-------
Figure 4. Locations of lower Bay stations (1) Mumfort Is., York R., (2)
Allen's Is., York R., (3) Guinea Marshes, U) Mouth of Severn R., Mobjack Bay,
(5) Four Point Marsh, Ware R., Mobjack Bay, (6) Vaucluse Shores off Hungars
Creek, (7) deep station.
-------
region of the spectrum at the vegetated sites (Fig. 5), despite the effects of
high blue attenuation due to runoff. A significant difference among sites
based en PAR attenuation coefficients was also observed in July; however, one
vegetated site (Four Point Marsh) had attenuation coefficients as high as the
unvegetated sites (Fig. 6). This was due to the increased attenuation of
wavelengths above 500 ran at the Four Point Marsh site during July. The only
general light quality differences between vegetated and unvegetated sites that
was evident from these preliminary analyses was the reduced attenuation in the
500-700 ran region at vegetated sites during May.*"
Kaumeyer et al. (1981) measured a significant difference in PAR
attenuation coefficient inside and outside SAV beds at Todd's Cove, Md. during
July, August, and September, 19eO. Kd (PAR) for the vegetated areas was from
0.4 m~ to approximately 2 m~l lower. Significant differences were r.ot found
in attenuation inside and outside grassbeds at their Parson's Island s.udy
site. Table 1 summarizes the results of their studies.
Histcrical Data Bases and Optical Properties of the Chesapeake Bay
Most of the historical light data for the Chesapeake Bay has been
col'ected by Secchi disc. This method is not ideal but can be us-.'J to indicate
trends. Heinle et al. (1980) reviewed Secchi disc light data for both mid-Bay
and the Patuxent River, which was chosen because of the extensive data base
(Fig. 7). Transparency has decreased since the 1930's, especially during the
winter in the mid-Bay (Fig. 7a). An increase in turbidity, as estimated by
Secchi disc measures, has been quite dramatic in the Patuxent (Figs. 7b, 7c).
Mid-1970's Secchi disc data for rivers in the upper Chesapeake Bay are
reported in Table II from Stevenson and Confer (1978). The values are
generally low «1.0 m) and are similar to those reported for the Patuxent
during the 19t>0's and 1970's (Figs. 7b, 7c).
Increases in chlorophyllous pigments due to phytoplankton blooms (which
can be caused by increased nutrients) may have a severe effect on light
attenuation in the photosyntheticaily critical blue and red spectral regions
(Fig. Ib, Id). Historical chlorophyll data for the Chesapeake Bay and
Patuxent River are summarized in Figures 8 and 9. Chlorophyll concentrations
have incre ^ed dramatically in the upper and mid-Bay since the early 1950's.
Concentrations as high as 100 to 200 vg'l" are not unusual. In contrast,
lower Bay — main stem — concentrations have not significantly changed (Fig.
8b). Concentrations in the Patuxent River have inceased significantly in both
the upper and lower portions (Fig. 9), especially during late spring and early
summer (Fig. 9b). Levels in excess of 100 ug*l were common in the summer
throughout the 1970's, these are TWICE the concentrations measured during the
previous decade.
^Subsequent measurements and analysis extend and corroborate this conclusion:
not only is the mean violet and blue attenuation lower at vegetated sites but
the variation is also less, (see Chapter 3, this volume).
-------
UNVEGETATED SITES
VEGETATtD SITES
'E
e
re
z
o
MUMFORT IS
GUINEA MARSH
RUNOFF
3-
ZH
•\v
\ JULY
\ \
-A
\
\
\
\
3 '
Z
Ul
4
O
5. 3H
O
Q
cn
\ "«
\ '
SEVERN R
JULT\
2-
\JULV
\0 \
RUMOFF\
FOUR POINT MARSH
MARCH
APRIL
MAY
JULY
\ \
\ ^^
•
\ MAY
\
400
1
500
600 700 400
WAVELENGTH IN NANOMETERS
i
500
600
700
Figure 5. Mean monthly diffuse downwelling spectral attenuation coefficients
for vegetated and unvegetated sites in the lower Chesapeake (March-July,
1981). All coefficients calculated for the depth interval 0.1 to 0.5 m.
Mumfort Island (York River) and Severn River sites: unvegetated. Guinea
Marsh and Four Point Marsh (Ware River) sites: vegetated.
-------
2.0
o
in
7e
z
UJ
u
u.
u.
UJ
o
o
=>
z
UJ
4
cr
i.o-
0.5-
MUMFQRT IS
•— ALLENS IS
GUINEA MARSH
— FOUR PT. MARSH
FOU» PT MARSH
(VEGETATED)
MARCH
APRIL
MAY
JUNE
JULY
Figure 6. Mean monthly downwelling PAR attenuation coefficient + 1 standard
error of the mean for vegetated and unvegetated sites in the lower Chesapeake
Bay.
-------
TABLE I. COMPARISON OF MEAN PAR ATTENUATION COEFFICIENTS INSIDE AND
OUTSIDE OF VEGETATED AREAS AT TODD'S COVE, MD. 1980
(KAUMEYER et al., 1981)
Month Location
June
July
August
September
SAV
Reference
SAV
Reference
SAV
Reference
SAV
Reference
2.6 + 0.20
2.5 +_ 0.75
2.5 + 0.30
2.9 jf 0.70
1.8 + 0.56
3.1 +_ 0.33
1.9 + 0.34
3.8 + 0.96
49
-------
4-
3-
2-
•S 2.0-
10-
o.
UJ
Q
; 0.5 H
u
o
UJ
I/)
O1974-77
• 1936-40
A '960, l»«l, I9«4
A MEDIAN 8/ZZpt*
O-£ RANGE WZSpt*
I I
I I
I
I
I
I
SECCHI DISC
14 I S E
(d f)
,936-1939
HEINLE/NASH
1968- 1970
FLEMERe* 01(1970)
j F M A M JJA'SON D
2.5-
2.0-
I 5-
1.0-
05-
0
1
6
8 10
SURFACE SALINITY (%o)
i
12
I
14
Figure 7. Historical Chesapeake Bay Secchi disc values (after Heinle et al.,
1980 and references therein), (a) monthly mid-Bay means, (b) monthly means
Patuxent River estuary, (c) Patuxent River Secchi dapth vs. salinity, July
(after Mihursky and Boynton, 1978).
50
-------
TABLE II. AVERAGE SECCHI DISC DATA (cm) BY RIVER SYSTEM, MARYLAND
CHESAPEAKE BAY, 1972-1976a, (as reported in Stevenson &
Confer, 1978).
River System
Elk & Bohemia
Sassafras
Howe 11 & Swan Points
Eastern Bay
Chop tank
Little Chop tank
James Island & Honga
Honga River
Bloods worth Island
Susquehanna Flats
Fishing Bay
Nanticoke & Wicomico
Manokin
Patapsco
Big & Little Annemessex
Gunpowder & Bush Headwaters
Pocomoke Sound, MD
Magothy
Severn
Patuxent
Back, Middle & Gunpowder
Curtis & Cove Point
South, West & Rhode
Chester
Love & Kent Points
Smith Island, MD
AVERAGE
1972
33.0
34.3
33.8
67.3
60.7
64.5
70.1
78.2
73.7
64.5
49.5
55.4
94.2
73.7
109.7
42.9
101.6
83.8
97.3
80.3
79.5
45.2
74.7
76.2
89.7
78.5
70.1
1973
35.1
52.3
75.4
62.5
62.5
59.4
64.0
67.3
87.6
65.5
77.0
58.9
94.7
80.0
92.7
38.3
82.0
97.3
70.4
80.8
75.7
77.0
66.0
73.4
74.7
76.2
71.1
1974
_
-
-
76.5
84.3
66.8
74.2
72.6
94.7
82.6
85.6
65.8
101.3
67.8
96.3
46.7
-
73.4
79.5
61.5
73.2
81.8
61.2
100.1
117.6
89.7
79.5
1975
25.7
29.2
61.2
54.6
61.5
63.8
67.1
68.8
177.0
33.8
75.7
61.0
107.4
-
88.1
-
96.8
-
-
66.8
75.4
58.9
48.5
87.9
72.1
139.4
76.2
1976
36.3
51.
57.7
75.9
64.3
78.5
73.4
67.8
83.3
76.5
54.1
58.9
81.0
70.1
85.1
53.8
85.9
74.4
86.4
62.7
61.2
73.7
67.1
85.1
89.9
87.6
71.4
51
-------
01
f
o
ovj —
70-
60-
50-
40-
30-
20-
10-
o-
i
0 CBI 1949-1951
a CBI 1964-1966
O CBI 1969 -1971
• EPA 1969 -1971
1
1
J.ltf,
Il2« •
(1970)
£
1
I"
ft) *H a OB^^S
i i 1 i
i
a
i
1
a
1
^
240
1
i
(1*631
1
A
1
t
O
1 i
a
<
c
1
i
i
a
I '
A> 4
i A
1
a
i
1123
1
t
.6
A
i
\j
i i
,
<
i
(
\
"»«' UPPER Q
BAY
.
t
^
1 1
y
c
i
1
i
2
A
i
2
.
1 1
1 1
,
&
M A A
A In-i
U ~ rf 1Kt
B "3
1 1 1
30-
20-
10-
0-
O FLEISCHER etal, (1976) 1973 DATA
O CBI 1949-1931 -Potomac to Rappahonnock (744)
• CBI 1949-1951 Lower Bay Below Rappahannock (724,707)
• PATTEN etal, (1963)
a CBI 1964-1967 (746)
D CBI 1969-1971 (744,7443)
• CBI 1969-1971(724,707) R
-------
I
100
- 80
o>
a
01 60
o 40
20
100
80
60
a 40
o
5
* 20
a
01
0>
a.
c.
a.
o
3
100
80
60
20
Q
Jon,F«b, Mar
• Lower Marlboro
O Btnedict Bridge
Q Queen Tree Landing
b
May, June, July
c
Aug ,Sept, Oct
1962 64 66
68 TO
YEAR
72
74 76
78
Figure 9. Summary of historical chlorophyll ^ data for three regions of the
surface waters of the Patuxent R., Md. (a) January-March (b) May-July (c)
August-October (after Heinle et al., 1980).
53
-------
In addition to the thoroughly documented increased chlorophyll-^
concentration in the Patuxent, there have also been increases in moat of the
other tributaries of the Bay. Chlorophyll-^ concentrations in the Choptank,
Chester, and Miles Rivers of the Middle Eastern Shore are 1.5 to 2 times
higher presently than earliest data showed. There have also been upstream
increases in the Magothy, Severn (Md.) and South Rivers. In the upper Potomac
concentrations up to 100 Ug'1"1 were measured in the mid-19601s.
Concentrations in the lower Potomac were generally higher in the 1960's than
1950, except during March and April (Heinle et al., 1980) (see Table III).
Increased chlorophyll a_ concentrations have also been measured in the
Rappahannock and York rivers during the last few years. The upper James has
had high concentrations similar to the upper Potomac since the mid-1960's but
the lower River still does not. Dense algal blooms have been noted in the
Elizabeth, Back, and Poquoson Rivers of the lower Bay.
Heinle et al. (1980) summarized the state of the Bay graphically in terms
of enrichment—which they defined as "deviations in concentrations of
chlorophyll _a from historic, natural periods of stability or steady state
concentrations." Figure 10 depicts the regions of the Bay which they
categorized as moderately or heavily enriched. Significantly, most of these
same areas have experienced declines in Bay grasses on a time scale
overlapping the enrichment (Orth and Moore, 1982).
Suspended particulates and dissolved materials also impede the amount and
quality of light reaching the benthos. Amounts of dissolved organic
materials, inorganic particuiace matter and allochthonous organic particulate
matter in the Bay are mainly determined by input (runoff) of freshwater to the
tributaries. Dramatic increases can result from storm events. Table IV
summarizes annual mean freshwater flow to the entire Bay and major storms
during the period, 1951-1979. In addition to adding large amounts of sediment
to the water column, major storm events increase nutrient loads originating
from agricultural fertilizers and other sources thus stimulating phytoplankton
blooms. There is also an apparent wet-year, dry-year cycle imposed on the
data. The five year flow averages suggest a mid-1960's depression followed by
an increase through the 1970's. Although these data have not been rigorously
analyzed, it is apparent that long term changes and or cycles in climatic
conditions (rainfall, temperature and major storms) influence water quality
and therefore the optical properties of Bay waters.
Suspended sediment transport and discharge of the Susquehanna River, the
major source of freshwater to the Bay, is given in Table V. Gr ss et al.
(1978) suggest that one-half to two-thirds of the suspended sediment discharge
of the Susquehanna is deposited behind the dams or in lower reaches of the
river during years of low flow and no major flooding. However, during major
floods these deposits are eroded and transported into the Bay. Thus, the dams
effectively increase the amount and variability of sediment discharged under
flood conditions possibly contributing to periodic stressing of the Bay.
54
-------
TABLE III. RANGES OF CONCENTRATIONS OF CHLORPHYLl. a_ (yg I"1) AT
SURFACE AND BOTTOM DEPTHS IN THE LOWER POTOMAC RIVER
DURING 1949-1*51, AND 1965-1966 (after, Heinle et al.,
1980).
Month
January
March-April
May
July
October-November
1949-1951
Surface Bottom
1-2
10-21
3-6
3-5
1-9+
1-2
12-27+
9-24+
1-2+
1-7
1965-1966
Surface Bottom
3.2-4.7
1.1-20.0
5.8-13.2
9.0-13.8
9.3-24.0
3.1-5.0
1.1-9.5
4.3-9.8
1.0-1.8
3.6-11.0
55
-------
Moderately Enriched
Heavily Enriched
Figure 10. Enriched sections of the Chesapeake Bay. Enrichment is defined as
an increase in chlorophyll a_ levels from historic, natural periods of
stability (after Heinle et al., 198C).
-------
H
TABLE IV. TOTAL ANNUAL MEAN FRESHWATER FLOWS TO THE CHESAPEAKE BAY
AND OCCURRENCE OF HURRICANES, 1951-1979 (after Heinle et
al.. 1980).
Year
1951
1952
1953
1954 Hurricane
1955 (2) Hurricanes
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1971
1972 Hurricane
1973
1974
1975
1976
1977
1978
1979 Hurricane
Bay Annual Average
(ft3^-1)
82,100
94,300
72,800
58,700
73,400
76,000
64,400
81,400
66,400
77,300
78,000
64,800
52,400
61,900
49,000
79,000
131,800
95,200
76 , 900
103,100
84,400
80,100
91,300
113,800
5-Year
Average
76,260
73,100
61,220
97,180
92,400
57 !
1
I
J
-------
TABLE V. SUSPENDED SEDIMENT TRANSPORT AND DISCHARGES OF SUSQUEHANNA
RIVER TO THE CHESAPEAKE BAY (after Gross et al., 1978).
Calendar Year
Annual suspended sediment discharged
(106 metric tons/yr.)
Above Dam Below Dam
1966
1967
1968
1969
1970
1971
1972
Agnes, 24-30 June 1972
1973
1974
1975
Eloise, 26-30 Sept. 1975
1976
1.5
1.7
>1.7**
nd
>2.0
>1.4**
11.3
7.6
3.2
1.7
>3.8
1.6
nd
0.7 (602)*
>0.3**
nd
0.32 (60%)*
>!.!**
1.0
33
30
1.2 (54%)*
0.8 (53%)*
11
9.9
1.2
nd * no data
* Percent discharged during annual spring flood
**Records incomplete for the year
58
-------
LIGHT AND PHOTOSYNTHESIS IN CHESAPEAKE BAY SAV COMMUNITIES
General Review of Photosynthesia
Photosynthesis is the process by which light is used as the energy source
for the synthesis of organic compounds. Three basic steps are involved in the
process: 1) absorption of light energy by photosynthetic pigments; 2)
processing the captured light energy to produce the compounds ATP and NADPH;
and 3) the reduction of C02 using ATP and NADPH and the production of
carbohydrates. The first two steps are light-dependent and collectively
referred to as the "light reaction." The third step is light-independent and
termed the "dark reaction."
Photosynthetic pigments have characteristic light energy absorption peaks
within the photosynthetically active region of the spectrum. Chlorophyll £
absorbs light more effectively at higher wavelengths ObOO nm) while accessory
pigments such as chlorophyll b_, carotenoids, and others are more effective
absorbs at shorter wavelengths (<600 nm). Chlorophyll _a_ and the accessory
pigments transfer the absorbed light energy at varying efficiencies to
specialized chlorophyll a_ molecules (P70U) where it is used directly for
biochemical reactions.
The photochemical reactions are driven by units of light energy termed
photons (quanta). Quantum energy is a function of wavelength; quanta of
shorter wavelengths contain more energy than quanta of longer wavelengths.
