked with bouys to identify stations for routine sampling
and experimental studies.
The contents of this chapter are divided into four major sections and
treated individually. These are 1) Plant Community Structure and Physical-Chemical
properties at the Vaucluse Shores study site, 2) Diel, Seasonal and Annual
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Studies of Community Productivity and Metabolism, and, 3) Light Relations
and Controls on Phytosynthesis and Community Metabolism, and 4) Nutrient
Dynamics.
Each section is presented separately with regard to methods, results
and discussion. The results of these various studies are summarized in a
concluding section. This format was adopted to more closely align with our
previously stated overall objectives. The time frame for the various studies
reported herein falls in the period July, 1978 through November, 1980. The
section of light relations is preliminary as many studies are continuing and
will not be completed until October, 1981.
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Figure 1. Vaucluse Shores Study Site.
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R = RUPPIA
Z = ZOSTERA
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CHESAPEAKE
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SECTION A
PLANT COMMUNITY STRUCTURE AND PHYSICAL-CHEMICAL
REGIMES AT THE VAUCLUSE SHORE STUDY SITE
INTRODUCTION
In order to determine strucutral properties of the SAV community and
physical-chemical regimes characteristic of the study area, routine studies
were carried out to investigate plant species distribution and relative
abundance, plant canopy structure and chemical characteristics, and various
sediment, chemical properties. All sampling was carried out at the Vaucluse
study site between transects B and C (Report Introduction, Fig. 1).
METHODS AND MATERIALS
Plant Distribution, Relative Abundance and Biomass;
Plant distribution and relative abundance was determined along transects
A, B and C in July, 1979 and along transect B in May, July and August, 1980
to determine areal coverage by species and distribution with water depth.
A line-intersect method was employed using two divers and is described in
detail by Orth, et al., (1979). Briefly, the transects, illustrated in Figure
1 were followed from the sandbar beginning at low tide and progressing toward
the shore. A 100 meter line marked at 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 repeated twice at each
sampling point. During each transect study, time and water depth were record-
ed at each station and later compared to a continuous, relative tide height
record kept near-shore. These data were used to calculate bottom depth
relative to mean low water at each sampling point (Orth, et al., 1979).
10
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These data also provided direct comparisons with the previous intensive mapping
effort (July, 1978) by Orth, et al. (1979) for identifying any major differences
in distribution between years and providing relative abundance and distribu-
tion information for the bed as a whole.
Plant biomass was determined at monthly intervals for the period April,
1979 to April, 1980 in the Ruppia, mixed, and Zostera dominated communities.
These data were collected by Orth and details of sample collection and process-
ing are given elsewhere (Orth, et al., 1981). However for comparison to
information given in this section, the results of these studies are presented
for continuity.
Plant canopy structure was determined in each vegetation zone at
approximately monthly intervals beginning in April, 1979 and continued through
August, 1980. Canopy structure was investigated by estimating leaf area
index (LAI), the ratio of leaf area to substrate area (Evans, 1972). Plant
samples of each species from the Ruppia, Zostera and mixed vegetation area
were collected by hand, returned to the laboratory and rinsed free of sediment.
Leaves were then removed and 3 replicate samples of 10 shoots each were
sectioned at 5 cm intervals from the base to apex of the leaves. Leaf area
(one-sided) was determined using a LI-COR Model LI13100/1+1 Leaf Area Meter.
LAI results are reported as m^ leaf surface (one-sided) per nr sediment surface
for 5 cm intervals for each species at each site as well as on a site basis.
Rooting depth was determined during July.1979 in the three major
vegetation areas by hand coring using a 10.26m (diameter) acrylic corer. Four
replicate cores were taken per area 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 contribu-
tion to total weight determined for each section.
11
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Chemical analyses of plant tissue consisted of wet weight:.-dry weight,
organic matter and C:N determinations. Samples were collected by hand with
2
a 0.033 m corer and separated by species into above and belowground fractions
at the Ruppia and Zostera site. At the mixed vegetation area, belowground
tissue was not separated by species. Leaf litter (aboveground, dead, un-
attached material) was also collected. Methods of analyses are as described
in the following section.
Physical-Chemical Characteristics of Plant Tissue and Sediments;
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 and February through May, 1980. Analyses performed
in relation to community type were, adenosine triphosphate (ATP), water
content, percent organic matter (POM), particulate organic carbon and nitrogen
(POC and PON). Duplicate sediment samples were taken by hand to a depth of
approximately 30 cm using a 5 cm (diameter) acrylic core tube. The cores were
capped underwater and sealed with tape for transport to the laboratory.
The cores were then extruded, split vertically and section into 0-2, 2-5,
5-10, 10-15, 15-20 and >20-30 cm horizontal intervals. For each core, processing
consisted of: duplicate 1 cc plugs extracted using boiling 0.1 M sodium
bicarbonate for ATP analysis (Bancroft, et al., 1974); the remaining sediment
fraction was frozen for later water content/organic matter and POC/PON
analyses.
Water content and organic matter content was determined on frozen
sediments by drying at 60°C to constant weight for water content and ashing
@ 550°C for organic matter determination. Wet weight:dry weight and organic
matter content for plant tissue samples were determined in a similar manner.
All weights were determined to the nearest 0.01 mg. POC and PON analyses
12
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for both plant tissue and sediments were performed using a Perkin-Elmer
Model 240B Elemental Analyzer..
