Seagrass Ecology

1996 Abstracts

Examining the Correlation Between the Presence of the Slime Mold Labyrinthula sp. and the Loss of Thalassia testudinum in Florida Bay

Jan H. Landsberg , Barbara A. Blakesley , Angela Baker , Gil McRae , Mike Durako , Jeff Hall , Ruth Reese , and Justin Styer, Florida Marine Research Institute, Florida Department of Environmental Protection, St. Petersburg, FL.

One of the primary factors considered in seagrass die-off has been that reduced freshwater inflow due to drought, diversion of upland runoff, and reduced rainfall over the bay changed Florida Bay from an estuary to a hypersaline marine lagoon. Lack of direct hurricane impact over the past two decades has reduced the frequency of periods of low salinity and allowed an increased accumulation of sediment on banks and in many basins (Zieman et al. 1989). These environmental changes are thought to have allowed Thalassia to increase to very high densities. The apparent density-dependent die-off was initially restricted to basins in the west such as Rankin, Johnson, and Rabbit that had dense Thalassia populations (Robblee et al. 1991). Low-density or chronically-stressed Thalassia populations on banktops in lower salinity basins in east, central, or south Florida Bay were unaffected (Durako & Kuss, 1994). It has been postulated that various environmental stressors or the seagrass pathogen, Labyrinthula (Porter & Muehlstein, 1989; Robblee et al. 1991) were involved in seagrass mortality in Florida Bay, but no field documentation was available on Labyrinthula distribution, speciation, or pathogenicity.

Although Porter & Muehlstein (1989) suggested that Labyrinthula could have a major role in seagrass mortality in Florida Bay, this causality has not been rigorously investigated. The slime mold Labyrinthula has generally been considered to be a secondary pathogen that invades stressed, weakened Thalassia. As Labyrinthula infection progresses, necrotic lesions spread to cover the Thalassia leaf tissue. The loss of leaf cover and limitation on photosynthesis in individual shoots may ultimately lead to seagrass mortality, particularly in turbid areas or in reduced light. Labyrinthula impairs photosynthesis and it has been postulated that the associated decrease of oxygen available to rhizomes leads to hypoxia (Durako & Kuss, 1994). High sediment sulfide concentrations are toxic to seagrass in certain geographical locations (Carlson et al. 1994). In combination with such a situation, Labyrinthula can be a primary factor in seagrass die-off. Accordingly, it would not be anticipated that Labyrinthula would play a major role in seagrass disease and mortality in high-light, low-turbidity, and well-flushed Thalassia beds. When Thalassia is stressed by low light, low dissolved oxygen, high sulfide sediments, phytoplankton blooms, and turbid or stagnant waters, this seagrass may be highly susceptible to disease and certain Labyrinthula species may be extremely virulent.

We wished to determine what role Labyrinthula plays in the Florida Bay seagrass mortality. Our research program is designed to answer the following questions:

Answering these questions is critical to clarifying the actual impact of Labyrinthula on seagrass in the Florida Bay system.

In April 1995, in conjunction with the FHAP program (Durako et al. this conference), we began to investigate the temporal and spatial distribution of Labyrinthula in Florida Bay Thalassia communities, and to determine its role in disease and associated mortality of seagrass. Twice yearly, in spring and fall, Thalassia shoots from sites in 10 basins are screened for lesion and labyrinthulid presence. These basins are representative areas of the bay system. They differ in their sediment profiles and characteristics, seagrass density and abundance, and are individually influenced by varying environmental factors. From west to east and south to north, these are Johnson, Rabbit, Rankin, Twin, Whipray, Madeira, Calusa, Crane, Eagle, and Blackwater. Health criteria of Thalassia sampled in 1995-1996 included: 1) the occurrence of and relative blade area covered by lesions, 2) the relative distribution and prevalence of Labyrinthula infection (presence/absence on individual blades), and 3) the association among the occurrence of lesions, prevalence of Labyrinthula infection, salinity, water clarity, and temperature.

