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 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, interrnediate 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 for 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.