RESULTS AND DISCUSSION
Sampling and analyses to characterize plant community structure and
general physical and chemical characteristics at the Vaucluse study site
were conducted over various intervals beginning in July 1978 and ending in
July 1980. The sampling was scheduled in such a manner to evaluate both
seasonal and annual cycles of the various parameters and to create a data
base for longer term monitoring efforts. More specifically, these data have
been used for designing studies around documented temporal events as well as
for designing sampling strategies and experimental approaches. These data
have been used to partition other data sets for both spacial and temporal
correlation with other measures.
The results and discussion of these efforts are divided into two
principal areas; 1) Plant Community Structural Characteristics and Dynamics,
and 2) Physical-Chemical Characteristics of Vegetated and Non-vegetated
Sediments.
Plant Community Structural Characteristics and Dynamics
Plant community structural characteristics and dynamics were investigated
by transect studies of species distribution and relative abundance (% cover),
monthly biomass determinations, rooting depth analyses, determination of
canopy structure (leaf area index ot LAI) and plant tissue analyses for
wet:dry weight ratio, organic content, and carbon:nitrogen ratios.
1. Relative Abundance and Distribution; Plant distribution and percent
cover (as an estimate of species relative abundance) was determined along
transects A, B, and C in July, 1979. Figure 2 illustrates the distribution
and percent cover by species relative to bottom depth at mean low water (MLW)
13
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Figure 2. SAV Distribution and Relative Abundance along Transects A, B,
and C, July 1979.
14
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TRANSECT B
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along the transects. In terms of areal coverage and relative abundance, the
most significant stands of Z. marina are located south of transect C and
occupy the deeper (>50 cm below MLW) areas of the grass bed extending
shoreward of the sand bar. Significant stands of R. maritima are located
along all transects but confined to the shallow (<50 cm below MLW) areas
at this time of year. There is a major zone of overlap for the two species
on all the transects that ranges laterally from ca. 250 m on transects A and B
to 100 m on transect C. The depth range of the overlap or mixed vegetation
areas is ca. 60 to 100 cm below MLW bottom depth on transects A and B. The
mixed area on transect C is confined to below 60 cm and the only monospecific
"L. marina stands are adjacent to the sand bar. These results for both
relative abundance and distribution follow closely the results obtained by
Orth, et al. (1979) in July ,1978 for the same transects as well as the lower
Bay generally.
The general pattern of depth distribution and abundance is characteristic
of SAV communities in the lower Chesapeake Bay (Orth et al., 1979). JR.
maritima is the dominant species in shoal benthic habitats less than 50 cm
deep (MLW) while monospecific stands of Z_. marina are found in habitats >100 cm
deep. The mixed Ruppia-Zostera vegetation zones are also characteristic;
the depth ranges (ca.. 60-90 cm!.below MLW) observed at the Vaucluse site
are typical of the lower Bay.
This generalized distribution pattern for lower Bay seagrasses raises
important questions and suggests hypotheses relative to factors governing
plant distribution and abundance. Apparent among these are the physiological
characteristics of each species and their response to environmental factors
such as light, temperature, nutrients and desiccation. Direct competition
*
between species for available substrate does not appear a natural determinate;
however, few data exist to evaluate the role this potential interaction might
play.
16
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Figure 3. SAV Distribution and Relative Abundance along Transect B, July,
1979; May and August, 1980.
17
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TRANSECT B
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The dynamic nature of the distribution and abundance of the plant community
was investigated by repeating relative abundance and distribution surveys
along transect B in May and August, 1980 (Fig. 3). Based on the results of
plant biomass studies (Orth, et al., 1981) our sampling times represented
months of maximum growth rates (May through July) and die-back (August) for
Z. marina and R. maritima at the mixed site. R. maritima in the shallow area
exhibits a fall period of maximum growth.
Species distribution along the depth gradient is similar for the three
sampling periods; however, the relative abundance (% cover) is dramatically
different (Fig. 3). During the late spring (May) Z_. marina is relatively
uniformly distributed in the deeper areas of the grass bed and has maximum
relative abundance during this period of time. By mid-summer (July), both
species are relatively uniformly distributed within their zonal limits
although Z_. marina is reduced in relative abundance. There is some indication
that R. maritima has invaded the deeper, Zostera-dominated areas of the bed
during this period but confirmation of this can only result from longer term
monitoring and more detailed studies. During the later part of the summer
(August), Z_. marina is significantly reduced in relative abundance at the
deeper stations indicating natural plant mortality, JR. maritima is more
patchly distributed and generally occupies the deeper areas within its range.
These results suggest not only the dynamic nature of the bed, but, also the
complementary nature of the growth and dynamics of the two seagrass species.
We observed that species dominance and abundance shift during the growing
season and these parameters are out of phase for the two seagrass species.
This indicates that, as suggested previously, physiological and morphological
differences between species rather than direct competitive interactions
influence species distribution. Consequently, from an applied standpoint,
criteria developed for management of lower Bay seagrasses may not universally
apply to all species and for all locations.
; 19
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2. Biomass Relationships and Rooting Depth; Above-ground plant biomass
(vegetative shoots) in the three vegetated zones was also determined to
provide information on growth, production and community dynamics. Detailed
analyses and methodologies are presented elsewhere (Orth, et al., 1981).
A summary of these data are provided in Figure 4. Over an annual period,
It. maritima exhibits a unimodal cycle of biomass. At the Ruppia site,
the peak biomass is in the fall (September-October) while the peak occurs
earlier (July-August) for Ruppia at the mixed site. _Z. marina exhibits a
bimodal cycle of annual growth and mortality with periods of maximum growth
occurring b'oth in the spring to early summer and also in the fall to early
winter; these periods of maximum biomass are followed by periods of shoot
die-back. The re-growth of Z^. marina is not as pronounced in the mixed site
as in the Zostera site. These data also indicate to some extent the year to
year variation in standing stock. For example, there was approximately a
50-60% increase in peak above-ground biomass for Ruppia between 1979 and 1980
and a corresponding 20-25% increase for Zostera..
