THE EFFECTS OF CHRONIC LIGHT REDUCTION ON THALASSIA TESTUDINUM
AT STATIONS ACROSS THE GULF OF MEXICO
Final Report
to the EPA Environmental Research Laboratory - Gulf Breeze
David A. Flemer, Project Monitor
Principal Investigator
J.C. Zieman
Co-Principal Investigators
Paul R. Carlson, Jr.
Kenneth H. Dunton
Michael J. Durako
James W. Fourqurean
Kenneth L. Heck, Jr.
Co-Investigators
T. Frankovich
Kun-Seop Lee
Cynthia A. Moncreiff,
Jill M. Zande
Department of Environmental Sciences
University of Virginia
Charlottesville VA 22903
(804) 924-0570; 982-2137 fax
Florida Marine Research Institute, FDEP
University of Texas at Austin
Florida Marine Research Institute, FDEP
Florida International University
Marine Environmental Sciences Consortium, Alabama
University of Virginia
University of Texas at Austin
Marine Environmental Sciences Consortium, Alabama
Marine Environmental Sciences Consortium, Alabama
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^- TABLE OF CONTENTS
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5 I- INTRODUCTION -1-
CONCEPTUAL APPROACH -3-
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Carbohydrate Carbon -40-
DISCUSSION -46-
In situ light requirements of Thalassia testudinum -48-
Changes in biomass and carbon budget -48-
VI. SEDIMENT SULFIDE AND PHYSIOLOGICAL RESPONSES OF THALASSIA
-TESTUDINUM JO SHADING -51-
INTRODUCTION -51-
METHODS : . . -51-
Sulfide -51-
Physiological Parameters -51-
RESULTS -51-
Spring 1993- -52-
Fall 1993 -52-
Spring 1994 -52-
Summer 1994 -53-
Pore Water Sulfide Concentrations -54-
VII. CHANGES IN PHOTOSYNTHESIS VERSUS IRRADIANCE CHARACTERISTICS OF
THALASSIA TESTUDINUM IN RESPONSE TO SHORT-TERM LIGHT REDUCTION -72-
MESOCOSM EXPERIMENTS -72-
FIELD EXPERIMENTS: Sunset Cove -73-
VII. Summary -80-
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List of Figures
Figure 1-1 General site map showing the locations of the main field sampling
stations across the Gulf of Mexico. These are at Florida Bay, FL, at the
southern tip of Florida, St. Joseph's Bay, FL, in the Florida panhandle,
and Corpus Christ! Bay, TX, on the south Texas coast -2-
Figure 3-1 ' Location map showing the two south Florida sites at Rabbit Key Basin
(RKB) and Sunset Cove (SUN) in Florida Bay -8-
Figure 3-2 Photosynthetically Active Radiation (PAR) for the Rabbit Key Basin
(RKB) site. The circles represent the average daily intensity for the
month depicted, and the triangles represent the maximum intensity
recorded for that month -10-
Figure 3-3 PAR values for both Rabbit Key Basin and Sunset Cove for the period
of the experiments. These values are monthly averages of the daily
integrated PAR. For each month, they represent the total flux of
photons received per day -10-
Figure 3-4 Monthly averaged temperature and salinity for the stations in Rabbit
Key Basin and Sunset Cove in Florida Bay -11-
Figure 3-5 Leaf length (cm), leaf width (mm), and leaf area Index rLAI, m2/m2) for
the stations at Rabbit Key Basin -13-
Figure 3-6 Leaf length (cm), leaf width (mm), and leaf area Index (LAI, m2/m2) for
the stations at Sunset Cove -14-
Figure 3-7 Short shoot density (ss rrv2), leaves per short shoot (I ss'1), and leaf
standing crop (g nr2) at Rabbit Key Basin -15-
Figure 3-8 Short shoot density (ss rtr2), leaves per short shoot (I ss'1), and leaf
standing crop (g rrv2) at Sunset Cove -16-
Figure 3-9 Photosynthetic Biomass (g nv2), non-photosynthetic biomass (g rrv2),
and total biomass (g rrv2) of Thalassia at Rabbit Key Basin -18-
Figure3-10 Photosynthetic Biomass (g nr2), non-photosynthetic biomass (g rrv2),
and total biomass (g rrv2) of Thalassia at Sunset Cove -19-
Figure 3-11 Turnover rate (% d'1), areal productivity (g rrv2 d'1), and the ratio of
Photosynthetic to non-photosynthetic biomass (%) of Thalassia at
Rabbit Key Basin -21-
Figure 3-12 Turnover rate (% d'1), areal productivity (g rrv2 d'1), and the ratio of
Photosynthetic to non-photosynthetic biomass (%) of Thalassia at
Sunset Cove ' -22-
Figure 4-1 Map showing study area location in the northeastern Gulf of Mexico on
the west shore of St. Joseph Bay -25-
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Figure 4-2 Water temperature, salinity, ambient nutrient levels, suspended
sediments, and particulate organic matter (POM) at the St. Joseph Bay
study site -27-
Figure 4-3 Standing crop (g dry wt nrv2), biomass production (g dry wt nrv2 d'1), and
turnover rate (mg g'1 d'1) for Thalassia testudinum at the St. Joseph
Bay study site -29-
Figure 4-4 Leaf area index (LAI), average blade width (cm), and total plant
biomass (g dry wt m~2) for Thalassia testudinum at the St. Joseph Bay
study site -30-
Figure 4-5 Rhizome biomass (g dry wt rrv2), root biomass (g dry wt nr2), and
above- to below-ground biomass ratio for Thalassia testudinum at the
St. Joseph Bay site -31-
Figure 4-6 Short shoot densities (nr2), shoot specific growth rates
(g dry wt ss'1 d"1), and calcareous epibiont coverage for Thalassia
testudinum, St. Joseph Bay site -32-
Figure 5-1 Average daily photo flux density (PFD) collected underwater (control,
14% SI and 5% SI treatment cages) and at the surface -37-
Figure 5-2 Pore water ammonium concentration in of control, 14% SI and 5% SI
treatment cages -39-
Figure 5-3 Shoot densities in control, 14% SI and 5% SI treatment cages -39-
Figure 5-4 Blade widths of Thalassia testudinum from control, 14% SI and 5% SI
treatment cages -40-
Figure 5-5 Chlorophyll a, chlorophyll b_ and total (chl §+b.) concentrations of
Thalassia testudinum leaves from control, 14% SI and 5% SI treatment
cages -41-
Figure 5-6 Daily leaf production on a shoot (A) and area! (B) basis in control and
treatment cages -42-
Figure 5-7 Changes in biomass partitioning of Thalassia testudinum into different
plant parts (leaf, short stem, rhizome and roots) as a result of light
manipulation between August 1993 and July 1994 -44-
Figure 5-8 Carbohydrate carbon concentration in different plant tissues of
Thalassia testudinum from control and light treatment cages in August
and December 1993 and April 1994 -46-
Figure 6-1 Rhizome ADH activity for all sites, Fall 1993 -60-
Figure 6-2 Rhizome protein and total carbohydrate concentration, Fall 1993 -61-
Figure 6-3 Rhizome sugar and starch concentration, Fall 1993 -62-
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Figure 6-4 Rhizome ADH and normalized ADH activity for all sites, Fall 1993 -63-
Figure 6-5 Rhizome protein and total carbohydrate concentration, Spring 1994. .. -64-
Figure 6-6 Rhizome sugar and starch concentrations, Spring 1994 -65-
Figure 6-7 Rhizome ADH and normalized ADH activity for all sites, Summer 1994. -66-
Figure 6-8 Rhizome protein and total carbohydrate concentration, Summer 1994. . -67-
Figure 6-9 Rhizome sugar and starch concentration, Summer 1994 -68-
Figure 6-10 Comparison of pore water sulfide (PWS) concentrations among sites
and treatments for Summer and Fall 1993 -69-
Figure 6-11 Comparison of pore water sulfide (PWS) concentrations among sites
and treatments for Spring and Summer 1994 -70-
Figure 6-12 Seasonal variation of pore water sulfide concentrations among
treatments at St. Joseph's Bay and Rabbit Key Basin -71-
Figure 7-1 Chlorophyll a-based photosynthesis versus irradiance characteristics
(respiration, Pmax, and a) of Thalassia testudinum in response to short-
term light reduction in FMRI mesocosms -74-
Figure 7-2 Chlorophyll a-based photosynthesis versus irradiance characteristics
(lc, lk, and Pmax gross) of Thalassia testudinum in response to short-
term light reduction in FMRI mesocosms -75-
Figure 7-3 Photosynthesis versus irradiance responses of Thalassia testudinum
exposed to ambient light, 10% shade and 20% shade for 12 days in
FMRI mesocosms -76-
Figure 7-4 Changes in chlorophyll a concentration of leaves of Thalassia
testudinum in response to short- and longer-term light reduction in
FMRI mesocosms -77-
Figure 7-5 Chlorophyll a-based photosynthesis versus irradiance characteristics
(Respiration, Pmax, and a) of Thalassia testudinum in response to short-
term light reduction in Sunset Cove, Key Largo -78-
Figure 7-6 Chlorophyll a-based photosynthesis versus irradiance characteristics
(lc, lk, and Pmax gross) of Thalassia testudinum in response to short-
term light reduction in Sunset Cove, Key Largo -79-
Figure 8-1 Trends in seagrass responses at the three sites, Florida Bay (FB), St.
Joseph's Bay (STJ), and Corpus Christi Bay (TX) -83-
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Table 4-1
Table 4-2
Table 5-1
Table 5-2
Table 5-3
Table 5-4
Table 5-5
Table 6-1
Table 6-2
Table 6-3
Table 6-4
Table 8-1
Table 8-2
List of Tables
Typical observed light reductions at St. Joseph Bay study site -26-
Short shoot densities and aboveground biomass of Thalassia testudinum
beds in Gulf of Mexico seagrass studies -34-
Daily average photon flux density (PFD), % of in situ ambient (% ISA), %
of surface irradiance (% SI) and the daily period of light saturated
photosynthesis (Hsal) in control and treatment cages -38-
Chlorophyll a:b ratio of Thalassia testudinum leaves from control, 14% SI
and 5% SI treatment cages at four different sampling times -42-
Biomass changes in total and individual plant parts as a result of light
manipulation in May (initial sampling date) and August 1993 and April and
July 1994 -43-
Below- to above-ground ratios of Thalassia testudinum at 46% SI
(control), 14% SI and 5% SI in August 1993 and April and July 1994. . . -45-
Soluble carbohydrate carbon content -47-
Analysis of variance in physiological parameters for all sites -56-
Comparison of physiological parameters among sites -57-
Comparison of physiological parameters among treatments -58-
Sediment pore water sulfide concentrations -59-
Comparison of Thalassia testudinum parameters at all sites for the
summer of 1993. South Florida uses Rabbit Key Basin data -81-
Incoming Photosynthetically Active Radiation (PAR) delivered to the top
of the canopy of the two light reduction treatments as a percentage of the
light received at the top of the canopy of the control plots at the three
sites '82~
VI
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ACKNOWLEDGEMENTS
This research was supported primarily through a grant from the Environmental Protection Agency. At the
north Florida site the-authors are indebted to the following individuals for field assistance and sample
processing in various forms throughout the project: K. Canter, P. Carlson, A. Foster, A. Gunter, E. Lores,
P. Harper, S. Rikard, L. Scarborough, D. Webb, M. Williams, and P. Bologna. Logistic field support was
provided by the Marine Environmental Sciences Consortium and made possible through a grant to KLH.
The authors from the Texas site are very grateful to Jim Kaldy and Sharon Herzka for their countless hours
of reading and discussions and extremely helpful comments. We thank Susan Schonberg and Kim Jackson
for providing solutions for computational problems and excellent assistance in the lab. This work was
supported by the Texas Higher Education Coordinating Board Advanced Technology Program (Grant No.
3658-419 and 3658-426) and Grant No. X-996025-01-1 (EPA, Region 6) and a Coastal Submerged Aquatic
Vegetation Initiative Grant from the U.S. Environmental Protection Agency.
From the south Florida site we thank the many who helped collect and process the samples over the seven
years of this study, including M.J. Absten, R.M. Price, R.T. Zieman, L. King, J. deDomenico, and a special
thanks for assistance in cage cleaning to Senator Bob Graham. Logistical support was provided by M.B.
Robblee of the National Biological Service and Everglades National Park. Funding for this work was
provided primarily by the Environmental Protection Agency, with additional support from the Southeast
Research Center of Everglades National Park and the Florida Department of Environmental Protection.
Finally, individually and collectively we wish to thank Dr. Dave Flemer of the EPA Gulf Breeze, FL.
Laboratory. He was instrumental in the initiation of this project, and in preventing its premature ending.
Although his guidance was invaluable, we would most of all like to thank him for his patience throughout all
aspects of the project.
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I. INTRODUCTION
For several decades, the role and importance of seagrasses as habitat and as a trophic source, whether
grazed directly, consumed as detritus, or acting as a means of support for epiphytic algae, has been
increasingly well-documented in the coastal zones of the world. However, the environmental factors that
influence the development and overall health of seagrass communities are much less well understood. While
we know that acute,-dramatic reductions in light levels have a negative effect on the growth and survival of
seagrasses, there is limited information on the responses of these plants to lesser but chronic light reductions
associated with phenomena such as persistent phytoplankton blooms and increased sediment loads.
There have been significant declines in seagrass coverage in many parts of the world related to reduced water
quality and increased turbidity (Orth & Moore, 1983; Cambridge & McComb, 1984; Giesen et al., 1990; Larkum
& West, 1990). The primary causes for increased turbidity are erosion of silt substrates, pollution, algal
growth, ship and barge traffic and dredging activities (Peres & Picard, 1975; Cambridge & McComb, 1984;
Cambridge et al., 1986; Onuf, 1994). Although seagrass growth and survival is largely related to light
availability, there is a considerable amount of variability among species. Duarte (1991) reported an average
minimum light requirement of 10.8% of surface irradiance (SI) for seagrasses from a worldwide survey of their
maximum colonization depth. However, Dennison et al. (1993) reported that the estimated minimum light
requirements for various seagrass species probably ranges from 4% to 25% SI. These variations are probably
a result of the unique physiological and morphological adaptations among species and locations.
Photoadaptive responses of seagrasses to reductions in irradiance have been reflected in some species by
increases in chlorophyll (chl) content and decreases in biomass, growth rate, shoot density and chl a : b ratios
(Backman & Barilotti, 1976; Wiginton & McMillan, 1979; Dennison & Alberte, 1982, 1985, 1986; Bay, 1984;
Neverauskas, 1988; Tomasko & Oawes, 1989; Abal et al., 1994).
Thalassia testudinum is one of the most important seagrass species along the coasts of the Caribbean and
the Gulf of Mexico and is the subject of this study. It is the climax species across the region, and typically
the community dominant, usually by a large amount. Thalassia consists of horizontal rhizomes which branch
at regular intervals, and erect short shoots (vertical rhizome) bearing foliage leaves and roots (Tomlinson &
Vargo, 1966). This species constructs very dense rhizome systems and has differentiated vertical rhizome
tissue (Duarte et al., 1994). Because of its relatively large leaves and basal leaf growth, it is much easier to
measure growth and production than any of the other Gulf and Caribbean seagrasses (Zieman, 1974). In this
study, we examined changes in leaf elongation, biomass, carbohydrate carbon content, blade width,
chlorophyll content and chl a : b ratios in response to in situ light manipulations in Thalassia. We also
investigated changes in biomass and carbohydrate carbon partitioning to different plant parts as a result of
changes in underwater irradiance to determine the effect of light reduction on the partitioning of carbon in
Thalassia. Continuous measurements of underwater quantum irradiance were made to document the amount
of light received by plants in each shaded treatment. We also monitored changes in pore water ammonium
and sulfide levels to assess sediment anoxia.
This research was undertaken as a multi-site study specifically addressing chronic light reduction and the
responses of the seagrass community to this stress. There is a vital need for information in this area in light
of the critical condition of the seagrass communities located throughout the Gulf of Mexico. The major field
sites were located at Florida Bay at the southern tip of Florida, St. Joseph Bay in the northern Florida
Panhandle, and Corpus Christi Bay on the western Texas coast (Fig. 1-1). The Florida Bay sites are
subtropical and in recent decades were clear water sites where the seagrasses were rooted in biogenically
derived carbonate sediments. St. Joseph's Bay is clear, but the seagrasses are in clastic sediments and the
climate is warm temperate. The Corpus Christi Bay site is midway in latitude between the other sites, but is
climatically temperate due to winter cold fronts. The sediments here are clastic and the water is historically
the most turbid of the three sites.. More complete site descriptions are given in Chapters II-IV. Throughout
the course of this project, each of the major field sites experienced one or more severe stresses, several of
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environmental conditions, initially caused the direct loss of over 4,000 ha of seagrasses, largely Thalassia,
totally denuded and an additional 23,000 ha severely affected (Robblee et al, 1991). While the initial dieoff
has subsided, secondary algal blooms and turbidity plumes are blanketing hundreds of square kilometers,
and causing general seagrass losses over wide areas. In Texas, protracted brown tides have for many
years now periodically covered seagrass sites. St. Joseph Bay experienced stress in the form of heavy
grazing pressure by sea urchins and a major rain event that dropped salinities to 10 ppt and the resulting
influx from the watershed produced increased color in the water for a protracted time. These facts show the
level of stress to which the communities in the coastal zone are being subjected around the entire periphery
of the Gulf of Mexico.
This project was a response to the Coastal Submerged Aquatic Vegetation Initiative RFP, where the overall
objective was to determine the response of seagrass communities to reduced levels of incoming light.
Recent workshops and reports (Kenworthy and Haunert, 1991; Neckles, 1993) have agreed that a) historic
standards of allowable light reduction are detrimental to seagrass communities, and b) chronic light
reduction, due to suspended sediments, eutrophic algal growth, or a combination of the two, is the most
important stress currently affecting submerged coastal vegetation. Historic estimates of compensation depth
for aquatic plants has been the depth where 1 to 5% of the surface incident light remains. These values
were based on studies of phytoplankton production (Ryther, 1956; Steemann-Nielsen, 1952). However
recent studies show that seagrasses and aquatic macrophytes have much higher light compensation levels
than phytoplankton because of the oxygen demand of roots, rhizomes, and lateral shoot bases (Chambers
and Kalff, 1985). The report of a recent workshop on the light requirements of seagrasses estimated
minimum light requirements of from 15 to 25% of the incident radiation (Abstracts and Summaries in
Corpus Christ! Bay
Fig. 1-1. General site map showing the locations of the main field sampling stations across the Gulf of
Mexico. These are at Florida Bay, FL, at the southern tip of Florida, St. Joseph's Bay, FL, in the Florida
panhandle, and Corpus Christ! Bay, TX, on the south Texas coast.
