MODELING SEAGRASS COMMUNITIES IN TROPICAL AND SUBTROPICAL BAYS AND ESTUARIES - A MATHEMATICAL-MODEL SYNTHESIS OF CURRENT HYPOTHESES

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 pro ductivity and biomass are based on relationships among three species o f seagrass (Thalassia testudinum, Halodule wrightii, and Syringodium f iliforme), epiphytes attached to seagrass, macroalgae, and several env ironmental factors, including light, temperature, salinity, sediment n utrients, and water-column nutrient concentrations. These relationship s were derived from the published literature and include both experime ntal 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 i nsolation, intermediat elevels of seasonal variability in temperature and salinity, and low water-column and intermediate-to-high sediment n utrient concentrations. Increasing the supply of nutrients to the wate r column stimulates the productivity of epiphyte on seagrass, resultin g in decreased light to seagrass blades and less Thalassia productivit y. Thalassia and epiphyte biomass undergo seasonal changes in abundanc e; however, epiphyte biomass lags Thalassia by about 40 days. Halodule dominates when sediment nutrients are high and when there are environ mental extremes of temperature and salinity. Syringodium is the commun ity dominant in areas with more oceanic influence, characterized by le ss variability in salinity and temperature and lower water-column and sediment nutrients. This model is still in an early developmental stag e. Preliminary sensitivity analyses identified important factors for c ommunity productivity and composition. The most important model parame ters for seagrass include the productivity/biomass relationships, diff erential tolerances to extreme salinities, and the P/I curves (especia lly for Thalassia). All of the relationships between environmental fac tors and epiphytes are important, and these are the least certain deri vations. 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 eventual ly be expanded to simulate seagrass dynamics across a spatial domain.