[DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] Blackened, necrotic lesions on 
Thalassia testudinum leaves are frequently associated with seagrass die-off in Florida 
Bay. A previously undescribed species of the marine slime mold, genus Labyrinthula, is 
the primary causal agent of these lesions. When Labyrinthula infection was present, 
variations in lesion coverage resulted in significant differences in dry-weight based 
photosynthesis versus irradiance (P/I) responses of Thalassia leaf tissue, reducing 
photosynthetic capacity and oxygen output. Maximum photosynthetic rate, P max , 
decreased to below zero when lesions covered 25% or more of the leaf tissue. In 
addition, respiration rates in infected leaves were up to three times greater than in 
adjacent, uninfected tissue. Alpha (a), the initial slope of the P/I relationship, exhibited 
little change with low lesion coverage but was usually reduced with higher lesion 
coverage. These results show that the presence of Labyrinthula lesions impair 
photosynthesis of Thalassia leaf tissues and might reduce oxygen available for 
transport to below ground tissues, possibly making Thalassia more susceptible to 
hypoxia and sulfide toxicity. Thalassia shoots were collected in Johnson Key Basin, 
Rabbit Key Basin, Rankin Lake and Sunset Cove. 
1994 0 
Fong, P., and M. A. Harwell (1994) Modeling seagrass communities in tropical and 
subtropical bays and estuaries: a mathematical model synthesis of current hypotheses. 
Bull. Mar. ScL 54(3):757-81. 
[DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] A preliminary simulation model 
was generated to predict changes in the biomass of five components of the autotrophic 
seagrass community that dominates tropical and subtropical bays and estuaries. 
Changes in productivity and biomass are based on relationships among three species of 
seagrass ( Thalassia testudinum, Halodule wrightii, and Syringodium filiforme), 
epiphytes attached to seagrass, macroalgae, and several environmental factors, 
including light, temperature, salinity, sediment nutrients, and water-column nutrient 
concentrations. These relationships were derived from the published literature and 
include both experimental data and current alternative hypotheses. The model predicts 
that Thalassia is the community dominant under "normal" bay or estuarine conditions in 
tropical and subtropical regions, including high solar insolation, intermediate levels of 
seasonal variability in temperature and salinity, and low water-column and 
intermediate-to-high sediment nutrient concentrations. Increasing the supply of 
nutrients to the water column stimulates the productivity of epiphytes on seagrass, 
resulting in decreased light to seagrass blades and less Thalassia productivity. 
Thalassia and epiphyte biomass undergo seasonal changes in abundance; however, 
epiphyte biomass lags Thalassia by about 40 days. Halodule dominates when sediment 
nutrients are high and when there are environmental extremes of temperature and 
salinity. Syringodium is the community dominant in areas with more oceanic influence, 
characterized by less variability in salinity and temperature and lower water-column 
and sediment nutrients. This model is still in an early developmental stage. Preliminary 
sensitivity analyses identified important factors for community productivity and 
composition. The most important model parameters or seagrass include the 
productivity/biomass relationships, differential tolerances to extreme salinities, and 
the P/I curves (especially for Thalassia). All of the relationships between 
environmental factors and epiphytes are important, and these are the least certain 
derivations. We need to conduct a thorough sensitivity analysis, validate the model with 
field data, and generate more information on the algal components of the community. 
This simple community model will eventually be expanded to simulate seagrass 
dynamics across a spatial domain. Data from Florida Bay seagrass studies is discussed 
in this paper. 
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