Light energy transferred to P700 is most efficient as it is used directly in
the photosynthetic system while light energy transfer via chlorophyll &_ and
accessory pigments is less efficient. Quantum yield, the moles of QI produced
or CC>2 fixed per photon of light absorbed, may be used to estimate the
transfer efficiency.
The light utilization spectrum of a particular species is termed the
action spectrum, a characteristic curve obtained by measuring the- relative
photosynthetic output of intact plant parts at discrete wavelengths. The
action spectrum is an important feature since it reflects the ability of a
species to adapt to various light quality regimes (Fig. Id). This is of
particular importance when considering photosynthesis of submerged plants. In
aquatic environments, spectral shifts in light energy result from the water
itself, suspended organic and inorganic materials, dissolved organic compounds
and other water column constituents. (See Chpt. 3 for a discussion of the
relationship between the light quality environment of the Chesapeake Bay and
its potential effect on photosynthesis).
A general approach to the investigation of photosynthesis as related to
total PAR radiation is to construct light saturation curves for various
species (Fig. lla). An examination of photosynthesis-light curve? (P-I
curves) shows that photosynthesis (P) increases with increasing irradiance to
an optimal point (lopt) where over a range of irradiance, the photosynthetic
59
-------
Pmox
M
01
2
o
1 I an4 2
Figure 11. Diaj,ramatic photosynthesis-1ipht relationships Cseo text for
descriptior. ot parameters).
-------
system is saturated and maximum photosynthesis (Pmax) occurs. At higher
irradianc", there may be a depression in the photosynthetic rate, termed
photoinhibition. The initial slope of the curve (AP/AI or a) and Pmax are the
two major parameter used in describing P-I curves (Jassby and Platt, 1976).
a is a function of the light reaction of photosynthesis and is an estimator of
the quantum yield. PTOax " a function of the dark reaction and is influenced
by environmental factors or the physiological state of the plants (Parsons et
al., 1977). The term 1^, proposed by Tailing (1957) is the irradiance at
which a linear extension of the initial slope intercepts Pmax. *k ^8 regarded
as indicative of the plant's adaptation to its light regime (Steeman Nielsen,
1975). I'fc is irradiance wht-re P - 0.5 Pmax and is similar to the
Michalis-Menten half-saturation constant. Ic is the irradiance at the
compensation point, where photosynthesis equals respiration (P • R).
Characteristic P-I curves are shown in Fig. lib. Plants adapted to high
and low light environments, termed respectively sun and shade species, exhibit
different P-I curves. Sun species (curve 3) generally exhibit higher Pnax
values than shade species, which exhibit greater a and lower Ic values (curves
1 and 2). In the aquatic environment, with reduced availability of light,
species exhibiting shade type photosynthesis (greater photosynthetic rates at
low light intensities) are at an advantage if they are not intertidally
exposed to high light levels.
Photosynthesis of Submerged Vascular Plants in Relation to Light and
Temperature
In situ studies of submerged angiosperms point to the role of light in
seagrass production and distribution (Jacobs, 1979; Mukai et al., 1980). In a
study of Zostera in Denmark, Sand-Jensen (1975) showed a positive correlation
between leaf production and insolation over a nine-month period. Biomass and
photosynthetic rates of Posidonia declined with depth near Malta (Drew and
Jupp, 1976); this probably was due to decreased light penetration with depth.
In before and after studies of an estuary that was closed to the sea, Nienhuis
and DeBree (1977) reported that the Zostera population increased in density
and extended to a greater depth; they suggested that this was probably due to
an increase in water transparency, van Tine (1977, 1981) found a correlation
between reduced light transmission and the biomass and diversity of benthic
macrophytes including Thalas*ia, Halodule and Sringodium in an estuary in the
Gulf of Mexico.
In situ light and manipulation experiments have provided evidence of the
importance of light to seagrass production. For example, at the end of a
nine-month study during which ambient light was reduced by 63%, in situ
Zostera densities were only 5* of that of the control (Backman and Barilctti,
1976). In similar studies, Congdon and McComb (1979) reported that lower than
ambient light levels resulted in lower Ruppia biomass; as the shading duration
increased, higher light levels were required to sustain a high biomass.
Studies involving the epiphytic community, those organisms directly
attached to submerged angiosperm blades, suggest that the epiphytes have a
detrimental effect on the seagrass hosts, primarily due fo shading of the
raacrophytes by the epiphytes. Sand-Jensen (1977) reported that Zostera
61
-------
photosynthesis was reduced by up to 312 due to a decreased penetration of
light and inorganic carbon through the epiphytic community to the seagrass
blades. Johnstone (1979) hypothesized that the rapid linear growth of Enhalus
leaves (up to 2 cm day"1) was related to a shading effect to epiphytes.
However, the net community productivity of the seagrass-epiphyte system may
not be reduced. It may be tnat the different pigment complexes of the various
epiphytes provide optimum light absorption during seasonally changing aquatic
light conditions. Both Kiorbe (1980) and Phillips et al. (1978) provided data
to indicate that epiphytic development suppressed macrophyte growth. The data
of Penhale and Smith (1977) suggested that an epiphytic community may be
beneficial in certain environments. For Zostera exposed at low tide, the
epiphytes prevented desiccation damage by trapping a film of water and
probably reducing the photoinhibitory effect of high light.
In addition to light, temperature also influences submerged macrophyte
distribution and productivity rates (Biebl and McRoy, 1971; Drew 1978) van
Tine, (1977, 1981) found reduced biomass and diversity of benthic macroalgae
and seagrasses in estuarine waters impacted by a lower plant induced thermal
plume. The biogeography of marine and brackish water plants points to a
temperature effect on world wide distribution; for example, genera such as
Zostera, Ruppia, Phyllospadix, and Posidonia, occur mainly in temperature
zones while genera such as Thalassia, Syringodium, and Halophila occur mainly
in subtropical and tropical zones. Drew (1979) reported that the Pmax of four
seagrass species collected near Malta increased in direct proportion to
temperature up to temperatures (30-35°C) where tissue damage occurred;
decreases were not observed at environmental temperatures. In contrast,
Penhale (1977) observed a decline in Pmax from 22 to 29°C for Zostera in North
Carolina where environmental temperatures reach 34°C. The co-existence of
species such as Ruppia and Zostera in the lower Chesapeake Bay may be a result
of differential responses to both temperature and light as apparently is the
case in a Myriophy1lum-Va1lisneria association described by Titus and Adams
(1979). They reported that a greater for Vallisneria, in conjunction with
the temperature dependence 01 photosynthesis, resulted in a temporal
partitioning of resources. Vallisneria was apparently favored by midsummer
conditions and Myriophyllum by spring and fall conditions.
Sun and shade
(Spence and Crystal
exhibit higher Pmax
lower Ic values and
to a wide range of
under high and low
the I
opt
value four
species have been described for submerged macrophytes
, 1970a, b; Titus and Adams, 1979). Sun species generally
values than shade species which exhibit greater and
lower dark respiration rates. Certain species can adapt
light conditions. Bowes et al. (1977) cultured Hydrilla
irradiances; subjecting the plants to high light increased
-fold. Plants grown under low lipht achieved Ic and Ij< at
lower intensities.
In seagrass systems, pigment relationships generally vary with light
quantity or with position within the leaf canopy. The adaptive capability of
seagrass pigment systems to the light environment has been shown in various
studies. For example, Wiginton and McMillan (1979) reported chat the total
chlorophyll content was inversely related to light for several Caribbean
seagrasses collected at various depths. For seagrasses cultured at several
light levels, the total chlorophyll content increased with deceasing quantum
-------
flux (McMillan and Phillips, 1979; Wiginton and McMillan, 1979). Within !
individual meter-long Zostera leaves, the chlorophyll £ to chlorophyll b_ ratio j
varied significantly, with the lowest ratio at the basal portion of the plant I
(Stirban, 1968). In a detailed study of chlorophyll relationships in a
Zostera system, Dennison (1979) observed no substantial variation in total -
chlorophyll content within the leaves as a function of depth of the leaf |
canopy in integrated samples along a depth gradient within the bed; however, ;
the chlorophyll a^ to chlorophyll b_ ratio decreased from the apical to basal '
portion of the leaves.
Although the physiological photosynthesis-light relationship ultimately
determines the light levels at which plants grow, the morphology of individual
plants and the community canopy structure may play an important role in
production and species distribution. In a study of Myriophyllum and
Vallisneria, Titus and Adams (1979) observed that the former had 682 of its
foliage within 30 cm of the surface while the latter had 622 of its foliage
within 30 cm of the bottom. Myriophyllum, an introduced species, has often
displaced the native Vallisneria; a contributing faactor is probably the
ability of Myriophyllum to shade Vallisneria. In a detailed community
structure analysis of a monospecif'c Zostera community across a depth
gradient, Dennison (1979) concluded that changing leaf area was a major
adaptive mechanism to decreasing light regimes.
PHOTOSYNTHESIS-LIGHT STUDIES IN CHESAPEAKE BAY
Investigations of photosynthesis-light relationships carried out through
the Chesapeake Bay Program can be categorized into three general experimental
designs. In the first, P-l curves were constructed for the four dominant
species in the Chesapeake Bay system, Myriophyllum spicatum and Potamogeton
perfoliatus in the upper Bay (Kemp et al., 1981) ancl Zostera marina and Ruppia
maritima in the lower Bay (Wetzel et al., 1982). These experiments utilized
whole plants or leaves subjected to various light intensities (created through
the use of r.eutral density screens) and various temperatures. Kemp et al.
(1981) utilized microcosms in which the effects of various concentrations of
phytoplankton and other suspended solids on light penetration and Potamogeton
photosynthesis were determined. Wetzel et al. (1982) made in situ community •
metabolism measurements und°r a wide range of natural light regimes. In
certain experiments, neutral density screens were used to shade the community
on a short term basis.
I
P-I Relationship of Major Species
P-I curves were constructed for whole plants of M_. spicatum and P_.
perfoliatus at 21eC (Kemp et a!., 1981) (Fig. 12). Both species exhibited the |
characteristic photosynthetic response of green plants to light with J
saturation occurring between 600 and 800 uE m~* s~l. Myriophyllum exhibited a
greater Pmax and greater 1^ than Potamogeton; however, the two species i
exhibited similarct. Althou&u these species occur in the same general locale,
they did not forn dense, mixed bed stands where they would be in direct
competition for light.
63
-------
a
a*
a>
E
UJ
I
V)
O
O
I
a.
Q.
a
P max
MYRIOPHYLLUM SPICATUM
Pmox
i 200
600
LIGHT INTENSITY, M EINSTEINS m's
1000
.-2.-1
Figure 12. Photosynthesis-light carves for two species of upper Chesapeake
Bay submerged vascular plants (after Kemp et al., 1981).
MHUHi
A
-------
We determiu.-id the photosynthetic response to light and temper.-Jiure for
isolated Z_. marina and _R_. maritime leaves. Since these species co-exist in
the lower ChespecVe Bay, an evaluation of the photosynthetic parameters of
each species might suggest competitive strategies. Experiments carried out at
six temperatures and under natural light indicate that light saturation of
Zostera occurs at about 300 yE m~^ s~* while for Ruppia light saturation
requires about 700 yE m~^ s"*. Differences in Pmax between Zostera and Ruppia
were observed and appear related to temperature. At wanner temperatures,
Ruppia exhibits a higher Pn,ax tnan Zostera while the situation is reversed at
colder temperatures (e.g. Fig. 13). A summary of the data shows that Ruppia
exhibits the greater Pmax at temperature >8°C (Table VI). A comparison
between the two species shows that Zostera generally exhibits a greater o;
this suggests a competitive advantage for Zostera at lower light levels.
The data from these experiments suggest mechanisms for the species
distribution of Ruppia and Zostera in the lower Chesapeake Bay: Ruppia forms
single species stands in shallow intertidal to shallow subtidal areas where
high light and high temperatures are prevalent during the summer. Ruppia is
generally more efficient at the higher light and temperature regimes in these
habitats. Zostera, which has the greater depth range, is adapted to much
lower light conditions as indicated by the lower light saturation point and
greater a. In the mixed bed areas, Ruppia is always somewhat shaded by the
longer leaved Zostera. During the winter periods of greater water clarity,
Ruppia receives sufficient light to survive. During summer periods, its
higher Pmax probably contributes to its survival capability during the period
of greatest light attenuation.
Kemp et al. (1981) compared values of photosynthetic parameters taken
from the literature on submerged angiosperms (Table VII). Despite the fact
that these parameters were obtained under a wide range of experimental
conditions and over a wide range of biogeographical areas, the values are
rather similar. Pmax> which is a function of the dark reaction under optimal
environmental conditions or a function of the inhibitor under suboptimal
conditions, ranged from 0.9 to 3.7 mg C g~* hr~*. l\ ranged from 110 to
225 yE m~2 s-1 and I'k from 70 to 350 yE m~2 s"1.
The fact that submerged angiosperms have similar photosynthetic patterns
is useful from the management point of view where decisions often must be
based on information from only one or two species. However, to answer detailed
questions concerning species competition or species adaptations, it is
necessary to determine the interrelationship of photosynthetic patterns,
pigment complement, plant morphology and community canopy structure.
Thus, features in addition to photosynthetic parameters help determine
plant community photosynthesis. Canopy structure and chlorophyll content were
determined for a Ruppia-Zostera bed in the lower Chesapeake Bay (Wetzel et
al., 1982). Both Ruppia and Zostera showed a concentration of leaf area
(surface available for light absorption) at the lower portion of the canopy
where less light penetrates (Fig. 14). This probably allows for a greater
overall net community photosvnthesis than if there were a uniform vertical
distribution of leaf area. Highly significant differences were observed
between the vertical stratification of leaf area of Ruppia and Zostera.
65
-------
AUGUST 29,1979
90-1
SO 40 SO «0 70
LIGHT LEVEL (Ptrctnl Ambiml)
•0
50
£ 30
«
v>
ui
5 zo
I
a. 10
0-1
JANUARY 29, I960
Sostrro
LIGHT *2)E m
10 20 30 40 90 10
LIGHT LEVEL (P»rt»(ll
TO W >0 100
Figure 13. Photosynthesis-light curves for two species of lower Chesapeake
Bay submerged vascular plants from A mixed bed (Light is total light flux
during 4 h **C incubations).
-------
TABLE VI. PHOTOSYNTHETIC PARAMETERS OF RUPPIA MARITIMA AND ZOSTERA
MARINA LEAVES AT VARIOUS TEMPERATURES. (LIGHT - total
light flux during the 4h ^C incubations).
TEMP
CO
1
8
12
18
21
28
LIGHT
(E in'2)
5.0
22.1
15.1
21.8
14.5
12.0
Pmax (°>g C
Ruppia
2.15
3.12
3.91
2.60
3.82
2.39
Zostera
2.66
3.25
2.15
2.15
3.55
1.31
INITIAL
Ruppia
0.18
0.41
0.16
0.35
0.27
0.52
SLOPE
Zostera
0.70
1.41
0.55
0.34
0.27
0.69
67
i
J
-------
TABLE VII. SUMMARY OF PHOTOSYNTHESIS-LIGHT EXPERIMENTS FOR SELECTED
SUBMERGED AQUATIC .JJGIOSPERMS3 (from Kemp et al., 1981).
I
Plant Species
Zostera marina
M ii
Thalassia testudinum
Cymodocca nodosa
Halcdule uninervis
Syringpdium fili forme
Ruppia maritima
Vallisneria americana
Ceratophy Hum demersum
ii ii
Ranunculus pseudof luitas
Myriophyllum spicatum
ii ii
ii it
Potamogeton pectinatus
Pmaxb
1.5
2.2
1.2
1.3
1.7
2.5
2.6
1.5
1.6
3.7
1.9
2.2
3.2
2.2
3.3
2.8
1.9
1.3
0.9
1.1
Light
I'k
140
170
167
184
225
170
140
130
140
225
123
130
1J5
130
115
215
110
200
195
140
Parameters0
iir IG^
K
28
220
280
345
320
210
220
175
220
290
236
100
80
230
150
180
70
290
350
230
Drew
145
50
40
50
120
30
30
20
25
30
60
25
Reference
1979
Penhale 1977
Me Roy 1974
Sand-Jensen 1977
Buesa 1975
Capone et al. 1979
Beer & Waisel 1979
Drew 1978
Bear & Waisel 1979
Buesa 1975
Nixon & Oviatt 1973
Titus & Adams 1979
Van et al. 1976
Guilizzoni 1?77
Westlake 1967
Titus & Adams 1979
Van et al. 1976
Kemp et al. 1981
Westlake 1967
Kemp et al. 198'
Most of these data were interpolated from graphical relations provided
by respective authors.
,nax
light-saturated photosynthetic rate in mg C g~l h~*, where 02
production data were converted to C assuming PQ * 1.2.
