Newell 9314584 Stable isotope studies have shown that in many salt-marsh metazoans obtain a large fraction of their carbon from cordgrass, Spartina alterniflora. However, the ribbed mussel, Geukensia demissa, a dominant metazoan in eastern salt marshes, has only a limited capacity to digest plant detritus. Thus, these scientists hypothesize that bacteria and other protists are important food-web components in mediating the flow of cordgrass carbon, and nitrogen, to higher organisms. To address this hypothesis, scientists will first determine the concentration of microheterotrophs, phytoplankton, and cordgrass carbon near mussel populations over seasonal and geographical scales and then integrate this information in a model with measurements of the metabolic carbon and nitrogen demands, feeding activity, and assimilation efficiencies of G. demissa. Such a model will allow these scientists to determine if the microheterotrophs are sufficiently abundant to act as a major food source and to explore how metazoans exert top- down control on microheterotrophs in salt-marsh ecosystems. To examine how general this concept is among eastern estuaries, Newell and Stoecker will transplant mussels from a mid-Atlantic marsh to northern and southern marshes for stable isotope analyses. The purpose is to demonstrate fundamental differences in patterns of carbon cycling in spatially separate salt marshes. When completed, this project will provide a more comprehensive understanding of food-web interactions in the detritus-based, salt-marsh ecosystems. ***