The above-ground biomass pattern observed for Zostera is similar to that
observed in other warm-temperate climates and probably reflects a negative
response to increased summer water temperatures. In North Carolina, Penhale
(1977) reported a peak Zostera shoot biomass in March followed by a general
biomass decline throughout the rest of the year; this decline occurred as the.
water temperature increased during the summer. Also in North Carolina, Thayer
et al. (1975) observed a dramatic Zostera biomass decline following peak
summer water temperatures in August. Biebl and McRoy (1971) suggested that
prolonged or frequent temperatures rises above 30°C could result Zostera
mortality. A temperature preference is also reflected in the arctic and
temperate distribution of Zostera which reaches its southernmost limit on
the eastern U.S. coast at Cape Fear, North Carolina (Thayer et al., 1975).
20
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Figure 4. Shoot Biomass of Rvippia (•) and Zbstera (O) at the three
Vegetated Sites. Data from Orth et al., 1981.
21
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In general, Ruppia appears most tolerant of higher temperatures than
Zostera, based on seasonal studies (Richardson, 1980 and references therein)
the geographic range of Ruppia which extends south to the Gulf Coast of the
United States. Nevertheless, the annual biomass cycle of Ruppia at Vaucluse
Shores may also involve a negative response to high summer temperatures as
well as to other parameters characteristic of habitats where Ruppia grows.
At the Ruppia site, the shallow water allows for greater light penetration
than in the mixed site; in fact, Ruppia at its shallower habitat may be
photo-inhibited during much of the summer. In addition, plants in this
zone are frequently exposed during extreme low tides. The low July-August
biomass of Ruppia here may be due to the combination of high temperatures,
high light, and desiccation damage; a more favorable light and temperature
regime is probably present during its fall biomass peak. At the deeper,
mixed site where the light and temperature regime is less extreme, Ruppia
biomass exhibits a peak in July followed by a decline during the warm
August period. Ruppia at the mixed site is shaded by Zostera which has
longer leaves; the lower annual Ruppia biomass here may reflect lower
productivity rates due to less light penetration.
Root:shoot dry weight ratios were calculated from the mean biomass data
of Orth, et al. (1981) (Fig. 5). Root:shoot ratios reflect the plant's
metabolic expenditure in terms of non-photosynthetic and photosynthetic
tissue. Root:shoot ratios less than 1 were noted during much of the year for
both species at the mixed site and for Zostera at the Zosfcera site. In
contrast, Ruppia at the Ruppia site exhibited a ratio consistently above 1,
which suggests a different strategy for plants at the Ruppia site. The below-
ground portion of the plant is considered an important site of nutrient uptake
for submerged angiosperms (Penhale and Thayer, 1980 and references therein).
It is possible that lower interstitial nutrient concentrations may characterize
23
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Figure 5. Root:Shoot Dry Weight Ratio of Rtippia (•) and Zbstera (O) at
the three Vegetated Sites. Values calculated from the data of
Orth, et al., 1981.
24
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Ruppia
M
A S
1979
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M A M J J
1980
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A
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the Ruppia site; thus, the plant may expend more energy toward underground
growth. At the deeper mixed and Zostera sites characterized by lower light
penetration, root:shoot ratios less than 1 may reflect an additional expenditure
toward more photosynthetic tissue.
Rooting depth analyses of the seagrasses in the three vegetated habitats
indicate that greater than 98% of the root-^rhizome system is located in the
upper 10 cm of the sediment (Table 1). At the Ruppia site, a:greater pro-
portion of below-ground biomass was located in the upper 2 cm than at the
3
other two sites. The mean total root-rhizome biomass in the 1,226 cm core
sections was highest at the mixed site, with considerably lower values at
the Zostera site and lowest values at the Ruppia site.
The rhizosphere is the portion of the sediment under the immediate
influence of the sphere of plant roots (Rovira and Davey, 1974). Seagrass
roots may release oxygen to the sediments (Oremland and Taylor, 1977;
lizumi, et al., 1980). Also, dissolved organic carbon compounds may be
released by seagrass roots (Wetzel and Penhale, 1980; Wood and Hayasaka,
1981). In the highly anaerobic sediments of communities of plants with
submerged roots such as seagrass systems, plant activity results in a
profound influence on physical, chemical, and biological characteristics of
this zone. In the Vaucluse Shores system, the rooting depth data suggest
that this dynamic zone of activity is concentrated in the upper 5 cm of the
sediment. In addition, plant influence on sediment processes may be greatest
at the mixed site due to its greater biomass of root-rhizome material.
3 Canopy Structure: Plant community growth involves the response of •'.
individual plants to the environment and to other individuals in the community.
LAI is a fundamental parameter in community analysis since community growth is
not simply determined by the photosynthetic capacity of individual leaves.
That is, community growth is the product of the net assimilation per leaf
26
-------
TABLE 1. ROOTING DEPTH ANALYSES (JULY, 1979). VALUES ARE x + SD.