Labyrinthulids are also isolated from Thalassia lesions and grown on agar plates and in liquid media. Isolated labyrinthulids are being speciated using growth characteristics at different temperatures, salinities, and light regimes; cell sizes; host specificity; and both scanning and transmission electron microscopy. Thalassia shoots grown axenically from seed are currently being used for transmission experiments of labyrinthulid isolates to determine pathogenicity, environmental requirements, and other factors required to induce mortality in seagrass. Once pathogenicity has been established, mechanisms for invasion will be explored experimentally in the laboratory and in the field.

To determine the distribution and relative frequency of occurrence of Labyrinthula, we examined leaf blades from over 8500 Thalassia shoots collected from 872 sites in spring and fall of 1995 and spring of 1996. Fall 1996 sampling has begun. In 1995, the prevalence of Labyrinthula infection in individual basins generally varied from high in the west, north-west, and south-central to low in the north-central, central, and east. In 1995, from spring to fall, there appeared to be a general increase in the prevalence of Labyrinthula in Rankin and Johnson through Rabbit to Twin; this has been further extended in spring 1996 to Crane. In 1996 in Crane, the prevalence of infected sites rose to 68.0% from 9.0% in 1995. In 1995, the prevalence of Labyrinthula in shoots was generally low, ranging from 0 to 13%. Data from spring 1996 indicate a significantly higher prevalence of shoot infection than in the previous spring (Wilcoxon Signed-Rank Test, P<0.0001). In spring 1996, basins in the west, north-west, central, and south (Johnson, Rabbit, Rankin, Twin, and Crane) had higher prevalences of shoot infection (5.7 - 30.4%) than basins in the north-central and eastern areas (Whipray, Calusa, Madeira, Eagle and Blackwater) which had low prevalences of infection (0.0 - 1.71%). Increased prevalences of shoot infection were observed in some basins, particularly within Johnson (30.4%), Rankin (18.5%), and Crane (17.6%). These three basins all experienced more than a 17% increase in Labyrinthula prevalence between spring 1995 and 1996. The frequency of lesions and the relative blade area with lesions were quantified for about 30,000 Thalassia blades in 1995-1996. The highest prevalence of blade lesions in spring 1995 was observed at the two western-most basins, Johnson and Rabbit, where the prevalence exceeded 30%. Significant increases of more than 21.0% prevalence of blade lesions from spring to fall 1995 were observed at Rankin and Twin, while the only significant decrease occurred at Johnson (Wilcoxon Signed-Rank Test, P<0.0001). By spring 1996, a blade lesion prevalence of 17.1% was observed at Crane where the lesion prevalence had increased by 13% from spring 1995. The prevalence of lesions at Johnson rose dramatically by 32.0% from the previous fall. Labyrinthula and the presence of lesions had apparently spread from the west/north-west to the south.

In individual basins, there appear to be differences between prevalence of infection levels in relation to site salinity. Other integrating environmental factors are likely to be important, but in general it appears that salinity is a critical controlling factor in Labyrinthula infections. Laboratory studies are in progress to test this hypothesis. Salinities in the fall of 1995 were lower at all basins compared to the spring of 1995. By spring 1996, salinities at all sites had increased to higher levels than those of the previous spring. The basins nearer the land-sea interface on the north-central and east side (Whipray, Madeira, Calusa, Eagle, and Blackwater) were at lower salinities (mean range = 11.9 - 21.3 ppt) in the fall of 1995 than the more westerly and southern basins (Johnson, Rankin, Rabbit, Twin, and Crane) (mean range = 19.9 -31.5 ppt). In the spring of 1996 the basins with lower fall 1995 salinities had much lower prevalences of infected sites (0 - 12.5%) than those with high fall 1995 salinities (prevalences of 33.3 - 71.4%). Rankin experienced mid salinities (19.0 - 21.0 ppt) in the fall 1995 sampling period, and had a high Labyrinthula prevalence (66.7%). Mean secchi disc readings at Rankin (31.1 to 35.9 cm depth, fall 1995 and spring 1996 respectively) indicate that this basin had the lowest water clarity of the 10 basins. In all cases, trends by individual basin need to be compared to historical salinity data, other environmental factors, phytoplankton bloom dynamics, and any potential effects associated with Thalassia abundance, age, and mortality.

