Changes in food web structure under the influence of increased anthropogenic nitrogen inputs to estuaries

Abstract

Anthropogenic nitrogen loads to shallow coastal waters have been linked to shifts from seagrass- to algae-dominated communities in many regions of the world, yet the influence that these shifts have on the structure of nearshore food webs remains unclear. We used stable C and N isotope ratios to assess the relative importance of phytoplankton, macroalgae, and eelgrass Zostera marina as food sources to consumers in estuaries of Waquoit Bay, Massachusetts, USA, that receive low versus high nitrogen loads from their surrounding watersheds. We found that, in general, the diets of herbi- vores, suspension feeders, and detritlvores (collectively referred to as primary consumers) in the Waquoit Bay estuaries are influenced by the dominant forms of production that they are exposed to. Phytoplankton and macroalgae are the major food sources of primary consumers under both low and high N loading conditions, but eelgrass is also an important food so'urce where N loading is low Most of the primary consumers at the estuary with low N loading have between 10 and 16 % eelgrass in their diets. Some species, however, appear to have no eelgrass in their diets, while others have diets consist- ing of as much as 31 % eelgrass. Eelgrass C and N are passed on to benthic 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 which land-derived nitrogen is cycled within estuar- ies, as well as its ultimate fate.