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report of a recent workshop on the light requirements of seagrasses estimated minimum light requirements
of from 15 to 25% of the incident radiation (Abstracts and Summaries in Kenworthy and Haunert, 1991), while
in a worldwide survey of maximum seagrass distribution, Duarte (1991) found the average maximum depth
penetration to occur at 10.8% of surface radiation. These variations must, in part, be due to differing light
attenuation components in the various waters and the differences in the architecture of the various seagrass
species. Thalassia testudinum is one of the most robust and productive of all seagrasses, and has a very high
investment in belowground tissue requiring considerable metabolic energy (Zieman, 1982; Fourqurean and
Zieman, 1991).
In addition, in many areas the water quality standards are inadvertently drafted in such a way as to allow
continued degradation. It might seem, for instance, that allowing certain environmental alterations that will
reduce water clarity by only 10% might have little consequence. However, these standards rarely, if ever,
reference the 10% reduction to historic water clarity, since it is often not known, and allow reduction from the
present levels, which may already be significantly degraded from historic levels. Kenworthy and Haunert
(1991) summarized this as follows:" Federal water color criteria and the Florida transparency standard utilize
the compensation depth for photosynthetic activity as the parameter to delineate the minimum allowable light
level. The standard and criteria stipulate that the depth of the compensation point not be reduced by more
than 10% (substantially) compared to natural background. Because the history of significant human impacts
to many coastal ecosystems is longer than the timeframe over which water quality monitoring has established
natural background values, the standards can only be used to maintain the status quo (Italics added). A more
comprehensive approach to water quality management must be adopted in order to increase light availability
in environments which will support seagrass habitat."
Little research has addressed the chronic, subtle form of reduction of light to seagrasses. It is very easy to
show negative effects on seagrasses by covering them with a blanket, and the effects occur rapidly. By
contrast the effects of reducing light levels of healthy seagrass beds by 10 or 20 percent have not been well
studied. This project coupled field and mesocosm research and examined physiological and ecological
responses of Thalassia to reduced light availability.
CONCEPTUAL APPROACH This research approach utilized two related hypotheses of seagrass death
following light stress: negative carbon balance and sulfide toxicity. Light quality and quantity affect seagrass
growth, establishment, and survival by controlling carbon balance. The carbon balance of seagrasses is more
complex than that of phytoplankton or macroalgae due to the increased structural complexity of the
seagrasses, where carbon fixation in the leaves must supply the respiratory and growth requirements of the
non-photosynthetic structures which make up as much as 90% of the biomass of Thalassia and can account
for up to 60% of the respiration. Two methods of evaluating carbon balance have been utilized recently. One
method measures P/l response of the whole plants oriented naturally in the light field (Fourqurean and Zieman,
1991). This method produces an ecologically relevant P/l curve. Another widely used technique (Dennison,
1987) measures the P/l response of leaf segments arranged perpendicularly to a light field and produces
physiologically relevant P/l curves. While most of this project investigated whole-plant dynamics in the field
cages, chapter VII utilized Thalassia leaf segments to estimate P/l relationships.
Sediment sulfide affects seagrasses in two important ways: 1) direct toxicity effects due to diffusion into roots
and rhizomes, and 2) indirect effects due to chemical oxygen demand of the sediments on belowground tissue.
The former effect is probably more acute, while chemical oxygen demand is a chronic burden on the oxygen
status - and therefore, the carbon balance - of belowground tissue.
Many of the concepts implicit in the sulfide toxicity hypothesis are derived from shading studies, community
metabolism measurements, and physiological studies by the Florida Department of Natural Resources
(FONR) of seagrass dieoff in Florida Bay: 1. Sulfide, a potent toxin produced as a major end-product of
heterotrophic microbial metabolism in marine sediments is the primary agent of sediment chemical oxygen
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demand in seagrass sediments. 2. Under non-limiting light levels, Thalassia is able to avoid sulfide toxicity
by maintaining an oxidized rhizosphere. 3. However, when light becomes limiting or the internal oxygen
supply of roots and rhizomes is interrupted by some other process, Thalassia may be affected by sulfide
toxicity. 4. Thalassia is more prone to sulfide toxicity effects than are Halodule or Syringodium because
Thalassia typically has a much higher root/shoot ratio than other species. In addition, experimentally elevated
sediment sulfide concentrations in Florida Bay killed Thalassia, but not Syringodium or Halodule. The greatest
sulfide toxicity effects are to be expected in carbonate sediment environments such as Florida Bay and least
effects in sediments with high iron content.
Ethanol concentrations and alcohol dehydrogenase (ADH) activities of rhizome tissue will be used in field and
mesocosm experiments as indices of sulfide-induced and shade-induced hypoxic stress within the
belowground tissue of Thalassia. Many plant species produce ethanol by fermentation of acetaldehyde under
anaerobic conditions, and ADH activity is often used to infer the intensity of hypoxic stress in plant tissues.
ADH activity in Florida Bay Thalassia rhizomes is much higher than activities in Thalassia rhizomes from other
Florida estuaries, perhaps as a result of higher sulfide-driven sediment COD in Florida Bay. Because
fermentation produces only 2 moles of ATP for one mole of glucose (rather than 38 moles of ATP produced
by aerobic respiration of one mole of glucose), we also infer from high ADH activity in Thalassia rhizomes that
carbohydrate reserves may be rapidly depleted.
The principal objective of the study was to determine the responses of existing Thalassia testudinum meadow
to chronic light reduction, and as a corollary to this determine the critical light level which determines the
distribution of this seagrass species in the Gulf of Mexico. A series of secondary objectives were also
addressed, including (1) a partial assessment of the physical and chemical requirements for optimal growth
of Thalassia , (2) a preliminary evaluation of the utility of biomarkers, such as leaf chlorophyll and rhizome
alcohol dehydrogenase (ADH) levels, as indicators of seagrass ecological health, (3) identification of factors
responsible for light attenuation, and (4) examination of the potential synergistic effects of chronic light
reduction and porewater sulfide levels on the survival and growth of Thalassia testudinum.
II METHODS AND MATERIALS - GENERAL
A common cage design and sampling protocol were developed for the program. Reduced light fields were
achieved utilizing PVC-constructed cages covered with plastic mesh shade cloth. The cages were 1.5 m X
1.5 m X 0.5 m in dimension. Fine (0.64 cm) and coarse (2.54 cm) mesh sizes were used as cage tops to
reduce PAR. The sides of the cages were covered with the coarse mesh in an attempt to keep out herbivorous
grazers, specifically sea urchins, that would be attracted to structure. Control plots were constructed of the
same area! dimensions with coarse mesh sides but without tops. The sediment at the perimeters of all nine
plots (including the controls) were cut with a large saw to a depth of 40-50 cm at the initiation of the project
to ensure physiological independence of the seagrasses within the plots (Hamett and Bazzaz, 1983; Tomasko
and Dawes, 1989; Czerny and Dunton, 1995).
Initially we tested 20 different sizes and textures of mesh. While heavy nylon and polyethylene meshes such
as Vexar have existed for some time, as this project began these materials were being made in the traditional
black, and a new, translucent polymer. We had hoped the clear polymer would offer 10-15% light reduction,
but the reduction was only 1-3%, useful for caging studies, but not for light reduction. The black Vexar worked
fine, but instead of a gradient of light reduction, it grouped into two groupings despite a wide variety of mesh
sizes and shapes. The reduction levels centered around 30% and 50% reduction, still quite useful for our
purposes. For the high light treatment we selected 3/4" (1.91 cm) diamond Vexar mesh, with 70%
transmission, and 1/4"(0.64 cm) diamond mesh with 50% transmission. All tests were done with Li-Cor
spherical quantum sensors and LI-1000 recorders. Originally, for the best light control, the cages were
installed at several sites with the meshes described above. We rapidly found however, that the 1/4" mesh size
used as side panels reduce water flow too much, and retrofitted those cages with 3/4" mesh panels, which had
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little effect on the flow.
The project was initiated in the spring of 1993. While there were no severe epiphyte problems at that time,
by the middle of the summer of 1993, the single biggest problem that faced all of the sites was the cleaning
of the cages. While this was not a serious problem in the spring months, it has become an immensely time-
consuming task-in the summer, the Texas site having to clean cages once a week, and in Florida the time
period was at least every two weeks. Workers at the south Florida sites had the added problem that a large
proportion of the fouling organisms were hydroids and the divers ended up working in a soup of shredded
nematocysts.
As the cages became fouled, the resulting light to the plants declined until the next cleaning. This resulted
in a much more severe light reduction, especially in the fine mesh cages than was originally intended and the
plants in these cages declined. By the fall of 1993 the plants in the fine mesh cages were either greatly
reduced (Florida Bay) or completely gone (Texas), and a joint decision was made to remove the cages, but
to maintain and monitor the plots for signs of recovery. Thus for the last year of the project we sampled the
control areas, the coarse cages, and the abandoned fine mesh areas, the last to determine if any short-term
recovery occurred.
Photon Flux Measurement Photosynthetically active radiation (PAR, 400-700 nm) was collected
continuously using a LI-193SA spherical quantum sensor at canopy level, which provided input to a LI-1000
datalogger (LI-COR Inc.) enclosed in underwater housing. An underwater sensor was placed within one
replicate of each of the three treatments and cleaned regularly to minimize fouling. Photon flux was measured
at 1 min intervals and integrated hourly. At the Texas site, coincident measurements of incident surface PAR
were made at The University of Texas Marine Science Institute (UTMSI) in Port Aransas, approximately 8 km
from the study site, using a LI-190SA cosine corrected quantum sensor and LI-1000 datalogger. At the St.
Joseph's Bay site, initial measurements were made both at the surface (ambient surface irradiance) and
immediately above the seagrass bed canopy during sampling and cleaning periods. Measurements were also
made within the treatment enclosures; these latter measurements were used in all light reduction calculations.
Later this site used continuous recording similar to the other two sites
Biological Parameters
Plant response. Clipped shoots from the three 100 cm2 areas within the enclosures and the three 47 cm2 area
cores from within each of these areas were used to provide information on leaf morphology, epiphyte loads,
and seagrass above- and below-ground biomass. The harvested marked shoots of Thalassia testudinum
were processed according to Zieman (1974); however, epiphytes were removed from the seagrass blades
by gently scraping them from the leaf surface with a razor blade. This information was used to calculate the
following plant response parameters: standing crop, area! production, turnover rate, leaf area index (LAI), and
above- to below-ground biomass ratios.
At each site, twelve Thalassia short shoots within each enclosure (chosen arbitrarily) were perforated at the
sediment-water interface with a 18 gauge syringe needle. The marked shoots were identified with surveyor's
flags and bird bands. The marked leaves were allowed to grow for approximately two weeks, after which all
marked short shoots were harvested at the sediment-water interface. All leaf material was gently scraped with
a razor blade to remove epiphytic growth and then washed in fresh water. Leaf morphology was measured
to the nearest millimeter. Unperforated leaves and portions of the leaves below the perforation, considered
to be new growth, were separated from the rest of the leaf material. All leaf material was oven-dried at 85°C.
Leaf turnover rate (g g'1 day1) was determined as the ratio of the dry weight of new growth to the total dry
weight of attached leaves (above-ground standing crop) divided over the time of the growth period measured.
At the time of leaf productivity harvesting, short shoot density was determined by counting the number of short
shoots within 4 arbitrarily placed 10 cm X 10 cm quadrats within each enclosure at each site. A real leaf
-5-
-------
production rates were obtained by multiplying the leaf production rate per shoot by the shoot density
Biomass. For biomass, three replicate samples from each cage were collected with a 9 cm diameter coring
device. Thalassia biomass was separated into tissue types according to the methods of Fourqurean and
Zieman (1991). Samples were thoroughly cleaned of epiphytes and sediments, separated into leaf (blade
and sheath), short stem (including vertical rhizome), rhizome and root and dried at 60°C to a constant weight.
Shoot density was estimated by counting the number of shoots inside a randomly thrown quadrat (0.05 m"2).
Halodule biomass was separated into leaves and below ground components.
Chlorophyll. For determination of blade chlorophyll content, six replicate samples from each cage were
collected and then cleaned of epiphytes in the laboratory. Pre-weighed leaf tissues were extracted for 4-5
days in glass screw cap tubes with 5 ml N,N-dimethyl formamide (DMF) following Dunton & Tomasko (1994).
Absbrbance of the extracts was measured at 750, 664 and 647 nm on a Shimadzu UV 160U
spectrophotometer. Chi a and b contents were determined using the equations of Porra et al. (1989).
Epiphytic organisms. Ten blades were clipped from within each enclosure on a quarterly basis; these samples
were preserved with 4% formalin; the presence of calcareous red algae at the study site dictated this as
freezing tends to dislodge this material. The 10 blades collected from within each enclosure and preserved
with 4% formalin were examined using a randomized grid system to determine the macroscopic epibionts
present at each intersecting point. Epiphytes were essentially identified as functional groups; categories of
macroscopic epibionts were as follows: green algae, brown algae, non-calcareous red algae, colonial
ascidians, spirorbids, serpulids, and a generic category for encrusting calcareous epibionts, which included
calcareous red algae and bryozoans.
Chemical Analyses On each sampling date four replicate sediment samples were collected to 10 cm depth
from each shading treatment cage with a 60 ml syringe. Sediment pore water was obtained by centrifugation
(5,000 xg for 15 min) and then diluted (1 : 5) with ammonium free seawater. Concentrations of NH/ were
determined using standard colorimetric techniques following the alternative method of Parsons et al. (1984).
Sediment pore water samples to determine sulfide concentration were collected with a pore water sampler
under anaerobic conditions (Zimmermann et al., 1978) at the end of the experiment (August 1994). Samplers
filled with nitrogen gas were inserted in the sediment and pore water surrounding the porous polyethylene frit
was collected by the vacuum created with a 50 ml syringe. Dissolved sulfide content of pore water was
determined colorimetrically according to Cline (1969). A 5-ml pore water sample was transferred to a test
tube, to which 0.4 ml of the mixed diamine reagent was added under a nitrogen atmosphere. Color
development was allowed to proceed in the dark for 30 min, after which the absorbance was determined
spectrophotometrically at 670 nm. Dilutions were made after color development with distilled water. The
concentrations of sulfide in the samples were calculated by standardization with known sulfide concentrations.
Dried plant materials from biomass samples were used to determine carbohydrate carbon content in different
plant parts. Soluble carbohydrates from leaf, short stem, rhizome and root were determined using the MBTH
(3-methyl-2-benzothiazolinone hydrazone hydrochloride) analysis (Parsons etal., 1984, Pakulski & Benner,
1992). Ground plant samples of 10 mg were hydrolyzed with 10 ml of 0.1 N HCI for 24 h at 100°C in a water
bath to determine soluble carbohydrates. For determination of total carbohydrates, a hydrolysis using 12 M
H2SO4 was conducted. The hydrolyzed samples were neutralized with 2 ml of 0.5 N NaOH, and 0.1 ml of the
sample was diluted with 10 ml of persulfate distilled water in a serum vial. This sample was reduced with 0.25
ml of 10% KBH4 for at least 4 hours in the dark, and acidified with 1 ml of 2N HCI to allow hydrogen gas to
evolve. Triplicate 1 ml aliquots of hydrolysate from each sample serum vial were transferred to acid-washed
and combusted (500°C, 4 hours) screw cap test tubes. Two additional 1 ml aliquots of hydrolysate were
transferred to serve as blanks. Periodic acid solution (0.1 ml) was added to each of the three sample tubes
and incubated for 10 min in the dark at room temperature. Sodium arsenite solution (0.1 ml) was added to
each sample tube in order to stop the oxidation reaction. For analytical blanks, 0.2 ml of the sodium arsenite
-6-
-------
and periodic acid mixture was pipetted into each of the two additional 1 ml of hydrolysate serving as blanks.
The triplicate samples and duplicate blanks were acidified with 0.2 ml of 2 N HCI. Freshly prepared 0.2 ml of
MBTH solution was added to both samples and blanks, after which the tightly-capped tubes were incubated
for 3 min in a boiling water bath. The tubes were cooled to room temperature with tap water. Once cooled,
0.2 ml of ferric chloride solution was added to the tubes, followed by a 30 min incubation at room temperature
in the dark for color development. After color development 1 ml of acetone was added to each tube and
absorbances were.measured immediately at 635 nm with a spectrophotometer. Mean corrected absorbances
calculated by subtracting analytical blanks were compared with a glucose standard and converted to
equivalent carbon values.
-7-
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Ill FLORIDA BAY, FLORIDA , SITE
J.C. Zieman, T. Frankovich, and J.W. Fourqurean
MATERIALS AND METHODS
Study Sites The seagrass shading experiment was conducted at two sites within Florida Bay during the
period from April 1993 to September 1994. Florida Bay is located at the southern tip of Florida between the
Florida mainland and the Florida Keys (Fig. 3-1). Both study sites are approximately 1.5 m (MLW) in depth and
are characterized by nutrient-limited seagrass meadows dominated by Thalassia testudinum with a sparse
understory of Halodule wrightii (Zieman and Fourqurean, 1989; Fourqurean et al., 1992). Both sites have been
affected by the recent and continuing Thalassia dieoff (Robblee et al., 1991) and associated algal blooms
(Phlips and Badylak, 1996). Historically, the waters of Florida Bay were very clear (mean light extinction
coefficient = 0.5 nr1, Fourqurean and Zieman, 1991), but recent and continuing phytoplankton blooms have
greatly reduced light penetration (Phlips et al., 1995). The Sunset Cove (SUN) site is located in eastern Florida
Bay approximately 100 meters from the shoreline of Key Largo. SUN is not openly connected with either the
Gulf of Mexico or the Atlantic Ocean, as such tidal influence is very limited. Sediment depths averaged 60 cm
and the Thalassia was moderately dense (mean shoot density = 580 shoots rrv2; mean standing crop = 90
g m'2). In contrast, the Rabbit Key Basin (RKB) site, in western Florida Bay, is affected by tidal influences from
both the Gulf of Mexico and the Atlantic Ocean. The sediments are much deeper (130 cm) and the Thalassia
was more dense (mean shoot density = 780 shoots nr2), but individual shoots were smaller (mean standing
crop = 82 g rrv2)
Vv- -'; A
RABBIT KEY BASM «
(RKB)
D SHALLOW CAMONATt v
MUO AMIt \
Fig. 3-1. Location map showing the two south Florida sites at Rabbit Key Basin (RKB) and Sunset Cove
(SUN) in Florida Bay.