Light variables: I'^ * half-saturation constant; l^ * intersection of
initial slope and Pmax» IG * 1^8nt compensation point where apparent
production approaches zero. Light data converted to PAR units (pE m~2
s"1) assuming 1 mW cm"2 » 2360 Lux - 0.86 cal cm"2 h"1 *
s"1 (Hansen & Biggs, 1979).
Values of Ig are not available for experiments using the
which cannot measure negative net photosynthesis.
46
~2
14
C method
68
-------
AUGUST I960
CO
ffi
co
50 H
40 H
30 H
20 H
IOH
Ruppia BED
Ruppia mari ti ma
MIXED BED
Rupp 10 maritima
"1
Zostera BED
Zostero manna
MIXED BED
Zoster a mar ma
L
I 20 I 2
LEAF AREA INDEX
Figure 14. Vertical distribution of one-sided leaf area index (m~2 plant m~2
substrate) for Ruppia and Zostera at three vegetated sites in the lower
Chesapeake Bay.
69
-------
Ruppia exhibits much greater leaf area than Zoatera at the lower canopy
(0-10 cm above substrate); this probably contributes to its success in the
mixed bed areas where it is shaded by Zostera.
Preliminary estimates of pigment content of Ruppia and Zostera suggest
differences between species (Fig. 15). The highest concentrations of
chlorophyll were at mid-canopy for Zostera and the top-canopy for Ruppia
(Wetzel et al., 1982). Ruppia also showed a higher total chlorophyll
concentration than Zostera. This higher chlorophyll concentration in
combination with its canopy structure is an adaptation which contribute to
Ruppia'a success in mixed bed areas.
Microcosm Studies
The microcosm studies of Kemp et al. (1981) showed a negative effect of
suspended sediments on Potamogeton photosynthesis (Fig. 16). Two
concentrations of fine sediment particles (< 64 ym in diameter), kept in
suspension with recirculatinv pumps, reduced light availability in the two
treatments and resulted in significantly lower photosynthesis of Potamogetcn
compared to a control. Kemp et al. attributed about half the decrease in
productivity of treated systems to the accumulation of epiphytic solids on the
plant leaves. Further consideration of the microcosm data involved
calculating regressions between chlorophyll ji or filterable solids and light
attenuation coefficients. From these, it was concluded that in the northern
Bay, the effect of light attenuation by phytoplankton would be small while the
effect of non-chlorophyllous suspended sediments on photosynthesis would be
more significant.
In Situ Studies of Community Response to Light
The effect of light on plant community metabolism was investigated in
upper and l^er nhcodye^'ke Bay grassbeds. In both areas, community metabolism
was estimated as oxygen production in large, transparent incubation chambers.
During these experiments, detailed measurements of light energy (PAR) reaching
the plants were made. In some experiments, neutral density screens were used
to decrease available light similar in design to the *^C studies on individual
species.
A summary of the upper Bay Potamogeton community response to light is
presented in Fig. 17, which includes estimates from both early (May) and late
(August) periods it. the grow'ng season (Boynton, unpublished data). The Ic of
the plant community occurs at about 200 uE m~2 s~* and the data suggest that
the community is not light-saturated in the ranges of measured in situ light
flux. An analysis of the seasonal trends suggested no differences in the
regression of light and community metabolism between seasons.
Based on these and other studies, Kemp et al. (1981) concluded that grass
communities in the upper Bay are often light limited. For example, actual
subsurface light data and three theoretical light extinction coefficients were
used to calculate light penetration to a depth of 0.5 m above the substrate; a
depth below which Potamogeton grows (Fig. 18a, b). Photosynthetic parameters,
Ic, I'fc and Pmax were calculated from a P-I curve (Fig. 18c). These
70
-------
40-i
mg Chi
Figure 15. Vertical distribution of total chlorophyll for Ruppia and Zostera
from a mixed bed area (Values + standard error, n » 3.
71
-------
ft
ui
»-
z
o
I
Q.
oc
<
Q.
'£
UJ
a.
E
in
1.0 -
40
20
150
7- 100
-(b)
50
(Q )
PMOTOSYNTHETIC
RESPONSE TO
SEDIMENT LOADING
CONTROL
HIGH TREATMENT (M>
LIGHT AVAILABILITY
- o --- o— -
(C) j.
SUSPENOED SOLIDS
24 6 6 10
TIME OF EXPERIMENT, DAYS
Figure 16. Effect of (c) total suspended solids (TSS) on (b) light
availability and (a) rate of photosynthesis of Potamogeton perfoliatus in
microcosms (after Kemp et al., 1961).
72
i:
-------
30-i
20-
o
?
u
u
0
o
-------
1500-
1000-
'E
IU
at.
x
u.
X
o
in
to
o
o
I
a.
2
a.
500 -4
AIR-WATER INTERFACE
lUplillllllllllllllllll
250-
500-
"T"~T ' "" " T
0900 1200 1500
TIME (hr)
P perfoliatus
200
1000
i i
LIGHT FLUX
m
s')
Figure IK. Oiagranatic representation of surface (a) snd underwater (b) light
flux at Tofld's Cove, upper (Jnesapcake Bay calculated for three lipht
extinction (K) coefficients.
Ic, I
and
1 r.ia x
calculated from P - I curve of
Potamogeton perfoliatus (c) (after Kaumeyer i-t al., 19S1).
-------
parameters are identified for each light penetration curve an«J suggest thai
for much of the daylight period, the plant community is light-limited. At the
early morning and dusk periods of the day, the community is apparently
heterotrophic.
In the lower Bay, community metabolism studies were carried out in three
areas: jluj>jp_ia-dominated, Zostera-dominated and a mixed Ruppia-Zostera area
(Wetzel et al., 1982). These studies were conducted under a wide range of in
situ light situations and under artificial shading conditions. The shallow
Ruppia areas exhibit higher light and temperature regimes than the deeper
Zostera areas; the mixed bed is intermediate between the two.
Short term shading expeiintents resulted in a general decease in community
metabolism for both Ruppia and Zostera communities. For the Ruppia site,
apparent productivity increased with increasing light to a midday peak and
decreased during the early afternoon (Fig. 19). Based on P-I curves, Ruppia
was light-siturated during much of the day and would not be photoinhibited.
The unexplained afternoon depression, which occurred while light was
increasing, nay be due to increased community respiration rates under these
summer high temperatures. A similar pattern was observed for the Zostera
site, where shading also resulted in decreased apparent productivity (Fig.
20). In contrast, the afternoon depression in productivity rates was not as
dramatic as in the Ruppia bed and this trend in Zostera seemed to follow the
decreasing light availability unlike the trend in Kuppia. These results are
similar to those found throughout the study and suggest differences between
the two communities.
Plots of apparent productivity vs. light flux at the top of the canopy
were used to compare all three habitats (Fig. 21). Differences among the
three sites were observed for these summer experiments. Both the Ruppia and
the mixed bed areas showed decreases in apparent productivity at the highest
light fluxes. The Zpstera site, which did not receive the high light that the
other sites received, showed no decrease in rates. P-I curves for the
seagrass species showed no photoinhibition, even at high summe: temperatures,
and suggested that the Pmax of Ruppia should be greater than Zosfera at this
time of the year. Zostera appears adapted to lower light levels as evidenced
by its high apparent productivity rates.
The erratic pattern of data points and greater number of negative rates
for Ruppia strongly suggest « different community behavior. This patrern mcy
be due to differences in comirunity respiration rat s, plant species
photorespiration rates or the photosynthetic pattern of other primary
producers such as macro- and microalgae. The mixed bed site shows an
intermediate pattern, suggesting an interactive effect of the presence of both
species of seagrass.
A summary of linear regression analyses of apparent productivity vs.
light flux at the top of the canopy for the three areas is presented in Table
VIII. At the community level, the correlation coefficient, r, is strongly
influenced by season, with the lower values generally observed for the winte-
months. These are the times of year of clearest water and the specific rate
asymptotically approaches Pmax« Therefore the linear relationship does not
; ,1
i
75
-------
1400
oeoo toco 1200 i4oo
TIME OF DAY (hr)
1600
1*00
Figure 19. Apparent proouctivity and light flux at the. canopy top vs. time of
day for Ruppia experiments at 100, 71, 50 and 30% of ambient light at the
canopy top.
76
-------
I8OO-
'- 1600-
ui 1400-
x 1200-
,_ 1000-
z
o
-1 8CXH
600-
LI6HT
Zot*ro SHADE
16 JUL 80
-5OO
-400
-3OO
-ZOO
-100
- 0
--IOO
--200
e
-------
o
«
o>
5
..'•1.
-10
V
X
-O
3
-l
»
, o
*J
-------
TABLE VIII. APPARENT 02 PRODUCTIVITY AND LIGHT:
ANALYSIS FOR LOWER BAY STUDIES.
LINEAR REGRESSION
DATE
14 Feb 80
21 Feb 80
19 Mar 80
29 Apr 80
2 May 80
2 Jun 80
5 Jun 80
9 Jul 80
16 Jul 80
19 Aug 80
23 Sep 80
7 May 80
11 Jul 80
21 Aug 80
25 Sep 80
26 Sep 80
5 May 80
14 Jul 80
[mg 02 in
AREA N
Zostera 33
36
H 31
20
11
" 20
30
57
" 76
" 16
H 27
Ruppia 10
" 83
11 26
" 10
" 16
Mixed 28
50
hr~* vs.
m
68.1
78.0
65.4
280
582
307
286
96.5
124
89.2
108.1
363
52.5
385
242.5
323.2
89.7
77.9
E m~2 hr'1
b
86.5
157
105
-183
-267
-472
-309
-147
- 67.1
- 8
-------
adequately describe the photosynthetic response. This is true for all
measures taken at or near Pmax.
In the Zostera community, maximum rates occur in the spring and early
summer. Over this period, the estimated community light compensation point
progressively increases, due to increased respiration, to the point that daily
community production is negative. This corresponds to the characteristic
mid-summer die-off of Zostera in these areas (Wetzel et al., 1982). Except
for winter and early spring (February and March), the community as a whole is
light-limited.
The Ruppia community dominates the higher light and temperature areas of
the bed. Maximum rates of apparent photosynthesis occur during the summer and
they corroborate the earlier conclusions that Ruppia has both higher Pmax and
Ic characteristics. Some data suggest that community respiration inceases in
early afternoon during high light and temperature conditions. These
conditions are prevalent at mid day low tides during July and August.
Overall, Ruppia dominated communities in the lower Bay appear adapted to
increased light and temperature regimes and do not appear light limited.
For the Chesapeake Bay system as a whole, these data and similar studies
completed in upper Bay suggest an extreme sensitivity of Bay grasses to
available light. These data also agree very well with information on other
geographical areas and species. Our general conclusion is that light and
factors governing light energy availability to submerged aquatic vascular
plants are principal controlling forces for growth and survival.
SUMMARY
The apparent optical properties of the Chesapeake Bay indicate a
light-limited environment for benthic photosynthesis. Water per se, suspended
particles and dissolved compounds all interact to selectively absorb those
wavelengths most important for autotrophic production. Plant oigment systems
are adapted for efficient light energy capture in relatively narrow bands. In
many cases, it is precisely rhese wavelengths that are most rapidly attenuated
in the estuarine water column. Diffuse downwe 11 ing attenuation coefficients
in upper and lower Bay communities indicate a severe attenuation of light
energy in the photosynthetically important violet-blue (400 to 500 nm) region
of the spectrum. There is a progressive increase in attenuation in these
spectral regions during the critical spring growing season for SAV.
Comparison of vegetated and non-vegetated areas in the Chesapeake Bay suggests
lower attenuation during spring in the vegetated areas (see Chpt. 3 for
details). Kaumeyer et al. (1981) also reported significant differences in PAR
attenuation for a vegetated site in the upper Bay.
There is a much larger data base on plant response to PAR light energy
for the Chesapeake Bay as well as other bodies of water. The dominant plant
species in the Bay show the classical, hyperbolic photosynthetic response to
increasing PAR. Specific plant response studies suggest physiological
differences among species. Tha dominant upper Bay species, Hyriophyllum
spicatum and Potamogeton perfoliatus, light-saturate between 600 and 800 vE
m ^ sec"* but differ in Pmax and *k' Q' spicatum appears to higher
80
-------
light conditions than £. perfoliatus. In a similar manner, the dominant lower
Bay species, Ruppia maritima and Zostera marina, appear physiologically
different with regard to lignt response. j*. maritima is adapted to high light
and temperature while Z_, marina is adapted to lower light regimes and is
stressed at higher, summer temperatures.
In situ studies of entire plant communities in both Maryland and Virginia
indicate that the communities generally operate under suboptical light
conditions. There was no apparent light saturation reached for upper Bay
communities, i.e., net apparent community productivity did not asymptotically
approach a maximum value. Studies in lower Bay communities suggest that 2,
marina is light-limited during most of its growing season. Only in the
shallower jl. maritima areas did the community phocosynthetic response become
light-saturated and perhaps photoinhibited at times. These results indicate
that at least in terms of total PAR energy and probably because of the extreme
attenuation in the 400 to 500 run region noted earlier, submerged plan''
communities in Chesapeake Bay are generally light-stressed.
Historical data relative to lignt (secchi disc, chlorophyll _a and
indirectly nutrients) and the past distribution and abundance of submerged
aquatics indicate progressive Bay wide changes in systems structure and
function. Heinle et al. (1980) and Orth et al. (1981) discuss these in
detail. In terms of Bay grasses and the light environment, twc overall
conclusions of these reports are particularly important. Heinle et al. (1980)
have noted and documented the generalized increase in nutrients (and loadings)
and chlorophyll concentrations in major tributaries of the Chesapeake Bay over
the past several decades. Orth et al. (1981, 1982) concluded for roughly the
same time scale that the general pattern of disappearance of submerged plant
communities followed a similar "down-river" pattern. It also appears that
upper Bay and western shore lower Bay communities have been the most severely
impacted. These conclusions together with our studies on the light
environment and photosynthesis-light relations in SAV ecosystems suggest that
factors increasing diffuse downwelling attenuation in the 400-500 run region
are critical in controlling plant growth and survival. The specific factors
that appear to have the greatest impact are organic and inorganic suspended
particles. The presence of these particles is directly related to land runoff
and indirectly to nutrient addition.
In summary, it appears that Bay grasses are living in a marginal light
environment and that progressive worsening of water quality will further
stress the plant communities. To conclude that light has been singularly
responsible for recent declines in the vegetation goes beyond the data
Available. But the data do indicate the extreme sensitivity of the vegetation
to changes in available light. The implicit assumption that over the past
several decades water quality throughout the Bay and particularly in the
tribuaries has progressively declined is a feasible explanation for the
corresponding decline of Bay grasses. Further changes in these parameters can
only affect Bay grasses in an adverse way.
81
-------
LITERATURE CITED
Anderson, F. E. 1980. The variation in suspended sediment and water
properties in the flood-water front traversing a tidal flat. Estuaries
3:28-37.
Backman, R. W. and D. C. Sarilotti. 1976. Irradiance reduction: effects on
standing crops of the eelgrass Zostera marina in a coastal lagoon. Mar.
Biol. 34:33-40.
Baker, K. S. and R. C. Smith. 1980. Quasi-inherent characteristics of the
diffuse attenuation coefficient for radiance. In, S. Q. Duntley, (ed.)
Ocean Optics VI, pp. 60-63, Proc. Soc. Photo-Optical Instrumentation
Engineers, Vol. 208.
Beer, S. and Y. Waisel. 1979. Some photosynthetic carbon fixation properties
of seagrasses. Aquat. Bot, 7:129-138.
Biebl, R. and C. P. NcRoy. 1971. Plasir.atic resistance and rate of
respiration and photosynthesis of Zostera marina at different salinities
and temperatures. Mar. Biol. 8:48-56.
Booth, C. R. and P. Dustan. 1979. Diver-operable multiwavelength radiometer.
In: Measurements of Optical Radiations. Soc. Photo-optical
Instrumentation Engineering 196:33-39.
Bowes, G., T. K. Van, L. A. Barrard, and W. T. Haller. 1977. Adaption to low
light levels by Hydrilla. J. Aquat. Plant Manage. 15:32-35.
Buesa, R. J. 1975. Population, biomass and metabolic rates of marine
angiosperms on the northwestern Cuban shelf. Aquat. Bot. 1:11-23.
Burt, W. V. 1953. Extinction of light by filter passing matter in Chesapeake
Bay waters. Science 118:386-387.
Burt, W. V. 1955a. Interpretation of spectrophotometer readings on
Chesapeake Bay waters. J. Mar. Res. 14:33-46.
Burt, W. V. 1955b. Distribution of suspended materials in Chesapeake Bay.
