Experimental and stable isotopic approaches to investigate controls of temperate and tropical seagrass food webs

Seagrass beds are highly productive ecosystems heavily influenced by human activities, such as coastline development and overfishing. The resulting forcing from the top-down and bottom-up may interact to alter seagrass habitats. Human activities may also modify seagrass food webs and change their stable isotope signatures. To better understand these changes I manipulated top-down and bottom-up controls in a tropical seagrass bed and studied seagrass food webs in a temperate and tropical system.;Growth and biomass of Thalassia testudinum in Jobos Bay, PR were not directly controlled from the bottom-up by nutrients. There were significant interactions, however, between effects of increases in nutrient supply and increased grazing pressure. Herbivorous fish consumed significantly more seagrass from experimentally nutrient enriched plots. Nutrient enrichment also increased abundance of fish of all trophic levels, with piscivores responding numerically to increases in larger prey, indicating a bottom-up cascading effect of nutrient enrichment from producers to higher trophic levels.;Macroalgae and seagrasses made up the base of the coastal benthic food web in Puerto Rico whereas mangroves did not appear to play a major role. Human development of the coastline increased delta15N of the food web. Changes in delta13C were best explained by the degree of marine influence, with more marine sites having food webs heavier in delta13C. There was no significant effect of precipitation.;The sub-estuaries of Waquoit Bay, MA are heavily influenced by land use on the watershed and represent a range of nitrogen loads and degrees of eutrophication. Eelgrass (Zostera marina) is highly sensitive to increases in nitrogen load and seagrass habitat only survives at the lower end of this range in nitrogen loads. I studied stable isotopes and structure of the food web in three Waquoit sub-estuaries with sufficiently low nitrogen loads to sustain eelgrass populations. The delta15N values of consumers were strongly affected even at these low nitrogen loads. Changes in producer composition, resulting from increased nitrogen loading, altered diets of consumers. Fauna shifted from a more seagrass and detritus based diet in the most pristine estuary to feeding on macroalgae in the heaviest loaded estuary.