-8-
-------
Experimental Design The seagrasses at both sites were exposed to three different light treatments (ambient,
30% and 60% of ambient in situ light) in order to monitor physical and biological changes resulting from a
reduction in the intensity of PAR reaching the seagrasses. The three treatments were conducted in triplicate
at each site. The shading cages were cleaned every two weeks, but bio-fouling reduced PAR to as little as 8%
(fine) and 16% (coarse) of the in situ ambient light. After six months of shading treatment, in October 1993, all
experimental plots at the Sunset Cove site were dismantled due to the death of all seagrass short shoots in
the shaded cages .(both fine and coarse). One month later, the shading screen tops were removed from the
three fine screen cages at the Rabbit Key Basin site due to nearly complete shoot mortality.
Underwater measurements of photosynthetically active radiation (PAR, 400-700 nm) were recorded
continuously in one plot of each treatment at each site. Three LI-193SA spherical (4n) quantum sensors
provided input to a LI-1000 datalogger (LICOR, Inc., Lincoln, Nebraska USA) at each site.
Leaf Mark Productivity and Short Shoot Density Thalassia testudinum leaf productivity was measured
during April, July, and September of 1993 at both sites and in April, July, and September of 1994 at just the
Rabbit Key Basin site. The leaf marking method (Zieman, 1974,1975; Frankovich and Zieman, 1994) was used
to determine leaf productivity, leaf biomass, and leaf morphology.
Biomass Seagrass biomass (above-ground and below-ground) was measured during August and October
of 1993 at both sites and during March, June, and September of 1994 at the remaining Rabbit Key Basin site.
Biomass was measured using a PVC corer (diameter = 28 cm).
RESULTS
Physical Parameters
Photosynthetically Active Radiation. The way in which light data is recorded, processed, and presented can
affect interpretation and comparison with other variables. In figure 3-2 PAR is plotted in the three treatments
as monthly averages and maxima. The maxima are the highest value recorded for that month, while the
averages are the mean of all daily averaged data for the month. At the controls, the means are about 40-50%
of the maxima, while at the coarse treatment the means ran 20 to 35% of the maxima.
The PAR data is depicted differently in figure 3-3. Here the data for both SUN and RKB are plotted in average
Einsteins or mol photon err2 d~1. This represents the daily and monthly integrated photon flux to the plants
What is most meaningful here, and is an important point is that the integrated flux values are attenuated to a
greater extent than the average or maximum values by the epiphytic buildup on the cages, and probably by
other forms of light attenuation, such as turbidity plumes and algal blooms, as well.
Temperature Temperatures throughout the project period were typical of Florida Bay, ranging from the upper
30's in the late summer to around 20^C in the winter. In the more open water of the basins, major upward
excursions from these means are not as common as sudden drops below the means with the passing of winter
cold fronts.
Salinity Over the period of this project, salinities at the two stations were moderate for Florida Bay. SUN
ranged from 27.5 to 39 psu while RKB was slightly higher on the whole but more constant. SUN is a relatively
enclosed cove with relatively poor circulation compared with RKB, and this is reflected by the rise in salinity
in August and September 1993 in response to the elevated temperatures from July to September of the same
year.
Of the temperature and salinity conditions monitored, the salinities should have been less stressful to the plants
than the high temperatures encountered in the summer of 1993.
-9-
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Monthly Maximum and Average Values
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3ontrol-A -- Coarse-A Fine-Average
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Fig. 3-2. Photosynthetically Active Radiation (PAR) for the Rabbit Key Basin (RKB) site. The circles
represent the average daily intensity for the month depicted, and the triangles represent the maximum
intensity recorded for that month.
35 -
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Mol Photora/m2/diy
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Fig. 3-3. PAR values for both Rabbit Key Basin and Sunset Cove for the period of the experiments. These
values are monthly averages of the daily integrated PAR. For each month, they represent the total flux of
photons received per day.
-10-
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Fig. 3-4. Monthly averaged temperature and salinity for the stations in Rabbit Key Basin and Sunset Cove
in Florida Bay.
-11-
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Biotic Parameters
Leaf length showed a general increase at the SUN control stations and an overall increase at the RKB controls
(Fig. 3-5 & 3-6). Initially there was little difference between the treatments, but progressively the shaded
treatments declined relative to the control. At SUN the length of the coarse treatments was reduced to about
60% of the length of the control while the fine treatment showed an increase in September after declining
strongly in the summer. RKB showed a seasonal variation, shorter in the cooler periods and longer in the
warmer samplings. Leaf length in the treatment pens was similar to that of the control throughout the first year.
By July of the second year, the length of leaves in the coarse treatment declined to near half the length in the
control pens, while leaf lengths began to increase in the former fine treatment pens.
Leaf Width was initially the same at both sites (Fig. 3-5 & 3-6). At SUN, leaf widths of the treatments
progressively declined, and by September 93 the control showed a decline also. In RKB, there was an initial
decline by both treatments. The coarse mesh treatment continued to decline relative to the control, while the
fine mesh treatment showed an increase in September 1993 and continued to remain the same as the control
when the cages were removed.
Leaf Area Index incorporates input from leaf length, width, and density (Fig. 3-5 & 3-6). At SUN, the original
LAI at initiation varied with the highest values at this time being in the coarse treatment. With time the LAI in
the control plots increased, due largely to an increase in leaf length, while the treatments progressively
declined, mainly due to a decrease in short shoot density. At RKB, the LAI in the control plots followed a
seasonal trend, While there was a rapid and steady decrease in the LAI of the treatments relative to the control
plots. When the fine mesh screens were removed, the LAI stabilized while it continued to decline in the coarse
treatments. This decrease in the coarse mesh treatments was due to progressive declines in leaf length and
short shoot density.
Short Shoot Density (Fig. 3-7 & 3-8) was actually higher at the treatment pens relative to the control pens in
SUN at the initial sampling. The density declined linearly the coarse treatment. At the fine treatment pens the
density was unchanged between April and July 1993 but dropped greatly by September 1993. At RKB all
treatments were of similar densities initially. At the control pens the density did not show a seasonal pattern
but increased in September 1993 and April 1994, but then decreased by September 1994 to a density similar
to the initial density. ( This seems to be inversely correlated with the PAR at RKB.) At the coarse treatment,
the density also rose in the middle two sampling periods relative to its initial value, but declined relative to the
control. By one year, the densities in the. course treatments were below their initial levels and significantly
below the density of the control pens. After 6 months treatment, the short shoot densities in the fine treatments
decreased relatively to the control and coarse treatment. Despite removal of the fine mesh pens, this
parameter continued to decline throughout the remainder of the experiment. This may indicate that short shoot
density is a good indicator of chronic stress, as many other parameters showed a relatively rapid recovery at
RKB following the removal of the fine treatment cages.
Leaves per Short Shoot In all cases, at the control stations, the lowest number of leaves per short shoot
occurred in the September sampling (Fig. 3-7 & 3-8). At SUN there was steady decline from April to
September. Both treatments decreased relative to the control and the fine mesh treatment decreased the most.
At RKB the treatments declined relative to the control in 1993. In 1994 the number of leaves in the coarse
treatment remained below the level of the control while the fine mesh treatment increased to greater than the
control once the fine mesh was removed.
Leaf Standing Crop (Fig. 3-7 & 3-8) At SUN, while the standing crop in the coarse treatment was the highest
in the initial sampling, by July 1993, both light-reduction treatments were below the control. By September
1993 the coarse treatment was only 50% of the control and the fine treatment was less than 25%. At RKB both
-12-
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1.5 -
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April '93
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April '93
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April '93
Leaf Area Index
Rabbit Key Basin
July'93 Sept.'93
April '94
Time
Blade Length
Rabbit Key Basin
July '93 Sept.'93
April '94
Time
Blade Width
Rabbit Key Basin
July '93 Sept.'93
April '94
Time
control
i N i coarse
^i fine
I
J
July '94 Sept. '94
T
July '94 Sept. '94
July '94 Sept. '94
Fig. 3-5. Leaf length (cm), leaf width (mm), and leaf area index (LAI, m2 /m2) for the stations at
Rabbit Key Basin.
-13-
-------
1.5 -
0.5-
0.0
Leaf Area Index
Sunset Cove
control
coarse
fine
April '93 July '93 Sept.'93
Time
Blade Length
Sunset Cove
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April'93 July '93' Sept. '93
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1
10.0 -,
8.0 -
6.0 -
4.0 -
2.0 -
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Blade Width
Sunset Cove
April '93 ju|y -93 Sept.'93
Time
Fig. 3-6. Leaf length (cm), leaf width (mm), and leaf area index (LAI, m2 /m2) for the stations at
Sunset Cove.
-14-
-------
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July '93 Sept.'93
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Blades per Short Shoot
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13
-------
treatments showed a decrease relative to the control in July 1993. In September 1993 there was an increase
in the standing crop at the coarse treatment, and it was equal to the control while the fine treatment continued
at less than 50% of the control values. In 1994 the standing crop values in the coarse treatment declined to
about 10 % of the control. Where the light reduction mesh had been removed the standing crop increased
until it was greater than 50% of the amount of the control after having been much reduced. At both sites, this
parameter showed one of the most direct and strongest responses to light reduction.
Photosynthetic Biomass In order to spread out the workload, the biomass samples were collected the month
following the other biotic measurements. There was no biomass collected in March 1993. The photosynthetic
biomass portion of the cored samples corresponds to the leaf standing crop, reported previously, but is from
the biomass cores and therefore directly corresponds to the accompanying below ground biomass. At SUN
there was an increase in photosynthetic biomass from the cores while the leaf standing crop reported no
significant change. This is the only point on which the two methods of green plant estimates differ. The
treatments showed significant decreases in photosynthetic biomass at the coarse treatment and a precipitous
decline at the fine treatment. At RKB the patterning of all responses was nearly identical to that of leaf standing
crop. The control plots showed a seasonal response pattern, the photosynthetic biomass from the coarse
treatments declined throughout the project, and the fine treatment plots showed increases after the netting was
removed.
Non-Photosynthetic Biomass. The nonphotosynthetic biomass at SUN showed little difference between control
and the coarse mesh treatments over the duration of the experiment, but this fraction steadily declined in the
fine mesh treatments. At RKB a seasonal effect was seen at all treatments, as was a persistent decline. By
the end of the project, the non-photosynthetic biomass from the coarse mesh treatments was less than half
of the control, while there was less decline in the recovering fine mesh treatments.
Total Biomass The total biomass of Thalassia is shown for completeness. Its behavior is primarily driven by
that of the non-photosynthetic biomass.
Leaf Turnover Rate At SUN the turnover rates of Thalassia initially increased from April to July, but then
declined in September. By September 1993, the treatment values had fallen below those of the control plots.
At RKB leaf turnover rate was higher in 1994 than 1993, and was very consistent within each year. RKB
showed a similar pattern to SUN in year one where initially the treatments were the same or higher than the
control, but by the end of the year had declined to significantly less than the control. In 1994, the turnover rate
of the control sites had increased, and the uncovered, former fine mesh treatments, increased similarly while
the coarse mesh plots, while higher than in 1993, were less than the control values.
Areal Leaf Productivity At SUN the areal productivity at the control station increased in July and decreased
by September. While the areal productivity at the coarse plots was significantly higher than the control initially,
this fell off rapidly as the summer progressed. The areal productivity in the fine mesh cages decreased even
more rapidly and was nearly zero by September 1993. The pattern of areal productivity at RKB for the control
plots was similar in behavior to leaf turnover rate, with higher average values in 1994 than 1993. The areal
productivity showed an immediate decline at both treatments relative to the controls, with significantly greater
declines at the fine mesh stations. In 1994 areal productivity at the coarse mesh treatments continued a
decline relative to the controls while the uncovered fine mesh plots increased to over 50% of the control values.
-17-
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Photosynthetic Biomass
Rabbit Key Basin
control
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fine
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Aug. '93 Oct. '93
March '94
Time
Non-Photosynthetic Biomass
Rabbit Key Basin
1
I
Aug. '93 Oct. '93
March '94 June '94
Time
Total Biomass
Rabbit Key Basin
I
Aug. '93 Oct. '93
March '94 June '94
Time
June '94 Sept. '94
Sept. '94
I
Sept. '94
Fig. 3-9. Photosynthetic biomass (g m"2), non-photosynthetic biomass (g m'2), and total biomass
(g m2) of Thalassia at Rabbit Key Basin.
-18-
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CN
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400 -
300 -
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Photosynthetic Biomass
Sunset Cove
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Aug. '93 Oct. '93
Time
Non-Photosynthetic Biomass
Sunset Cove
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Aug. '93 Oct. '93
Time
Total Biomass
Sunset Cove
Aug. '93 Oct. '93
Time
Fig. 3-10. Photosynthetic biomass (g nr2), non-photosynthetic biomass (g fir2), and total biomass (g nr2) of
Thalassia at Sunset Cove.
-19-
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Photosynthetic to Non-Photosynthetic Biomass Ratio This is the ratio of photosynthetic to supporting tissue.
Historically Thalassia from Florida Bay has had a low ratio, with green tissue being only 10-15% of the total
plant biomass, and the high values seen at SUN are anomalously high for Thalassia. At the control plots the
photosynthetic tissue ranged from 25 to 31% of the non-photosynthetic tissue. While these values are high,
they declined rapidly with the stress of light reduction. The seagrass beds at SUN have developed to their
1987-1993 abundance in a relatively short time. It is believed that the trapping of rich nutrient material due to
the physiographjc shape of the cove, possibly coupled by enhanced nutrient runoff from development, has
aided in developing an extremely rich bed that does not have the normal sediment development of the rest of
Florida Bay, thus yielding the very high above/below ground ratios. By comparison with SUN, the ratios in RKB
at the control plots are characteristic of Florida Bay, averaging about 10%. In 1993 the effects of the shading
are seen to decrease the green tissue in both treatments, with greater decreases in the fine treatment. By 1994
the coarse treatment continues to decline, but the percentage of green tissue increases in the fine plots that
have had their cages removed.
DISCUSSION
Physical parameters. Shortly after the cages were installed, integrated light readings showed that the coarse
mesh cages were receiving about 50% of the light entering the control areas, while in the fine mesh cages the
PAR was 30 % of the control. In the summer, PAR at the control sites increased from 20 to 30 mol photons nv2
d'1 (MPD), while the PAR in the experimental cages was, on the average, only slightly higher than initial values
(Fig. 3-3). Throughout the first summer and fall the coarse treatments received 49 % of the MPD as the
controls and the fine treatment 21%. In the winter months, the PAR in the control cages fell to roughly 50%
of summer values while the PAR in the coarse mesh treatments declined by slightly less than 50%. Thus from
beginning in May 1993 until March 1994, the coarse treatments were receiving around 50% of the PAR in the
controls. By March 1994, PAR began to increase to summer values in the control, but there was little increase
in the experimental plots at this time so that by the summer of 1994 the experimental treatments were receiving
only about 30% of that of the control plots. Over the course of the entire experiment the coarse treatments
received 43% of the MPD of the controls.
At the end of the previous decade, from 1986 thru 1988 especially, Florida Bay was an exceedingly stressful
environment for seagrasses with salinities in the north-central core of the bay reaching 72 psu for several
months and exceeding 50 psu for over 14 months. From long-term records, the summer and fall of 1987 was
exceptionally hot and the waters were abnormally warm. In the late summer and fall of 1987 the beginnings
were detected of the Florida Bay seagrass dieoff which has led to the loss of many thousands of hectares of
seagrass. In addition the late summer of 1987, with its hot water, saw the largest coral bleaching event to date
in the Florida Keys. Robblee etal (1991) documented a total loss of over 4,000 ha and extensive damage and
loss to an additional 23,000 ha. This number has subsequently increased many times, but there is no currently
accurate assessment of the extent, due in large part to persistent algal blooms and turbidity plumes.
By comparison with the physical conditions in the late 1980's, the 1993-1994 time period were relatively benign.
The greatest apparent physical stressor during the time of the experiments would have most likely been
temperature and not salinity. Temperatures were quite warm in the summer of 1993, exceeding 30°C from July
to September (Fig. 3-4). 1994 saw lower temperatures in the summer, and the temperature data correlated
well with the PAR data, which also showed both higher average MPD and a longer duration of higher MPD in
the summer of 1993 than in 1994. Note that this does not correlate with the absolute highest PAR intensity.
A comparison of figures 3-2 and 3-3 shows that while the highest integrated photon flux values occurred in the
summer of 1993, the highest intensities occurred in March of 1994.
-20-
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03
0>
vP
ro
CN
T3
O
2.5 -,
on
2.0 -
1.5 -
E 0.5 -|
3.5 -,
3.0 -
2.5 -
2.0 -
1.5 -
1.0 -
0.5 -
0.0
Leaf Turnover Rate
Rabbit Key Basin
control
r\ \ i coarse
fine
I
April '93
July '93 Sept.'93
April '94
July '94 Sept. '94
Time
Areal Leaf Productivity
Rabbit Key Basin
I
I
April '93
July '93 Sept.'93
April '94
July '94 Sept. '94
Time
1
Q.
35 -,
30 -
25 -
20 -
15 -
10 -
5 -
0 -
.
Percent Photosynthetic Biomass
Rabbit Key Basin
March '94 June '94 Sept. '94
Fig. 3-11. Turnover rate (% d'1), areal productivity (g rrv2 d'1), and the ratio of photosynthetic to
non-photosynthetic biomass (%) of Thalassia at Rabbit Key Basin.
-21-
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o
3
Leaf Turnover Rate
Sunset Cove
control
i \ \i coarse
i fine
^.3 -
S 2.0 -
o3
°- 1.5 -
"*5
ID 1-° -
1 0.5 -
3
h nn
T
J.
-
X
T^
i
i
|
April '93
3.5 -,
3.0 -
2.5 -
2.0 -
1.5 -
1.0 -
0.5 -
0.0
35 -,
30 -
25 -
15 -
10 -
5 -
0
July '93 Sept.'93
Time
Areal Leaf Productivity
Sunset Cove
April '93
July '93 Sept.'93
Time
Percent Photosynthetic Biomass
Sunset Cove
I
Aug. '93 Oct. '93
Time
Fig. 3-12. Turnover rate (% d'1), areal productivity (g nrr2 d'1), and the ratio of photosynthetic to non-
photosynthetic biomass (%) of Thalassia at Sunset Cove.
-22-
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Salinities throughout the experimental period were near seawater strength for both stations. RKB was nearly
constant at 36-37 psu, while SUN varied from 28-38 during the time of the experiment there. Neither of these
salinity regimes should have been a significant source of stress to Thalassia, although the variations may well
have caused some of the differential responses seen between the years 1993 and 1994.