J. Mar. Res. 14:47-62.
Burt, W. V. 1958. Selective transmission of light in tropical Pacific
waters. Deep-Sea Res. 5.51-61.
Capone, D. G., P. A. Penhale, R. S. Oremland and B. F. Taylor. 1979.
Relationship between productivity and N2 (C2H2) fixation in a Thalassia
testudinum community. .•imnol. Oceanogr. 24:117-125.
82
H
i|
-------
Champ, M. A., G. A. Gould, III, W. E. Bozzo, S. G. Ackleson, and K. C. Vierra.
1980. Characterization of light extinction and attenuation in Chesapeake
Bay, August 1977. In: V. S. Kennedy (ed.), Estuarine Perspectives,
Academic Press, Inc., N.Y. pp. 263-277.
Clarke, C. L. and G. C. Ewing. 1974. Remote spectroscopy of the sea for
biological production studies. In: N. G. Jerlov and E. Steemann-Nielsen
(eds.), Optical Aspects of Oceanography. Academic Press, N.Y. pp.
389-413.
Clarke, G. L. and H. R. James. 1939. Laboratory analysis of the selective
absorption of light by seawater. J. Opt. Soc. Am. 29:43-55.
I
Congdon, R. A. and A. J. McComb. 1979. Productivity of Ruppia: seasonal j
changes and dependence on light in an Australian estuary. Aquat. Bot.
6:121-132.
Dennison, W. 1979. Light adaptation of plants: a model based on the seagrass i
Zostera marina L. M. S. Thesis, Univ. Alaska, Fairbanks. 69 pp. j
t
Drew, E. A. 1978. Factors affecting photosynthesis and its seasonal !
variation in the seagrasses Cymodocea nodosa (Ulcria) Aschers, and
Posidonia oceanica (L.) Delile in the Mediterranean. J. Exp. Mar. Biol.
Ecol. 31:l"73-194.
Drew, E. A. 1979. Physiological aspects of primary production in seagrass.
Aquat. Bot. 7:139-150.
Drew, E. A. and B. P. Jupp. 1976. Some aspects of the growth of Posidonia
oceanica in Malta. In: E. A. Drew, J. N. Lythgoe and J. D. Woods
(eds.), Underwater Research, Academic Press, London, pp. 357-367.
Dubinsky, Z. and T. Beraian. 1979. Seasonal changes in the spectral
composition of downwelling irradiance in Lake Kinneret (Israel). Limnol.
Oceanogr. 24(4):652-663.
Evans, G. C. 1972. The quantitative analysis of plant growth. Univ.
California Press, Berkley. 734 pp.
Fleischer, P., T. A. Gosink, W. S. Hanna, J. C. Ludwick, D. E. Bowker and W.
G. White. 1976. Correlation of chlorophyll, suspended matter, and
related parameters of waters ir. the Lower Chesapeake Bay area to
Landsat-1 imagery. Inst. of Oceanography, Old Dominion University
Technical Report No. 28, Norfolk, Va. 125 pp.
Fletner, D. A., D. H. Hamilton, C. W. Keefe, and J. A. Mihursky. 1970. Final
report to Office of Water Resources Research en the effects of thermal
loading and water quality of estuarine primary production. Contract No.
14-01-0001-1979. Office of Water Resources Research. U.S. Dept. of
Interior, Washington, D.C. NRI Ref. No. 71-6, Chesapeake Biological
Laboratory, Solomons, MD.
83
?
j
-------
Gates, D. M. 1971. The flow of energy in the biosphere. Sci. Amer.
224:88-103.
Cinsburg, R. N. and H. A. Lowenstam. 1957. The influence of marine bottom
communities on the depositional environment of sediments. J. Geol.
66:310-318.
Gross, M. G., M. Karweit, W. B. Cronin, and J. R. Schubel. 1978. Suspended
sediment discharge of the Susquehanna River to Northern Chesapeake Bay,
1966 to 1976. Estuarie* 1:106-110.
Guilizzoni, P. 1977. Photosynthesis of the submergent macrophyte
Ceratophyllum demersum in Lake Wingra. Wise. Acad. Sci. Arcs Lett.
65:152-162.
Hartog, C. den. 1970. The seagrasses of the world. North Holland,
Amsterdam. 275 pp.
Hartog, C. den. and P. J. G. Polderman. 1975. Changes in the seagrasses
population of the Dutch Waddenzee. Aquat. Bot. 1:141-147.
Heinle, D. R., C. F. D'Elia, J. L. Tatt, J. S. Wilson, M. Cole-Jones, A. B.
Caplins and L. E. Cronin. 1980. Historical review of water quality and
climatic data from Chesapeake Bay with emphasis on effects of enrichment.
Report to U.S. Environmental Protection Agency, Chesapeake Bay Program.
Chesapeake Research Consortium, Inc. Pub. No. 84. Univ. Md. Ctr.
Environmental and Estuarine Studies No. 80-15 CBL.
Hurlburt, E. A. 1945. Optics of distilled and natural water. J. Opt. Soc.
Amer. 35:689-705.
Idso, S. B. and R. G. Gilbert. 1974. On the universality of the Poole and
Atkins Secchi disk-light extinction equation. J. Appl. Erol. 11:399-401.
Inada, K. 1976. Action spectra for photosynthesis in higher plants. Plant
Cell Physiol. 17:355-365.
Jacobs, R. P. W. M. 1979. Distribution and aspects of the production and
biomass of eelgrass, Zostera marina L., at Roscoff, France. Aquat. Bot.
7:151-172.
James, H. R. and E. A. Birge. 1938. A laboratory study of the absorption of
light by lake waters. Trans. Wise. Acad. Sci. 31:154 pp.
Jsssby, A. D. and T. Platt. 19?6. Mathematical formulation of the
relationship between photosynthesis and light for phytoplankton. Limnol.
Oceanogr. 21:540-547.
Jerlov, N. C. 1976. Marine Optics. Elsevier Oceanography Series 14,
Elsevier Scientific Publ. Co., N.Y. 231 pp.
84
-------
Johnstone. I. M. 1979. Papua New Guinea seagasses and aspects of the biology
and growth of Enhalus acoroides (L. f.) Royle. Aquat. Bot. 7:197-208.
Kalle, K. 1966. The problem of Gelbstoff in the sea.
Ann. Rev. 4:91-104.
Oceanogr. Mar. Biol.
Kaumeyer, K., W. R. Boynton, L. Lubbers, K. Staver, S. Bunker, W. M. Kemp and
J. C. Means. 1981. Metabolism and biomass of submerged macrophyte
communities in northern Chesapeake Bay. In: W. M. Kemp, W. R. Boynton,
J. C. Stevenson, and J. Means (eds.), Submerged aquatic vegetation in
Chesapeake Bay: its ecological role in Bay ecosystems and factors leading
to its decline. Final Report, U.S. EPA, Chesapeake Bay Program.
Chapter 13.
R.
Kemp, W. M., J. J. Cunningham, J. Perron, S. Garbrandt, M. R. Lewis,
Hanson, R. Twilley, W. R. Boynton, and J. C. Stevenson. 1981.
Experimental observations of turbidity/light relations and their
influence on submerged macroph\tes in northern Chesapeake Bay. In: W.
M. Kemp, W. R. Boynton, J. C. Stevenson and J. Means (eds.). Submerged
vegetation in Chesapeake Bay: its ecological role in Bay ecosystems and
factors leading to its decline. Final Report, U.S. EPA Chesapeake Bay
Program. Chapter 1.
Kiefer, D. A. and R. W. Austin. 1974. The effect of varying phytoplankton
concentration on submerged light transmission in the Gulf of California.
Limnol. Oceanogr. 19:55-64.
Kiley, K. P. 1980. The relationship between wind and curent in the York
River estuary, Virginia, April 1973. Masters Thesis, School of Marine
Science, The College of William and Mary. 195 pp.
Kiorbe, T. 1980. Production of Ruppia cirrhosa (Petagna) Grande in mixed
beds in Ringkobing Fjord (Denmark). Aquat. Sot. 9:125-143.
Kranck, K. 1980. Variability ot participate matter in a small coastal inlet.
Can. J. Fish. Aquat. Sci. 37:1209-1215.
McCluney, W. R. 1975. Radiometry of water turbidity measurements. J. Water
Pollut. Control Fed. 47:252-266.
Mcrtillian, C. and R. C. Phillips. 1979. Differentiation in habitat response
among populations of New World seagrasses. Aquat. Bot. 7:185-196.
McRoy, C. P. 1974. Seagrass productivity: carbon uptake experiments in
eelgrass, Zostera marina. Aquacuiture 4:131-137.
McRoy, C. P. and C. McMillan. 1979. Production ecology and physiology of
seagrasses. In: C. P. McRoy and C. Helferrich (eds.), Seagrass
Ecosystems: A Scientific Perspective. Marcel Dekker, Inc., New York.
pp. 53-87.
1
85
-------
Mihursky, J. A. and W. R. Boynton. 1978. Review o." Patuxent River d«
-------
phytopiankton pigments, dissolved organic matter, and other participate
materials. Limnol. Oceanogr. 26:671-689.
Riaux, C. and H. L. Douville. 1980. Short-term variations in phytopiankton
biomass in a tidal estuary in Northern Brittany. Estuarine Coastal Mar.
Sci. 10:85-92. •
j
Sand-Jensen, K. 1977. Effect of epiphytes on eelgrass phorosynthesis. :
Aquat. Bot. 3:55-63.
Sauberer, F. and F. Ruttner. 1941. Die Srahlunesverhaltnisse der
Binnengewasser. Akademic Verlag, Berlin.
Scoffin, T. P. 1970. The trapping and binding of subtidal carbonate
sediments by marine vegetation in Bimini Lagoon, Bahamas. J. Sed.
Petrol. 40:249-273.
Scott, B. C. 1978. Phytopiankton distribution and light attenuation in Port
Hacking Estuary. Aust. J. Mar. Freshwater Res. 29:31-44.
Seliger, H. H. and M. E. Loftus. 1974. Growth and dissipation of
phytopiankton in Chesapeake Bay. II. A statistical ana.ysi.s of
phytopiankton standing crops in the Rhode and West Rivers and adjacent
section of the Chesapeake Bay. Chesapeake Sci. 15:185-204.
Spence, B. H. N. and J. Chrystal. 197Ca. Photosynthesis and zonation of ,
fresh-water macrophytes. I. Depth distribution and shade tolerance. New '„
Phytol. 69:205-215. f
i
Spence, 3. H. N. ar.d J. Chrystal. lV70b. Photosynthesis and zonation of t
fresh-water macrophytes. II. Adaptability of species of deep and shallow •
water. New Phytol. 69:217-227. •
Steeman Nielsen, E. 1975. Marine photosynthesis with special emphasis on the
ecological aspects. Elsevier, Amsterdam.
Stevenson, J. C. and N. M. Confer. 1978. Summary of available information on
Chesapeake Bay submerged vegetation. Fish and Wildlife Service
Biological Services Program. FWS/OBS/78/66, U.S. Dept. of Interior. 335
pp.
Stirban, M. 1968. Relationship between the assimilatory pigments, the
intensity of chlorophyll fluorescence and the level of the photosynthesis
zone in Zostera marina L. Rev. Rouro. Biol. Serv. Bot. 13:291-295.
Tailing, J. F. 1957. Photosynthetic characteristics of some fresh-water
plankton diatoms in relation to underwater radiation. New Phytol.
56:29-50.
Thompson, M. J., L. E. Gilliland and L. F. Rosenfeld. 1979. Light scattering
and extinction in a highly turbid coastal inlet. Estuaries 2:29-50.
87
-------
Titus, J. E. and M. S. Adams. 1979. Coexistence and the comparative light
relations of the submersed macrophytes Myriophyllum spicaturn L. and
Vallisneria americana Hichx. Oecologia (Berl) 40:373-386.
Van, T. K., W. T. Haller and G. Bowlec. 1976. Comparison of the
photosynthetic characteristics of three submersed aquatic plants. Plant
Phys-iol. 58:761-768.
van Tine, R. F. 1977. An ecological comparison of the benthic macroflora of
a power plant impacted estuary and an adjacent estuary. Masters Thesis,
Dept. of Botany, Univ. of Florida, Gainesville, Fla.
van Tine, R. F. 1981. Ecology of seaweeds and seagrasses in a thermally
impacted estuary in the Eastern Gulf of Mexico. In: G. E. Fogg and W.
D. Jones (eds.), Proc. VHIth Intl. Seaweed Symp., Aug. 1974, The Marine
Science l.abs Menai Bridge, Univ. College of North Wales, Wales, U.K. pp.
499-5U6.
van Tine, R. F. and R. L. Wetzel. 1983. Seasonal variation of underwater
spectral attenuation in shallow areas of the Chesapeake Bay. Paper
presented at The 7th Internalicnal hstuarine Research Conference, Oct.
1983, Virginia beacn, Va.
Westlake, C. F. 1967. Son-e effects of low velocity currents, 01 the
metabolism of aquatic macrophytes. J. Exp. Bot. 18:187-21)5. '
Wetzel, R. G. ar•.' P. A. Penhale. 19 >0. Transport of carbon and excretion of ,
dissolved organic caroon by leaves and roots/rhizomes in seagrass and j
their epiphytes. Aquat. Bot. 6:149-158. >
i
Wetzel, R. L., K. L. Webb, P. A. P.-nhale, ... J. Orth, D. F. Boesch, G. W. j
Boehlert and J. V. Merriner. 1979. The functional ecology of submerged 1
aquatic vegetation in the Lower Chesapeake Hay. Annual Grant Report U.S. «
EPA R805974, Ches.-.peake Bay Program, Ar.naoolis, Md.
Wetzel, R. L., L. Murray, R. F. van Tine and P. A. Ppnhale. 1982.
Photosynthesis, light resoonse and metabolism of submerged macroph"te
communities in the Lower Chesapeake Bay. Ir: R. L. Wetzel fed.),
Structural and Functional Aspects of the Ecology of Submerged Aquatic
Macrophyte Communities in the Lower Chesapeake Bay, Vol. I. Final Draft
Report Grant N'os. R8059"4 and X0032'o-01 Envirorunenta 1 Protection Agency,
Annapolis, Md.
Wiginton, J. R. and C. McMillan. 1979. Chlorophyll composition under
controlled Light conditions as related tr> the distribution of soagrasses
in Texas and the U.S. Virgin Islands. Aquat. Bot. 6:171-184.
Williams, J. 197(.'. Optical Propertios of the Sea. U.S. Naval Institute,
Annapolis, MD. 123 pp.
Williarrs, S. F. 1977. Seagrass productivity: the effects of light on carbon
uptake. M.S. Thesis, Vniv. of Alaska, Fairbanks. 95 pp.
Ver.tsch, C. S !9tJ. The influence ot phvtoplankton pigments on the colour
of seawater. Deep-Sea Res. 7:1-9.
8R
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''en
Chapter 3
THE UNDERWATER LIGHT ENVIRONMENT OF SHALLOW REGIONS
OF THE LOWER CHESAPEAKE BAY, ITS RELATIONSHIP TO SEAGRASSES
AND ITS POTENTIAL FOR BENTHIC PRIMARY PRODUCTIVITY
R. F. van Tine and R. L. Wetzel
Virginia Institute of Marine Science
School of Marine Science
College of William and Mary
Gloucester Point, VA 23062
L
I
-------
INTRODUCTION
All plants, whether aquatic or terrestrial, differentially absorb the
energy of tpecific ranges of light of different wavelengths via characteristic
complements of photoreactive pigment molecules located within subcellular
systems. The energy thus absorbed by quantum amounts is utilized, with
varying spectral efficiency, to drive the reactions of photosynthesis—- i.e.,
the synthesis of complex organic compounds from simple inorganic compounds
using fnt ohoton energy oi sunlight.
Since "he driving force of all ecosystems* is light, the energetic basis
of life, the success or failure of an ecosystem ultimately depends on the
ability of its primary producers to utilize efficiently the specific mix of
light energies available. Gveen plants, both terrestrial and marine,
photosynthesize most efficiently in tha violet-blue (400-500 ran) and
orange-red (600-700 nm) regions of the spectrum (Halldal, 1974). Inada (1976)
summarized the action spectra literature for a diverse taxonomic group of
terrestrial angiosperms and found quite a consistent pattern. The pattern
for the common estuarine green alga Ulva is remarkably similar (Levring, 1947,
1966; Haxo and Blinks, 1950; Halldal, 1974). All show the highest rates of
photosynthesis in the above mentioned spectral region*. Of course this is no
surprise. Green plants appear green because they absorb blue and red light
and reflect green light!