Site
Depth (cm)
N5
Total Root & Rhizome
Dry weight (mg)
Total Wt. %0-5 cm
%0-10cm
Ruppia (n = 4) 0-2
2-5
5-10
10-15
Mixed Bed (n = 3) 0-2
2-5
5-10
10-15
Zostera (n = 3) 0-2
2-5
5-10
10-15
2803 + 876 67 +
22 +
9-1-3
2 + 0
9750 + 2273 55 +
28 +
15 +
2 + 2
4540 + 908 51 +
42 +
6 + 2
0.7 +
9.7 89 + 3.0
10.0
.3
.96
4.6 83 + 11
15.2
10.1
14 93 + 3.2
15.9
.6
0.58
98 + 1.0
98 + 0.6
99 + 0.6
-------
area and the LAI. An increase in plant density affords a greater potential
for community production but mutual shading reduces the light penetration to
a point where net production decreases. Many factors such as light intensity,
leaf morphology, and leaf orientation (i.e., as influenced by tides and
currents f or seagrasses), play a role in determining the optimal LAI of a
community.
The results of our leaf area index (LAI) studies showed differences among
vegetated sites (Fig. 6). Analysis of variance of one-sided LAI by date and
site indicated significant differences among sites (p=0.016). The maximum
one-sided LAI values for the three sites in this study (Ruppia = 3.4, mixed =
4.2, Zostera = 3.2) were within the range of maximum values reported for other
seagrass communities. For example, maximum one-sided LAI values reported for
seagrass systems include 16.8 (Dennison, 1979), 9 (Jacobs, 1979), and 3.3
(Aioi, 1980) for Zostera; 9.3 for Thalassia (Gessner, 1971), and 8.3 and 1.4
for Posidonia and Cymodocea, respectively (Drew, 1978). Few data exist for
canopy structure in submerged angiosperm communities compared to terrestrial
communities. LAI values for terrestrial communities generally range from 1
to 12 and high LAI values tend to be positively correlated with high annual
productivity (Leopold and Kreideman, 1975).
In a detailed community structure analysis of a monospecific Zostera
community across a depth gradient, Dennison (1979) concluded that changing
leaf area was a major adaptive mechanism to decreasing light regimes. He
observed that the LAI increased with increasing depth to the deepest portion
of the bed where the LAI decreased. At these stations with the lowest
light penetration, further increases in LAI probably could not be maintained
due to the lowered net photosynthesis with less available light. This is
similar to the results obtained in this study in which LAI values generally
increase from the Ruppia to the mixed site and then decline at the deep
Zostera site.
28
-------
Figure 6. One-sided Leaf Area Index for the three Vegetated Sites, 1979-1980.
29
-------
4-
X 3
UJ
o
2-
UJ
Ruppia SITE
A ' M ' J ' J ' A ' S ' 0 ' N ' D | J
1979
A ' M r J ' J ' A '
1980
-------
The seasonal pattern of LAI reflects various trends within the three
sites (Fig. 6). At the Ruppia site, the decrease in LAI during fall-winter,
1979 mirrored a decrease in shoot densityi.leaf length and shoot biomass
(see Orth et al., 1981 for density, length and biomass data). The summer
increase in LAI corresponded to increasing insolation and reflected increasing
density, biomass and leaf length. At the mixed site, which generally
exhibited the highest LAI values, the steady decrease in LAI from August,
1979 to February, 1980 was a result of decreasing biomass, length and density
of Ruppia. Although the density of Zostera increased during this period of
new shoot growth, the leaf length and biomass decreased. The rapid LAI
increase in spring, 1980 reflected rapid increases in biomass and length of
Zostera and in density, length and biomass of Ruppia. At the Zostera site,
the steady LAI decline from April to October mirrored a decrease in leaf
length. The sharp decline in LAI at all three sites from April to June, 1980
reflected an unexplained decrease in shoot density.
The interrelationship of these community parameters is reported in other
studies* In a Zostera community in France, Jacobs (1979) observed a seasonal
trend in which increases in LAI paralleled increases in shoot density and the
number of leaves per shoot. Aioi (1980) observed that bqth the LAI and shoot
biomass exhibited a similar unimodal seasonal pattern with a maximum in May;
he related this pattern to day length. Center (1981) observed a continuous
trend toward maximum LAI of a freshwater Eichhornia community in which
increases in plant density in response to available space were followed by
increases in plant size in response to competition for light.
The results of the canopy structure studies also showed differences in
LAI between Ruppia and Zostera. Using Duncan's multiple range,test the
analysis indicated that the mean LAI values of Ruppia and Zostera were
significantly different (p=0i05). The data were further analysed using a
31
-------
least squares test for differences among means by species for the various
sites (Table 2). For Zostera, the mean LAI was not significantly different
between the mixed and Zostera sites. The mean LAI for Ruppia was significantly
different between the Ruppia and mixed sites; both of these were significantly
different from Zostera.
One-sided LAI values were calculated for 5 cm vertical sections of leaf
material in order to obtain information on the vertical stratification of
leaf area at the three vegetated sites (Figures 7,8,9,10). Scatter plots
of the data collected over the 17-month study show that for both Ruppia and
Zostera, maximum leaf area was concentrated in the lower portion of the canopy.
The Ruppia canopy exhibited the greatest concentration of leaf area from 0
to 5 cm above the substrate. Analysis of variance of the vertical distribution
of LAI showed highly significant differences between species (p = 0.0001).
Examples of the vertical stratification of leaf area of Ruppia and Zostera
at the three sites are presented in Figures 11, 12, and 13. Although the
LAI changes during the year, the relative distribution of leaf area does not
change. Over the yearly cycle, the LAI for Ruppia and for Zostera in their
pure stands was generally higher than that of each species at the mixed site.
The three examples illustrate a period of major biomass decline of Zostera
(August, 1979; Fig. 11), a period of low biomass of both species (January,
1980; Fig. 12), and a period of generally high biomass and long leaf length
of both species (July, 1980; Fig. 13).