Biotic Parameters At the beginning of the experiment, the standing crop in RKB averaged 82 g m2 and SUN
averaged 89 g m2. In 1983-84 the bay-wide average for Thalassia was 67 gm2 (Zieman et al 1989). While
the initial values for these stations are somewhat higher than the bay-wide average, they are typical of the
specific habitats involved. While the responses of the control plots are seen to be within normal variation in the
fall samplings (comparison of September 1993 with September 1994), the progressive loss of standing crop
in the coarse mesh treatment after year 1 shows the effects of the long-term light reduction. The pattern at
SUN was essentially the same. The following discussion will be based mostly on the RKB station because of
its longer data base, but note will be given where the behaviors differ significantly. Comparing September
1993 with 1994, the control plots lost about 13 % of their standing crop while the coarse mesh plots lost 78%,
a very significant loss. The fine mesh cages showed a greater rate of decrease while under treatment, and
rebounded somewhat following shade removal.
This pattern is mirrored with the LAI response, as LAI is a parameter that is closely coupled with standing crop.
What is instructive is analyzing how these changes are produced. Leaf width showed virtually no change at
the control plot or the fine mesh treatments, but did progressively decrease in the coarse mesh treatments.
Decreasing leaf width has been shown to be a parameter that is sensitive to stress in seagrasses. The
Thalassia leaves at the control site increased in length throughout the experiment. In the coarse treatment the
leaf length increased initially but then declined to a length that was similar to that at the start of the experiment.
The number of leaves per short shoot tended to vary similarly across all treatments, but the short shoot density
showed very pronounced behavior. At the control plots, the short shoot density increased in the middle of the
experiment, but by the end was similar to the density at the beginning. There was some initial increase in short
shoot density in the coarse treatment plots, but in 1994 these showed a strong progressive decline so that by
September 1994 they were over 30% less than the densities in the control plots. Compared with most other
parameters measured, the short shoot density, which had declined by the fall of 1993, continued to decline
even after the screening was removed in the fine mesh plots.
At the control site, the leaves maintained their standing crop and LAI by becoming longer and slightly wider
as the short shoot density declined slightly. This may be a response of the plant to produce larger, and
especially longer leaves in response to a better light field. All of these parameters declined in the coarse
treatments resulting in the great decline in standing crop and LAI seen there. While the short shoot density
continued to decline in the fine treatment plots, the plants responded with longer, wider leaves and increased
the number of leaves per short shoot by 50%.
Throughout the experiment, the patterrvof photosynthetic biomass from plants from core samples showed very
similar response to leaf standing crop from the productivity samples. This was similar across all treatments.
During this time there was somewhat of a decrease in total biomass, which was generated by the change in
below ground biomass. By the end of the experiment, the total biomass in the coarse mesh treatment was less
than 50% of the controls, while the former fine treatments were only about 30 % less. The result of these
changes is best illustrated in the ratio of photosynthetic to non-photosynthetic biomass. This parameter shows
a slight increase throughout time at the controls, declines greatly in the coarse mesh treatment and declines
and then rebounds at the fine treatments.
The major differences in the responses between RKB and SUN were brought about by the greatly reduced
belowground biomass at SUN. While the non-photosynthetic biomass at RKB decreased from 1993 to 1994,
the same fraction at SUN was only 25-30% of the amount at RKB. For Florida Bay, the ratios found at RKB
are much more typical of the system than those at SUN. It is believed that this is the result of rapid expansion
into a very nutrient rich environment at SUN. This appears to be a combination of natural trapping of sediments
-23-
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by the physiographic makeup of the site, with the potential of nutrient input from lawns and nearby
development.
The turnover rate is a measure of the dynamic performance of the plant. It is produced dividing the areal
productivity by the standing crop. As the number is then normalized, it is a useful measure to compare
seagrass meadows that vary greatly in biomass. While the turnover rate for localities tends to be consistent
over time, there were major differences in all treatments between 1993 and 1994. At the control plots the
turnover rate (here expressed as % d'1) was 1-6-1.7 % d'1 in 1993 and near2 % d'1 in 1994. By September
1993, the treatment rates had declined relative to the control. In 1994, the coarse mesh treatment remained
lower than the control, but the former fine mesh plots rebounded and were growing at a rate equal to the
controls.
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IV ST. JOSEPH'S BAY, FLORIDA, SITE
Cynthia A. Moncreiff, Kenneth L Heck, Jr., Jill M. Zande
MATERIALS AND METHODS
Study area. St. Joseph Bay, located in the northeastern Gulf of Mexico, is a protected, shallow coastal
embayment with extensive seagrass habitat dominated by monospecific stands of Thalassia testudinum Banks
ex Konig (Iverson and Bittaker 1986), The study site selected for light manipulations was located on the west
side of St. Joseph Bay near Eagle Harbor (29°46'N, SS^AW) (Fig. 4-1). This embayment has no significant
sources of freshwater input (Valentine and Heck 1991) other than local rainfall. Mean tidal range is
approximately 0.4 m, with a daily to mixed frequency; local winds often overwhelm this microtidal regime (Tidel
Tide Prediction Software 1995). Salinity varied from 26 to 38%o and temperature from 6.5 to 33°C over the
course of the study; these values were slightly outside of the reported ranges of 30 to 36%o for salinity and 8
to 30°C for temperature (Valentine and Heck 1991). Peak production rates for seagrasses are reported to be
nearly 75 g C m'2 y1 (Iverson and Bittaker 1986). The site is relatively pristine, with minimal influences from
industrial development and limited commercial fishing activities; the area is a component of the T. H. Stone
Memorial St. Joseph Peninsula State Park. Nuthent, suspended sediment, and paniculate organic matter
levels are generally low; water clarity is high, with visibility generally well in excess of 2 m (6.6 ft). Water in St.
Joseph Bay has a history of being quite clear, due in large part to the coarse sand sediments in the bay
(Iverson and Bittaker 1986)
Port SI. Jo*
Out **
N
Fig. 4-1. Map showing study area location in the northeastern Gulf of Mexico on the west shore of St.
Joseph Bay.
-25-
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Experimental design. A series of enclosures, constructed of a 1.75 m x 1.75 m (5.7 ft) x 0.6 m (2 ft) height
schedule 40 PVC frame covered with neutral density plastic mesh (extruded Vexar Diamond mesh, Internet,
Inc., Minneapolis, MN) secured to the frame with cable ties, were designed so that the tops could be removed
for sampling and enclosure maintenance. Two different mesh sizes (fine and coarse) were used to achieve
the desired light level reductions: 6.4 mm (1/4") mesh (fine), which resulted in a reduction to 30-40% of surface
irradiance; and 19.4 mm (3/4") mesh (coarse), which produced a reduction to 60-70% of surface irradiance.
Control enclosures (no reduction) were used to assess any potential caging effects. Mist netting was used to
cover the tops of the enclosures for the purpose of exclusion of the short-spined sea urchin, Lytechinus
variegatus (Lamarck), from the enclosures, as sea urchin herbivory has been demonstrated to affect seagrass
growth responses (Valentine and Heck 1991). Three replicate enclosures were constructed for each treatment
for a total of 9 enclosures.
The enclosures were placed in a continuous bed of Thalassia testudinum with an average depth of 1 m and
secured to the substrate; the perimeters of each enclosure were cut to a depth below the rhizome layer
(approx. 40 cm) to ensure the separation of the plants from the rest of the meadow with respect to physiological
resources (Tomasko and Dawes 1989). Enclosures were cleaned every two weeks or as needed to control
epibionts; this was critical as any great degree of growth of material on the neutral density plastic mesh had
a measurable effect on transmittance and attenuation of light reaching the enclosed seagrasses. Enclosure
tops were removed during this process to ensure complete cleaning and to minimize the impact of adding
epibiont material to the enclosures.
Sampling schedule and design. Enclosures were deployed on 20 March 1993; initial samples were collected
from within each enclosure area on 21 March, and pore water sulfide collected on 24 March. The remaining
quarterly samplings were conducted on 11-12 June and 25-26 September 1993. Short shoots of Thalassia
marked using a modification of Zieman's (1974) leaf marking technique were harvested no more than 3 weeks
post-marking. During the first year of the study, samples were collected as described in the original protocol,
with the modifications detailed below.
RESULTS
Light. As stated earlier, the enclosures produced reductions in light levels reaching the seagrass canopy at
the site of 60-70% of that normally reaching these submerged plants within the 19 mm (3/4") mesh enclosures
(reduction to 60-70% ambient light), and a reduction to only 30-40% of normal light levels with the 6.4 mm (1/4")
mesh (reduction to 30-40% ambient light). Actual measured light levels representative of what was observed
throughout the study are shown in Table 4-1, Degree of light attenuation varied as a result of levels of epibiont
accumulation on the enclosures used to shade the seagrass. Light levels at times approached the hypothetical
minimum of 10% of surface irradiance required to support the growth of Thalassia testudinum (Iverson and
Date
24-VII-93
6-X-94
Enclosure
Control
Coarse (3/4")
Fine (1/4")
Control
Coarse (3/4")
Recovery (former 1/4")
Surface irradiance
2800
2766
2667
2360
2367
Light reaching enclosed canopy
1033H15
740 ± 40
467 ± 24
563 ±23
240 ± 20
Table 4-1. Typical observed light reductions at St. Joseph Bay study site. Values shown are expressed
in nEm'2s'1.
-26-
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Water Temperature and Salinity
Mar May Jul Sep Nov Jan Mar May Jul
1993 1994
Ambient Nutrient Levels
Sep
May Jun Jul Aug May Jun Jul Aug Sep Oct
1993 1994
POM and Suspended Sediment Concentrations
25
H- POM
-A- sediment
Mar Apr May Jun Jul Aug Sep Mar Apr May Jun Jul Aug Sap Oct
1993
1994
Fig. 4-2. Water temperature, salinity, ambient nutrient levels, suspended sediments, and
particulate organic matter (POM) at the St. Joseph Bay study site.
-27-
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Bittaker 1986). Another consideration is the wavelengths of PAR reaching the substrate; Calem and Pierce
(1993) found that distribution of T. testudinum was limited to areas where blue light reached the sediment
surface. Light attenuation by the Vexar mesh, especially when heavily epiphytized, may have potentially
affected this parameter. Other physical and chemical parameters. Water temperature, salinity, and ambient
nutrient levels are shown graphically in figure 4-2. Temperature ranged from a low of 6.5°C in January 1994
to a high of 33°C, observed in July of both years of the study. Salinity ranged from a low of 19 ppt following
an intense local rainfall event in August 1994 to a high of 38 ppt in January, which coincided with the observed
temperature minimum. Nutrient levels were consistently low over the course of the study; NO3 ranged from
0.03 to 2.73 um/l, PO< from 0.02 to 0.14 um/l, and NH4 from 0.5 to 2.58 um/l. Suspended sediment and POM
levels were also consistently low, with suspended sediment concentrations generally averaging less than 20
mg/l and POM less than 5 mg/l dry weight. Lower values were observed during the calmest observed field
conditions.
Thalassia response. Plant response parameters are shown graphically in figures 4-3 through 4-6. Standing
crop of Thalassia testudinum showed expected seasonal trends, with lowest values being observed in March
1994 prior to the onset of spring growth. A significantly lower standing crop was observed in September 1993
for the 6.4 mm (1/4") mesh treatment (p<0.05). A similar pattern was observed in areal leaf production (Fig.
4-3) and in leaf area index (Fig. 4-4). Turnover rates (Fig. 4.3) were lowest in September 1993 for all
treatments. The zero value observed for the 6.4 mm (1/4") mesh treatment in March 1994 was due to complete
grazing of all marked shoots within the enclosures by rogue sea urchins. Compensatory growth appeared to
be occurring in the shaded and recovery treatments in September 1994, with the 6.4 mm (1/4") mesh turnover
rate being significantly different from the control (P<0.05).
Urchin grazing of marked shoots in March 1994 resulted in zero values for leaf area index (LAI) (Fig. 4-4). LAI
was significantly reduced in the 1/4 mesh treatment in both June and September 1993 (P<0.05); differences
at all other times were not significant. During the 1994 recovery phase for the 6.4 mm (1/4") mesh treatment,
LAI returned to pre-shading levels.
Average seagrass blade widths showed a trend towards a decrease in response to shading; however,
differences among treatments were significant only in March 1994 (p<0.05); the recovering 6.4 mm (1/4") mesh
treatment was different from the control (p<0.05). Total plant biomass (Fig. 4-4) was highest in all treatments
at the initiation of the study, and showed a decrease from March to June that may have been a result of
severing rhizomes to eliminate contributions to the enclosed plant material from outside the enclosures. Total
biomass remained relatively constant throughout the remainder of the study. Rhizome and root biomass (Fig.
4-5.) showed similar trends. Above- to below-ground biomass ratios were lowest in March of both years of the
study, with the zero value observed in the recovering 6.4 mm (1/4") mesh treatment resulting from nearly
complete grazing of aboveground biomass by urchins. This ratio was markedly higher in control treatments
in June of both years, and sufficiently higher in June 1994 to suggest that a shading response was beginning
to occur in the 19 mm (3/4") mesh treatment.
Short shoot densities (Fig. 4-6) were significantly lower in the 6.4 mm (1/4") mesh treatment in September 1993
in comparison the control and 19 mm (3/4") mesh treatments (P<0.05), and also throughout 1994. Shoot
specific growth rates were significantly depressed in the 6.4 mm (1/4") mesh treatment in September 1993 and
in June 1994 relative to the control. Additional evidence for compensatory growth during the recovery phase
for this treatment is suggested by the high rate observed in September 1994.
Calcareous epibiont coverage was examined only at the St. Joseph Bay site due to problems with spontaneous
separation of epiphytes from leaves during shipment of samples from the other sites (Corpus Christi Bay,
Texas and Florida Bay, Florida). This information is shown in figure 4-6. There appear to be fewer encrusting
organisms present on seagrass blades during June of both years. Differences among treatments were not
significant.
-28-
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Leaf Standing Crop
Control
Coarse
Fine
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Areal Leaf Productivity
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Leaf Turnover Rate
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Fig. 4-3. Standing crop (g nrr2), biomass productivity (g rrv2 d'1), and turnover rate (mg g'1 d'1) for
Thalassia testudinum at the St. Joseph Bay study site.
-29-
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Leaf Area Index
CM
O
Control
Coarse
Fine
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Blade Width
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Total Biomass
2000
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Fig. 4-4. Leaf area index (LAI, m2/m2), blade width (cm), and total biomass (g nrr2) for Thalassia
testudinum at the St. Joseph Bay study site.
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Rhizome Biomass
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Root Biomass
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Aboveground/Belowground Biomass Ratio
Mar 93 June 93 Sept 93 Mar 94
Time
June 94 Sept 94
Fig. 4-5. Rhizome biomass (g m *), root biomass (g nrr2), and above-to below-ground biomass ratio
for Thalassia testudinum at the St. Joseph Bay study site.
-31-
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Short Shoot Density
1000
Control
Coarse
Fine
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Shoot Growth Rates
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Calcareous Epibiont Coverage
Mar 93 June 93 Sept 93 Mar 94 June 94 Sept 94
Time
Fig. 4-6. Short shoot densities (rrv2), shoot specific growth rates (g ss'1 d"1), and calcareous
epibiont coverage for Thalassia testudinum at the St. Joseph Bay study site.
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Mesograzer abundances were also examined only at the St. Joseph Bay site due to problems with shipment
of samples from the other sites (Corpus Christi Bay, Texas and Florida Bay, Florida). These samples were
sorted and the organisms found identified within a series of functional groups. As for the calcareous epibionts,
there appeared to be fewer organisms present among the seagrass blades during the second year of the study,
likely as a result of decreased short shoot densities. There may also have been some enclosure effects, but
they were not detected. Differences among treatments were not significant.
DISCUSSION
The primary purpose of this study was to examine the response of Thalassia testudinum to chronic reductions
in light. Seagrasses have the potential to respond to light reductions in a number of ways, and exhibit a wide
range of growth responses on both seasonal and annual scales (Zieman and Zieman 1993, Marba et al. 1994);
these variations must be considered when interpreting observed results. Previous studies have indicated that
stunted growth, decreased short shoot densities, and reduced biomass were common responses, and that light
levels equivalent to 20% of surface irradiance were necessary for continued seagrass survival (Short et al.
1974, Congdon and McComb 1979, Dennison 1987, Kenworthy and Haunert 1991). The findings of this study
appear to support these observations.
Light. As explained previously, the enclosures produced reductions in light levels reaching the seagrass
canopy of 60-70% within the 19 mm (3/4") mesh enclosures, and a reduction to only 30-40% of normal light
levels with the 6.4 mm (1/4") mesh enclosures. Actual measured light levels at times approached the
hypothetical minimum of 10% of surface irradiance required to support the growth of Thalassia testudinum
(Iverson and Bittaker 1986). Based on observed plant responses in the 6.4 mm (1/4") mesh treatment,
reduction in light to levels that are 30-40% of normal ambient light were sufficient to significantly suppress
growth to a level that was not tolerated by T. testudinum. Olesen and Sand-Jensen (1993) observed that zero
growth occurred in Zostera marina at 11% of surface photosynthetically active radiation (PAR); the shading in
this study approximated delivery of 15% of surface PAR with the 6.4 mm (1/4") mesh treatment for a growth
response close to zero. However, by removing the shading stress during the second year of the study, we
demonstrated that in St. Joseph Bay, the plants were able to recover within less than one year to pre-stress
conditions.
Other physical and chemical parameters. Water temperature and salinity were generally within the optimal
ranges reported elsewhere for Thalassia testudinum (Phillips 1960). Suspended sediment and POM levels
were also consistent with suspended sediment concentrations generally considered acceptable for submerged
aquatic vegetation (Dennison et al. 1993). Macauley and others (1988) found that I. testudinum responded
most strongly to water temperature, although light is often thought to be a more critical parameter (Iverson and
Bittaker 1986, Robbleeetal. 1991).
Thalassia response. Values observed for short shoot densities and standing crop (aboveground biomass)
of Thalassia testudinum in Gulf of Mexico seagrass beds are shown in Table 4-2 for purposes of comparison
with the present study. Observed values for control enclosures fell well within the range observed for other
studies in northern Gulf of Mexico seagrass meadows. Iverson and Bittaker (1986) reported short shoot
densities of roughly 600 rrr2 in April to 1100 rrv2 in September; observed values for all treatments were closer
to the value reported in their study for April. However, they indicate that short shoot numbers were relatively
constant in St. Joseph Bay, which was observed in all but the 6.4 mm (1/4") mesh treatment.
Compensatory growth which appeared to occur in both the shaded 19 mm (3/4") mesh and recovery treatments
in September 1994 is consistent with plant responses to shading reported in the literature (Olesen and Sand-
Jensen 1993, Gordon et al. 1994). However, use of leaf marking as a technique to monitor plant response in
shading studies is perhaps inappropriate. Leaf elongation occurs in response to reduced light levels, which can
give a false indication of plant response when assessing growth energetics (Olesen and Sand-Jensen 1993;
Czerny and Dunton 1995).