Although we are unaware of any reported seagraas action spectra, it seems
reasonable to assume that these plants are aimi ar to their terrestrial
taxoaonic cousins and their marine ecological cousins with similar pigment
complexes — i.e., chlorophyll a, b and accessory pigments 6-carotene and
xarthophylls. The chlorophylls are solely responsible for the absorption of
energy above 600 nm but that below 500 nm is due to both accessory pigments
and chlorophylls a and b (Zscheile'and Comar, 1941; Zscheile et al., 1942;
Govindjee and Govindjee, 1975).
Seagrasses exist in an environment characterized by drastic temporal and
spatial fluctuations in light energy. As light passes through the water
column it is attenuated by absorption and scattering due to the water itself,
dissolved inorganic and organic substances, and suspended particles.
Differential spectral attenuation results in light quality shifts — color
changes. These shifts can hove profound implications for benthic plants with
their genetically determined finite range of usable light energies.
Scattering — the change in direction of light propagation caused by
diffraction, refraction and reflection due to particles, water molecules and
dissolved substances — is wavelength dependent, but in an irregular and
*With the exception of those few driven by chemosynthesis.
90
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complex manner (Jerlov, 1976). Only 4-112 of incident irradiance between
300-700 nm it reflected from the surface or backscattered out of the oceanic
water column (Clark and Ewing, 1974). Absorption is a thermodynamically
irreversible process wherein photons are converted into thermal, kinetic or
chemical energy. Absorption accounts for moat of the observed attenuation of
light. Forward and lateral scattering effectively increases the path-length
of light thereby exposing it to additional absorption. The major particles in
estuaries are clays and silts with small diameters which tend to scatter light
of the shorter wavelengths of the visible spectrum (blue end) more than the
longer wavelengths. Therefore one would expect greater attenuation of blue
light than red light in estuaries.2 Much of the attenuation of the energy
contained in the long wavelengths O600 r.^i) is due to either the water
molecules themselves, as shown by James ant Birge (1938) for pure water, or to
the water plus its dissolved salts (Clarke and James, 1939). There is little
difference in attenuation between pure water and filtered seawater (Yentsch,
1960); the effect of sea salts is insignificant. The energy contained in the
lower and upper PAR, violet-blue and orange-red, respectively, is particularly
susceptible to absorption by particulate matter (Burt, 1958; Prieur and
Sathyendrauath, 1981). Chlorophyll pigments in the water column associated
with phytoplankton and the breakdown products of plants also absorb most
strongly in the blue and red. Dissolved organic compounds ("yellow substance"
or Gelbatoff) greatly attenuate the shorter wavelengths (Kalle, 1966). Thus,
since estuaries are loaded with a myriad of autochthonous and allochthonous
dissolved and suspended substances, the light energy reaching the benthic
plants of an estuary is likely to be reduced in both the red and especially
the blue regions of the spectrum ~ exactly those portions to which green
plants respond most efficiently photosynthetically.
The small amount of available Chesapeake Bay data on diffuse downwelling
2 1 irradiance attenuation indicates a severe attenuation of light energy in
the photosynthetically important 400-500 nm (violet-blue) region of the
spectrum. Attenuation in the short wavelengths is particularly marked in the
turbidity maximum region of the Bay at the mouth of the Sassafras River, and
at the mouth of the Patuxent River during August (Champ et al., 1980). The
mean Bay attenuation coefficients calculated by Champ et al. (1980) are about
1.0 m~l higher than Jerlov's (1976) most turbid coastal water classification.
A comparison of attenuation coefficients reported for the Chesapeake Bay
and its tributaries is presented in Figure 1 along with Jerlov's (1976)
standard k ( X) curve representing his most turbid coastal water classification
(Type 9). For the Chesapeake Bay, the earliest measurements of k(X) were made
by Hurlburt (1945) (Fig. la). His values fall in the lower range of more
recent in situ measurements. Champ et al. (1980) conducted a light
characterization survey of the Chesapeake Bay during August, 1977. Their mean
values are shown in Fig. la. Specific site measurements made by them in and
near the mouths of the Saaaafrass, Patuxent, Potomac and Chester Rivers appear
in Fig. Ic. Their attenuation measurements in the turbidity maximum zone at
the mouth of the Sassafras River are the highest reported for the Bay: there
Chapter 1 for a more detailed discussion of marine optics.
91
-------
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i» nearly no available light below SOU nm. Pierce et ai. (1981) intensively
monitored the Rhode River during 1980 and 1981. Their annual mean attenuation
values for an uprivcr station and one at the mouth of the river are plotted in
Fig. Ib. The upriver station was found to be consistently more turbid,
presumably due to ita proximity to autochthonous sources. Maximum
penetration was at 575 ran and minima at 775 anrt 425 nm. Attenuation
coefficients derived from 4t irradiance measurements from the Rhode River
(Seliger and Loftus, 1974) are also shown in the figure.
The purposes of the work reported herein were to (1) describe the
spectral light environment of shallow areas of the Lower Chesapeake Bay, (2)
determine the differences, if any, occurring between vegetated and unvegetated
sites, and (3) relate any differences to potential benthic photosynthesis.
METHODS
Downwelline diffuse 21 spectral irradiance was measured as
quanta*nm~^*cm~^*8~^ at 12 biologically significant wavelengths (410, 441,488,
507, 520, 540, 570, 589, 625, 656, 671, 694, run ^ 5 nm). The measurements
were made with a Biospherical Labs model Her-1000 multiwavelength
apectroradiometer (Booth and Dustan, 1979), calibrated against U. S. Bureau
of Standards lamps every 6 months. Calibration curves changed less than 0.5Z
indicating an extremely stable system.
Measurements were taken at 6 shallow sites «2m) and at one relatively
deep site in the lower Chesapeake Bay (Fig. 2). The sites were chosen for
their vegetational history - all but one having been vegetated by macrophytes
in the recent past (Orth, et al., 1979). Five of the sites were located on
the western shore of the Lower Chesapeake Bay in the York River and Mobjack
Bav. The remaining 2 sites were across the Bay on the eastern shore of
Virginia at Vaucluse Shores near Hungar's Creek. The Mumfort Is. (York
River) and Severn River (Mobjack Bay) sites were unvegetated but had
previously been part of seagrass beds. There were healthy seagrass beds
(Zostera marina and Ruppia maritime) at the Guinea Marsh, Four Point Marsh
(Mobjack Bay) and Vaucluse Shores sites. Measurements at the later site were
made in corroboration with in situ productivity studies (Wetzel et al., 1982;
Murray and Wetzel, 1982). The Allen's Is. site (York River) represented a
transitional vegetative state - natural populations had disappeared yet Orth
and colleagues (Orth, et al., 1979) had successfully transplanted it.
Furthermore, it appears that a natural population may be returning to this
site (K. Moore, personal communication, 1981). The deep site station was
located across a sand bar and about 1 mile west of the Vaucluse Shores
vegetated site. It was chosen as a reference station.
The diffuse attenuation coefficient (kj) for downwelling 2 1 irradiance
was chosen as the parameter most suitable for characterizing the light
environments at each site due to its quasi-inherent nature. It has been found
to be relatively insensitive to changes in solar zenith angle (Baker and
Smith, 1979), except for very large angles.
93
-------
Figure 2. Locations of lover Chesapeake Bay sites. (1) Muafort Is., York R.
(unvcgetated), (2) Allen's Is., York R. (transitional), (3) Guinea Marsh
(vegetated), (4) Severn R. (unvegetated), (5) Four Point Marsh, Ware R.
(vegetated), (6) Vaucluse Shores (vegetated) (7) deep "Bay" Station 1 ni.
vest of aitt 6.
J
-------
The diffuse downwelling attenuation coefficient is defined as
-1 dE (z)
kd(z)
E.(z) dz
d
or
- in (-kd(z2-Zl))
where k•
estimated for a body of water with known kj as follows,
Ed(z2) - Ed(Z1)-e-kd(z2 ~ «1>
where _e is the base of the natural logs. The attenuation coefficients
reported in this work were calculated between depths of 0.1 and 0.5 m as a
estimator of water column attenuation not associated with air-water or
water-substrate interface phenomena. Hence, these values are a function of
the inherent optical properties of the water bodies concerned. Since Kd is
not a constant for water bodies leas than 10 m deep, all comparisons between
sites and seasons were made for the same depth interval, i.e., 0.1 to 0.5 m.
All such calculations were based on irradiance measures taken at these depths
between 1000 and 1400 e.s.t. Variations in k
-------
r
chemical energy in the form of organic matter through photosynthesis. The
relative potential PSR thus expresses the relative potential efficiency of
photosynthesis given a specific irradiance distribution and a specific
absorption complex (set of pigments).
All irradiance measurements were taken on the sunny side of the boat well
away from the boat shadow. Each measurement recorded represented the mean of
from 250 to 500 scans taken over approximately 30-60 sees. This was done to
smooth out the effects of water waves, patchy clouds, and patchiness of
suspended particulates.
The western shore stations were monitored several times each jeason on
paired dates approximately 1 week apart selected to coincide with the
confluences of high tide with solar noon and low tide with solar noon. The
Vaucluse Shore site was monitored at least every other month at times chosen
to accommodate productivity studies being made there (Wetzel et al., 1982;
Murray and Wetzel, 1982).
RESULTS
In order to facilitate an understanding of the possible trends in the
spectral distribution of underwater light in the shallow Lower Bay, the data
for the seven Stations monitored has been summarized variously by month,
season, site and vegetational state. All mention of attenuation coefficients,
unless otherwise noted, refers to k
-------
WINER
<=>
o
Legend
A BAY
x JNVEGETXTtD,
D VECETA1E.D,
400 450
500 550 600
Wavelength (nm)
650
Figure 3. Mean winter spectral attenuation at vegetated and unvegetated sites
in the lower Chesapeake Bay. (Bay • Deep water station).
97
-------
SPRING
£ 3
s>-,
2-
o
400 450 bOO 550 bOO
Wavelength (nm)
Legend
x UN VEGETATED.
O VEGETAtO,
650 TOO
Figure 4. Mean spring 1981 spectral attenuation at vegetated and unvegetated
sites in the lower Chesapeake Bay. (Bay • deep water station).
98
-------
SUMMER
o 2-
o
400 450
500 560 600
Wavelengtn (nm)
Legend
A BAY
x UHVEGFA1.0.
D
TOO
Figure 5. Mean summer 1981 spectral attenuation at vegetated and unvegetated
aitea in the lower Chesapeake Bay. (Bay • deep water station).
99
-------
AUUMN
.0
"a
I
Legend
A BAY
x uNVEGGAt
0 VtCEIAtQ.
400 450
SOO bt:0 feOO
Wavelength (nrn)
b50 TOO
Figure 6 Mean autumn 198) spectral attenuation at vegetated and unvegetated
sites in the Lower Chesapeake Bay. (Bay • Deep water station).
100
-------
TABLE I. MEAN SEASONAL SHALLOW WATER SPECTRAL ATTENUATION (a"1) LOWER
CHESAPEAKE BAY, 1981 (z • .1, .5 m)
Winter
(nm)
410
441
488
507
520
540
570
58*
625
656
671
694
Veg.
1.14
.892
.566
.510
.460
.404
.371
.407
.563
.644
.731
.834
Unveg.
1.04
.868
.503
.435
.397
.364
.283
.317
.497
.522
.623
.679
Spring
Veg.
1.91
1.-2
1.14
1.02
.919
.813
.711
.735
.907
.956
1.06
1.11
Unveg.
2.48
2.04
i.45
1.28
1.14
.973
.841
.858
.987
1.05
1.18
1.22
Summer
Veg.
3.07
2.70
1.79
1.60
1.41
1.22
1.08
1.09
1.24
1.28
1.41
1.43
Unveg.
3.08
2.62
1.78
1.58
1.40
1.19
1.00
1.01
1.15
1.21
1.38
1.34
Fall
Veg.
2.21
1.85
1.28
1.15
1.03
.887
.787
.798
.935
1.01
1.14
1.19
Unveg.
2.04
1.64
1.05
.918
.802
.674
.581
.599
.754
.842
.961
1.02
101
-------
Mean Seasonal Spectral Attenuation
Legend
A WINiER
*
D SUMMER
• AU1UMN
400 450 500 550 600 650
Wavelength (nm)
TOO
Figure 7. Mean seasonal spectral attenuation for shallow waters of the lower
Chesapeake Bay. (All sites combined).
102
-------
large difference between the mean deep-water (bay site) coefficients and the
shallow water coefficients possibly due to lesser resuspension of particulates
in the deep site.
~ i
The mean autumnal values were similar to those of the sprint mean \
coefficients for the vegetated sites, ranging from about 0.79 m~* at 570 nm to \
about 2.2 m"1 at 410 nm and the unvegetated mean from 0.58 m~* to 2 ra"1 at the
same wavelengtha.
A significant difference between the mean spectral attenuation
coefficients for vegetated and unvegetated sites waa found for spring (Fig.
471The mean spring violet-blue (500 nm) unvegetated k(X) is more than 1
standard deviation higher than the mean k(X) for violet-blue light at
vegetated sites (cf. Figs. 8, 9). Both shallow water vegetated and
unvegetated means were higher than that for the deep water (bay site). Mean
spring vegetated k(X) ranged from 0.71 m~* at 570 nm to 1.9 m~* at 410 nm.
Corresponding values for the unvegetated sites were 0.84 and 2.5 m~*,
respectively. That ia, violet light of 410 nm waa reduced 85Z per meter at the j
average vegetated site and over 92Z per meter at the average unvegetated site
during spring. There was little or no mean difference for wavelengtha greater
than 550 nm (yellow, orange, red). The deep water station ranged from about 1
to 1.5 m'1.
Monthly Mean k(X)
If one compares the monthly mean spectral attenuation for vegetated sites
(Figs. 10, 11, 12) with that for unvegetated sites (Figs. 13, 14, 15) the most
obvious difference to be found, as discussed above, is the higher attenuation
of the shorter wavelengths at the unvegetated sites during the spring of the
year. Th-» unvegetated sites are characterized by elevated attenuation over
the entire spectrum from May through October, whereas the vegetated sites do
not show consistently elevated attenuation coefficients over the same time
period. Attenuation of the violet and blue wavelengths (400-500 nm) at the
vegetated sites increases gradually, reaching the maximum during September.
Minor peaks occur during April and July. The attenuation coefficient for the
remainder of the spectrum (500-700 nm) also exhibits these minor peaks but
there is no increase towards the September maximum.
There does however appear to be a strong seasonal pattern to the
attenuation coefficient at both vegetated and unvegetated sites, differing
mainly in the timing of commencement of high values. The onset of high
attenuation, especially of short wavelengths, appears to differ from year to
year, as can be seen by comparing the March 1982 values (month 15 on graphs)
with the March and May 1981 values (Figs. 10, 13). The correspondences at
each type of site between the March 1982 (month 15 on the figures) short
wavelength attenuation coefficients and those for the proceeding May (month 5)
are quite striking. The relationship between the vegetated and unvegetated
sites for the March 1982 attenuation coefficients is also analagous to their
relationship in May, 1981. That is, at the average vegetated site the mean
violet coefficients (410, 441) for May 1981 and for March 1982 were both
betv -n about 1.5 and 2.0 m"* whereas those for the average unvegetated site
were between 2 and 3 m~l for both months.
103
-------
MEAN SPRNG VIOLET-BLUE K for VEGETATED SUES
3-
•
1
o
Legend
A Mean
x Moon T. t S.D.
x Mean - 1 S.D.
400 420 440 460 480
Wavelength (nm)
500
Figure 8. Mean spring 1981 violet-blue attenuation +_ 1 S. D. at vegetated
sites.
104
-------
MEAN SPRING VIOLET-BLUE K for NON-VEGETATED SES
J. "
JC
2-
o
Legend
A Mean
X Meon + 1 S.D.
x Mean - 1 S.O.
400 420 440 460 480
Wavelength (nm)
500
Figure 9. Mean spring 1981 violet-blue attenuation ^ 1 S. D. at unvegetated
sites.
105
-------
2-
VIOLET-BLUE
Legend
A WL-410
X
D Wb-488
8 10
MONIH
12 14
16
Figure 10. Mean monthly violet-blue attenuation at vegetated site*.
106
-------
X
,- Li-
's
u
I H
j
< 0.3-
GREEN-YELLOW
Legend
A WL«6Cf7
x WL-520
a WL-540
• WL-570
B WL-589
8 10
MONiH
12 14
16
Figure 11. Mean monthly green-yellow attenuation at vegetated sites.
107
-------
ORANGE-RED
o
Legend
6 3 iO
i4 '.6
Figure 12. Mean monthly orange-red attenuation at vegetated aites.
108
-------
§a
C
.0
~c
6
VIOLET-BLUE
Legend
A WI-H10
X WI-441
D
8 10
MONTH
12 14 1b
Figure 13. Mean monthly violet-blue attenuation at unvegetated site*.