The canopy structure at the three sites probably reflects adaptations
to the specific light regimes. In the shallow, high light environment at
the Ruppia site, the upper portion of the canopy may be photoinhibited during
much of the year; a greater leaf area near the substrate where light is
reduced may allow for a greater net canopy photosynthetic rate. For both
species at the mixed site and for Zostera at the Zostera site, concentration of
32
-------
Table 2. Analysis of Variance (Least Squared Means).
Group Number
Group Species Site I/J 1 234
1 Ruppia Mixed * 0.0001 0.0031 0.0001
2 Zostera Mixed 0.0001 * 0.0288 0.2412
3 Ruppia Ruppia 0.0031 0.0288 * 0.001
4 Zostera Zostera 0.0001 0.2412 0.001 *
Prob> ITI HO: x (I) = x (J)
33
-------
Figure 7. Scatter Plot of the Vertical Distribution of LAI for Ruppia at
the Ruppia Site, 7 Observations hidden.
34
-------
E
u
o
ui
CO
35-i
30-
25-
Ruppia maritima
Ruppia BED
a.
o
15 -
999
10 -
5J
0.5
1.0
1.5
2.0
LEAF AREA INDEX
-------
Figure 8. Scatter Plot of the Vertical Distribution of LAI for Rvippia at
the Mixed Bed Site, 15 Observations hidden.
36
-------
E
u
o
Ul
v>
LL
O
O.
O
35-i •
30-
25-
20-
15 -I
10 -
0
Ruppia maritime
MIXED BED
•• • ••
0.5
i
1.0
i
1.5
2.0
LEAF AREA INDEX
-------
Figure 9. Scatter Plot of the Vertical Distribution of LAI for Zbstera
at the Mixed Bed Site, 30 Observations hidden.
38
-------
50-
40-
E
2 30-
z
o
K
0
UJ
OT 20-
UL
O
Q. -
10-
o-
c
Zostera marina
MIXED BED
• •• •
•«• ••
• •••• ••
• •• •••• • ••
•• •••• •• • • •
•
1 i i i
) 0.4 0.8 1.2 1.6
LEAF AREA INDEX
-------
Figure 10. Scatter Plot of the Vertical Distribution of LAI for Zostera
at the Zostera site, 32 Observations hidden.
40
-------
50 ^
_. 4O-
6
g
V)
a.
o
Q.
O
20-
10-
0J
Zostera marina
Zoster Q BED
0.4
0.8
\
1.2
I
1.6
LEAF AREA INDEX
-------
Figure 11. Vertical Distribution of LAI for Riippia and Zoster a at the three
Vegetated Sites, August, 1979.
42
-------
LU
50-
40-
30-
20-
Ruppia BED
Ruppia maritime
Zoster a BED
Zostera marina
\
2
MIXED BED
Ruppia maritime
MIXED BED
Zostera marina
0 I
LEAF AREA INDEX
i
2
-------
Figure 12. Vertical Distribution of LAI for Ruppia and Zostera at the three
Vegetated Sites, January, 1980.
44
-------
Ul
cr
i-
(O
CD
3
to
5
o
DC
U.
(O
(E
UJ
t-
LU
UJ
O
50-
40-
30-
20-
10
50-
40-
30-
Ruppia BED
Ruppia moritima
MIXED BED
Ruppia maritimo
Zoster a BED
Zoster a marina
MIXED BED
Zoster a marina
2 0
LEAF AREA INDEX
-------
Figure 13. Vertical Distribution of LAI for Ruppia and Zostera at the three
Vegetated Sites, July, 1980.
46
-------
50-
40-
30-
20-
Ruppia BED
Ruppia maritime
Ul
H
<
cc
CO
ID
o
tr
u.
tr
LJ
i-
UJ
UJ
o
50-
40-
30-
20-
10-
Zoster a BED
Zoster a marina
MIXED BED
Ruppia maritime
10
i
20
MIXED BED
Zostera marina
\
10
i
20
LEAF AREA INDEX
-------
photosynthetic area in the lower canopy would compensate for the decreased
light with increasing mean depth. Species differences in the vertical
stratification of leaf area would be particularly important at the mixed
site where Ruppia is shaded by Zosteraj these differences probably contribute
to successful co-existence.
Although other factors are involved, competition for light in submerged
communities is probably the most important factor in determining community
structure. Haller and Sutton (1975) characterized the community structure
of Hydrilla, an introduced species which often displaces native species in
Florida. They proposed that Hydfilla communities, by forming a dense canopy
at the water surface (which can reduce light penetration by 95% in 0.3 m)
have effectively replaced native Vallisneria communities which are less
dense. In a study of Myriophyllum and Vallisneria, Titus and Adams (1979)
observed that the former had 68% of its foliage within 30 cm of the surface
while the latter had 62% of its foliage within 30 cm of the bottom. Like
Hydrilla, Myriophyllum has been successful in establishing dominance in
competitive situations.
4. Wet;Dry Weight Ratios of Plant Tissue; Wet:dry weight ratios of Ruppia
and Zostera were calculated for samples collected in May, 1981. Ratios were
6.31 +_ 0.76 (n=5) for Ruppia leaves and 8.72 +_ 0.47 (n=5) for Ruppia roots
and rhizomes. Ratios were 7.2*) + 0.65 (n=4) for Zostera leaves and 8.69 +
0.73 (n=4) for Zostera roots and rhizomes. These values are similar to ratios
reported by Neinhiis and DeBree (1977) for Zostera marina (leaves, 6.7 to 11.1;
yoots and rhizomes, 5.5-16.7).
