| The effects of stressors related to canal discharges on near-shore, shallow-water tropical macroalgae occurring in Thalassia testudinum (turtlegrass) ecosystems were investigated. Three functional groups of macroalgae (drift algae, rhizophytic algae, and seagrass epiphytes) contribute to seagrass system structure and function. Drift algae and epiphytes have the potential to shade seagrasses under conditions of elevated water-column nutrients. Within Biscayne Bay, seasonal and spatial changes in the macroalgal community may be related to canal discharges. Drift algae dominated in canal influenced sites, while rhizophytic algae were dominant in oceanic sites. Epiphytes were found in both conditions with filamentous species in canal sites, whereas calcareous forms dominated in oceanic sites.; The physiological responses by the macroalgae to environmental variables including light, temperature, salinity, nutrients, and hydrodynamic regime were investigated. Temperature was found to drive seasonal changes in abundance, whereas salinity and nutrients were important determinants of the spatial distribution of the three functional groups within the Bay. Low-salinity stress from canal discharges had a negative impact on growth, while nutrient addition (primarily ammonia-nitrogen) had a stimulating effect. Bay-wide tidal circulation patterns were found to affect the distribution of drift algae, with accumulation occurring along the mainland coastline, and removal of drift algae occurring in the high-flow conditions typical of the oceanic inlets.; A simulation model of algal productivity was developed to augment a pre-existing primary production model for seagrass systems. The algal productivity models were modified and parameterized with data from field and experimental investigations in Biscayne Bay, as well as from pre-existing literature data available for similar tropical seagrass systems. The models were validated for a select number of sites (canal, sheetflow and oceanic) representing the range of environmental regimes present in Biscayne Bay. Predicted simulation results agreed closely with the observed field data for both drift and rhizophytic algae, but less so for the epiphytes. In canal-influenced portions of the Bay, rhizophytic algae had reduced biomass because of low-salinity stress, drift algae bloomed under favorable temperature conditions because of the high nitrogen loadings, and filamentous epiphytes were favored over calcareous forms. |
