Coupling of estuarine benthic and pelagic food webs to land-derived nitrogen sources in Walquoit Bay, Massachusetts, USA

The fact that land-derived sources of nutrients promote eutrophication in the receiving coastal waters implies coupling between land and marine environments. Increasing nitrogen inputs in the estuaries are followed by major shifts in biota composition and abundances. In the present paper we used N and C isotopic ratios to analyze the coupling of benthic and pelagic components of food webs to estuaries receiving different N loads from their watersheds. We found that primary producers, benthic taxa, and fishes were coupled to the watersheds and estuaries where they were collected. In contrast, zooplankton was uncoupled. Primary consumers and predators feeding on benthic prey within the estuaries were also coupled to the watershed and estuaries, but predators feeding on zooplankton were not. We hypothesized that short water residence time in these estuaries uncoupled plankton from terrestrial influence. Stable isotopic measurements of N in producers, consumers, POM, and sediment in different estuaries of Waquoit Bay, Massachusetts, USA, demonstrate a consistent link between land-use on contributing watersheds and the isotopic ratio in all the benthic components and food webs.. The remarkably consistent link suggests that the benthos was tightly coupled to land-derived inputs, and that these components, particularly macrophytes, could be good indicators for monitoring increases in land-derived N inputs. Our results showed that stable isotopes of N and C have the potential for use in basic research and applied monitoring, but need to be applied considering the features of estuaries that might couple or uncouple organisms regarding dependency on land, such as hydrodynamic exchanges.