48
-------
5. Elemental carbon and nitrogen analysis; Elemental analysis of plant
materials is useful for a number of purposes. Tissue analysis may provide
estimates of nutrient availability for growth (Gerloff and Krombholz, 1966)
or give an indication of possible food quality for other organisms
(Godshalk and Wetzel, 1978). Carbon:Nitrogen atomic ratios in plankton are
considered to be about 6.6 according to the Redfield Model (Redfield,
1958). Goldman et al. (1979) have suggested that phytoplankton of
composition with greater ratios are deficient in nitrogen, and grow slower
than maximally. Angiosperms contain more structural carbohydrates, such as
cellulose, than phytoplankton (e.g. Almazan and Boyd, 1978), and thus these
considerations are not as straightforward as in the phytoplankton although
similar analogies have been used. Carbon:nitrogen ratios seldom approach
the Redfield Model value of 6.6 in seagrasses and reported values range
from 10 to 80 with values of 10 to 20 being ususal for Zostera marina
(.Table 3).
Seasonal changes might be expected in the nitrogen and carbon content
of seagrasses since these plants, at least in temperate climates, undergo
seasonal periodicity in growth. Vinogradov (1953), reviewing earlier work,
suggested a seasonal change in nitrogen content of Zostera marina, with the
lowest nitrogen content occuring toward fall or during winter. Harrison
and Mann (1975) report a distinct sinusoidal character with a peak leaf H
percentage of 4.8 in March and a minimum of 1 .2 in September. Our seasonal
data set for percentage nitrogen in Zostera marina leaves (Table 5) is
extensive, but, as of this writing, appears inadequate to establish good
corelations with season or with biomass (Figure 4). Hopefully with
expansion of the data set or more complete analysis, a meaningful
intrepretation will be available. However, it appears extremely unlikely
tnat any textbook picture such as is presented by Harrison and Mann (1975)
49
-------
Table 3. Weight percent C and N and atomic C:N ratios in seagrasses
Species
Amphibolis griffithii
Cymodocea nodosa
C. serrulate
H
M
Enhalus acoroides
Halodule uninervis
Halophilia decjpiens
H. havaiiana
H. ovalis
ii
H. ovata
H. spinulosa
II
Phyllospadix scouleri
Posidonia oceanica
n
P. ostenfeldic
It
P. sinuosa
riant Part
leaves
steins
leaves
leaves
leaves
rhizomes
leaves
rhizomes
leaves
leaves
rhizomes
leaves
rhizomes
leaves
leaves
leaves
roots
rhizomes
leaves
roots/rhizomes
leaves
" - roots
Syringodium isoetifolium leaves
Thalassia hemprichii
T. testudinum
Zostera capricorni
rhizomes
leaves
leaves
mizoiaes
leaves
rhizomes
leaves
Location
W. Australia
u
Corsica
N. Queensland
it
N. Queensland
Palau
N. Queensland
n
Hawaii
W. Australia
N. Queensland
n
n
•i
California
Corsica
n
n
W. Australia
ii
ii
N. Queensland
Barbados
N. Queensland
%c
30.
3Q
37.
38.
36.
39.
>!?.
36.
24.
16.
29.
36.
33.
38.
35.
33.
34.
22.
21.
31.
32.
*N
1.3
0.96
1.59
1.25
1.70
2.18
0.94
1.21
1.13
0.63
1-.45
1.98
1.57
0.76
0.93
1.04
0.73
0.80
0.55
1.63
1 .11
C:N
26.8
36.4
27.2
21.6
35.4
67.1
19.8
82.
24.7
20.8
43.4
33.2
27.7
34.6
24.4
24.8
29.8
51.7
77.6
46.5
23.3
21 .2
24.5
58.2
43.7
36.9
54.0
32.
44.9
25.5
77.5
22.2
13.9
30.1
20.5
58.1
33.6
Ref.
1
1
1
2
1
2
2
2
1
1
1
2
2
1
2
1
1
2
2
2
1
1
1
1
1
1
1
1
1
2
2
1
3
2
2
1
Table 3 continued on next page.
50
-------
Table 5 cont.
Zostera marina leaves
leaves
leaf-sheath
leaf-blade
" rhizomes
root
" dead- leaves
" leaves-green
" leaves-brown
leaves
" leaves
M rhizome
root
" leaves
California
Rhode Island
Japan
It
n
n
Wash. , Alaska
Denmark
n
Germany
Japan
n
ii
Canada
58.
31.
57.4
40.
58.
29.
29.1
55.9
50.7
58.5
56.0
54.1
26.2
55-45
6.14
2.05
2.0
2.29
1.4
1.4
1.4
2.25
1.5
2.75
2.6
2.8
2.9
1.2-4.8
7.2
17.8
18.
18.6
25-
18.
18.8
12.9
17.5
12.0
11.9
10.6
7.8
9-50
1
1
4
4
4
4
5
6
6
6
7
7
7
8
References 1; Atkinson and Smith, 1981; 2) Birch, 1975; 3) Patriquin, 1972;
4) Aioi and Mukai, 1980; 5) Godshalk and Wetzel, 1978; 6) Vinogradov, 1955;
7) Seki and Yokohama, 1978; 8) Harrison and Mann, 1975.
will be found. An estimated median value for percent nitrogen for Zostera
marina leaves from the literature (Table 5) is 2.5$ and apparently higher
tnan the mean of 1.8 from our data. Percent organic carbon in Zostera
marina leaves appear to be quite similar for our data (mean of 56$) and the
literature (median 56$); thus the C:H ratio of our data (25) is almost
twice the median value (15) from the literature. In contrast to the
conclusion from the evaluation of interstitial nutrients (Section D,
Nutrient Cycling), these data would seem consistent with the suggestion
that the Vaucluse grass beds are nitrogen deficient.