109
-------
I '
J
"o
I
~ 0.5
GREEN-YELLOW
8 10
MONIH
Legend
A Wt-507
x WL-520
a WI-640
• WI-S70
• Wl-589
12 14 16
f.
Figure 14. Mean monthly green-yellow attenuation at unvegetatrd sites,
110
-------
>''/
JE,
JC
1-
o
i
~ 0.5 H
ORANGE-RED
b 8 10
MONIH
* 1 -U=-Jon-D«cai, IJ-IS^J
Legend
A Wl-felS
x WL-fe56
D WU-671
• WU-694
12 14 16
Figure 15. Mean monthly orange-red attenuation at unvegetattd sites.
Ill
-------
For unvegetated aicea the year 1981 was broken up into two distinct light
environments with little transition, while in the vegetated sites there was a
shorter high attenuation period with a more gradual transition from low to
high values.
The variability (expressed as _+! standard deviation) of the monthly mean
attenuation for vegetated and unvegetated sites is shown for wavelengths of
441 run and 671 run in Figures 16, 17, 18 and 19. These wavelengths are near
the photosynthetic action peaks for marine green plants (Halldal, 1974). The
variation in violet attenuation (441 run) is consistently greater during the
high turbidity season (May through October, 1981) for unvegetated sites than
for vegetated sites (c.f. Figs. 16, 17). A comparison of the variability of
red attenuation between vegetated and unvegetated sites yields much less
difference. (The high variability during September for both wavelengths at
the "average" vegetated site is mathematically due to the high attenuation at
the Vaucluse Shore site, included in the mean, compared to the relatively low
attenuation at the vegetated western shore sites). Nonetheless, constancy of
light quality may be of consequence to benthic plants.
Since May 1981 seems to have been not only a pivotal month for the
relative light environment in the unvegetated sites of the Lower Bay, but is
also the month with the highest observed net seagrass community productivity
(Murray and Wetzel, 1982) let us look more closely at the radiant energy
attenuation calculations for that month. The mean vegetated and unvegetated
k(X) for May are shown in Fig. 20. The unvegetated mean is significantly
higher at all wavelengths. There is a difference of more than 1.0 m~* at 410
nm, and a difference of about 1.0 m~l at 441 nm. The difference decreases to
about 0.5 at 520 nm. Below 540 nm there is a constant difference of less than
0.4 m~*. A 1.0 m~l difference in attenuation coefficients represents a
relative irradiance reduction of 63Z over a 1.0 m path. That is, during May
approximately 632 less violet light was able to pass through a meter of water
at the "average" unvegetated site than at the "average" vegetated site. Over
a 2.0 meter path the relative reduction would be 882.
Individual sites
A comparison of the mean May k(X) for each site is presented in Fig. 21.
The two definitely unvegetated sites (Mumfort Is. in the York R. and the site
in the mouth of the Severn R.) had the highest attenuation at all wavelengths.
The Severn R. site was especially high below 500 nm. The mean k(X) for the
violet wavelengths (410, 441 nm) was between 3.5 and 4.0 m"* for this site. A
unit of radiant energy would be reduced 97-98Z passing through a meter of
water with an attenuation coefficient between 3.5 and 4.0 ttT1. A reduction of
greater than 99Z would result from the passage of light through two meters of
water with an attenuation coefficient greater than 3.5 m"1. That is, there
was essentially no violet light below the surface of the water at the Severn
R. site during the average Hay sampling period.Blue light (488 nm) was also
greatly attenuated at this site (reduced about 902 per meter). Mean
attenuation at the Mumfort Is. site ranged from a high of about 3.0 m~^ af 410
nm about 1.5 m~l at 507 nm and no higher than that for the remainder of the
spectrum. The red end of the spectrum was even more attenuated at the Mumfort
Is. site than in the Severn R.
112
-------
Unvegetated Sites (Lambdo=441nm)
1
3-
§aH
o
Legerxi
A Mean
x M«on •*- 1 S.O.
< Mean - 1 S.D.
8 10
MONIH
12 14 16
Figure 16. Mean monthly variability of attenuation of light of 441 nm at
unvegetated sites.
113
-------
Vegetated Sites (Lambda=441nm)
a
1 H
v/1 L \
7
Legend
A M»on
X
x Mean - 1 S.D
24
6 8 10 12 14 16
MONTH
Figure 17. Mean monthly variability of attenuation of ligbt of 441 ran at
vegetated sites.
114
-------
Unvegetated Sites (Lambda-671nm)
•*=>
i.
.Q
1-
Legend
A Mean
x Meon r 1 S.D.
x Mean - 1 S.O
b 8 10
MQNIH
12 14
16
t
i
Figure 18. Mean monthly variability of attenuation of light of 671 ran at
unvegetated sites. "~"~"
115
-------
>
*-
' 3-
"""
Attenuation coefl.
0 -• N>
• . i
Vegetated Sites (t_ambda-671nm)
"\
' \
. W
A r\ '/ V .
/ . \ / ^\ ./ \\ x*^
/ /\ v y /^^s V ' \
// \V- 8 10 12 14 16
MOMTri
Figure 19. Mean monthly variability of attenuation of light of 671 nc at
vegetated sites.
116
-------
Mean May Spectral Attenuation by Vegetation
i3
enuati
Legend
A UNVEGETATED,
400 450 500 550 600
Wavelength (nm)
650 700
Figure 20. Mean May 1981 spectral attenuation at unvegetated and vegetated
sites.
117
-------
Mean Spectral Attenuation by Site
o
1H
400 450 500 550 600
Wavelength (nm)
begend
O WtCA.MMCM
• igDMK.
* fo>«fo»y.M<>i!ei.t..
650 700
Figure 21. Mean May 1981 spectral attenuation at individual sites.
118
-------
All Bean May coefficients at the vegetated site* - Guinea Marsh, Pour
Point Marsh and Vaucluae - were below about 2 ra~^ except those for violet
light at Four Point Marsh. However, the attenuation of yellow, orange and red
light at the Four Point Marsh site was extremely low - less than 0.5 m .
This is a reduction of only about 402 per meter. This site and the unvegetated
Severn R. site are located very close to each other, both in Mobjack Bay and
have a very similar pattern of spectral attenuation but at different
magnitudes. Note how the k curves track each other. The mean k( X) for the
unvegetated Severn R. site is consistently about 1 m"1 higher than that for
the vegetated Four Point Marsh site during May. Violet attenuation at the
Guinea Marsh and Vaucluae Shore vegetated sites was between about 1.5 and
2 m"1. Attenuation of the rest of their spectra being between 1 and 1.5 nT1.
The Vaucluse site had the lowest violet-blue mean attenuation during May while
the Four Point Marsh site had the lowest yellow to red attenuation; lower than
even the mean winter values at all other sites! Notice that with the
exception of the violet light at Four Point Marsh the mean May attenuation for
all vegetated sites at all wavelengths was less than about 2 m~l. Thus, the
extremely low May attenuation of the longer wavelengths at Four Point Marsh
may compensate for the high attenuation of the short wavelengths in terms of
total light energy available for photosynthesis.
The spectral attenuation at the Allen's Is. site is intermediate to that
of the vegetated and unvegetated sites. That is interesting because this site
is intermediate with respect to its vegetation. It was vegetated in the past,
but lost its bay grasses with the rest of the lower Bay. However, it has been
successfully transplanted by R. J. Orth and associates and has some natural
plants coming back (personal communication K. Moore, VIMS). Since this site
appears in transition the irradiance measurements taken there have not been
included in either the "vegetated" or "unvegetated" means reported herein, but
has been treated separately.
A comparison of the mean seasonal k( X) at individual sites (Figs. 22 thru
28) reveals both differences between vegetated (Vaucluse, Guinea, Four Point)
and unvegetated (Mumfort, Severn) sites and reveals individual site
idiosyncracies. The seasonal means plotted on these graphs were calculated
fron 1981 and 1982 measurements combined. Therefore "winter" (defined as
Jan.-Mar.) at some sites represents values from both years. Since, as
explained earlier, the high turbidity season started in March during 1982 and
in May during 1981, the resulting combined winter mean includes both high and
low values at those sites which were monitored both years.
Mean seasonal spectral attenuation values were at or below 2 m~* for all
seasons except summer at the western shore vegetated sites (Figs. 24, 26). As
noted previously, attenuation at the red end of the spectrum was particularly
low at Four Point Marsh (Fig. 26). Note how similar autumn, winter and spring
appear at this site. Winter and spring mean values were also less than 2.0
m~l at the eastern shore vegetated site (Vaucluse Shores, Fig. 27), but autumn
was the most turbid season for this site as it also was for the deep site on
the Eastern Shore (Bay, Fig. 28). None of the western shore sites showed this
pattern. The light environment, not surprisingly, appears to be quite
different in the different masses of water on opposite sides of the Bay. With
the exception of fall the mean seasonal attenuation at the deep site (Fig. 28)
119
-------
Mean Seasonal Spectral Attenuation
-<=>
A
O
Legend
A WM1ER
x SPRiNG_
a SUMMER
• AUIUMN
400 450 500 550 600
Wavelength (nrn)
650 TOO
Figure 22. Mean seasonal spectral attenuation at Mumfort Is. (York R.).
120
-------
Mean Seasonal Spectral Attenuation
.Q
Legend
A WINTER
X SPRING
a SUMMER
• AU1UMN
400 450 500 550 600 650 700
Figure 23. Mean seasonal spectral attenuation at Allen's Is. (York R.),
121
-------
Mean Seasonal Spectral Attenuation
3'
a-
'•
Legend
A WINIER
X SPRMC_
D SUMMER
• AU1UMN
400 450 500 550 600 650 700
Figure 24. Mean seasonal spectral attenuation at Guinea Marsh.
122
-------
/
Mean Seasonal Spectral Attenuation
o
\
\
JB--B
Legend
A WNIE.R
x SPRING
O SUMMER
• AUIUMN
400 450 500 J>50 600 650 700
Figure 25. Mean seasonal spectral attenuation at the mouth of the Severn R.
(Mob jack Bay).
123
-------
Mean Seasonal Spectra! Attenuation
\
\
--a
Legend
A WMER
x SPRING
0 SUMMER
• AlflUMN
400 450 500 550 600 650 700
Figure 26. Mean seasonal spectral attenuation at Four Point Marsh (mouth of
Ware R., Mobjack Bay).
124
-------
,-/• : • /
''/ S/'
/'
Mean Seasonal Spectral Attenuation
o-
Legend
A WIN1E.R
x SPRiNO
a SUMMER
400 4bO bOO 550 600 6bO 700
i
Figure 27. Mean seasonal spectral attenuation at Vaucluse Shores Zostera bed
(Eastern Shore).
125
-------
Mean Seasonal Spectral Attenuation
A3^
2-
o
~o
1
Legend
A WINIER
x SPRING
O SUMMER
• AUOJMN
400 450 500 550 600 650 700
Figure 28. Mean seasonal spectral attenuation at the Deep Station (Bay) at
VaucLuse Shores (Eastern Shore).
126
J
-------
was consistently lower than the corresponding values at any other site. Note j
the relative color shift from blue to red betveen spring to summer at this
site (the intersection of the 2 seasonal curves). This shift also occurs in
modified form at the eastern shore seagrass site (Vaucluse, Fig. 27). Here j
the penetration of blue light is reduced significantly but the penetration of j
red light is not increased. ;
The western shore unvegetated sites (Mumfort, Severn, Figs. 22, 25) both
had mean spring violet attenuation values between 2 and 3 m~l, definitely
higher than the corresponding values at any vegetated site. Each of the
seasonal mean curves for Mumfort Is. (Fig. 22) are high compared to the other
site*. The autumn and winter mean k(x) for the Severn R. site are, however,
similar to the corresponding curves for vegetated sites.
The Allen's Is. site (Fig. 23) is once again difficult to classify. Its
mean spring k( X) is intermediate, summer low, winter high and fall it, about
average.
The mean monthly water column attenuation for selected wavelengths at
individual sites ie presented in Figs. 29 through 35. The wavelengths
presented in these figures (410, 441, 488, 570, 671, 694 ran) were selected not
only for their biological relevance with respect to photosynthetic action
spectra and in vivo pigment absorption peaks, but to outline the extremes and
means of the full set of 12 wavelengths measured with more clarity and less
confusion than is possible by showing the entire set measured. A spline
interpolation was used to connect the discrete measurements.
A seasonal pattern of turbidity at all sites is most obvious. As
mentioned previously the high turbidity at the unvegetated sites (Figs. 29,
32) clearly starts in Nay and continues through October, during 1981. This is
especially obvious for the shorter wavelengths. The onset of high turbidity
appears earlier in 1982, during March, with violet attenuation approaching or
exceeding 3.0 m ~* at the unvegetated sites. At Guinea Marsh (vegetated, Pig.
31) the violet attenuation increased gradually from a low of about 1.5 m~' in
March of 1981 to a peak of between 3.5 and 4.1 m~* during September, declining
dramatically to winter levels in November. As of March 1982, no increase was
evident. The attenuation of longer wavelengths at Gui.tea Marsh peaked sharply
in September and declined during October to reach otherwise constantly low
values during November. A similar pattern of attenuation for the longer
wavelengths of light was found at the other western shore vegetated site, Four
Point Marsh (Fig. 33). However, May 1981 values were extraordinarily low (0.5
ro"*) for the red end of the PAR at this site. Simultaneously, the violet-blue
attenuation during May was higher than the other vegetated sites. The Allen's
Is. site (Fig. 30) shows much less monthly variation than any other western
shore site.
The eastern shore vegetated site at Vaucluse (Fig. 34) reached its
maximum short wave attenuation during October, but also peaked in April. The
minima occurred during March, May and June 1981 and January 1982. The long
wave attenuation followed the same pattern but with much leas magnitude.
March, May and June, 1981 were also the months of maximum net productivity for
the benthic Zostera community at this site (Murray & Weteel, 1982). The same
127
-------
Mean Monthly Attenuation by Waveiengin
3-
-------
Mean Monthly Attenuation by Wavelength
.
§
o
1,
10
Month
to
Legend
A WL-41Q
X
a w?-£88
• Wl-570
X Wlr-671
* Wb-694
20
V-U-.
Figure 30. Mean monthly attenuation of selected wavelengths at Allen's Is.
(York R.).
129
-------
Mean Monthly Attenuation by Wavelength
D
3
o
'a
b 10
Month
15
Legend
3 Vfc-488
a WU-670
B
20
< •
i
Figure 31. Mean monthly attenuation of selected wavelengths at Guinea Marsh,
130
-------
Mean Monthly Attenuation by Wavelength
8 2
O
'.^
O
1-
10
Montn
15
Legend
A Wb-410
a WL-488
B WU-570
Wl=694
20
-1t>-=>.una2-Ngf 8
Figure 32. Mean monthly attenuation of selected wavelengths at the mouth of
the Severn R. (Mobjack Bay).
131
-------
Mean Monthly Attenuation by Wavelength
Legend
A WL~*10
X
D Wb-488
• WL-570
• ^Bse^
« WL-69*
10 12
Montri
14
16
Figure 33. Mean monthly attenuation of selected wavelengths at Four Point
Marsh (mouth of Ware R., Mobjack Bay). Note: Horizontal axis differs from
other figures in thif series.
\
132
-------
Mean Monthly Attenuation by Wavelength
§»
-
a
o-
:0
Month
Legena
A Wli-410
X
a *u-570
WL--694
20
Figure 34. Mean monthly attenuation of selected wavelengths in the Zostera
bed at Vaucluse Shores.
133
-------
Mean Monthly Attenuation by Wavelength
coe
K
1
uati
At
10
Montn
15
Legend
A Wl-410
X Wb-441
a WL-488 _
• WL-570
WL-671
20
Figure 35. Mean monthly attenuation of selected wavelengths at Deep Station
(Bay) at Vaucluse Shores.
134
-------
pattern of attenuation, but with less extreme oscillation is apparent at the
deep water station (Fig. 35) adjacent to the Vaucluse site.
In summary, attenuation across the entire spectrum at unvegetated sites
appears to begin earlier in the year and increase at a more rapid rate than at
vegetated sites.
Relative Potential Benthic Photosynthesis
Potential benthic production is determined both by the light available
for photosynthesis at the bottom of the water column and by the inherent
photosynthetic response of the plants present. Limiting our discussion to
green plants and using published photosynthetic action spectra, mean seasonal
attenuation coefficients and incident irradiance measurements we can calculate
the relative potential jghotoaynthetically £torable jradiation, PSR (Morel,
1978; Smith, 1979). The results of these calculations for spring are
presented in Figure 36. Spring was chosen for this example not only because
it is the season with the greatest difference in attenuation between vegetated
and unvegetated sites, but because it is the season of highest net seagrass
community productivity (Murray and Wetzel, 1982; Wetzel et al., 1982).