-33-
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Location
Edmont Key, FL 1981
1982
1983
1985
PerdidoKey, FL1993
1994
Boca Ciega Bay, FL
St. Joseph Bay, FL
St. Joseph Bay, FL
1994
Short Shoots rrv2
505
788
560
500
684
825
200
564
700
g dry wt rrr2
...
82.3
49.5
480
94.9
111.5
Reference
Durako and Moffler 1987
Hecketal. 1994
Taylor et al. 1973
Valentine and Heck 1991
This study
Table 4-2. Short shoot densities and aboveground biomass of Thalassia testudinum beds in Gulf of Mexico
seagrass studies.
Observed above- to below-ground biomass ratios were somewhat different than responses seen in other
seagrass species. In Zosfera marina, more biomass was apportioned to leaves than to rhizomes with reduced
light, resulting in an increase in this ratio (Olesen and Sand-Jensen 1993). This ratio was only observed to
increase in this study in the control treatment, primarily as a seasonal growth response.
Short shoot densities were lower for both shade treatments at the conclusion of the study, even though the 6.4
mm (1/4") mesh treatment was allowed to recover. Similar results have been observed for Posidonia sinuosa
following a 5-month period of intense shading (> 80% light reduction; Gordon et al. 1994). This response is also
significant in light of the relative constant short shoot densities observed over time in St. Joseph Bay (Iverson
and Bittaker 1986; this study).
Calcareous epibiont growth on seagrass blades was anticipated to respond to changes in leaf elongation rates
and to short shoot growth rates, in general, with epibiont load increasing in the shaded treatments over time in
response to eventual slower growth rates. Although decreased shoot specific growth rates were observed, an
increase in epibiont coverage was not.- Lower percent coverage of leaf surfaces was observed in the 6.4 mm
(1/4") mesh treatment throughout 1994, although it was not statistically significant due to large standard errors
in this parameter. Differences may have resulted from the decreased availability of colonizing organisms in the
6.4 mm (1/4") mesh treatments in conjunction with decreased numbers of short shoots in these enclosures,
even during the recovery phase or decreased growth of epibiont due to shading.
As stated earlier, mesograzer abundances were determined on the basis of functional groups. It was expected
that the mesograzer community would respond to decreases in seagrass biomass and decreased short shoot
densities as observed in other studies (Connolly 1995). As for the calcareous epibionts, there appeared to be
fewer organisms present among the seagrass blades during the second year of the study, likely as a result of
decreased short shoot densities. There may also have been some enclosure effects, but they were not
detected.
Results of this study indicate that Thalassia testudinum meadows in St. Joseph Bay, which are presently not
impacted by chronic reduced light levels or other physicochemica! stresses such as high sulfide levels or
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reduced salinities, have a high potential for use in future studies of reductions in light that are more subtle than
that achieved with the 6.4 mm (1/4") mesh treatments. Observations of plant responses to longer-term
reductions in light than those observed in this study will provide the type of information that will allow the
questions resulting from observed cause-and-effect relationships among the environmental parameters that
potentially can affect the survival and growth of seagrasses in critical coastal habitats.
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V. CORPUS CHRISTI BAY, TEXAS
Kun-Seop Lee and Kenneth H. Dunton
MATERIALS AND METHODS
Study Site The study site is located in eastern side of Corpus Christi Bay (27° 49' N, 97° 7' W). This site
has been the focus of several recent investigations on south Texas seagrasses (Dunton, 1990,1994; Czemy
& Dunton, 1995). Thalassia testudinum, Halodule wrightii and Syringodium filifbrme are the dominant
seagrass species in this area. This study was conducted on a monotypic meadow of Thalassia testudinum
with an average water depth of 1.2 m. Water temperature ranged from 34°C in July and August to 13°C in
January, while salinity varied from 27 to 32 %o.
In situ Light Manipulation Light shading cages (1.5m x 1.5m x 0.5m) were placed in a monotypic meadow
of Thalassia testudinum to achieve artificial in situ light reduction. Coarse mesh (1.91 x 1.91 cm) reduced
irradiance to 14% of surface irradiance (SI) while fine mesh (0.64 x 0.64 cm) reduced irradiance to 5% SI.
Cages without a top screen were placed on the seagrass bed as controls. Three replicate cages for each
treatment were deployed in a random manner in the seagrass bed. The shading mesh was replaced with
cleaned mesh every one or two weeks to minimize the effects of fouling on light transmissivity. The perimeter
of each cage was cut to a sediment depth of about 30 cm to physiologically isolate plants located within and
outside of the cages. Shading was initiated in late April 1993 and terminated in August 1994. The experiment
lasted a total of 490 days. No data exist for fine mesh cages (5% SI) after November 1993, since all plants
within these cages died by that date.
Biological Measurements Quarterly measurements of plant density, biomass, leaf chlorophyll content, blade
width and leaf elongation rates were made in the experimental cages.
RESULTS
Underwater Irradiance The annual quantum flux at the surface was 12983 mol rrv2 yr1, and ranged from
an average of 55.7 in July to 18.4 mol nrv2 day1 in December 1993 (Fig. 5-1). The annual quantum flux at the
seagrass canopy was 5207 mol nr2 yr1, which corresponded to 46% SI. Two light manipulation treatments
using coarse and fine mesh significantly reduced (P <0.001) underwater irradiance to 1628 (14% SI) and 864
mol nr2 yr1 (5% SI) respectively. In control cages, underwater photon flux density (PFD) ranged from 9 to
22 mol nr2 day1 (average 14.3 mol m~2 day1). Average PFD in the cages shaded with coarse and fine mesh
was 4.5 and 2.4 mol m"2d'1 respectiveJy (Table 5-1). Unlike surface measurements of PAR, underwater PFD
did not exhibit a seasonal sigmoidal curve.
Pore water Ammonium and Sulfide Sediment pore water ammonium concentrations were measured three
times (September 1993 and April and July 1994) during the course of this study. Pore water ammonium
concentrations in 14% and 5% SI cages were significantly (P <0.001) higher than controls (46% SI). Pore
water ammonium concentrations for control cages ranged from 62 uM NH4* in April 1994 to 101 uM NH4* in
July 1994. The concentrations in 14% SI treatment cages ranged from 141 uM NH4* in April 1994 to 179 uM
NH4* in July 1994 (Fig. 5-2). There was no significant difference in pore water ammonium concentrations
between sites receiving 14% SI and 5% SI in September 1993, shortly before all plants at 5% SI died.
Pore water sulfide concentrations of the control and 14% SI cages in August 1994 were 107 uM and 179 uM
sulfide, respectively. The concentration of sulfides in the shaded cages was significantly (P=0.01) higher than
in the controls.
-36-
-------
I I I
Feb April June
1994
Fig. 5-1. Average daily photo flux density (PFD) collected underwater (control, 14% SI and 5% SI
treatment cages) and at the surface (The University of Texas Marine Science Institute in Port Aransas).
Data collection at the 5% SI treatment was terminated in November 1993 following the death of all plants
in these cages.
Shoot Density and Blade Width Shoot densities in control cages (46% SI) ranged from 457 to 785 nv2.
Shoot densities in 14% and 5% SI cages were significantly (P <0.001) lower than controls throughout the
experiment (Fig. 5-3). In August 1993, after 116 days shading, shoot densities in the various treatments were
785 nr2 (control), 296 nr2 (14% Sl)rand 168 rrv2 (5% SI). All plants exposed to 5% SI died after 200 days
of shading treatment and over 99% of plants receiving 14% SI died by the end of the experiment (490 days).
Blade widths in Thalassia testudinum decreased significantly (£ <0.001) as a result of light reduction, with
blade width decreasing more rapidly in plants at 5% SI than those at 14% SI (Fig. 5-4). After only 36 days of
shading (May 1993), blade widths of plants receiving 5% SI had already decreased significantly (P <0.001)
to 6.0 mm compared to plants in the control and at 14% SI. In August, after 128 days of shading, blade widths
of plants at 5% SI averaged 4.7 mm compared to plants at 14% SI, which were 6.6 mm in May 1993, but
decreased to 4.8 mm in April and July 1994. Blade widths of control plants ranged from 6.4 to 7.0 mm during
the entire period.
Chlorophyll Content Total chlorophyll (chl § + chl bj from control plant leaves ranged from 5.0 mg chl g'1
dry wt in July 1993 to 6.7 mg chl g'1 dry wt in July 1994 (Fig. 5-5). Total blade chlorophyll and chl b content
increased significantly (P=0.019 and P_ <0.001 respectively) with decreased levels of PAR. Chl a levels also
-37-
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Average PFD
(mol photons rrr2 day1)
% ISA
%SI
HMl(h)
Total irradiance
(mol nr2 yr1)
Control
(in situ ambient)
14.27
100
46.4
8.5
5207
Light Manipulation
Coarse mesh
4.46
31.4
14.3
3.3
1628
Fine mesh
2.37
16.1
5.4
0.9
864
Table 5-1 Daily average photon flux density (PFD), % of in situ ambient (%ISA), % of surface irradiance (%%
SI) and the daily period of light saturated photosynthesis (Hsat) in control and light manipulation cages. Hsal
values based on a saturation irradiance of 140 nmol m~2 s~1 for Thalassia testudinum (Dunton, unpub. data).
showed an increasing trend with light reduction, but it was not statistically significant (£=0.11). Total blade
chlorophyll concentrations in 5% SI plants ranged from 5.4 mg chl g'1 dry wt in September 1993 to 6.5 mg
chl g'1 dry wt in July 1993, while chlorophyll concentrations were lowest (6.0 mg chl g~1 dry wt) in July 1993
and highest (8.3 mg chl g'1 dry wt) in July 1994 for plants at 14% SI.
The chl a: b ratios of blades from control cages ranged from 2.7 in September 1993 to 3.4 in July 1994 (Table
5-2). Chl a : b ratios of blade tissue decreased significantly (P<0.001) as a result of the two light reduction
treatments. The ratios were highest in plants receiving 46% SI and lowest in the plants receiving 5% SI. Chl
a: b ratios of 14% SI plants was highest (2.7) in April 1994 and lowest (2.1) in July 1994, while plants at 5%
SI showed chl a : b ratios of 2.4 in July 1993 and 2.5 in September 1993.
Leaf Production Rates Leaf production rates were highest during summer and lowest during the cooler
months (Fig. 5-6), ranging from 1.5 mg shoot1 d'1 (0.7 g nr2 d"1) in April 1994 to 5.0 mg shoot"1 d"1 (2.4 g rrv2
d'1) in July 1994 in control plants. Leaf production rate per shoot (mg shoot1 d"1) and areal leaf production rate
(g m'2 d'1) decreased significantly (£_<0.001) with shading. In May and August 1993, leaf productivities of
plants receiving 14% SI were 3.8 and 2.5 mg shoot"1 d'1, compared to plants at 5% SI, which were 1.9 and 1.2
mg shoof1 d'1. Areal leaf productivity at 5 and 14% SI dropped to nearly zero after about one year of shading
as a result of extremely low shoot densities.
Biomass Biomass decreased significantly (P <0.001) with light reduction. In August 1993, 116 days
after shading, the biomass in cages at 14% SI decreased to less than half that of controls and was less than
a third of control biomass within the 5% SI treatments (Table 5-3). All plants receiving 5% SI died by
November 1993, after 200 days of reduced PAR. The biomass of the plants receiving 14% SI decreased to
20% of control biomass by April 1994 (after 345 days of shading), and to 1.4% of control biomass by July 1994
(after 457 days of shading).
Relative to controls, leaf biomass decreased more rapidly than biomass of below-ground tissues under light
reduction. In August 1993, after 116 days of shading, the leaf biomass of plants at 5% SI decreased 95%
-38-
-------
i i Control
r^^l 14% SI
" 5% Sl
1.200 -
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July-1994
Fig. 5-2. Pore water ammonium concentration in sediments of control, 14% SI and 5% SI treatment cages.
Values are means ± SE (n=3).
1000 -,
Aug-1993 April-1994 July-1994
Fig. 5-3. Shoot densities in control, 14% SI and 5% SI treatment cages. Values are means ± SE (n=3).
-39-
-------
8 -
T
I | control
14% si
5% SI
T
GO
T
May, 1993
Aug,1993
April, 1994
July.1994
Fig. 5-4 Blade widths of Thalassia testudinum from control, 14% SI and 5% SI treatment cages. Values are
means ± SE (n=3).
compared to a corresponding drop of 50% in below-ground biomass. After loss of leaf material, root biomass
decreased rapidly. In April 1994, after 345 days of shading, root biomass decreased 90% at 14% SI, while
short stem and rhizomes maintained 20-30% of their biomass relative to controls. Although rhizome material
was the most durable plant part, biomass of this component dropped 98% by the end of the experiment.
In control plants, below/above-ground ratios changed significantly with season (P=0.0015). The ratio was
lowest (1.3) in August and highest (5.8) in April (Table 5-4). Percentage of leaf (above-ground) biomass as
a function of total biomass was highest in August 1993, while that of rhizome was highest in April 1994. The
percentages of short stem and root biomass were fairly constant. Leaf biomass accounted for 45% of total
biomass in August, while accounting for only 17% in April. Rhizome biomass was 20-40% of total biomass
in July and August and about 50% of total biomass in April (Fig. 5.7).
Below/above-ground ratios significantly (P <0.001) increased with light reduction (Table 5-4). In August 1993
the ratio of plants receiving 46% SI was 1.3 while those of plants receiving 14% and 5% SI were 3.0 and 14.7,
respectively. By July 1994 the below/above-ground ratio of control plants was 2.1 compared to 14.0 for plants
at 14% SI; this difference was reflected by rhizome tissues, which constituted nearly 70% of the total plant
biomass in plants at 14% SI compared to 40% for control plants (Fig. 5-7).
Carbohydrate Carbon Soluble carbohydrate carbon content of plants at 46% SI was highest in rhizomes
(130-136 mg C g'1 dry wt) and in short stem (102-152 mg C g'1 dry wt), and relatively low in leaf (50-66 mg C
g-1 dry wt) arid in root tissue (57-74 mg C g'1 dry wt) (Fig. 5-8). Shading treatments significantly (P <0.001)
lowered the soluble carbohydrate carbon content of leaf, rhizomes and short stem. However, the content of
root tissue did not change significantly (P=0.53) with light reduction. Soluble carbohydrate levels in rhizomes
-40-
-------
and short stem decreased more rapidly with reduced light than that of leaf material. In both shading
treatments rhizome carbohydrate carbon content was 50% lower and leaf carbon content was about 15%
lower than controls. In April 1994, total carbohydrate carbon content in control plants ranged from 107 mg C
g'1 dry wt in leaf to 158 mg C g~1 dry wt in rhizome tissues (Table 5-5). Total carbohydrate carbon content
decreased to 91 mg C g'1 dry wt in leaf and 127 mg C g~1 dry wt in rhizome tissues with light reduction.
Structural carbohydrate carbon content was estimated by subtraction of soluble carbohydrate carbon content
from total carbohydrate carbon content. In leaf tissues of control plants, about 50% of total carbohydrates was
attributed to structural carbohydrate, while only 20% in rhizome tissues was structural. Structural carbohydrate
carbon content in plant tissues did not decrease with light reduction.
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Chl bChl aTotal Chl bChl aTotal Chl bChl aTotal Chl bChl aTotal
July, 1993
Sept, 1993
April, 1994
July, 1994
Fig. 5-5. Chlorophyll a, chlorophyll b and total (chl a+b) concentrations of Tha/ass/a testudinum leaves from
control, 14% SI and 5% SI treatment cages. Values are means ± SE (n=3).
-41-
-------
Chlorophyll a:b ratio
Sampling Date
July 1993
Sept. 1993
April 1994
July 1994
Control
2.74 ± 0.09
2.72 ± 0.05
2.73 ± 0.07
3.44 ±0.10
14% SI
2.51 ±0.08
2.52 ± 0.06
2.67 ± 0.05
2.10 ±0.07
5% SI
2.37 ±0.08
2.48 ± 0.06
nd
Table 5-2. Chlorophyll a:b ratio of Thalassia testudinum leaves from control, 14% SI and 5% SI treatment
cages at four different sampling times. Values are means ± SE (n=3). nd: no data
I I Control
fT\n 14% SI
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May,1993 Aug.1993 April,1994 July ,1994
Fig. 5-6. Daily leaf production on a shoot (A) and area! (B) basis in control and treatment cages. Values are
means ± SE (n=3).
-42-
-------
Biomass (g dry wt m':)
Sampling Date
(Days shaded)
Total
Control
14% SI
5% SI
Leaf
Control
14% SI
5% SI
Rhizome
Control
14% SI
5% SI
Short shoot
Control
14% SI
5% SI
Root
Control
14% SI
5% SI
May 1993
(0)
577.0 ± 79.3
nd
nd
240.6 ± 28.0
nd
nd
182.6 ±36.7
nd
nd
98.2 ± 17.8
nd
nd-
55.6 ±9.7
nd
nd
August 1993
(116)
971.2 ± 111.2
421.4 ±40.4
307.9 ± 56.0
445.3 ± 68.2
122.1 ±24.4
30.1 ±8.0
200.7 ± 38.6
114.6 ± 15.1
129.6 ± 11.5
195. 7 ±25.9
110.8 ± 20.4
103.6 ± 32.1
129.6 ± 11.0
73.9 ± 8.5
44.6 ± 14.3
April 1994
(345)
367.5 ± 34.4
73.8 ± 19.0
nd
63.4 ± 10.5
7.8 ±2.5
nd
185.6 ± 12.1
36.3 ± 9.6
nd
87.4 ± 17.9
25.9 ±7.8
nd
31.3 ± 5.1
3.7± 1.1
nd
July 1994
(457)
561.5 ± 85.5
21.1 ± 11.8
nd
181.2 ± 25.6
0.6 ± 0.4
nd
216.8 ± 29.4
14.3 ± 10.1
nd
123.3 ± 32.3
6.0 ± 6.0
nd
40.2 ± 5.1
0.2 ± 0.2
nd
Table 5-3. Biomass changes in total and individual plant parts as a result of light manipulation in May
(initial sampling date) and August 1993 and April and July 1994. No plants survived in the 5% treatment
cages after November 1993. Values are means ± SE (n=3). nd: no data.
-43-
-------
Control
14% SI
5% SI
August
1993
308 g m
971 g m
April
1994
July
1994
368 g m2
562 g m2
74 g m2
.1%
21 g m2
Fig. 5-7. Changes in biomass partitioning of Thalassia testudinum into different plant parts (leaf, short stem,
rhizome and roots) as a result of light manipulation between August 1993 and July 1994. Circle area
corresponds with total plant biomass listed for each site/date combination.