Preliminary indications suggest that medium-to-high salinity basins with persistent or new Labyrinthula infections (prevalences of 1-20%) from spring 1995 through spring 1996 were associated with areas of declining seagrass density. Loss in Thalassia abundance in certain areas in Twin and Rabbit from spring 1995 to spring 1996 (Durako et al. this conference) are almost paralleled by the distribution of Labyrinthula and high lesion cover in these basins during the same time period. These seagrass losses, the presence of Labyrinthula, and percentage blade lesions present will be statistically evaluated. The increased prevalence of Labyrinthula in Crane and Johnson in spring 1996 may foreshadow Thalassia losses in these basins. Sampling this fall will enable us to determine if there is a significant impact by Labyrinthula. In other areas such as Rankin, where seagrass die-off and slow recovery have been occurring for some time, and where seagrass density is low, these latter factors in combination with high turbidity, lowered light, and medium salinities appear to allow persistent Labyrinthula infections. At Twin and Crane where environmental stressors are apparently minimized (deep basins, medium density of seagrass, good flushing, low turbidity, and good light penetration) the role of Labyrinthula in inducing disease and subsequent mortality of seagrass may be determined within the next few seasons of sampling.

Basins with widely fluctuating salinities with pulses of low salinity, wide salinity ranges, or low salinities from 8-18 ppt appear to have low-level prevalence of Labyrinthula, minimal lesion cover, and are not experiencing heavy die-off. Determining the potential for Labyrinthula to act as a primary pathogen that is controlled by specific environmental cues or seagrass resiliency still remains a challenge for Florida Bay research in the near future. Comparisons of laboratory results and field-collected data are beginning to provide a clearer picture of the role of Labyrinthula in seagrass mortality in Florida Bay. Understanding the role of Labyrinthula as a major contributing factor in seagrass die-off, particularly if the dynamics of the pathogen response are controlled by salinity, will enable managers to develop appropriate water management strategies and to recommend adaptive restoration options.

Carlson, P.R., Yarbro, L.A. & Barber, T.R. 1994. Relationship of sediment sulfide to mortality of Thalassia testudinum in Florida Bay. Bull.Mar.Sci. 54:733-746.

Durako, M.J. & Kuss, K.M. 1994. Effects of Labyrinthula infection on the photosynthetic capacity of Thalssia testudinum. Bull.Mar.Sci. 54:727-732.

Porter, D. & Muehlstein, L.K. 1989. A species of Labyrinthula is the prime suspect as the cause of a massive die off of the seagrass, Thalassia testudinum in Florida Bay. Mycol.Soc.Am.Newsl. (abstract). 40:43.

Robblee, M.B., Barber, T.R., Carlson, P.R., Durako, M.J., Fourqurean, J.W., Muehlstein, L.K., Porter, D., Yarbro, L.A., Zieman, R.T. & Zieman, J.C. 1991. Mass mortality of the tropical seagrass, Thalassia testudinum in Florida Bay (USA). Mar.Ecol.Prog.Ser. 71:297-299.

Zieman, J.C., Fourqurean, J.W. & Iverson, R.L. 1989. Distribution, abundance, and productivity of seagrasses and macroalgae in Florida Bay. Bull.Mar.Sci. 44:292-311.

Mesocosm Studies of Biscayne Bay Seagrass Communities and the Relevance to Florida Bay Seagrass Dynamics

Elizabeth A. Irlandi and Mark A. Harwell, University of Miami, Rosenstiel School of Marine and Atmospheric Science, Center for Marine and Environmental Analyses, University of Miami, Miami, FL.