Curve "a" (Fig. 36) represents the mean incident spectral irradiance
during clear spring days at noon. Notice how the quantum distribution
decreases rapidly below 500 nm. The estimated total attenuation coefficients
for both vegetated and unvegetated sites are shown as curves "b". These
represent the spring spectral attenuation from just above the water's surface
to a depth of 1.0 m for calm, clear days around noon. The attenuation at the
unvegetated sites is higher at all wavelengths than that at the vegetated
sites during spring: especially for wavelengths less than 500 nm. This
difference increases from .23 m ^ at 507 nm to .51 m"1 at 410 nm. There is a
difference between site types of about .1 m~* from 570 to 700 nm.
The resulting estimated benthic irradiance for both vegetated and
unvegetated sites is shown as curves "c". (Note that the verticle axis is
less than one-fifth that of the verticle axis for curve "a"). Compare these
with curves "a" to ascertain the dramatic decreases in irradiance through just
1.0 u of water. The benthic irradiance at the average spring unvegetated site
ranges from 0.21 cm"2^"1 at 410 nm to 2.13 cm~2's~* at 570 nm. The
corresponding irradiance for vegetated sites is .349 and 2.37 respectively.
The relative photosynthetic action spectrum for Diva taeniata, a typical
shallow water estuarine green algal specie is plotted as curve "d" after Haxo
and Blinks (1950) and HalIda11 (1974). The photosynthetic pigment complex of
green algae is very similar to that of seagrasses. The useable light energies
of highest photosynthetic efficiency fall between 400-500 nm and between 650
and t>80 nm.
The potential PSR for the average spring vegetated and unvegetated site
is presented as curve "e". This is simply the normalized (0-100) product of
curves "c" and "d". The resulting potential PSR curve is a function therefore
of incident radiation, total attenuation and the inherent spectral efficiency
of "green marine plant" photosynthesis. This PSR spectral distribution
135
A
-------
10 -i
«o
UJ
u
z
<
o
oc
cc
8-
u 6-
4 -1
— — — - Unvegetated
400
1
500
600
X Inm)
700
Figure 36. Relative potential spring PSR for vegetated and unvegetated sites.
Solid lines represent vegetated areas, dashed lines represent unvegetated.
(a) Mean spring surface irradiance at noon on a clear day, (b) total
attenuation, (c) benthic irradiance at 1.0 m, (d) relative photosynthetic
action spectrum, (e) relative potential photosynthetically storable radiation
(PSR).
136
-------
includes two peaks; one broadly centered about 490 nm in the blue end and the
other extending from about 590 to about 690 nm. The blue peak has been
shifted from about 440 nm in the action spectrum to about 490 nm in the PSR
due to the combined effects of the high attenuation of violet and the low
original violet insolation. Much of the difference between the mean vegetated
and mean unvegetated PSR curves lies within the blue peak. The unvegetated
blue peak is quite insubstantial (below 50) whereas the vegetated peak is from
12 to 16 relative units higher. In a marginal light environment this
difference in potential atorable radiant energy may be quite important. In
the red peak region the difference between the two curves is much lower - from
5 to 8 units between 590 and 670 nm.
DISCUSSION
In the lower Chesapeake Bay the light available to benthic plants may not
include great quantities of energy at the wavelengths which can be most
efficiently used by those plants. The greatest loss of potential energy
appears to be in regions of the spectrum most significant for photosynthesis.
Whether the difference in light quality between vegetated and unvegetated
sites ie causal, and the direction of that causality cannot be determined by
the work reported here. But, there does appear to be a negative correlation
between light quality and the presence of seagrasses. A consideration of the
direction of the causality may be irrelevant and analagous to asking the
question, which came first, the chicken or the egg? That is, does the
baffling effect of seagrasses cause settling of the fine suspended material
thus reducing the scattering and absorption (lowering attenuation) and
allowing more light energy to reach the benthos? Or, does a water body with
relatively little suspended material provide a light environment suitable for
seagrasses to survive, grow and thus baffle the water and maintain water
clarity so light can continue to reach the benthos — etc.?
Yes—to both. Ecosystems are comprised of dynamically interconnected
biological and physical components which interact materially and
energetically. The morphogenesis of ar ecosystem from simple pioneer
beginnings to a mature dynamically stable climax stage involves a continual
reciprocal induction process between its physical and biological parts. This
is achieved through sensitive feedback mechanisms. A mature ecosystem is a
homeostatic entity capable of internal adjustment to a range of external
conditions - within limits.
Let's define the benthic seagrass community and its overlaying water
column an ecosystem. The water clarity is affected by the baffling effect of
the plants (Ginsburg and Lowenstain, 1958; Scoffin, 1970; Wanless, 1981;
Boynton and Heck, 1982) and the plants are certainly affected by the water
clarity. A minimal water clarity is necessary for a seedling of seagrass to
successfully colonize a suitable barren substrate and to grow vegetatively to
some minimal size necessary to provide the baffling necessary to induce
settling (Boynton and Heck's (1982) "critical bed size") and trapping of
enough of the fine particles to clear the water column to maintain a tolerable
light environment for continued growth. If during the early seedling stage
the water clarity is insufficient than the ecosystem will not succeed to the
137
-------
self regulatory, homeostatic stage and a seagrass community will not be
established.
A model of a seagrass ecosystem including the water column is shown in
Figure 37 as an aid to the conceptualization of the sensitive feedback
relationship between the seagrass community and water clarity. Of particular
interest is the relationship between critical bed size and sedimentary
baffling and attenuation.
Due to the dynamic nature of the littoral zone and coastline, normal
variations in physical parameters may often exceed an established seagrass
community's ability to adjust and survive on a local scale. Rapid
recolonizing - both vegetatively and sexually (via seeds) would be expected _i_f
water clarity permits, but, if—due to nutrient enrichment and subsequent
plankton blooms or particle runoff, or both—the light environment becomes
unsuitable for the re-establishment of a new seagrass community, than the net
seagrass ecosystem size may dimish despite survival of the established beds
not affected by normal local disturbances. That is, since there appears to be
thresholds below which the critical feedback between biological and physical
parameters of the system cannot be established (critical bed size and a
minimal light environment) — replacement of seagrass communities lost due to
normal processes may be impossible during periods of reduced water quclity.
The hiotorical pattern of increasing nutrient enrichment of the
Chesapeake Bay from agricultural runoff and municipal sources and its presumed
contribution to excess planktonic productivity (Heinle et al, 1980) coupled
with the particulate load associated with runoff may have altered the water
clarity enough to account for the decline in submerged aquatic vegetation via
the mechanism discussed above. Our measurements of spectral irradiance and
attenuation in the littoral zone of the lower Chesapeake would certainly lead
us to believe that there isn't much light to spare—especially at the
frequencies most efficiently used by green plants for photosynthesis.
138
-------
REFLECTED
a
BACK-
SCATTERED
CRITICAL BED SIZE
Figure 37. Conceptual model of critical feedback between biological and
physical components of the seagrass ecosystem. (Symbols after Odum, 1983),
139
-------
CONCLUSIONS <
i
1. A seasonal pattern of spectral attenuation occurs in the shallow waters of I
the Lower Chesapeake Bay. Summer has the highest, spring and fall are
intermediate and winter has the lowest.
2. The onset of high attenuation differs from year to year.
3. The seasonal pattern of attenuation differs between vegetated and :
unvegetated sites. The transition from low winter attenuation to high
summer attenuation is more abrupt at unvegetated sites. There is a
shorter high attenuation- season at vegetated sites.
A. There is a significantly greater attenuation of violet light in
unvegetated sites during spring, especially during May 1981 and March
1982. During May, 1981, 632 less /iolet light was able to pass through a
meter of water at the average unvegetated site than at the average
vegetated site; 882 less through 2.0 meters. (A difference of 1.0 m~l at
nm).
5. The variability of violet attenuation was greater at unvegetated sites
during the high turbidity seasons. i
6. The pattern and magnitude of spectral attenuation differed on opposite ' }
sides of the Bay. j ;
i .
7. There is a reduction of potential photosynthetically storable radiation j '.
(PSR) at unvegetated sites. Less light is available at those wavelengths H
most efficiently used by green marine plants for photosynthesis. • ;
8. A critical feedback between the biological and physical components of
the seagrass/water column ecosystem must be established if the system is
to maintain homeostasis.
140
i
A
-------
LITERATURE CITED
Baker, K. S. and R. C. Smith. 1980. quasi-inherent characteristics of the
diffuse attenuation coeificient for irradiance. In, S. Q. Duntley, (ed.)
Ocean Optics VI. pp. 60-63, Proc. Soc. Photo-optical Instrumentation
Engineers, Vol. 208.
Booth, C. R. and P. Dunstan. 1979. Diver-operable multiwavelength
radiometer. In, Measurements of Optical Radiations. Soc. Photo-optical
Instrumentation Engineers 196:33-39.
Boynton, W. R. and K. L. Heck, Jr. 1982. Ecological role and value of
submerged macrophyte communities: A scientific summary. In, E. C.
Macalaster, D. A. Barker and M. Kasper, (eds.), Chesapeake Bay Program
Technical Studies: A Synthesis, U. S. Environmental Protection Agency,
Washington, D.C. pp. 428-502.
Burt, W. V. 1958. Selective transmission of light in tropical Pacific
waters. Deep-Sea Res. 5:51-61.
Champ, M. A., G. A. Gould, III, W. E. Bozzo, S. G. Ackleson and K. C. Vierra.
1980. Characterization of light extinction and attenuation in Chesapeake
Bay, August, 1977. In, V. S. Kennedy, (ed.), Estuarine Perspectives,
Academic Press, Inc., N. Y. pp. 263-277.
Clarke, G. L. and H. R. James. 1939. Laboratory analysis of the selective
absorption of light by seawater. J. Optical Soc. Am. 29:43-55.
Clarke, G. L. and G. C. Ewing. 1974. Remote spectroscopy of the sea for j
biological production studies. In, N. G. Jerlov and E. Steeman Nielsen
(eds.), Optical Aspects of Oceanography, Academic Press, N. Y. pp.
389-413.
Ginsburg, R. N. and H. A. Lowenstam. 1958. The influence of marine bottom
communities on the depositional environment of sediments. J. Geol.
66:310-318.
Govindjee and Govindjee. 1975. Bioent>rgatic8 of Photosynthesis. Academic
Press, N. Y.
Halldal, P. 1974. Light and photosynthesis of different marine algal groups.
In, N. G. Jerlov and E. Steeman Nielsen, (eds.) Optical Aspects of
Oceanography pp. 343-360, Academic Press, N. Y.
Haxo, F. T. and L. R. Blinks. 1950. Photosynthetic action spectra of marine
algae. J. Gen. Physioi. 33(3):389-422.
141
-------
Heinle, D. R., C. F. D'Elia, J. L. Taft, J. S. Wilson, M. Cole-Jones, A. B.
Caplins and L. E. Cronin. 1980. Historical review of water quality and
climatic data from Chesapeake Bay with emphasis on effects on enrichment.
Report to U.S. Environmental Protection Agency, Chesapeake Bay Program.
Chesapeake Research Consortium, Inc. Pub. No. 84. Univ. MD. Center for
Environmental and Estuarine Studies No. 80-15 CBL. i
i
Hurlburt, E. A. 1945. Optics of distilled and natural water. J. Optical !
Soc. Amer. 35:689-705.
Inada, K. 1976. Action spectra for photosynthesis in higher plants. Plant
Cell Physiol. 17:355-365.
James, H. R. and E. A. Birge. 1938. A laboratory study of the absorption of
light by lake waters. Trans. Wise. Acad. Sci. 31:154 ->p.
Jerlov, N. G. 1976. Marine Optics. Elsevier Oceanography Series Vol. 14,
Elsevier Scientific Pub. Co., N. Y. 231 pp.
Kalle, K. 1966. The problem of rhe Gelbstoff in the Sea. Oceanogr. Mar.
Biol. Annu. Rev. 4:91-104.
Levring, T. 1947. Submarine daylight and the photosynthesis of marine algae.
GHteborgs Vetensk Samh. Handl., IV Ser., B, 5/6:1-89.
Levring, T. 1966. Submarine light and algal shore zonation. In, R. i
Bainbridge, G. C. Evans, and 0. Rackham, (eds.), Light as an Ecological j
Factor, pp. 305-318. British Ecol. Soc. Symp. Vol. No. to, Blackwell Sci. , j
Pubs. Ltd., Oxford, G. B. '
Morel, A. 1978. Available, useable and stored radiant energy in relation to
marine photosynthesis. Deep-Sea Research 25:673-688.
Murray, L. and R. L. Wetzel. 1982. Compartmental studies of community oxygen
metabolism. In, R. L. Wetzel (ed.) Structural and Functional Aspects of
the Ecology of Submerged Aquatic Macrophyte Communities in the Lower
Chesapeake Bay Grant Nos. R6U5974 and XUU3245-01, U.S. Environmental
Protection Agency, Washington, D.C.
Odum, H. T. 19b3. Systems Ecology: An Introduction. John Wiley & Sons,
N.Y., 644 pp.
Orth, R. J., K. A. Moore and li. H. Gordon. 1979. Distribution and abundance
of submerged aquatic vegetation in the lower Chesapeake Bay. E.P.A.
Report No. 600/8-79 -029/SAVl.
Pierce, J. W., D. L. Correll, M. A. Faust, W. H. Klein and B. Goldberg. 1981.
Spectral quality of underwater light in a turbid estuary, Rhode River,
MD, U.S.A. Unpub. manuscript.
142
-------
Prieur, L. and S. Sathyendranath. 1981. An optical classification of coaetal
and oceanic waters based on the specific absorption curves of
phytoplankton pigments dissolved organic water, and other particulate
materials. Limnol Oceanogr 26:671-669.
Scoffin, T. P. 1970. The trapping and binding of subtidal carbonate
sediments by marine vegetation in Bimini Lagoon, Bahamas. J. Sed.
Petrol. 40:249-273.
Seliger, H. H. and M. E. Loftus. 1974. Growth and dissipation of
phytoplankton in Chesapeake Bay. II. A statistical analysis of
phytoplankton standing crops in the Rhode and West Rivers and adjacent
section of the Chesapeake Bay. Chesapeake Sci. 15:185-204.
Smith, R. C. 1979. Bio-Optics. In, S. Q. Duntley, (ed.) Ocean Optics VI,
pp. 47-53, Proc. Soc. Photo-Optical Instrumentation Engineers, Vol. 208.
Wanless, H. R. 1981. Fining - upwards sedirentary sequences generated in
seagrass beds. J. Sep. Petrol. 51(2):445-454.
Wetzel, R. L., R. F. van Tine and P. A. Penhale. 1981. Light and submerged
microphyte communities in the Chesapeake Bay: A scientific summary.
Special Report No. 260 in Applied Marine Science and Ocean Engineering,
Virginia Institute of Marine Science, Gloucester Point, VA. 54 pp.
Wetzel, R. L., L. Murray, R. F. van Tine and P. A. Penhale. 1982.
Photosynthesis, light response and metabolism of submerged uacrophyte
communities in the Lower Chesapeake Bay. In, R. L. Wetzel (ed.)
Structural and Functional Aspects of th Ecology of Submerged Aquatic
Macrophyte Communities in the Lower Chesapeake Bay, Vol I. Final Draft
Report Grant No. R805974 and X003245-01 U.S. Environmental Protection
Agency, Washington, D.C.
Yentsch, C. S. 1960. The influence of phytoplankton pigments on the colour
of the sea water. Deep-Sea Res. 7:1-9.
Zschiele, F. P. and C. L. Comar. 1941. Influence of preparative procedure on
the purity of chlorophyll components as shown by absorption spectra.
Botan. Gazette 102:463-481.
Zscheile, F. P., J. W. White, Jr., B. W. Beadle, and J. R. Roach. 1942. The
preparation and absorption spectra of five pure carotenoid pigments.
Plant Physiol 17:331-346.
143
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Chapter 4
SPECTRAL DISTRIBUTION AND ATTENUATION OF UNDERWATER LIGHT
IN A TROPICAL MANGROVE CREEK AND SEAGRASS BED,
LACUNA DE TERMINOS, CAMPECHE, MEXICO: A PRELIMINARY ANALYSIS.1'2
R. F. van Tine and R. L. Wetzel
i
Virginia Institute of Marine Science j
School of Marine Science
College of William and Mary
Gloucester Point, VA 23062
1. This research was supported in part by the U.S. Environmental Protection
Agency, Chesapeake Bay Program Grants R805975 and X003245-01 the College
of William and Mary, Virginia Institute ot Marine Science, Department of
Wetlands Ecology.
2. Contribution no. , the Virginia Institute of Marine Science, Gloucester
Point, VA 23062.