-44-
-------
Sampling Date
August 1993
April 1994
July 1994
Control
1.3 ±0.2
5.8 ±0.9
2.1 ±0.2
Below/above-ground ratio
14% SI
3.0 ±0.6
12.4 ±2.9
14.0 ±3.9
5% SI
14.7 ±6.0
nd
nd
Table 5-4. Below-to above-ground ratios of Thalassia testudinum at 46% SI (control), 14% SI and 5% SI in
August 1993 and April and July 1994. Values are means ± SE (n=3). nd: no data.
-45-
-------
150
120
90 -
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,g 30
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S3 14% SI
August 1993
T
December 1993
T
T
April 1994
Leaf Short stem Rhizome
Root
Fig. 5-8. Cartohydrate carbon concentration in different plant tissues of Thalassia testudinum from control
and light treatment cages in August and December 1993 and April 1994. Values are means ± SE (n=3).
-46-
-------
Carbohydrate carbon
(mg C gdw1)
Total
Soluble
Structural
Leaf
107.0
±4.8
49.7 (46)
±1.1
57.4 (54)
±4.2
Control cage
Rhizome
157.5
±4.4
130.5(83)
±6.1
27.0(17)
±10.6
Short stem
115.2
±4.4
101.6(88)
±5.4
13.6(12)
±4.8
Root
107.4
±2.0
70.9 (66)
±1.4
36.5 (34)
±3.5
Leaf
91.0
±4.5
45.0 (49)
± 1.6
46.0(51)
±3.4
14% SI
Rhizome
127.1
±3.2
64.7(51)
±1.9
62.4 (49)
±3.0
Short stem
90.4
±3.3
56.2 (62)
± 1.8
34.2 (38)
±1.2
Root
1171
±2.1
67 1 (57)
± 1.3
50.0 (43)
±2.1
Table 5-5. Total, soluble and structural carbohydrate carbon content of different plant tissues of
Thalassia testudinum from control and 14% SI cages in April 1994. Values are means ± SE (n=3).
Numbers in parentheses represent the percent soluble or structural carbohydrate carbon of total.
-------
DISCUSSION
In situ light requirements of Thalassia testudinum Light reduction resulted in decreases in shoot density
and biomass in Thalassia testudinum. Plants in the control cages (46% SI) remained healthy throughout the
experiment; in contrast, all plants at 5% SI died within 7 months, and most shoots at 14% SI died after 16
months. Czerny & Dunton (1995) also demonstrated that Thalassia testudinum did not tolerate a light
reduction equivalent to 14% SI. This finding is consistent with the minimum light requirements (15-25% SI)
reported by Dennison et al. (1993) for Thalassia testudinum from Florida and the Caribbean. Further long-term
measurements of in situ PAR in Thalassia testudinum beds at variety of depths is needed to establish the
minimum light requirements for Texas plants as has been done for Halodule wrightii (Dunton, 1994).
Thalassia testudinum showed various morphological and physiological adaptations in response to changes
in underwater light availability. Seagrasses can respond to light reduction by increasing chlorophyll content
and decreasing their chl a: b ratio (Wiginton & McMillan, 1979; Dennison & Alberte, 1982, 1985; Abal et al.,
1994). We found agreement with these trends as shown by increases in chlorophyll concentrations and
decreases in chl a : b ratios in response to light reduction, although some of these changes were not
statistically significant. Wiginton & McMillan (1979) reported that chl a : b. ratios were correlated with depth
distribution of seagrasses; additionally, they suggested that the chl a : b ratios controlled distributional
differences among species. Seagrass occurring in deep areas had low chl a : b ratios, but seagrass occurring
at shallow depths had a higher ratio. They suggested that differences in chl a : b ratios were a response to
reduced PFD at depth, and not to changes in light quality. However, measurements of underwater spectral
irradiance (Weidemann & Bannister, 1986; McPherson & Miller, 1987) indicated that the wavelengths absorbed
by chl a decreased more rapidly than the wavelengths absorbed by chl b with increasing water depth. Thus,
although plants receiving less light may increase their total chlorophyll concentration to increase light
absorption efficiency, rapid increases in chl b relative to chl a would allow more efficient use of the more
abundant wavelengths at depth.
Seagrass blade width has been correlated with environmental factors that are ultimately related to underwater
light regimes as noted by several investigators (McMillan, 1978; McMillan & Phillips, 1979; Phillips & Lewis,
1983). For example, McMillan (1978) noted that Thalassia populations from turbid bays were characterized
by having narrower leaves compared to plants in clear water. Phillips & Lewis (1983) found that Thalassia
blade width decreased with increasing depth, and suggested that light attenuation was the causal factor. Our
results indicate that decreased light availability has a significant and almost immediate effect on blade width,
which decreased about 2 mm (to 4.7 mm) as a function of light reduction and shading duration. Since
attainment of the minimum 4.7 mm width was subsequently followed by plant death, decreases in blade width
may be a convenient indicator of light stress in Thalassia testudinum.
Changes in biomass and carbon budget Many studies suggest that whole plant carbon balance is a major
factor determining the growth and distribution of seagrasses (Dennison & Alberte, 1982, 1985; Marsh et al.,
1986; Zimmerman et al., 1989; Fourqurean & Zieman, 1991; Zimmerman et al., 1991). Carbon balance has
often been estimated from rates of respiration and photosynthesis vs. irradiance curves constructed for above-
ground tissues only (Dennison & Alberte, 1985; Marsh et al., 1986; Zimmerman et al., 1991 ), but there are
several other factors that must be considered. These factors include a knowledge of carbon and biomass
partitioning into different plant parts, carbon metabolism of below-ground tissues, storage of photosynthate and
root anoxia.
-48-
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Leaf biomass from control cages accounted for almost 50% of total biomass of Thalassia testudinum during
the warm growing season, and decreased more rapidly than biomass of below-ground tissues as a result of
reduced light. Percent leaf biomass as a function of total biomass decreased from 45% to 9.3% in plants at
5% SI during the first four months. Decreased leaf biomass in the shaded cages is probably a product of both
defoliation and decreased leaf growth. Defoliation is a normal response of terrestrial and submerged plants
to reduced light levels (Addicott & Lyon, 1973; Backman & Barilotti, 1976; Neverauskas, 1988), but defoliation
during the active growing season may seriously impact seagrass survival through decreases in the production
and transport of oxygen to below-ground tissues.
Root biomass decreased more rapidly than rhizome biomass with light reduction. Plants receiving 14% SI for
345 days lost about 90% of their root biomass, while rhizomes still maintained 20~30% of their biomass relative
to controls. Results from a litter bag decomposition experiment indicate that Thalassia rhizome is more
resistant to decay than root tissues (Kenworthy & Thayer, 1984), which is in agreement with our findings. The
increase in the decomposition of below-ground material under low light conditions may also contribute to
increases in pore water ammonium and sulfide levels.
The below-ground tissues of seagrasses generally exist in an anoxic environment (Penhale & Wetzel, 1983).
In addition to lack of oxygen for aerobic respiration of below-ground tissues, sulfide is produced in anaerobic
sediments by bacteria using sulfate as a terminal electron acceptor (Sorensen et al., 1979). Sulfide inhibits
respiration, oxygen release and nutrient uptake by plant roots (Bagarinao, 1992). In Florida Bay, pore water
sulfide concentrations were considerably higher in die-off areas than in healthy Thalassia beds (Carlson et al.,
1994) suggesting that sulfide toxicity may play a role in the loss of seagrass. Photosynthetically produced
oxygen is secreted into sediment through roots (Smith et al., 1984), supporting aerobic metabolism and
creating an oxidized zone around the roots where pore water sulfide and ammonium oxidation can occur. In
this study, the concentration of pore water ammonium and sulfide in shaded cages were significantly higher
than that from control cages. This suggests that below-ground tissues in shaded cages were exposed to
anoxic conditions more frequently than controls. Thalassia testudinum is more vulnerable to anoxia than other
seagrass species because of the relatively high ratio of below-ground biomass to above-ground biomass and
its deep rooted growth habit.
Soluble carbohydrate carbon content was highest in rhizome tissues and likely serves as an energy reserve
for plant sustenance during the winter period (Dawes & Lawrence, 1980; Durako & Moffler, 1985). In this study
levels of soluble carbohydrates decreased significantly in all plant tissues (except roots) with reduced light;
however, different plant parts showed distinctive decreasing patterns. Soluble carbohydrate levels in rhizomes
and short stems decreased to half that of control plants with light reduction, while levels decreased slightly in
blades but remained constant in roots. However, structural carbohydrate levels in plant tissues did not
decrease with light reduction. Stored carbohydrate in rhizome tissues can be used to meet the respiratory
demands of the plant and can contribute to new growth in above-ground tissues during periods of low
photosynthetic production (Dawes & Lawrence, 1979, 1980; Pirc, 1985; Dawes & Guiry, 1992). In addition,
when below-ground tissues respire anaerobically, carbon demand increases to meet the metabolic
requirements of plants, further decreasing carbohydrate reserves. We found that the demands on stored
carbon reserves depleted carbohydrates in rhizome tissues to the levels equivalent to that in the leaves and
roots, and consequently the plants were not capable of providing reduced carbon compounds to meet their
daily metabolic energy requirements.
In summary, in situ light reduction resulted in a rapid decrease in leaf biomass in Thalassia testudinum through
-49-
-------
defoliation and low leaf elongation rates. The drop in leaf biomass enhanced anoxia in sediments through
decreases in photosynthetic oxygen production and transport to below-ground tissues, ultimately raising the
concentration of toxic sulfides and promoting root anaerobic fermentation. Utilization and rapid depletion of
stored carbohydrate reserves in rhizome tissues, combined with low productivity and high concentrations of
sediment sulfides, resulted in plant loss at light levels equivalent to 5 and 14% SI (864 and 1628 mol m"2 yr1,
respectively). Our results also indicate that the effects of light reduction in Thalassia testudinum are reflected
in a variety of parameters which are conveniently monitored, including shoot density, blade width, leaf growth,
chl a : b ratio and chlorophyll content. These parameters are potentially valuable indicators of seagrass health
based on their sensitivity and relatively rapid response to changes in underwater irradiance, and consequently
can be important tools in the management of submerged aquatic vegetation (Neckles, 1994).
-50-
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VI. SEDIMENT SULFIDE AND PHYSIOLOGICAL RESPONSES
OF THALASSIA TESTUDINUM TO SHADING.
Paul R. Carlson, Jr.,
INTRODUCTION
Drastic declines in the distribution and abundance of estuarine seagrass and submerged aquatic
vegetation (SAV) communities have occurred in many estuaries throughout the Gulf of Mexico as the result
of concurrent declines in water quality. Benthic aquatic plants are particularly sensitive to decreased light
availability due to phytoplankton blooms, sediment resuspension, and nutrient-stimulated epiphytes.
This report describes our contribution to a collaborative research project funded by the U. S. Environmental
Protection Agency and coordinated by Dr. J. C. Zieman of the University of Virginia. The overall objective of
this project was to determine the effects of light attenuation on the survival and growth of turtle grass (Thalassia
testudinum). We focused on the physiological responses of Thalassia to light attenuation and the potential
role of sediment sulfide as a synergistic stressor which might amplify the effects of light attenuation on
Thalassia.
METHODS
Sulfide- Pore water sulfide samples were collected by two methods. From March 1993 until November 1993
pore water samples were collected monthly using suction lysimeters, or "sippers." From December 1993 until
September 1994, pore water sulfide concentrations were determined quarterly on sediment cores collected by
hand coring from each enclosure. Sulfide concentrations from sipper samples and sediment cores were
determined using a sulfide ion-specific electrode (Orion Model 95-01).
Physiological Parameters- Rhizome samples were collected quarterly from each enclosure. Rhizome
segments were transferred to scintillation vials, frozen with liquid nitrogen, and transported to FMRI on dry ice.
Prior to analysis, samples were stored at -70 deg. C. To avoid "oversampling" within enclosures, we limited
quarterly collection of belowground tissue to one core per enclosure. As a result, we analyzed four mature
rhizome segments per enclosure, instead of two mature rhizome segments and two rhizome apices as we
originally planned. In fall 1993, we began collecting Thalassia rhizomes outside enclosures at the Rabbit Key
Basin (RKB) and St. Joseph's Bay (SJB) sites to serve as outside controls.
Four analyses were performed on each rhizome segment. Alcohol dehydrogenase (ADH) analyses were
performed by the procedure of Bergmeyer (1974). ADH activity was calculated two ways: 1. raw activity
divided by tissue fresh weight and 2. activity normalized to tissue protein concentration. Protein was
determined by the Coomassie Blue procedure as modified by Appenroth an Augsten (1987). Extractable
sugars and starch were determined by a sequential extraction procedure (Zimmermann et al., 1989) which
uses 80% ethanol to extract sugars and 1N NaOH to extract starch from rhizome tissue. For purposes of
discussion, the sum of extractrable sugars and extractable starch is described below as total carbohydrate.
Statistical analyses (tests of normality, analyses of variance, and multiple range tests) were conducted using
SAS Release 6.03 (SAS Institute, 1988). To facilitate comparison among sites, separate analyses of variance
(ANOVA) were performed for fall 1993, spring 1994, and summer 1994 collections. Spring 1993 (time-zero)
-51-
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comparisons among sites were made using initial collections from each site: March 1993 at St. Joseph's Bay,
April 1993 at Rabbit Key Basin and Sunset Cove; and June 1993 at Corpus Christi Bay.
RESULTS
Spring 1993- Significant differences among sites were observed for all physiological parameters in the time-
zero samples (Table 6-2). Corpus Christi Bay samples had protein, sugar, starch, and total carbohydrate
concentrations and ADH activities which were consistently lower than those of other sites. For most
parameters, values were highest at Rabbit Key Basin and slightly lower at St. Joseph's Bay; for some
parameters the difference was statistically significant, for others it was not. Sunset Cove values for most
parameters were intermediate between those for St. Joseph's Bay and those for Corpus Christi Bay. The
pattern of protein concentrations among sites differed from the pattern for other parameters: values were
highest at St. Joseph's Bay, lower at Sunset Cove and Rabbit Key Basin, and lowest at Corpus Christi Bay.
Fall 1993- When data from all treatments were pooled for each site, significant differences among sites were
noted for rhizome ADH activity, protein, sugar, starch, and total carbohydrate concentrations (Table 6-1). ADH
activity ranged from 2.68 umol/min/gFW at St. Joseph's Bay to 1.117 umol/min/gFW at Corpus Christi Bay
(Table 6-2). Normalized ADH values showed a similar trend; values were highest at St. Joseph's Bay, slightly
lower at Rabbit Key Basin and Sunset Cove, and significantly lower at Corpus Christi Bay. Protein
concentrations were highest at St. Joseph's Bay and Corpus Christi Bay, lower at Rabbit Key Basin, and lowest
at Sunset Cove.
Total carbohydrate was much higher at Rabbit Key (71 mg/gFW) than at other sites (Table 6-2). Values at
Corpus Christi Bay and St. Joseph's Bay were intermediate, while Sunset Cove values (21 mg/gFW) were
lowest. Similar patterns were seen in extractable sugar and starch concentrations.
Significant treatment effects were seen for all physiological parameters when data from all sites sampled in fall
1993 were pooled (Table 6-1). ADH activities (raw and normalized) were significantly higher in outside control
and control treatments than in coarse or fine mesh treatments (Table 6-3). ADH activities in fine and coarse
mesh treatments were not significantly different from one another.
Total carbohydrate concentrations in outside controls (115 mg/gFW) were significantly higher than those of
control enclosures (69 mg/gFW). Total carbohydrate concentrations of both coarse and fine mesh treatments
were not significantly different from one another but were significantly lower than those of control enclosures.
Similar patterns were seen in extractable sugar and starch concentrations.
ADH activity exhibited a strong treatment effect at three of the four sites sampled in fall 1993. ADH activity of
control enclosures was significantly higher than that of coarse and fine treatment enclosures at the .Corpus
Christi Bay (CCB) and Sunset Cove (§UN) sites (Fig. 6-1). At the Rabbit Key Basin site (RKB), ADH activity
of outside controls and control enclosures was significantly higher than that of the fine mesh enclosures. While
the ADH activity of RKB coarse mesh enclosures was lower than that of controls and higher than that of fine
mesh enclosures, the differences were not statistically significant. Differences among treatments at the St.
Joseph's Bay (SJB) site were not significantly different. Patterns for normalized ADH activities were similar
to those for raw ADH activity. However, fewer significant differences among treatments were noted for
normalized ADH activities.
Although protein concentrations exhibited significant differences among treatments, the pattern is difficult to
interpret. Protein in the coarse mesh treatment is significantly lower than that of both control and outside
control treatments, but concentrations in the fine mesh treatment are not significantly different from those of
any other treatment (Table 6-3). Examining protein concentrations site by site (Fig. 6-2), values in the coarse
mesh treatment appear to be lower (but not significantly lower) than those of other treatments at each site.
Carbohydrate parameters (sugar, starch, and total carbohydrate) exhibited marked treatment effects in fall
1993. At all sites, total carbohydrate in the control treatment was higher than values in either (coarse or fine
-52-
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mesh) shaded treatment (Fig. 6-2). However, coarse and fine mesh treatments values were not significantly
different at any site. At St. Joseph's Bay and Rabbit Key Basin (the only two sites where outside control
samples were collected), total carbohydrate was significantly greater in outside controls than in control
enclosures.
Extractable sugar concentrations (Fig. 6-3) exhibited a pattern similar to total carbohydrate. For all sites except
St. Joseph's Bay, sugar concentrations in control treatments were significantly higher than values in either
coarse or fine mesh treatments. At all four sites, sugar concentrations in the two shaded treatments were not
significantly different from one another. As noted for total carbohydrates, sugar concentrations in outside
control samples at Rabbit Key Basin and St. Joseph's Bay sites were significantly higher than values in control
enclosures.
Fewer significant differences among treatments were noted for starch concentrations than for total
carbohydrate and extractable sugars. Outside control starch values were higher than control enclosure values,
and shaded enclosure values were uniformly low, but starch concentrations from control enclosures were not
significantly higher than those of the shaded enclosures for three of four sites.
Spring 1994- Protein, sugar, and total carbohydrate concentrations, as well as ADH activities (raw and
normalized), varied significantly among sites in spring 1994 (Tables 6-1, 6-2). Values for most parameters at
the Corpus Christi Bay site were significantly lower than those of St. Joseph's Bay or Rabbit Key Basin.
However, protein concentrations at both St. Joseph's Bay and Corpus Christi were significantly lower than at
Rabbit Key Basin.
Significant treatment effects were noted for ADH activities (raw and normalized), extractable sugar, and total
carbohydrate. Protein and starch concentrations, however, did not vary significantly among treatments (Table
6-3).
