Abstract
Nutrient loading can dramatically alter benthic communities and has been implicated in the worldwide decline of seagrass beds. Ongoing changes in food webs caused by overfishing could also contribute to seagrass decline. However, the interaction of these factors and the role of small invertebrate grazers in mediating them are poorly understood. We examined the relative impacts of nutrient loading and food web alteration on eelgrass Zostera marina L. community structure in Chesapeake Bay by manipulating nutrients, predatory crabs, and invertebrate grazers in field enclosures over 28 d in summer. Nutrient loading increased epiphyte accumulation early in the experiment, decreased eelgrass biomass, and reduced the abundance of the colonial tunicate Botryllus schlosseri. Grazers decreased epiphyte accumulation, altered the recruitment of sessile invertebrates, and sometimes damaged eelgrass via overgrazing. Crabs reduced the abundance of eelgrass, and changed the species composition and abundance of grazers and sessile invertebrates. On average, the impacts of food web alterations and nutrient loading were comparable in magnitude and tended to be additive, rather than interactive. However, the distinct responses of different taxa in the community to the experimental treatments indicated that food web structure interacted with both bottom-up and top-down forces to determine overall community organization. These results highlight the importance of incorporating food web dynamics into seagrass conservation and management efforts.