The University of Miami's Center for Marine and Environmental Analyses (CMEA) operates experimental microcosm and mesocosm facilities located at the Rosenstiel School of Marine and Atmospheric Science campus on Virginia Key. Financial support for construction and operation of the facility currently comes from contracts and grants that CMEA administers from the U.S. Army Corps of Engineers and the National Oceanic and Atmospheric Administration Coastal Ocean Program. The experimental work conducted in the facilities along with field manipulations and observations provide empirical data to CMEA scientists for incorporation in policy-relevant models of seagrass and hard-bottom communities in Biscayne Bay.

The mesocosm facility consists of 12 fiberglass tanks (3.6m long, 2m wide, 1.4m deep, ca. 6159 liters or 1625 gallons). Each tank is half buried to aid in temperature control and stability, and can be partitioned into four separate and isolated 0.9m x 2.0m quadrants. Nine of the tanks are currently dedicated for seagrass experiments, two more will be used for hard-bottom and coral reef work, and the third will be used as a holding tank and nursery for seagrass seedlings. The nine seagrass tanks currently have a 40-cm deep sediment layer.

Water is supplied from Biscayne Bay and is settled (not filtered) before entering the mesocosms. Each quadrant has its own water supply and drain. Water is delivered at a rate of 5-6 gpm to each mesocosm tank via a 1" pipe which is diverted to 3/4" pipes at each quadrant. Standpipes (1.5") drain and skim each quadrant. These standpipes are currently set to allow a water column of 85 cm and are adjustable down to 30 cm. Water is circulated in the tanks by 1/12 hp (530 gph) powerheads placed across from the skimming standpipes.

In June 1996 construction of the mesocosm facility was completed and Thalassia testudinum cores (20-cm diameter) were planted in each tank at 12" centers. We are currently testing the facility to determine the best size tank (partitioned or not) to simulate natural field conditions. After two months of adjustment and acclimation to the transplanting process, monthly measures of water column and porewater nutrient levels, and water column chlorophyll concentrations have been taken from partitioned (e.g., 0.9 x 2.0m, 1.8 x 2.0m) and non-partitioned (2.0 x 3.6 m) tanks for comparison with natural seagrass beds. We are also making measures of growth, productivity, and microbial decomposition rates for comparison. Once we have established the size of tank that best simulates natural field conditions, we will begin long-term experiments to examine changes in seagrass and macroalgal community structure in response to freshwater discharge from canals. At this time, the tanks will be planted with a mixture of Talassia testudinum, Halodule wrightii, and Penicillus capitatus (the most abundant rhizophytic alga based on field sampling). Recruitment of non-rhizophytic algae occurs naturally via the settled water supplied to the facility. Half of the tanks will be pulsed (i.e., salinity dropped by 50% for a 24-hour period followed by an infusion of new, flow-through seawater) once a week with aerated domestic tap water that contains a slurry of sediments (to increase turbidity of the water column). This treatment will mimic a canal discharge event by introducing low-salinity, turbid water that is high in nutrients (levels of nitrogen and phosphate in domestic tap water are considerably higher than Biscayne Bay water). The control tanks will be pulsed with aerated seawater. All tanks will be stocked with grazers (e.g., pinfish and gastropods) at natural densities to control excessive epiphyte growth. Over time we will monitor growth and percent cover of the plant species. We anticipate these experiments to be log-term manipulations that will assess community- level responses to the collective effects of salinity, nutrient, and light changes associated with canal discharge.

Our microcosm facilities will allow controlled experiments examining how the different species of seagrass and macroalgae respond to changes in salinity, light levels, and nutrients separately and in combination. We are also assessing how salinity fluctuations affect grazing rates of herbivores. The CMEA flow-through microcosm facility contains thirty 30-gallon aquaria. The aquaria are housed in an 18' x 20' greenhouse. The system can be used for flow-through or recirculating conditions, depending on the desired treatments. The system also contains four 240-gallon head tanks, which serve as reservoirs for seawater or freshwater and as scrubbers or holding tanks.