-------
ACKNOWLEDGEMENTS
We want to extend thanks to all our Mexican colleagues who accepted and
worked closely witn our contingent of fourteen scientists, especially Ramiro
Roman, Director of the Centro de Ciencias del Mar y Limnologia (UNAM) who
provided us lodging, space, logistical support and a friendly atmosphere anr?
Drs. Alejandro Yanez and Vivianne Solis of the Universidad National Autoroma
de Mexico (UNAM) for their many contributions.
Thanks also to our fearle-s leader, Dr. J. W. Day, Jr. for organizing the
expedition, and to Chris Madden and Linda Deegan for their help in the field.
We also thank Ms. Carole Knox and Nancy White for their expert
secretarial services and Ms. Melissa van Tine for her editorial assistance.
145
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INTRODUCTION
There has been a paucity of systematic experimental studies of spectral
irradiance in marine environments (Jerlov 1976) and even fewer studies have
been reported for estuarine waters (Wetzel et al. 1981; Champ et al. 1980;
Pierce et al. 1981). None, to our knowledge, exist for La gun a de Terminos
except a qualitative description of turbidity patterns (Day and
Yanez-Arancibia 1980), depicting an east to west gradient corresponding with
the predominate wind-driven circulation of lagoon water (Gierlof f-Emden 1977).
The present work to describe the light environment, was designed as part
of a Winter 1981 multi-disciplinary, U.S. -Mexican ecosystem study of the
lagoonal seagrass beds. Wetzel et al. (1982) provide a description of the
study site, Estero Pargo (see Fig. 1). Estero Pargo Creek drains an extensive
mangrove swamp dominated by Rhizophora mangle, and the mouth opens on an
extensive seagrass bed dominated by Thalassia testudinum.
METHODS
In situ measurements of downwelling 2 U spectral irradiance were made as
quanta iun~^ cm"* s~* at 12 biologically significant wavelengths ^ 5 run (410,
441, 488, 507, 520, 570, 589, 625, 656, 671, 694) using a Biospherical Model
MER-1000 submersible spectroradiometer (Booth and Duns tan 1979) that had
recently been calibrated using U.S. Bureau of Standards lamps. Each
measurement taken was the mean of 250 individual scans made over a
several-second interval to eliminate wave-crest refraction distortions,
effects of non-random distribution of particles in the water column, and
nonunifonn sky conditions. Measurements were made at the mouth of Estero
Pargo Creek over a Thalassia testudinum bed, at a site approximately 1 km up
the creek off the U.N.A.M. Centro de Ciencias del Mar y Limnologia dock and at
an intermediate midstream site. Measurements at the Thalassia site were made
periodically throughout the daylight hours during 3 days coinciding with other
studies (Wetzel et al. 1982). Measured depths ranged from .75 to 1.1 m. The
diffuse vertical attenuation coefficient (k) was assumed to be an exponential
function of depth (Jerlov 1976) and calculated for each wavelength as:
m-ln (g,/E0)
where Er is downwelling irradiance at depth z and Eo is insolation just above
the water surface.
RESULTS
The downwelling spectral irradiance for noon and 1:00 p.m., both at the
surface (incident irradiance) and at a depth of just less than a meter
(benthic irradiance) are presented in Fig. 2 along with their corresponding
attenuation coefficients, for a clear sky day at the Thalassia site. The noon
-------
Figure 1. Map of Study Site
147
-------
r INCIDENT
/ IRRAOIANCE
BENTHIC
IRRADIANCE
400
500
600
700
A tnm)
Figure 2. Downwelling spectral irradiance and spectral attenuation
coefficients over a Thalassi" testudinum bed off the mouth of Estero Pargo
Creek on a clear day. (A) 1200 C.S.T., 10-15 knot sw wind, z - .88 m, (B)
1300 C.S.T., Calm, z " .85 m. Shaded area represents difference in
attenuation which may be due to wind driven resuspension. (Each point is the
mean of 250 scans).
148
-------
TABLE I
Daily Mean k
Color
Violet
Blue
Green
Yellow
Orange
Red
(run)
410
441
488
507
520
540
570
589
625
656
671
694
PAR
(400-700)
n
z(m)
t
Feb. 6
3.64
3.02
2.22
2.05
1.90
1.75
1.63
1.66
1.86
1.94
2.06
2.11
2.01
2250
.98-1.1
1430-1730
Feb. 8
3.48
2.77
1.92
1.76
1.66
1.54
1.40
1.40
1.52
1.53
1.58
1.65
1.74
1250
.85-. 91
1115-1400
Feb. 10
5.27
4.26
2.99
2.74
2.55
2.34
2.10
2.08
2.26
2.25
2.34
2.40
2.54
500
.76
1115-1200
Grand Mean
4.13
3.35
2.38
2.18
2.04
1.87
1.71
1.71
1.88
1.91
1.99
2.05
2.10
400
.76-1.1
1115-1730
149
-------
measurement (A) was made during a 10-15 knot s.w. wind but an hour later (B)
the wind had diminished. There is a sharp attenuation of blue and violet
light below about 500 nm, while the lowest attenuation occurs in the yellow
region between 550-600 nm. As indicated, although little appreciable
difference exists between the two insolation curves, there is a considerable
increase in attenuation, across the spectrum, between the windy noon and the
calm 1:00 p.m. values (shaded area of Fig. 2). Noontime PAR
(Photosynthetically Available Radiation) insolation is about 23 x 1016
quanta-cm"2^'!, while benthic PAR irradiance is only 3-6 x
Mean daily spectral attenuation coefficients are shown in Table I for
three of five days at the Thalassia site off the mouth of the creek.
Attenuation per meter of the shortest wavelengths ranged from a mean daily low
of 3.64 to a high of 5.27, representing the highest values observed at the
site. The mid-band yellow light (570-590) was least attenuated, ranging from
1.40 m~l to 2.10 m~l. The grand weekly mean attenuation coefficients
represent the average of 4,000 scans. Climatic conditions ranged from windy
and cloudy to clear and calm and covered the time period, 1115 to 1730 C.S.T.
Depths ranged from .76 to 1.1 m. The grand mean (Fig. 3-(l), curve C) should
well characterize spectral attenuation at this site during the "El Norte"
season, as the 5-day measurement period was an interim between two such storm
events.
Comparison of spectral attenuation coefficients along a 1 km upstream
transect (Fig. 3-(D) revealed a dramatic decrease of violet and blue light
(400-500 nm). Attenuation was higher at all wavelengths upstream (A) than at
the mouth (C), the difference assymptotically increasing to almost 3.0 m"1 at
the violet end of the spectrum and approaching zero in the red region (650-700
nm). Midstream attenuation (B) vas intermediate between the extremes.
DISCUSSION
As light passes through a body of water, its energy content and spectral
quality are changed by absorption and scattering due to the water itself,
dissolved substances, and suspended particles. The combined effect of these
processes is termed attenuation. Backscattering, the change in direction of
light propagation caused by diffraction, refraction, and reflection, is
wavelength dependent in an irregular and complex manner. Scattering is of
less importance in determining attenuation- in shallow water since usually no
more than 0.5Z of the incident irradiance is back-scattered out of the medium
(Clark and Ewing 1974). Lateral and forward scattering increase the path of
light thereby exposing it to more absorption. The resulting absorption the
thermodynamically irreversible process of photon conversion into thermal,
kinetic, or chemical energy, e.g. photosynthesis), accounts for most of the
apparent attenuation observed in natural bodies of water.
Much of the attenuation of long wavelengths is due to water itself (James
and Birge 1938); the effect of sea salts is insignificant (cf., Clarke and
James 1939). The energy of blue and red wavelengths is selectively absorbed
by particles (Burt 1958; Prieur and Sathyendranath 1981). The shorter
wavelengths also are strongly attenuated by dissolved organic material and
150
-------
6-
5-
'• 4
3
o
O
O
2H
(I)
ui
_i
o
H 2
oc
I -
(2)
400
500
I
600
700
A (nm)
Figure 3. (1) Comparison of diffuse downwelling spectral attenuation
coefficients at three sites in Estero Pargo Creek. (A) Upstream off the
U.K. A.M. dock (B) midstream, (C) off the nouth in a Thalassia bed. Data for
curves A and 3 represent the mean of 250 scans. Curve C represents the mean
of 4000 scans taken during 3 days. (2) The difference in downwelling spectral
attenuation coefficient between the upstream site and the mouth of Estero
Pargo Creek. The curve represents the arithmetic difference between curves A
and C, Fig. 3-(l).
151
-------
complexes of this material, or Gelbstoff ("yellow substance"), the collective
name given these complexes by Kalle (1966). Gelbstoff is formed from
carbohydrates produced by the decomposition of organic matter. At Estero
Pargo much potential organic material is provided in the wet season by
mangrove litter fall, which ranges from about 1 to 4 gm'tn~2*day and takes
several months to decompose (Day and Yanez-Arancibia 1980). Further,
absorption in the blue and red regions of the spectrum by chlorophyll-bearing
phytoplankton also contributes to the total spectral attenuation that
characterizes a specific body of water.
Figure 4 is a schematic representation of spectral energy flow through a
marine environment. The relative proportions of the various constituents
discussed above determine the ultimate light quality and light quantity
available to power photosynthetic reactions. The diagram uses the typical
noon clear sky irradiance distributions measured above and below water at the
Estero Pargo seagrass site. Typical curves of spectral absorption due to
dissolved organic matter (including Gelbstoff), non-green particulate matter,
chlorophyll and pure seawater are shown impinging on the path of light from
surface to bottom. Rain and wind ("Nortes") cause runoff with increased
dissolved organics and resuspension of particulate matter and benthic
chlorophyll-bearing microalgae, thus increasing particle scattering,
absorption and consequently attenuation. The specific spectral energy
distribution at a depth thus depends on both the physical and biological
characteristics of the water column and the forcing functions impringing on
the system.
In Estero Pargo Creek the dominating influence se^ras to be from dissolved
organics* If the spectral attenuation coefficients for the mouth of the creek
are subtracted from those at the upstream site (curve A - curve B, Figure
3-(D), the resultant curve (Fig. 3-(2)) may represent the attenuation due
solely to the constituents ui the water column present upstream but not
downstream. If one compares the resultant curve with the specific absorption
curves in Figure 4 it can be seen that it matches the shape of the dissolved
organic curve very closely. The extreme attenuation of the short wavelengths
decreases downstream as the diluting effect of the waters from the lagoon
become more apparent. The creek waters no doubt affect the light environment
in the grassbed at its mouth, contributing to the high violet-blue
attenuation.
An example of the effects of resuspension caused by wind can be seen in
Figure 2. The shaded area of the figure represents the decreased attenuation
corresponding to a decrease in wind from 10-15 knots to calm during a 1 hour
interval. Both sets of measurements were taken during a clear sky. Notice
that although the incident irradiances at the mid-spectral region (550-625 nm)
are almost identical, the attenuation coefficients and benthic irradiances
differ significantly. This is probably due to resuspended particulate matter
and benthic micro-algae.
SUMMARY
Laguna de Terminos is a relatively large tropical estuary ( 2500 km2)
that supports one of the most extensive nearshore fisheries in the Gulf of
152
-------
INCLUDES
BACKSCATTER
AND REFLECTANCE
Figure 4. Schematic representation of spectral attenuation through an
estuarine water column. (Q » quanta'nm"'*cm~2*s~*; X • wavelength, 400-700
nm; k * specific absorption coefficients, m~^; numbers for irradiance are PAR
integrals, quanta'PAR'cm~2*s~l. Energy circuit language after Odum 1972;
specific absorption curves redrawn from Prieur and Sathyendranath, 1981).
153
-------
Mexico. In part, Che support is provided by the extensive seagrass beds
(Thaiassia testudinum) occupying the shallower lagoon areas. As part of a
joint U.S.-Mexico study of these vegetated communities, we present our
preliminary analysis of submarine light quality and quantity and several
factors possibly controlling light energy distribution in this estuarine
environment.
At the Estero Pargo study site, which represents an area of seagrassee
that are probably light-stressed (Wetzel et al. 1982), there is a significant
attenuation (2 to 4 oTM of the photosynthetically important short
wavelengths, 400-500 run. Incident noon PAR irradiance was about 23 x 10^
-»9 I f* — 9
quanta cm a"1 and benthic irradiance (z»im) was 3 to 6 x 10° quanta cm"**
s~l. In the adjacent roangro?e swamp channel (Estero Pargo Creek) there is
extreme blue-violet attention which is probably due to dissolved organics
originating from mangrove litter fall and decomposition.
At the Thaiassia study site, wind events, even relatively mild conditions
(10-15 kts), effect both light quality and quantity reaching the plant canopy.
Since the greatest attenuations occurred in a spectral region (400-525 ran)
which is extremelv imponant for absocpf >n bv chlorophylls and accessory
pigments in hicher plants, ttv observed benthic irradiance distribution has
significant »mpi '.cations fr>r soagi iSs r^rnmnni ty productivity.
!1
lr.4
A
-------
LITERATURE CITED
Booth, C. R. and P. Du»tan. 1979. Diver-operable mult iwave length radiometer.
In, Measurements of Optical Radiations. Soc. Photo-optical
Instrumentation Engineers, 196:33-39.
Burt, W. V. 1958. Selective transmission of light in tropical Pacific
waters. Deep-Sea Res. 5:51-61.
Champ, M. A., G. A. Could, III, W. E. Bozzo, S. G. Ackleson, and K. C. Yierra,
1960. Characterization of light extinction and attenuation in Chesapeake
Bay, August, 1977. In, V. S. Kennedy (ed), Estuarine Perspectives,
Academic Press, Inc., N.Y. pp. 263-277.
Clark, G. L. and H. H. Jame.-. 19.T*. Laboratory analysis of the selective
absorption of light by seawater. J. Opt. Soc. Am. *9:43-55.
Clark, G. L. and G. C. Owing. l^7i. Hemcte spectroscopy of the sea for
biological production studies. In. N. G. Jerlov and E. S Iceman-Nielsen ,
(eds.), Optical Aspects of Oceanography. Academic Press, N.Y. pp.
389-413.
Day, J. W., Jr. and A. Yinez-Arancibia. 1980. Coupling of physical and
biological processes in the Laguna dj Terminos, Campeche, Mexico.
Unpublished manuscript.
Gier lof f-Emden, H. G. 1977. Lac/ina de Terminos and Cnmpeche Bay, Gulf of
Mexico: Water mass interaction and l.igoonal-oceanic visibility due to
sedment luden waters. In, Ivaltor lie Geruyter (ed.), Ornital Hptnote
Sensing of Cog s tat and 0 ffshore Environments, A Maunal of Interpretation .
pp. 77-89, BerTiru
James, H. R. an»! E. A. Birge. 19JM. A laboratory study of the absorption of
by lake waters. Trans. Wisconsin Acad. Sc i . 11: 1^4 pp.
Jerlov, N. G. l^/h. la r IIP Qp_t i "s . F ispvier (-ceancgr iphv Series, Vol. t<*,
Elsevier Sciencitic Publ. Co., N. Y. 2'U pp.
Kaile, K. !9f>f>. Thf prt)t)l.-m of Golhsioff in the sea. Ocoanogr. Mar. Biol.
Rev. 4:91-10^.
Odum, H. T. 19/2. An energy curcuit Idn^ua^e lot ecological and social
systems: Its physical basis. lit, 13. .. I'.jilt'i1. (od.), Systems
and Simulation!- in EcnI»Ky, Voi. II. AcJ(Un«ic Press, N.Y.
-------
Pierce, J. W., D. L. Correll, M. A. Faust, W. H. Klein, and B. Gilbert. 1981.
Spectral quality of underwater light in a turbid estuary, Rhode River,
Maryland, U.S.A. Unpublished manuscript.
Prieur, L. and S. Sathyendranath. 1981. An optical classification of coastal
and oceanic waters based on the specific special absorption curves of
phytoplankton pigments, dissolved organic matter, and other particulate
materials. Limnol. Oceanogr. 26:671-o89.
Wetzel, R. L., R. F. van Tine anl P. A. Penhale. 1981. Light and submerged
macrophyte communities in the Chesapeake Bay: a scientific summary.
Special Report No. 260 in Applied Marine Science and Ocean Engineering,
Virginia Institute of Marine Science, Gloucester Point, VA., U.S.A. 58
pp.
Wetzel, R. L., L. Murray, R. F. van Tine, .T. W. Day, Jr. and C. J. Madden.
1982. Preliminary studies or community metabolism in a tropical seagraas
ecosvstem: Laguna de Terminos, Mexico. In, R. L. Wetzel (ed.)
Structural and functional Aspects of the Ecology of Submerged Aquatic
Macrophyto Communities in the Lower Chesapeake Bay, vol. I. Final Report
Grants #8803974 submitted Lo the U.S. Environmental Protection Agency,
Wash., U.C.
ISh
J
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