Nitrogen loading alters seagrass ecosystem structure and support of higher trophic levels

1. Anthropogenic‐derived nutrient inputs to coastal environments have increased dramatically worldwide in the latter half of the 20th century and are altering coastal ecosystems. We evaluated the effects of nitrogen loading on changes in macrophyte community structure and the associated fauna of a north temperate estuary. We found that a shift in primary producers from eelgrass to macroalgae in response to increased nutrient loading alters habitat physical and chemical structure and food webs. As nitrogen load increased we found increased macroalgal biomass, decreased eelgrass shoot density and biomass, decreased fish and decapod abundance and biomass, and decreased fish diversity. 2. The central importance of macroalgae in altering eelgrass ecosystem support of higher trophic levels is evident in the response of the ecosystem when this component was manipulated. Removal of macroalgae increased eelgrass abundance and water column and benthic boundary layer O2 concentrations. These changes in the physical and chemical structure of the ecosystem with lower macroalgal biomass resulted in higher fish and decapod abundance and biomass. 3. Both a 15N tracer experiment and the growth of fishes indicated that little of the macroalgal production was immediately transferred to secondary consumers. δ15N values indicated that the most abundant fishes were not using a grazing food web based on macroalgae. Fish tended to grow better and have a greater survivorship in eelgrass compared to macroalgal habitats. 4. Watershed‐derived nutrient loading has caused increased macroalgal biomass and degradation and loss of eelgrass habitat, thus reducing the capacity of estuaries to support nekton. Copyright © 2002 John Wiley & Sons, Ltd.