The plumbing for this facility was designed to allow three separate water treatments at any given time. Each aquaria drains into one of three sump tanks (A,B, or C) where the water is taken up by a pump and returned to the aquaria (recirculating). The aquaria are arranged 10 to one of three tables, and each table is equipped with the three separate water supplies to allow randomization of treatments and to minimize any bias in temperature and/or lighting differences associated with position of the aquaria in the greenhouse. When recirculating, water is pumped through a particulate filter, through a chiller/heat pump, and then to the aquaria feed lines. Water flow to each aquaria is controlled by a ball valve located above each tank from each of the three water feeds (A,B or C). Each table has connections to all three (A,B, or C) drain systems leading to the desired sump tank. Water drains back into the particular sump (A,B,or C) where it is then recirculated. The pump, filter, and chiller unit can bebypassed allowing flow-through of seawater. When the flow-through system is activated, seawater is gravity fed from a settling tank (40' head) into the microcosm facility.

Each system has a feed from the header/holding (H/H) tanks. Each H/H tank has freshwater (domestic tap water and R/O), saltwater, and air supply lines. In acute salinity applications, freshwater can be diverted from the H/H tanks to the recirculating pumps in the A, B, or C lines or some combination of the three lines. The volume of water equivalent to that entering drains out of the system. The system water is then diluted and recirculated at this new salinity.

We are currently running experiments in this facility to assess the effect of pulses of reduced salinity water (R/O water with negligible nutrient levels) on growth of macroalgae. Such experiments are currently underway with Laurencia, a common species of drift algae in Biscayne Bay. Following the assessment of reduced salinity stress on algal growth, we subject the algae to grazing pressures to determine if stressed algae is consumed by grazers at equal rates as unstressed algae.

CMEA also operates a static microcosm facility. This system consists of two greenhouses with two 200-gal tanks in each house. These tanks serve as water baths for individual experimental containers (aquaria, buckets, etc.). The system enables experimental units to be housed with ambient light, aeration, and constant temperature control. However, depending on the duration of experiments, water in the containers may have to be changed periodically to avoid stagnation and nutrient depletion. Unlike the flow-through and recirculating systems, this system will allow us to manipulate multiple factors simultaneously (e.g., temperature, salinity, light, and nutrients in various combinations). This will allow us to examine the effects of multiple factors and their interactions on seagrass and macroalgal growth.

While our research focuses on the effects of freshwater discharge from drainage canals on seagrass ecosystem function in Biscayne Bay, the results from our studies will have relevance to Florida Bay as well. All of the species we are working with occur in Florida Bay, and they are exposed to similar sorts of environmental stresses.

Seagrass Dieoff in Florida Bay (USA): Long-Term Trends in Abundance and Growth of Thalassia testudinum and the Role of Hypersalinity and Temperature.

Joseph C. Zieman, Department of Environmental Sciences, University of Virginia, Charlottesville VA; James Fourqurean and Thomas Frankovich , Southeast Research Program, Florida International University, Miami FL.

Beginning in late 1987 Florida Bay experienced a large and unprecedented dieoff of Thalassia testudinum. The dieoff occurred only in stands of dense T. testudnium. The largest dieoff zones were in western Florida Bay, but with some dieoff in the denser beds of the eastern Bay. The abundance and productivity of Thalassia testudinum was measured at 5 stations associated with the seagrass dieoff and 3 control stations, including one on the seaside of Key Largo, outside of Florida Bay, from 1989 to 1995. Early in the study salinity was very high, exceeding 46 psu, and decreasing to 29-38 psu in recent years. Seagrass standing crop and either short shoot density or mass per short shoot declined at nearly all stations. Areal productivity declined at 3 dieoff stations while mass-specific productivity increased at all dieoff stations and 1 control station. Seasonality was pronounced, detrended standardised residuals showed responses for all of the parameters to be greater than the yearly mean in spring and fall and less than the mean in fall and winter. Detrended residuals also showed decreased productivity to be correlated with increased salinities in the summer. Temperatures showed the greatest deviations from long-term means in the years of dieoff initiation.

