Geukensia demissa, the ribbed mussel, consumes particulate organic matter (POM) consisting of varying proportions of macrophyte and phytoplankton depending on its location in salt marsh ecosystems (1). Estuaries subject to different nitrogen loads, however, are often dominated by different primary producers (2). Thus, the mix of POM available to G. demissa can change not only along gradients within estuaries, but among them. In Waquoit Bay, Massachusetts, increased N loads from anthropogenic activity have resulted in decreased eelgrass (Zostera marina) biomass, and increased macroalgal and phytoplankton biomass (2). This study takes advantage of the different N and C stable isotope signatures found in these major producers to examine the sources of particles consumed by G. demissa living in estuaries subject to different nitrogen loads.