Differences in patterns among treatments in fall 1993 and spring 1994 result from the removal of shade screens
from the fine mesh enclosures after fall 1993 samples were collected. Six months later, levels of most
parameters in the open fine mesh enclosures were not significantly different from values in control enclosures.
However, sugar, total carbohydrate, and ADH activity were significantly lower in coarse mesh cages than in
control enclosures. Although sugar, starch, total carbohydrate, and ADH activity were higher in the outside
controls than in the control enclosures, the difference was not significant. Protein and starch concentrations
did not vary significantly among treatments at any site in spring 1994.
At the Corpus Christi site, ADH activity of the control treatment was significantly higher than that of the shaded,
coarse-mesh enclosures. At Rabbit Key Basin, ADH activity was significantly greater in control enclosures than
in coarse or fine mesh enclosures; outside control values were variable and generally lower than control
enclosures. At St. Joseph's Bay, normalized ADH concentrations of fine mesh (open) enclosures was not
significantly lower than values for outside controls. Differences between ADH values in control enclosures and
coarse mesh enclosures were not significant at St. Joseph's Bay.
Although starch concentrations did not vary significantly among treatments at any site in spring 1993, the
general pattern of sugar, starch, and total carbohydrate concentrations shows recovery of fine mesh enclosures
at Rabbit Key Basin to levels not significantly different from controls and outside controls. Although differences
in sugar, starch, and total carbohydrate concentrations among treatments at St. Joseph's Bay were not
significant, fine mesh enclosures appeared to recover more slowly at this site.
Summer 1994- Although St. Joseph's Bay and Rabbit Key Basin were sampled in fall 1994, the last sampling
period with data from all three remaining sites was summer 1994. At that time, significant differences among
sites were noted for all parameters sampled. In contrast, of all the parameters sampled, only ADH activities
exhibited significant differences among treatments at that time (Table 6-1).
Reversing the trend of fall 1993 and spring 1994, ADH activity at the Corpus Christi site was significantly higher
-53-
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than at the other two sites in summer 1994 (Table 6-2). In fact, ADH activities at Corpus Christi increased
steadily from spring (summer) 1993 to summer 1994, while activities at Rabbit Key and St. Joseph's Bay were
generally high in spring 1993 and spring 1994 and low in fall 1993 and summer 1994. Protein concentrations
were significantly higher and starch concentrations were significantly lower at St. Joseph's Bay than at the other
two sites.
Normalized ADH activity and starch concentrations exhibited similar variations among treatments. Values were
highest for outside controls and significantly lower for coarse mesh enclosures. Levels in recovering fine mesh
enclosures were not significantly lower than values in outside controls. Sugar concentrations and raw ADH
activity were lower in control enclosures than in outside controls, but only the difference in ADH activity was
significant. Protein concentrations were remarkably uniform among treatments at each site in summer 1994.
The significant difference in ADH activity among treatments apparently reflects the impact of outside controls
and a repeating pattern of insignificant differences for all three sites (Fig. 6-7). At each site, outside controls
had the highest ADH activity and coarse mesh enclosures had the lowest. ADH activities in recovering fine
mesh enclosures at Rabbit Key Basin and St. Joseph's Bay were as high as those of control enclosures.
Pore Water Sulfide Concentrations- Pore water sulfide (PWS) concentrations varied significantly among sites
for all but the last quarterly sample of this study (Table 6-4). Significant interaction of site and treatment effects
occurred for four sequential quarterly samples (fall 1993 through summer 1994), but treatment effects alone
were significant for only one quarterly sample (winter 1993). ANOVA results for site effects, in particular, reflect
the variable number of sites sampled each quarter (Table 6-4.B.).
At the beginning of the experiment, there was almost a twenty-fold difference between PWS concentrations
St. Joseph's Bay and Sunset Cove (Table 6-4). In general, the two Florida Bay sites had high PWS
concentrations, while the other two sites had low PWS concentrations. By the end of the experiment, however,
Florida Bay (RKB) PWS values were only 50% higher than PWS concentrations for St. Joseph's Bay.
When PWS concentrations for all sites were averaged in spring and summer 1993, there were no significant
differences among treatments (Table 6-4). In fall 1993, a significant treatment effect results from extremely
high PWS values in control enclosures at Rabbit Key Basin (Fig. 6-10) possibly as the result of a severe
phytoplankton bloom which occurred at that time.
Significant treatment effects occurred in winter 1993, as well as in spring and summer 1.994. For these three
quarterly samples, PWS vafues were significantly higher in coarse and fine mesh enclosures than
concentrations in control enclosures and outside controls, despite the fact that screens had been removed from
fine mesh enclosures after sampling in fall 1993. When PWS concentrations for each site are considered
separately (Fig. 6-11), significant treatment effects for spring and summer 1994 result from large differences
among treatments at Rabbit Key Basin, no significant differences at Corpus Christi Bay, and marginally
significant differences at St. Joseph Bay. Elevated PWS values for St. Joseph Bay outside controls in summer
1994 were caused by sea urchin grazing and other factors discussed below. By fall 1994, when only Rabbit
Key Basin and St. Joseph Bay sites were sampled, treatment effects were no longer significant.
In previous studies (Carlson et al. 1994), we have found that PWS concentrations in Florida Bay seagrass beds
are generally lowest in spring, increase from spring through summer, peak in fall, and decline through winter.
Rabbit Key Basin and Sunset Cove PWS values follow that trend fairly well (Table 6-4, Fig. 6-12). PWS values
at Corpus Christi Bay and St. Joseph's Bay, however, increased steadily during the course of the experiment.
A number of factors might have contributed to the steady increase in PWS concentrations at St. Joseph's Bay.
Outside control PWS values jumped between spring 1994 and summer 1994 as the result of urchin grazing
impacts. The trend continued for all treatments at St. Joseph's Bay during summer and fall, possibly as the
-54-
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result of torrential rainfall in the Florida Panhandle during this period. Salinity dropped to less than 10 ppt at
the head of St. Joseph's Bay and organic color stained surface water dark brown through much of the summer
and fall. PWS values for each site also might have been affected by removing the tops of fine mesh enclosures
in fall 1993.
-55-
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TABLE 6-1 : Analysis of Variance for All Sites. Data are F-Ratios, and significant
effects are noted by asterisks.
Physiological
Parameter
ADH Activity
(umol/min/g FW)
Normalized ADH
(umol/min/mg
protein)
Protein
(mg/g FW)
Extr. Sugar
(mg/g FW)
Extr. Starch
(mg/g FW)
Sugar-*- Starch
(mg/g FW)
Independent
Variable
Site
Treatment
Site x Tmt
Site
Treatment
Site x Tmt
Site
Treatment
Site x Tmt
Site
Treatment
Site x Tmt
Site
Treatment
Site x Tmt
Site
Treatment
Site x Tmt
Fall
1993
5.00**
11.67***
1.96
2.52
4.54**
1.37
7.71**
3.21*
0.94
19.61***
63.83***
3.75**
14.61***
15.48***
1.35
18.90***
30.25***
1.61
* P< 0.05
Spring
1994
3.72*
6.43***
1.73
5.55**
6.91***
3.21*
8.88**
1.13
2.40
12.14***
5.14**
0.83
1.92
2.52
0.76
5.29**
3.90*
0.84
** P< 0.01
Summer
1994
2.98
3.27*
1.02
5.19**
3.75*
0.67
33.27***
0.98
1.06
0.97
2.35
1.19
5.10**
1.02
0.97
3.06
1.56
1.09
***P< 0.001
-56-
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TABLE 6-2: Comparison of Physiological Parameters Among Sites. Data are mean
values for each parameter. Values with the same letter subscript are not significantly
different.
Physiological
Parameter - .
ADH Activity
(umol/min/g FW)
Normalized ADH
(umol/min/mg
protein)
Protein
(mg/g FW)
Extr. Sugar
(mg/g FW)
Extr. Starch
(mg/g FW)
Sugar+Starch
(mg/g FW)
Site
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Corpus Christi
Rabbit Key
St. Joseph
Sunset Cove
Spring
1993
0.69 c
3.46 a
3.44 a
1.72b
0.66 b
2.61 a
2.06 a
1.11 b
1.16c
1.40 be
1.75 a
1.57ab
9.94 c
32.47 a
24.70 b
12.27 c
22.09 c
51. 40 a
35.94 b
42.1 Sab
32.03 c
83.86 a
60.63 b
54.40 b
Fall
1993
1.17c
2.03 ab
2.68 a
1.45 be
0.78 b
1.81. a
1.91 a
1.59ab
1.47ab
1.28b
1.58 a
1.02c
12.47 b
22.03 a
14.77 b
7.55 c
29.22 b
49.01 a
31.70b
20.89 b
41. 70 be
71. 04 a
45.74 b
28.45 c
Spring
1994
1.89b
3.43 a
3.35 a
1.32b
2.00 ab
2.59 a
1.25b
1.80 a
1.36b
7.68 b
25.83 a
19.05 a
25.66 a
30.05 a
32.85 a
33.34 b
64.87 a
51. 90 a
Summer
1994
3.54 a
2.03 b
1.91 b
2.95 a
2.35 a
0.98 b
1.12b
0.88 b
2.20 a
28.51 a
23.58 a
20.06 a
39.86 a .
45.09 a
16.51 b
68.37 a
68.67 a
36.57 a
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TABLE 6-3: Comparison of Physiological Parameters Among Treatments. Data are mean
values for each parameter averaged for all sites. Values with the same letter subscript are
not significantly different. Sampling at Sunset Cove site ended fall 1993. Tops of fine
mesh cages removed after fall 1993 sampling.
Physiological .
Parameter
ADH Activity
(umol/min/g FW)
Normalized ADH
(umol/min/mg
protein)
Protein
(mg/g FW)
Extr. Sugar
(mg/g FW)
Extr. Starch
(mg/g FW)
Sugar+Starch
(mg/g FW)
Site/
Treatment
Outside Control
Control
Coarse Mesh
Fine Mesh
Outside Control
Control
Coarse Mesh
Fine Mesh
Outside Control
Control
Coarse Mesh
Fine Mesh
Outside Control
Control
Coarse Mesh
Fine Mesh
Outside Control
Control
Coarse Mesh
Fine Mesh
Outside Control
Control
Coarse Mesh
Fine Mesh
Fall
1993
3.31 a
2.89 a
1.42b
1.18b
2.30 a
2.08 a
1.78ab
0.94 b
1.46a
1.47 a
1.11 b
1.33ab
43.11 a
23.76 b
10.38 c
7.96 c
70.51 a
44.73 b
33.81 be
24.68 c
114.723
68.48 b
44.21 c
32.64 c
Spring
1994
3.89 a
3.88 a
1.76b
2.96 a
2.98 a
2.42 ab
1.12c
1.98b
1.39 a
1.59 a
1.47 a
1.63 a
26.93 a
22.24 ab
12.15C
18.21 be
41. 06 a
39.44 a
26.61 a
27.83 a
67.99 a
61.68ab
38.76 c
46.03 be
Summer
1994
3.87 a
2.13 b
1.32b
2.36 ab
3.37 a
1.96ab
0.98 b
2.00 ab
1.46 a
1.23 a
1.50 a
1.32 a
31.17a
22.70 a
15.86 a
30.10 a
37.38 a
35.20 a
24.66 b
39.17 a
68.55 a
57.90 a
40.53 a
69.27 a
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TABLE 6-4: Sediment Pore water Sulfide Concentrations. Data are mean values for each
parameter. Values with the same letter subscript are not significantly different.
Site/
Treatment
Spring
1993
Summer
1993
Fall
1993
A. Analysis of Variance- Data are F-Ratios
Site
Treatment
Site x Tmt
70.4"*
1.97
2.14
B. Comparisons amonj
Corpus
Christi
Rabbit Key
St.Joseph's
Bay
Sunset
Cove
125.7C
450.7b
31. 7c
616.2a
129.2"*
0.67
1.69
112.0*"
2.35
4.94"
Winter
1993
Spring
1994
86.96"*
7.05"
7.22**
8.57*"
2.55
2.86*
Summer
1994
6.21"
1.66
2.75*
Fall
1994
0.61
1.45
0.53
Sites- Data are Mean Pore water Sulfide Concentrations (uM)
112.9b
723.8 a
107.5 b
800.5 a
104.0C
1276.0 a
57.0 c
847.0 b
NS
762.8 a
105.7 b
NS
173.7 b
297.2 a
107.3 b
NS
225.4 b
421.0 a
260.5 b
NS
NS
464.0 a
364.0 b
NS
C. Comparison among Treatments- Data are Mean Pore water Sulfide Concentrations (uM)
Outside
Control
Coarse
Mesh
Fine Mesh
NS
214.8a
261. 9a
272.9a
NS
370.0 a
418.6 a
388.5 a
NS
701 .6 a
478.1 b
476.2 b
126.2 b
240.6 b
608.2 a
549.7 a
133.8 b
145.6 b
227.4ab
289.9 a
282. 1ab
210.7 b
238 8ab
355.8 a
314.0 a
249.7 a
552.0 a
540.9 a
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FALL 1993
CCB
RKB SJB
SITE
SUN
FALL 1993
CCB
RKB SJB
SITE
SUN
Fig. 6-1 Rhizome AOH activity for all sites, Fall 1993. Upper graph shows raw ADH activity. Lower
graph shows ADH normalized to protein concentration. Site names are abbreviated on the horizontal
graph axis CCB = Corpus Christt Bay; RKB = Rabbit Key Basin; SJB = St. Joseph's Bay, SUN = Sunset
Cove. Treatments are abbreviated along the right side of the graph: OUT = outside controls; CON =
control enclosures; CRS = enclosures with coarse mesh; FIN = fine mesh enclosures. Letters within or
adjacent to vertical bars indicate results of multiple range tests of means; values with same letters are .?ot
significantly (P<0 05) different.
-60-
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FALL 1993
CCB
RKB SJB
SITE
SUN
r
CJ
cc
<
h
CO
»
cr
<
a
D
CO
120
FALL 1993
CCB
RKB SJB
SITE
SUN
Fig. 6-2. Rhizome protein and total carbohydrate concentration, Fall 1993. Letters within or adjacent
to vertical bars indicate results of multiple range tests of means; values with same letters are not
significantly (P<0 05) different. Abbreviations for sites and treatments are as described for Fig 6-1
-61-
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FALL 1993
50
CC8
RKB SJB
SITE
SUN
FALL 1993
£
u.
05
i
HI
CCB
RKB
SJB
SUN
SITE
Fig. -3. Rhizome sugar and starch concentration, Fall 1993. Letters within or adjacent to vertical bars
indicate results of multiple range tests of means; values with same letters are not significantly (P<0 05)
different. Abbreviations for sites and treatments are as described for Fig. 6-1.
-62-
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SPRING 1994
CCB
RK8
SJB
SPRING 1994
CCB
RKB
SJB
Fig. 6-4. Rhizome AOH and normalized ADH activity for all sites, Fall 1993. Letters within or adjacent
to vertical bars indicate results of multiple range tests of means; values with same letters are not
significantly (P<0.05) different. Abbreviations for sites and treatments are as described for Fig. 5-1
-63-
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SPRING 1994
CCB
RKB
SJB
SPRING 1994
OJ
x
o
cc
2
co
+
QC
<
O
CO
100Y
80-
60-
CCB
RKB
SJB
Fig. 6-5. Rhizome protein and total carbohydrate concentration, Spring 1994. Letters within or
adjacent to vertical bars indicate results of multiple range tests of means; values with same letters are
not significantly (P<0 05) different. Abbreviations for sites and treatments are as described for Fig. 6-1
-64-
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SPRING 1994
0>
o
CO
40i
CC8
RKB
SJB
SPRING 1994
CC8
RKB
SJB
Fig. 6-6. Rhizome sugar and starch concentrations, Spring 1994. Letters within or adjacent to vertical
bars indicate results of multiple range tests of means; values with same letters are not significantly
(P<0.05) different. Abbreviations for sites and treatments are as described for Fig. 6-1.
-65-
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SUMMER 1994
CCB
RKB
SJB
SUMMER 1994
CCB
RKB
SJB
Fig. 6-7. Rhizome ADH and normalized ADH activity for all sites, Summer 1994. Letters within or
adjacent to vertical bars indicate results of multiple range tests of means; values with same letters are not
significantly (£<0.05) different. Abbreviations for sites and treatments are as described for Fig. 6-1
-66-
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SUMMER 1994
CCB
RKB
SJB
SUMMER 1994
CCB
RKB
SJB
Fig. 6-8. Rhizome protein and total carbohydrate concentration, Summer 1994. Letters within or
adjacent to vertical bars indicate results of multiple range tests of means; values with same letters are not
significantly (P<0.05) different. Abbreviations for sites and treatments are as described for Fig. 6-1
-67-
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SUMMER 1994
40i
E,
tr
o
CO
±
CCB
RKB
SJB
SUMMER 1994
CT
X
(J
cc
en
a:
6
LLl
80t
CCB
RKB
SJB
Fig. 6-9. Rhizome sugar and starch concentratipn, Summer 1994. Letters within or adjacent to vertical
bars indicate results of multiple range tests of means; values with same letters are not significantly
(P<0.05) different. Abbreviations for sites and treatments are as described for Fig. 6-1.
-68-
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g 200
Q.
SUMMER 1993
CCB
RKB
SJB
SUN
FALL 1993
CCB
RKB
SJB
SUN
Fig. 6-10. Comparison of pore water sulfide (PWS) concentrations among sites and treatments for
Summer and Fall 1993. Letters within or adjacent to vertical bars indicate results of multiple range tests
of means; values with same letters are not significantly (P<0.05) different. Abbreviations for sites and
treatments are as described for Fig. 6-1.
-69-
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500
SPRING 1994
CCB
RKB
SJB
600-r
SUMMER 1994
CCB
RKB
SJB
Fig. 6-11. Comparison of pore water sulfide (PWS) concentrations among sites and treatments for
Spring and Summer 1994. Letters within or adjacent to vertical bars indicate results of multiple range
tests of means; values with same letters are not significantly (£<0.05) different. Abbreviations for sites
and treatments are as described for Fig. 6-1.
