CHANGES IN FOOD-WEB STRUCTURE UNDER THE INFLUENCE OF INCREASED ANTHROPOGENIC NITROGEN INPUTS TO ESTUARIES

Anthropogenic nitrogen loads to shallow coastal waters have been linke d to shifts from seagrass- to algae-dominated communities in many regi ons of the world, yet the influence that these shifts have on the stru cture of nearshore food webs remains unclear. We used stable C and N i sotope ratios to assess the relative importance of phytoplankton, macr oalgae, and eelgrass Zostera marina as food sources to consumers in es tuaries of Waquoit Bay, Massachusetts, USA, that receive low versus hi gh nitrogen loads from their surrounding watersheds. We found that, in general, the diets of herbivores, suspension feeders, and detritivore s (collectively referred to as primary consumers) in the Waquoit Bay e stuaries are influenced by the dominant forms of production that they are exposed to. Phytoplankton and macroalgae are the major food source s of primary consumers under both low and high N loading conditions, b ut eelgrass is also an important food source where N loading is low. M ost of the primary consumers at the estuary with low N loading have be tween 10 and 16% eelgrass in their diets. Some species, however, appea r to have no eelgrass in their diets, while others have diets consisti ng of as much as 31% eelgrass. Eelgrass C and N are passed on to benth ic as well as pelagic secondary consumers. With losses of eelgrass as a consequence of nitrogen loading, an important pathway through which land-derived nitrogen enters food webs in the Waquoit Bay estuaries is eliminated. This fundamental change probably affects the rate at whic h land-derived nitrogen is cycled within estuaries, as well as its ult imate fate.