Roots and rhizomes of Zostera marina for Vaucluse clearly contain less
carbon and nitrogen than do the leaves and show a higher C:N ratio (Table 4).
This relationship seems to generally be borne out by the literature as well .
(Table 5) with the data set of Seki and Yokohama (1978) a possible
exception. Our data are for combined root and rhizome tissues and the data
of Seki and Yokohama (1978) suggest that it is specifically the root tissue
that may contain lower C concentrations.
-------
TABLE 4. Percentages carbon and nitrogen of the total root and rhizome
material of Zostera marina and the atomic ratio of carbon:nitrogen from the
pure stand plant material. Numbers are the mean, number
the standard deviation.
DATE
FEE 80
MAR 80
JUN 80
AUG 79
Auu bU
bitf 79
OUT 79
MEAN
TABLE 5.
Zostera
ORGANIC CARBON
PERCENTAGE
33.6 (2) 0.67
29.1 (1)
30.1 (4) 2.43
27.5 (5) 7.11
31.3 (8; 3.46
28.5 (3) 3-38
34.8 (3) 0.435
30.6
Percentages carbon
marina and the atomic
ORGANIC NITROGEN
PERCENTAGE
1.59 (2) 0.075
2.01 (1)
1.08 (4) 0.217
1.27 (5) 0.456
1.31 (8) 0.194
1.35 (3) 0.0411
1.18 (3) 0.0705
1.26
of observations and
CARBON NITROGEN
ATOMIC RATIO
24.6 (2) 0.66
16.9(1)
33.7 (4) 9.20
26.1 (5) 3.35
28.8 (8) 5.7
24.6 (3) 3.32
34.5 (3) 1.69
28.4
and nitrogen of the total leaf material of
ratio of carbon: nitrogen from the pure stand
plant material. Numbers are the mean, number of observations and the
standard deviation.
DATE
FEB 80
JUN 80
AUU 79
AUU 80
BEP 79
UCT 79
ORGANIC CARBON
PERCENTAGE
35.9 (D
34.1 (6) 4.22
37.2 (10) 1.74
37.5 (5) 0.67
34.5 (3) 0.242
38.6 (2) 3.41
ORGANIC NITROGEN
PERCENTAGE
3.03 (D
1.54 (6) 0.417
2.18 (10) 0.336
1.42 (5) 0.135
1.50 (3) 0.129
2.01 (2) 0.0449
CARBON NITROGEN
ATOMIC RATIO
13.8 (1)
27.0 (6) 5.09
20.3(10) 2.49
31.2 (5) 3.38
26.9 (3) 2.38
22.4 (2) 2.48
MEAN
36.4
1.81
24.9
-------
Since seagrasses are consumed primarily through the detrital food
chain, there has been considerable interest in decompostion of the plant
material. Nitrogen and carbon percentages and atomic C:N ratios in dead
Zostera marina leaf material is shown in Table 6. The means are all lower
than those from fresh leaf material. Harrison and Mann (1975) reported
nitrogen percentages from 1 to 1 .5 in contrast to our values of 1 to 2 and
carbon percentages from 26 to 32 in contrast to our values of 20 to 40 for
dead leaves of Zostera marina. Although no seasonal cycle is apparent for
either N or C percentages, the atomic C:N ratios show a minimum of 15 in
March and a maximum of 27 in September (Harrison and Mann, 1975) in
contrast to our data of maxima in September and February of 31 and 41
respectively. The comparison of C and N percentages of live vs dead leaves
of Zostera marina is in agreement with the hypothesis that soluble C
leaches from or is withdrawn from living leaves before they die to a
greater extent than soluble N. A consideration of changes in epibiota
during senescense and death of leaves may alter this intrepretation.
E b. Percentages carbon and nitrogen of the total detrital leaf
material of Zostera marina and the atomic ratio of carbon:nitrogen from the
pure stand plant material. Numbers are the mean, number of observations
and the standard deviation.
MTE
FEE
JUN
AUG
SEP
OCT
80
80
80
80
79
ORGANIC CARBON
PERCENTAGE
36.4
28.3
28.4
30.8
35.2
(3) 1
(4) 4
(5) 3
(6) 1
(3) 1
.43
.06
.9
.57
.95
ORGANIC NITROGEN
PERCENTAGE
1
1
1
1
2
.07
.39
.31
.20
.06
(3) 0
(4) 0
(5) 0
(6) 0
(3) 0
.231
.233
.186
.207
.079
CARBON NITROGEN
ATOMIC RATIO
40.9
23.8
25.9
30.7
20.0
(3)
(4)
(5)
(6)
(3)
9.54
2.08
5.9
5.7
1.85
27.3
1.20
22.5
-------
TABi.fi 7. Percentages carbon and nitrogen of the total material of Ruppia
maritime and the atomic ratio of carbon:nitrogen. Numbers are the mean,
number of observations and the standard deviation.