Seagrass Elemental Content and Epiphyte Loads Along the Nutrient Availability Gradient in Florida Bay

James W. Fourqurean and Thomas A. Frankovich, Southeast Environmental Research Program and Department of Biology, Florida International University, Miami, FL.

Loss of seagrasses around the world has been linked to eutrophication. The mechanism of this loss has often been found to be the overgrowth of seagrasses by epiphytes in areas of enhanced nutrient availability. Seagrass loss in Florida Bay began in 1987, and continues to the present. While the causes of the initial loss of seagrasses is as yet poorly understood, it was not associated with declines in water clarity or in increased epiphytism. Four years after the initial seagrass dieoff, algae blooms and increased turbidity led to continued seagrass decline owing to competition for light with phytoplankton. It has been suggested that nutrients that had supported seagrass biomass in 1987 were released from the sediments and fueled the phytoplankton blooms in 1991 to present. The degree to which this phenomenon has led to a change in the spatial pattern of benthic nutrient availability and increases in epiphytism of seagrasses is not known.

In this study, we examined the response of the elemental content and epiphyte load of Thalassia testudinum, the dominant seagrass in Florida Bay, to the naturally occurring gradient of nutrient availability across Florida Bay. This nutrient availability gradient has been documented for both the water column and for the seagrass communities of the bay. Phosphorus availability is highest in north western Florida Bay, and decreases to the east. In contrast, nitrogen availability is highest in central Florida Bay. We hypothesized that areas of greater nutrient availability in Florida Bay would have higher epiphyte loads on the seagrasses. Additionally, we compared the patterns of benthic nutrient availability from before the seagrass dieoff, as indicated by the tissue nutrient concentrations of nitrogen and phosphorus, to the post-dieoff patterns as measured at 24 water quality monitoring stations in the spring of 1994.We also examined seagrass epiphyte loads with respect to distance from a point source of nutrient input in the oligotrophic eastern part of Florida Bay. This point source is a bird colony island; the point source of nutrients results from the defecation of thousands of birds.

Total epiphyte standing stock, seagrass short shoot size, P content of seagrasses, and total P concentration in the water column were all correlated. All of these variables were highest in the northwestern portions of Florida Bay and decreased to the east. Total epiphyte loads, measured as the mass of epiphytes per mass of seagrass leaves, were not correlated with any measure of nutrient availability, however. There was a statistically significant, but relatively weak (r squared =0.18) relationship between epiphyte chlorophyll loads and measures of P availability. On a baywide scale, there was a relationship between the plant component of the epiphyte community and nutrient availability, but this relationship did not explain a large proportion of the variation in the load of epiphytic plants on seagrasses. Total epiphyte load was not related at all to the availability of P or N. This result suggests that some factor other than nutrient availability, perhaps grazing pressure, is the primary control on epiphyte loads in Florida Bay. These data were collected in 1994, during the time when seagrasses were being lost due to light stress. Apparently, whatever enhancement of nutrient availability that was caused by the initial seagrass dieoff did not lead to baywide increases in epiphyte loads.

The measurement of total epiphyte loads and observations of epiphytic species composition along a transect adjacent to a point source of nutrients revealed that the effect of nutrient enrichment on epiphyte levels is pronounced but very localized. Epiphyte loads were much higher close to the point source, but epiphyte loads were not elevated above background levels further than 30m from the shore. Differences in epiphytic species composition were also observed along this transect. Various "fleshy" rhodophytes and chlorophytes (Chondria sp., Ceramium spp., Laurencia spp., and Derbesia sp.) were abundant close to the island (15m), while epiphyte composition furthest from the island (45 m) was dominated by animal epiphytes(especially the epiphytic bivalves Pinctada imbricata and Brachidontes exustus). In contrast to the short distance from the point source that had altered epiphyte communities, previous work has shown that seagrass standing crop was enhanced up to 200 meters from the bird island and the nutrient content of the seagrass biomass showed elevated phosphorus as far as 90 meters from the bird island.