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ST JOSEPH'S BAY
LU
Q
CO
-------
VII. CHANGES IN PHOTOSYNTHESIS VERSUS IRRADIANCE CHARACTERISTICS OF
THALASSIA TESTUDINUM
IN RESPONSE TO SHORT-TERM LIGHT REDUCTION
Michael J. Durako and James W. Fourqurean
MESOCOSM EXPERIMENTS
The effects of short-term light reduction on the photosynthetic capacity of Thalassia growing in the FMRI
mesocosms was assessed during July 1993 by comparing the P/l characteristics of individual 1-cm-long leaf
segments from short-shoots growing in duplicate mesocosms with ambient light, 10% reduction from ambient,
and 20% reduction from ambient (six mesocosms total). Four 1-gallon pots containing 4-8 short-shoots with
natural sediment were used for P/l sampling (eight pots/light treatment). At each sample interval, the
youngest, fully-developed leaf from a short-shoot in each of three randomly chosen treatment pots was
harvested. Samples were harvested 3 days before shades were installed on the mesocosms (pretreatment),
and then after 3, 6, 12, and 110 days of light reduction. Three, 1 cm long leaf segments were cut from the
mid-section of each sampled leaf while submerged in a petri dish filled with ambient seawater from the
mesocosms. One segment was used for the P/l measurements, the second and third segments were rinsed
3 times in Dl water and placed in separate, numbered wells of multiwell plates and frozen for subsequent dry
weight and chlorophyll determinations.
Leaves to be used in the P/l measurements were placed in individual wells of a 6-well multiwell culture plate
containing 5 ml filtered (0.45 urn) seawater from the mesocosms. Leaf segments were removed from the
multiwells in random order and placed in 2 ml of N 2-sparged filtered seawater in a well-stirred, temperature-
controlled glass reaction chamber of a Hansatec DW/1 oxygen-electrode system. The Clark-type
polarographic oxygen electrode was calibrated using N 2(gas) and air-saturated filtered seawater.
Photosynthesis versus irradiance (P/l) relationships were determined for each randomly chosen leaf segment
at ambient temperatures using neutral-density filters and a Kodak projector lamp. Rates of photosynthesis
and respiration were measured as the change in dissolved oxygen concentration over a standardized
measurement interval (2 min) in the closed (no gaseous head space) reaction chamber. For each P/l run,
leaf tissues were first incubated in the dark (10-min equilibration, 2-min respiration measurement interval).
The tissues were then subjected to 12 light levels (=10 to =700 uE m ~2s~1 photosynthetically active radiation
[PAR = 400 to 700 nm] as measured by a Li-Cor2rr quantum sensor) in increasing order (1-min equilibration,
2-min photosynthetic measurement interval). At the end of each P/l run, leaf tissues were removed from the
chamber, rinsed three times in deionized water, placed in a well of a multiwell plate and frozen for subsequent
chlorophyll determinations. A complete P'/l treatment series (three replicate/treatment x three treatments =
nine runs) was run at each sample interval.
Chlorophyll was extracted by grinding leaf segments frozen by liquid N 2 in a liquid N2-chilled mortar in 90%
spectrophotometric grade acetone. Chlorophyll a_ content was then measured spectrophotometrically
according to Sternan (1988). Dry weight was determined by drying leaf segments for 48 h at 60 °C.
Photosynthetic and respiratory rates are expressed as pmole O 2 mg"1 chl a h"1.
Respiration rates and the P/l characteristics a and P max were determined for each P/l run (Fourqurean and
Zieman, 1991). The initial slope (a) of the P/l curve, which indicates photosynthetic efficiency, was calculated
by linear regression of the dark respiration rate plus the photosynthetic rates at the first four light levels.
Light-saturated photosynthetic rate (P max) was calculated by averaging the net photosynthetic rates at light
levels >300 uE rrrV PAR.
Figures 7-1 and 7-2 summarize the P/l characteristics over the 110 day experimental period. Little in the way
of treatment-related trends are evident until the +12 day post-treatment sampling. At this time, all six P/l
characteristics exhibited a stepwise decrease with decreasing light. After 110 days, all of the P/l
characteristics in the shaded treatments were lower than at 12 days. P max and lk were also lower in the
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characteristics exhibited a stepwise decrease with decreasing light. After 110 days, all of the P/l characteristics
in the shaded treatments were lower than at 12 days. Pmax and lk were also lower in the controls, suggesting
some overall stress of the plants in the mesocosms. These trends suggest that the plants may be shutting
down metabolic and photosynthetic processes in response to the reduction in light. Figure 7-3, which shows
the actual P/l data points after 12 days of light reduction, also illustrates this trend. The response curves in
figure 7-3 were generated by fitting the data points to the hyperbolic tangent model of Jassby and Platt (1976),
modified to include respiration. This figure illustrates that the segments did not reach light saturation at the
intensities used here, and hence, Pmax was underestimated. The stepwise decrease in a, Pmax, and respiration
with decreasing light, after 12 days, is partially due to the stepwise increase in chlorophyll a concentration in
the leaves (Fig. 7-4). However, the same trends were observed when the P/l characteristics were calculated
on a dry weight basis so the changes are more than just pigment-based.
FIELD EXPERIMENTS: Sunset Cove
The effects of short-term light reduction on the photosynthetic capacity of Thalassia growing in situ was
assessed during September 1993 by comparing the P/l characteristics of individual 1-cm-long leaf segments
from short-shoots growing in 1-m2 treatment plots in Sunset Cove, Key Largo (25°05.34N, 80°27.057W).
Triplicate plots with ambient light, approximately 10% reduction from ambient (coarse screen), and
approximately 20% reduction from ambient (fine screen) were established in a random 3x3 plot design. Shade
screens were attached to PVC frames with legs and were placed at a level just above the leaf canopy. At each
sample interval, the youngest, fully-developed leaf from one randomly chosen short-shoot from within each
of the three treatment plots was harvested. This was accomplished by cutting the leaf blade then placing it in
a prelabeled ziploc bag. Time 0 samples were harvested the day the shades were installed (9/16/93), samples
were then harvested as described above 3, 6,12, and 25 days after the establishment of the treatment plots.
Procedures for the P/l measurements were the same at those for the mesocosm experiments. All material was
collected the morning of the P/l runs.
Figures 7-5,7-6, and 7-7 summarize the P/l characteristics and leaf chlorophyll levels over the 25 day
experimental period. Unlike the mesocosm experiments, there are no treatment-related trends evident in any
of the measured characteristics from Sunset Cove Thalassia.
This lack of treatment-induced trends may be due to the heterogeneous nature of the experimental site. Sunset
Cove has experienced significant seagrass die-off so our site selection was limited to an apparently healthy
patch-bed adjacent to the long term shading plots. In addition, September-October is a period of relatively high
and constant water temperature, but decreasing day length. This is a time of year that may already be stressful
to seagrasses possibly masking any treatment effects. This may explain the overall increasing trend in
respiration rates over the 25-day experimental period.
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AmbUnt
10»
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I E
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0.10
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0.00
80
60
40
20
0
-2
-4
-8
-8
-10
-3 +3 +8 +12 +110
Period of light treatment (days)
Fig. 7-1. Chlorophyll a-based photosynthesis versus irradiance characteristics (respiration, Pmax, and a) of
Thalassia testudinum in response to short-term light reduction in FMRI mesocosms.
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Ambient
10*
20*
o
k.
o»
u
I
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E
o*
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900
400
300
200
100
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33
30
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20
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-I
-3
I
II.
i\
Period of light treatment (days)
Fig. 7-2. Chlorophyll a-based photosynthesis versus irradiance characteristics (lc, lk, and PmM gross) of
Thalassia testudinum in response to short-term light reduction in FMRI mesocosms.
-75-
-------
-J
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100.00 r
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20% shade
200
400
PAR (/iE m~ s )
600
800
-------
Ambient
10*
20%
8
01
E
o
6
4-110
Period of light treatment (days)
Fig. 7-4. Changes in chlorophyll a concentration of leaves of Thalassia testudinum in response to short- and
longer-term light reduction in FMRI mesocosms.
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Ambient
10X
20X
o
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0.7
0.6
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160
140
120
100
80
60
40
20
0
-3
-10
-15
-20
-25
-30
I
0 +3 +6 -1-12 +25
Period of light treatment (days)
Fig. 7-5. Chlorophyll a-based photosynthesis versus irradiance characteristics (respiration, Pmax, and a) of
Thalassia testudinum in response to short-term light reduction in Sunset Cove, Key Largo.
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Ambltnt SSS9 10*
20*
n
o
o
£
O
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150
125
100
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50
25
0
400
300
200
100
50
6 30
ui
^^
20
10
+ 3
+ 6
12
+ 25
Period of light treatment (days)
Fig. 7-6. Chlorophyll a-based photosynthesis versus irradiance characteristics (lc, lk, and P,,
Thalassia testudinum in response to short-term light reduction in Sunset Cove, Key Largo.
, gross) of
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VIII. SUMMARY
The carbon balance of seagrasses is more complex than that of phytoplankton or macroalgae due to the
structural complexity of seagrasses (Fourqurean & Zieman, 1991). Root/shoot ratios are very important to
seagrass carbon budgets because below-ground non-photosynthetic tissues must be supported by
photosynthetic carbon production in the leaves. The below-ground portion of Thalassia testudinum (turtle
grass) can account for 50 to over 90% of the total biomass ( Zieman, 1982; Powell et al, 1989; Fourqurean
& Zieman, 1991). Below-ground tissue is generally a photosynthate reservoir that supports growth .and
maintenance of other tissues during periods of low photosynthetic production (Dawes & Lawrence, 1980; Pirc,
1985) in addition to its role in anchoring the plant and the sediments and for nutrient uptake to the seagrass.
The photosynthate produced in leaf tissues and transported to subterranean tissues is critical in processes
involving new shoot growth, carbohydrate storage and respiration (Ralph et al., 1992). The strong seasonal
variation in stored carbon reserves in rhizome tissues are largely a function of photosynthate production in
summer and the utilization of these reserves for growth and respiration in winter and early spring (Dawes &
Lawrence, 1980; Pirc, 1985). Below-ground carbohydrate reserves are also critical for survival and regrowth
during extended periods of light reduction and following artificial defoliation (Dawes & Lawrence, 1979; Drew,
1983; Dawes & Guiry, 1992). Since plant use of stored carbon reserves in below-ground tissues is most often
related to unfavorable light conditions, decreased rhizome carbon reserves could be a reliable indicator of
impending decline in seagrass distribution and biomass.
Determining potential effects of reduced light at response levels that are relatively easily measured, such as
shifts in resource allocation, plant biomass and density, or effects on the associated faunal community
structure, will allow us to assess changes and perhaps avoid a serious decline in seagrass populations in other
locations. The primary experiments in this study utilized experimental shade treatments as an analogue to
nonwavelength-specific water-quality related reduction of light to Thalassia testudinum meadows. Shading
decreases the total amount of photosynthetically active radiation (PAR), and consequently the rates of
photosynthetic carbon fixation of seagrasses. Shading may also influence seagrass carbon budgets by
inducing morphological changes in the structure of the plants, or by inducing changes in the physiological
response of the photosynthetic apparatus of the plant. It is not presently known if chronic light reduction affects
seagrass respiration or allocation of plant resources into leaves versus non-photosynthetic structures.
Reduction in light may have short-term and long-term effects on seagrass communities, and photoadaptation
may occur at both morphological and physiological levels. Plants adapted to low-light environments tend to
have greater proportions of their biomass allocated to green leaves, the photosynthetic portions of the plants,
at the expense of non-photosynthetic structures like roots and rhizomes. Thalassia from naturally occurring
populations can exhibit a great deal of variation in the relative amount of biomass allocated to leaves
(Fourqurean and Zieman 1991). Thalassia growing at the deep, light-limited edges of seagrass beds can exhibit
higher leaf area indices and above-to-belowground ratios than nearby plants from higher light environments
(Dawes and Tomasko 1988). Thalassia plants from dense meadows have a higher proportion of their biomass
allocated to leaves than plants from sparse meadows as a consequence of low light from self-shading in the
dense meadows (Fourqurean et al, 199 ).
Other typical responses of plant to shading are physiological. The photosynthesis versus irradiance (P/l)
response of submerged macrophytes can change in response to changes in light availability. This physiological
photoadaptation can occur quickly (in a matter of days) after the change in light availability (Goldsborough and
Kemp 1988). Typically, the initial slope of the P/l curve, increases in response to shading, while the
asymptotically approached maximum photosynthetic rate, Pmax, decreases., but it is unclear whether this
physiological photoadaptation can occur rapidly enough in Thalassia to compensate for new reductions in
incident light to established meadows.
Throughout the course of this project, each of the major field sites experienced one or more severe stresses,
several of which are clearly chronic. Both Port Aransas and Florida Bay were heavily affected by storms in the
winter and spring of 1993. Florida Bay and much of the surrounding waters were effected by massive turbidity
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plumes and algal blooms resulting from the combination of seagrass dieoff and the frequent and intense
storms. The 'Storm of the Century' which occurred around 12 March 1993, just prior to the initiation of this
project, had sustained winds of >60 mph that lasted, in some areas, up to 36 hrs, and gusts to 95-107 mph.
Much of the damage and turbidity was because these high winds lasted much longer than winds from a
hurricane do, and the direction from which they came (the southwest). The turbidity had not yet fully settled,
when two weeks later another intense storm hit. In Florida Bay, seagrass die-off, which was likely the result
of a suite of environmental conditions, initially caused the direct loss of over 4,000 Ha of seagrass, largely
Thalassia (Robblee et al, 1991). While the initial dieoff has subsided, secondary algal blooms and turbidity
plumes are blanketing hundreds of square kilometers, and causing general seagrass losses over wide areas.
In Texas, protracted brown tides have for many years now periodically covered seagrass sites. St. Joseph Bay
experienced stress in the form of heavy grazing pressure by sea urchins and a major rain event that dropped
salinities to 10 ppt and the resulting influx from the watershed produced increased color in the water for a
protracted time. These facts show the level of stress to which the communities in the coastal zone are being
subjected around the entire periphery of the Gulf of Mexico.
Table 8-1. Comparison of Thalassia testudinum parameters at all sites for the summer of 1993.
South Florida uses Rabbit Key Basin data.
Turnover Rate
Areal Productivity
Leaf Production/shoot
Standing Crop
Biomass
Above/Below Ratio
Shoot Density
Blade Length
Blade Width
LAI
Latitude
%/d
g/m2/d
mg/ss/d
g/m2
g/m2
ss/m2
gm
mm
m2/m2
Rabbit Key Basin
South Florida
25.0 N.
1.6-1.7
2.5
2.3-2.8
146-163
370-417
0.1
880-1080
17.0
8.0
1.2-1.4
Texas
27.5 N.
2.4-2.8
2.4-3.3
4.2-4.4
101-123
560-917
0.1-0.15
506-785
15-21
7.0
2.4-2.8
St. Joe Bay
Florida
29.3 N.
1.8-3.0
2-3
3.4-6.5
100-111
450-790
.16-.28
490-590
24.0
11.0
.9-1.2
The accompanying tables and figure summarize the findings across all of the field sites. Table 8-1 shows the
variation in the parameters of the seagrasses at the three research sites. The numbers represent the
conditions from the initial sample through the first summer. While many are given in ranges, they still show
some significant differences in the structure at the different sites. Turnover rate had the least variation in south
Florida, probably due to its lower seasonal climatic variation, but south Florida had the lowest turnover rate
which is unusual as turnover usually increases towards the equator. Area! productivity was quite consistent
among the sites. The south Florida site also had the least production per shoot, but areal production remained
high as shoot numbers in south Florida were very high. The density of shoots in south Florida was significantly
higher than the other sites, and was twice the density at the northern Florida site. Standing Crop was highest
in south Florida by 30-50%, and Texas and north Florida were nearly identical. Although standing crop was
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highest in south Florida, total biomass was the lowest, with both north Florida and Texas having 50-100%
higher total biomass
As a result, the above/below ground ratios were lowest in south Florida with only 10% above ground material.
Texas ranged from 10-15% and the north Florida site was 16-28% above ground biomass. Short shoot
densities were typically high in south Florida, and were typically about twice the shoot density in north Florida.
The densities in Texas were intermediate. The Thalassia blades at the north Florida site were very robust.
Blade lengths in north Florida were 40% longer than in south Florida and Texas, although Texas showed
much more variation. Blade widths in south Florida and Texas were relatively narrow and were nearly 50%
greater at the north Florida site. Although the individual leaves were robust in north Florida, they were
relatively less abundant than elsewhere, and the LAI for the two Florida sites was just over 1 while the Texas
site had LAI's of 2.4 to 2.8.
Table 8-2 shows the very wide variation in light fields found across the region. Fundamentally it shows the
propensity for fouling at each of the three localities. In all cases the mesh, mesh size, and cages were
identical. The St. Joseph's Bay Florida site was the northern most site, and had the least light reduction effect
from the cages . The resulting light levels there were near design levels, and that is reflected in the plant
response.
Table 8-2. Incoming Photosynthetically Active Radiation (PAR) delivered to the top of the canopy of the two
light reduction treatments as a percentage of the light received at the top of the canopy of the control plots
at the three sites.
Rabbit Key Basin Texas St. Joe Bay
South Florida Florida
Latitude 25.0 N. 27.5 N. 29.3 N.
Coarse Mesh (3/4 in) 43% . 14% 60-70%
(range 30-49%)
Fine Mesh (1/4 in) 21% 5% 30-40%
Both the south Florida and Texas experienced much higher than anticipated levels of fouling. This was so
severe at the Texas site that the decrease in light reaching the canopy was about 5 times greater than the
decrease in north Florida and over 3 times greater than the south Florida site.
Despite the wide differences in the basic makeup of the seagrass meadows tested, and the generally greater
than designed light attenuation, some very consistent patterns emerged from the sites. This response is
summarized in figure 8-1. This figure shows the basic patterns of response to light attenuation seen at the
three sites. The upper lines from the three site depictions are the control plots. In south Florida and St.
Joseph's Bay, there was a slight upward trend in the principal parameters. In general this is within the normal
variation found on a site. The Texas site was different. There was a very pronounced downward trend in the
data, even at the control plots. This was possibly due to the effects of the chronic brown tides that have
plagued that area for several years.
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FB STJ TX
£ .?.-.
"
1 - control 2 - coarse 3 - fine
Fig. 8-1. Trends in seagrass responses at the three sites, Florida Bay (FB), St Joseph's Bay (STJ), and
Corpus Christ! Bay (TX).
At the coarse mesh treatments, the south Florida site and the Texas site, with their greater light attenuation
due to fouling showed pronounced downward trends, as would be expected under those circumstances. By
contrast, the north Florida site showed very little response to the decreased light until the second year, at
which time the plant parameters began to decline with continued light reduction. For the fine mesh treatments
there was also a lag period at the north Florida site, but declining plant performance was detected in the first
year. In these treatments, with their high light reduction, the plants declined relatively rapidly. When the fine
mesh cages were removed at the end of the first summer, the seagrasses at the Florida sites began a general
rebound, but the Texas site was completely destroyed by that time.
These results show that there is no good level of light reduction. With 30% of less attenuation from
background the St. Joseph's Bay stations showed a lag before beginning declines, indicating a use of stored
reserves to weather a short-term stress. At the stations that suffered severe light reduction, but where
seagrasses still survived, there was recovery after the removal of the light reduction stress.
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