The effects of nutrient enrichment on seagrass communities and their associated fauna in Florida Bay, Florida, USA

Seagrass loss has accelerated worldwide, with coastal nutrient enrichment often cited as a main cause. As such, it is important that the processes that structure seagrass ecosystems are understood. In my first experiment, I tested the relative importance of top-down and bottom-up effects by evaluating the combined and separate effects of nutrient enrichment and grazer species composition on epiphyte communities and seagrass condition. Although enrichment was achieved, no significant effects on seagrass and epiphyte biomass, as well as no strong grazer effects and very few significant nutrient-grazer interactions, were observed. This may have been a result of the grazing activities of unwanted amphipod grazers present in control cylinders which lowered epiphyte biomass to the same level as experimental grazers. If true, this experiment demonstrates that grazing effects are stronger than nutrient effects on epiphytic algae, and relatively low densities of grazers can control epiphyte biomass even at high nutrient loading rates.;In my second experiment, I measured the separate and interacting effects of nutrient enrichment and interspecific competition on the growth, fecundity, and stoichiometry of three invertebrate grazers and determined if enrichment altered competitive interactions between these grazers. Both crustacean grazers were significantly affected by interspecific competition but not by nutrient enrichment. The gastropod Turbo castanea was positively affected by nutrient enrichment but negatively affected by competition. A significant interaction between enrichment and competition occurred and although T. castanea achieved greater growth in enriched treatments, competition with other grazers limited that growth, demonstrating that nutrient enrichment can alter competitive interactions between grazer species.;In my third experiment, I examined the effects of long-term nutrient enrichment on seagrass and faunal composition, biomass, and stoichiometry. While enrichment altered seagrass composition, it had no effect on faunal composition and abundance, and few effects on faunal biomass. Phosphorous tissue concentrations of amphipods and killifish were higher near nutrient sources. It seems likely that amphipod phosphorous concentrations were elevated by grazing on enriched food sources, and that killifish phosphorous levels were elevated, in turn, by feeding on these amphipods. Thus, this experiment provides evidence that nutrient enrichment affected the stoichiometry of higher order consumers.;Overall, this research has demonstrated the strong effects consumers can have on epiphyte biomass under enriched conditions as well as the effects of nutrient enrichment on the competitive interactions, growth, and stoichiometry of the consumers themselves.