Previous studies have documented the spatial pattern in benthic nutrient availability using the C:N:P ratios of seagrass biomass that existed before seagrass dieoff. There was a striking pattern in C:P ratios, with low C:P in northwest Florida Bay and high C:P in eastern Florida Bay. C:N of seagrass leaves was minimum in the center of the bay. These patterns have been argued to be a response to the nutrient availability gradients. A comparison of the maps of C:N:P of seagrass leaves from pre- and post-dieoff indicates that the spatial pattern of benthic nutrient availability has changed very little since the dieoff.

Although epiphyte load is often touted as an indicator of nutrient availability in seagrass ecosystems, these findings suggest that seagrass biomass, species composition, and nutrient content are more sensitive indicators of nutrient availability than epiphyte load. We also found that, despite the increase in phytoplankton biomass and turbidity of Florida Bay, benthic nutrient availability was the same in 1994 as pre-dieoff in 1987.

These results have been submitted for publication in the journal Marine Ecology - Progress Series as: Seagrass epiphyte loads along a nutrient availability gradient, Florida Bay, FL, USA, by Thomas A. Frankovich and James W. Fourqurean.

The Status and Trends of Seagrass Communities in Florida Bay

Michael J. Durako, Margaret Hall, and Jeff Hall, Florida Marine Research Institute (FDEP), St. Petersburg, FL; and Lee Hefty , Jason Bacon, and Susan Kim, Dade County Department of Environmental Resources Management, Miami, FL.

The Florida Bay landscape continues to exhibit dramatic changes following the onset of seagrass die-off in the late 1980's and the initiation of widespread and chronic turbidity in late 1991. FMRI's Florida Bay Fisheries Habitat Assessment Program (FHAP) and DERM's C-111/Taylor Slough water quality and biological monitoring program in northeastern Florida Bay are providing spatially-extensive, qualitative and quantitative data to assess variation in macrophyte species distribution and abundance, community structure and population dynamics in relation to the multiple stressors affecting the Bay. Between these two programs, over 400 stations in 14 basins are currently being sampled seasonally for seagrass and macroalgal distribution and abundance.

Thalassia testudinum remains widely distributed in Florida Bay, with highest abundances generally occurring in the southwestern Bay. Analyses of recent (1995/96) Braun-Blanquet (B-B) cover/abundance changes indicate that the most significant losses of Thalassia are occurring in western (Rabbit Key Basin, RKB) and southern (Twin Key Basin, TWN) Florida Bay (Figure 1). These basins are somewhat far-removed from the land/sea margin, and salinities dropped only slightly following the high rainfall and freshwater inflow into the Bay that occurred during summer 1995. Although all basins sampled by FHAP exhibited patchy mosaics of gains and losses, average Thalassia cover/abundance values were stable or increased from spring 1995 to spring 1996 in all but one (TWN) of the ten basins sampled. Basins in central-to-northeast Florida Bay, which had 5 to 15% reductions in salinities between April and November 1995, exhibited stable or increasing Thalassia abundance. In spring 1995, about 39% of the area sampled by FHAP had less than 5% cover of Thalassia; the amount of area with little or no cover dropped to approximately 25% by spring 1996. Thalassia is presently least abundant in central Florida Bay (Rankin Lake, RNK and Whipray Basin, WHP); this region experienced the most prolonged and severe seagrass die-off. Distribution and abundance of Halodule wrightii showed little change from 1995 to 1996, except in northern Johnson Key Basin (JKB) where substantial increases were observed. Quantitative (shoot density) and qualitative (Braun-Blanquet) abundance data for spring 1995 were strongly correlated (r20.7) on a Bay-wide scale. The strength of the relationship varied from basin to basin, and was weakest in areas with generally high B-B values.

