Modeling the dynamics of three functional groups of macroalgae in tropical seagrass habitats

Abstract

A model of three functional groups of macroalgae, drift algae, rhizophytic calcareous algae, and seagrass epiphytes, was developed to complement an existing seagrass production model for tropical habitats dominated by Thalassia testudinum (Turtle-grass). The current modeling effort simulates annual biomass dynamics for each of the three functional groups under a variety of stress regimes imposed by nutrient-laden freshwater discharges into a well-flushed coastal bay in South Florida. The model is parameterized based on multiple years of experimental data collected from Biscayne Bay, as well as literature values reported for other Florida and Caribbean seagrass habitats. Calibration of the model yields a good fit of predicted-to-observed biomass ( r 2>0.85) for sheet-flow and oceanic-influenced sites, but a poor fit ( r 2=0.13) under canal-influenced conditions. This is hypothesized to be related to the lack of adequate observed data on seagrass epiphyte dynamics to parameterize the model. Sensitivity analysis showed that the model is easily perturbed by changes in intrinsic growth parameters of the algae (daily growth rate, mortality rate, carrying capacity), while it is much less sensitive to changes in the functional form of the response curves that are used to characterize the stress tolerances of the algae to light, temperature, salinity, and nutrient conditions. This model is considered suitable for use in seagrass habitats similar to those occurring in South Florida, as these are the conditions to which the model was calibrated.