| Anthropogenic nutrients from the bottom of the food web up have led to eutrophication and frequent macroalgal blooms in coastal waters. Humans have also over-exploited marine consumers by exerting control from the top of the food web down. To understand the changes taking place in estuarine benthic communities at species, community, and ecosystem levels in response to such bottom-up and top-down controls, I surveyed macroalgae and fauna, defined trophic linkages, and carried out an experiment in Waquoit Bay, MA. This site has a unique combination of estuaries that represent a range of nitrogen loads and levels of eutrophication.; Macroalgal biomass increased with increasing nitrogen supply. Compared with past decades, species composition of macroalgal communities have also changed. Opportunistic species of macroalgae have out-competed other species for light and nutrients, and have dominated canopies in estuaries with high nutrient regimes.; Net oxygen consumption by high macroalgal biomass led to frequent hypoxia, which in turn lowered number and species of consumers. Grazers, mainly small crustaceans, decreased more than other groups. In the eutrophic estuary, lower numbers of grazers, and hence lower prey availability, forced some omnivores to shift from feeding mainly carnivorously to feeding as herbivores, taking advantage of the abundant macroalgal resource.; Bottom-up and top-down forces interact to control growth of bloom-forming macroalgae. In situ experiments showed that bottom-up effects on macroalgal growth were nutrient- and species-specific. Bottom-up mechanisms also lowered grazer abundance, an interaction of bottom-up and top-down controls acting on macroalgae. Rapid growth of macroalgae in response to high nitrogen loads overwhelmed the ability of grazers to control algal biomass from the top down. Where algal growth was low, grazers were able to control algal biomass.; Eutrophication restructures estuarine benthic communities and food webs, and changes the relative influence of bottom-up and top-down controls on macroalgal biomass. With increasing nutrient inputs to coastal waters, losses of benthic species will likely increase, and many of these species are commercially important. To effectively mitigate the changes to benthic communities that are mainly driven by bottom-up mechanisms in eutrophied estuaries, conservation strategies should focus on reducing nutrient inputs to coastal waters. |