In the transitional basins and coves of extreme northeastern Florida Bay, average B-B cover/abundance of Thalassia and Halodule increased from fall 1995 (' =1.30 ñ 1.15 s.d. and 0.32 ñ 0.38 s.d. for Thalassia and Halodule, respectively ) to spring 1996 (' = 1.67 ñ 1.38 s.d. and 0.63 ñ 0.77 s.d. for Thalassia and Halodule, respectively). Thalassia was the most widely distributed seagrass and typically dominated were it occurred in these transitional basins. Thalassia was present at approximately 75% of the 72 sampled stations, but was noticeably absent from the isolated "upstream" basins of Joe Bay and Highway Creek. Halodule was also widely distributed, occurring at about 70% of the stations. Ruppia maritima occurred at 20% of the stations during the low salinity conditions (9.6% ñ4.8) of fall 1995, but frequency of occurrence dropped to 13% by spring 1996, when conditions were more mesohaline (20.5% ñ 2.9 s.d.). Ruppia distribution was generally isolated to "upstream" basins (Joe Bay, and Highway Creek) where it appeared to be seasonally dominant. Basin averages for Ruppia abundance ranged from 50% during months with lower salinities, to <5% in late spring when salinities were higher. Quantitative (Thalassia shoot density) and qualitative (Braun-Blanquet) abundance data for the transitional basins are also well correlated (r20.6). This value represents all Thalassia data for the period December 1995 through June 1996.

Quantitative (seagrass shoot density) data varied at six fixed, long-term stations in the transitional basins sampled during fall 1995 and spring 1996. Seagrass composition and abundance at two sites in Little Madeira Bay remained unchanged, and were consistent with historical trends. At two other sites (Trout Cove, and Long Sound), Thalassia shoot density decreased from fall 1995 to spring 1996, falling below the historical ranges for each station. Halodule density at the Long Sound station increased from approximately 75 shoots m-2 in fall 1995 to about 175 shoots m-2 in spring 1996. Shoot density of Ruppia decreased substantially over this same period at the Highway Creek station. Recent qualitative abundance data (Braun-Blanquet, June 1996) support many of the observations from the fixed, long-term monitoring stations.

Recent losses of Thalassia in western Florida Bay have occurred where there has been persistent turbidity. The turbidity is caused by widespread, persistent phytoplankton blooms as well as resuspended carbonate sediments exposed by seagrass die-off, especially on the western Florida Bay banks (e.g., Sandy Key). Seagrass declines in the western Bay may be the result of light-stress induced mortality in addition to die-off. Losses in southern Florida Bay (TWN) do not appear to be related to turbidity, and the patterns of loss are similar to those observed during the die-off events of the late 1980's.

The distribution of Thalassia loss between 1995 and 1996 shows a relatively high spatial coincidence with the distribution and abundance of the marine slime mold Labyrinthula; reductions in Thalassia abundance are paralleled by elevated infection rates and lesion coverage (Landsberg et al., this conference). The occurrence of this marine slime mold also appears to be higher in chronically-turbid areas, and lower in basins with reduced salinities; both turbidity and low salinity are stressful to Thalassia.

Shoot-specific structural and dynamic characteristics of Thalassia generally increased between spring 1995 and spring 1996. Standing crop increased in 9 of the 10 basins, decreasing only in WHP. Shoot-specific productivity of Thalassia increased in all basins except TWN and WHP (Figure 2). Turnover rates increased in 7 of the 10 basins, with highest increases occurring in RNK and Crane Key Basin (CRN). These increases in turnover may be due partially to a shift in sampling time from May in 1995, to June in 1996. However, the general pattern of increased distribution, abundance, and production suggests some recovery of the seagrass community, and perhaps reflects an improvement in water quality conditions in Florida Bay.

Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov