Abstract
The ribbed mussel, Geukensia demissa (Dillwyn, 1817), is an abundant bivalve in New England salt marshes, and has been found to be a long-term integrator of anthropogenic nitrogen loading to a watershed. The response of G. demissa morphometrics, condition index, density, and nitrogen and carbon isotope values were examined at Spartina alterniflora (Loisel, 1807) creek edge, creek wall, and mudflat habitats from two pairs of nutrient-enriched and reference creeks at Plum Island Estuary, Massachusetts, USA. Ribbed mussels from the creek wall had greater shell weight, and condition index than mussels from the S. alterniflora creek edge or mudflat habitats. Ribbed mussel density was general greater at the creek wall, and unexpectedly greater at the unfertilized Nelson Creek than the fertilized Clubhead Creek in 2013. Few G. demissa were collected from the mudflat in any creek. Ribbed mussels collected from creek wall habitat in fertilized creeks demonstrated lower nitrogen isotope values than reference creeks resulting from the fractional contribution of the nitrogenous fertilizer in the experimental creeks. Significant differences between carbon isotope values were not consistent between the nutrient enriched and reference creek pairs. Carbon isotope values indicated the main food source of G. demissa, a primary consumer, was phytoplankton. Despite significant differences in G. demissa nitrogen and carbon isotope values between enriched and reference creek pairs, the < 2‰ difference indicates that moderate levels of nutrient enrichment may not have been sufficient to elicit a bottom-up response in the phytoplankton that transferred to the local mussels. This study highlights the temporal and spatial complexities between the response of primary producers to nutrient over-enrichment and response of G. demissa condition and distribution in tidal salt marshes along the Atlantic coast.