DATE ORGANIC CARBON ORGANIC NITROGEN CARBON NITROGEN
PERCENTAGE PERCENTAGE ATOMIC RATIO
LEAF MATERIAL FROM RUPPIA SITE
36.5 (4) 3.53 2.48 (4) 0.118 17.2 (4) 0.876
LEAF DETRITUS
28.8 (1) 1.91 (1) 17.6 (1)
ROOTS/RHIZOMES FROM RUPPIA SITE
22.Q (2) 5.23 0.85 (2) 0.078 30.1 (2) 4.6
RUOTS/RHIZOMES FROM MIXED SITE
54.3 (2) 3.04 1.37 (2) 0.177 29.7 (2) 6.5
A consideration of N and C values from Zostera marina in comparison to
those of Ruppia maritime from the pure stands as well as the intermediate
mixed bed may increase our understanding of these species response to
environmental conditions as well as factors which influence their
distribution with depth. Since we discussed our mixed bed data base (Ocean
City Meeting, Oct. 1980) we have decided that there are technical reasons
which cast doubt o'n their validity and we will not use those data in
question. Our present small data base from Ruppia maritime is reported in
xable 7; we do have more samples to analyze. Carbon content of Zostera
marina and Ruppia maritime are essentially the same, especielly in the
leeves. Ruppie maritime leaves eppear to be richer in nitrogen then leeves
of Zostere marina. Until we expend our date bese we will not attempt
intepretations of these dete.
-------
Physical-Chemical Characteristics of Vegetated and Non-vegetated Sediments
Physical-chemical characteristics of vegetated and non-vegetated sediment
were investigated by studies of organic matter and adenosine triphosphate
(ATP) content of sectioned 30 cm cores taken from study sites at Vaucluse
Shores. The interstitial nutrient content and C:N ratios of these cores is
discussed in Section D, Nutrient Cycling.
1. Organic Matter Content; Overall, the organic matter content of the
sediment was low at all sites; most values were less than 1% of the dry
weight and reflect the sandy nature of the substrate. The organic content
of sectioned cores at four sites during July, 1979 is presented in Figure
14. Several factors contribute to the organic matter pool in the sediments:
living and dead plant and animal tissue, microbial autotrophs and hetero-
trophs, dissolved organic matter, etc. The influence of the plant community
is seen in the total amount and depth distribution of organic matter content
at the four sites. The organic content is lowest in the non-vegetated
sand patch; in addition, the depth distribution of organic content is
relatively uniform in the sand patch. In contrast, the sediment organic
matter content was generally higher in the three vegetated sites. Here,
the organic content was concentrated in the upper 10 cm of the sediment arid
generally decreased with depth. The upper 10 cm portion of the sediment
corresponds to the zone of 98% of the root-rhizome biomass (Table 1).
These results reflect the highly dynamic nature of the biological interactions
in the rhizophere of submerged plants. The organic matter content in the
upper 10 cm of the Ruppia site was lower than that of the mixed and Zostera
sites. At the latter two sites, the root-rhizome biomass in July was 3-4
times greater than at the Ruppia site; thus,the underground portion of the
plant community appears to exert a considerable influence on organic matter
content.
55
-------
Figure 14. Vertical Distribution of Sediment Organic Matter (% of Dry Weight)
during July, 1979.
56
-------
JULY 1979
% ORGANIC MATTER
E
u
Q.
UJ
Q
0.
Q.
ID
cr
x
o
o
z
o
UJ
x
oc
UJ
to
o
M
(X rangei n = 2)
-------
2. ATP Content; The results of the ATP content analyses also reflect the
influence of the seagrass rhizosphere. ATP concentrations are generally
used as an estimator of heterotrophic biomass, although the ATP from any
viable cell is included in these values. Higher percentages of total ATP
in the upper 10 cm of 30 cm cores were generally observed in the vegetated
sites compared to the non-vegetated sand patch (Table 8). The vegetated
sites presumably contain greater concentrations of substrates (such as dead
plant matter and dissolved organic matter secreted by the roots) to support
heterotrophic growth than the non-vegetated sites.
The seasonal distribution of ATP concentration with depth during
1979 is presented in Figure 15. The ATP concentration at all sites was
higher in the upper portion of the sediment; this trend was generally more
pronounced at the three vegetated sites. The ATP concentration tended to
be greater during the summer, which corresponds to the period of highest
community respiration (see Section B, Community Productivity and Metabolism).
In addition, root-rhizome biomass tended to be high in the summer. This
seasonal trend of increased ATP concentration in the summer is shown for
the 0-2 cm portion of the sediment in Figure 16. Further discussion of the
sediment organic matter content and ATP concentration will follow in Section
D, Nutrient Cycing, where the results of interstitial nutrient concentration
measurements are presented.
58
-------
TABLE 8 . Percent total ATP in 0-5 cm and 0-10 cm vertical core sections.
Habitat
Ruppia
Mixed
Zostera
Sand Patch
Depth
(cm)
0-5
0-10
0-5
0-10
0-5
0-10
0-5
0-10
Feb
72
81
69
79
80
88
49
64
Mar
44
59
64
75
59
72
-
April
45
90
51
66
59
81
43
59
June
44
61
58
73
60
74
65
90
July
63
73
49
65
40
53
48
62
Aug
36
53
41
57
-
-
Sept
45
62
37
53
85
91
-
Oct
23
34
37
53
36
52
34
50
X
46
64
51
65.
60
73
48
65
59
-------
Figure 15. The Depth Distribution (cm) of ATP concentration in the Sediments
at Vaucluse Shores, 1979-1980.
60
-------
FEB.
MAR.
APR.
/y>vxy<
r
f
*?
I
d
i i i i f
1
if
i i
1 1 j 1 1
a
1
''III
1
i i i i i
i i 1 i i
ad.
I
1.0 0 1.0 0
i i i i i
j i
1.0 0
I I
1.0 0
I I
SEP. OCT.
1.0 0 1.0
i i i
n.d.
n.d.
n.d.
-------
Figure 16. The Sediment ATP Concentration in the Upper 2 Centimeters at
Vaucluse Shores, 1979-1980.
62
-------
u
-------
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