Ribbed Mussel Nitrogen Isotope Signatures Reflect Nitrogen Sources in Coastal Salt Marshes

Abstract

The stable nitrogen isotope ratio in tissue of the ribbed mussel (Geukensia demissa) was investigated as an indicator of the source of nitrogen inputs to coastal salt marshes. Initially, mussels were fed a diet of 15N-enriched algae in the laboratory to determine how the tissue nitrogen isotope ratio (δ15N) changed with time. Steady-state times were calculated and found to be size dependent, ranging from 206 to 397 d. This indicated that mussels are long-term integrators of δ15N from their diet and may reflect nitrogen inputs to a marsh. Next, indigenous mussels were collected from 10 marshes with similar hydrology and geomorphology in Narragansett Bay, Rhode Island, USA, and mussel δ15N values were evaluated as indicators of nitrogen source. Significant positive correlations were observed between δ15N in mussels and the fraction of residential development in the marsh watersheds. In contrast, mussel isotope ratios showed significant negative correlations with the fraction of combined agricultural and recreational land use. These correlations suggested that the mussel nitrogen isotope signature is influenced by nitrogen derived from human activities in the adjoining marsh watershed. A more detailed examination of these relationships indicated that land use practices in close proximity to marshes and estuarine characteristics may also influence the observed nitrogen isotope signature. A simple, empirical model based on the 10 watersheds was developed to predict mussel δ15N from land use characteristics. The predictive ability of the model was tested with data from 12 additional marshes having similar geomorphology as the original 10, but differing in hydrology and mode of nutrient input. The model showed that ribbed mussel nitrogen isotope signatures may provide information on the source of nitrogen to coastal areas. This could be of use in developing general policies or strategies for monitoring and assessing coastal eutrophication. In addition, the isotopic ratio of mussels is useful as a proxy for watershed land use practices when assessing ecological responses to nutrient enrichment in coastal marshes.