Comparison of Guekensia Demissa Populations Along a Nitrogen Loading Gradient in Narragansett Bay, RI
Authors: Hall Janis 
Published: February 2018

Anthropogenic eutrophication of coastal estuaries impacts these vital ecosystems by increasing primary production, hypoxic conditions, pathogen concentration, and greenhouse gas emissions, all of which are leading to the degradation of shorelines, disease transmission, and hypoxia-related fish kills. Narragansett Bay is a prominent feature of Rhode Island, making up over 500 km of coastline and acting as a watershed for over 2,000 square meters of land in both Massachusetts and Rhode Island. This estuary is important to both the economy of the state of Rhode Island and its fringing ecosystems are necessary for a healthy shoreline. The beaches of Narragansett Bay revenue over $5 billion a year in tourism, stimulate more than 40,000 jobs, and are a source of many economically important marine organisms such as oysters, mussels, hard shell clams, finfish, and lobsters. The numerous fringing habitats of Narragansett Bay, including rocky intertidal zones, seagrasses, and estuarine marshes all play important roles for the coastline of Rhode Island, with salt marshes accounting for more than 600 ha of the shoreline.

Salt marshes along Narragansett Bay serve many ecological roles including water quality maintenance, storm surge reduction, erosion control, and habitat and food to fish and wildlife. The ribbed mussel, Geukensia demissa is the biomass-dominant benthic invertebrate in coastal marshes and is a foundation species, providing habitats for other organisms. Ribbed mussels have also been found to be a useful indicator species of nitrogen levels within Narragansett Bay, exhibiting increases in nitrogen-loads with greater biomass, density, and growth rates, and the δ 15N signatures of G. demissa tissues reflect anthropogenic-derived nitrogen. The purpose of my thesis was to compare ribbed mussel populations in some of the same salt marshes along the well-documented nitrogen-loading gradient, 14 years after the previous studies and following the installation of one phase of a two-part wastewater reduction program into the bay. In addition, my work is part of a larger effort to investigate changes in greenhouse gas emissions by plants, sediment microbiota, and gut microbiota in the ribbed mussels, in response nitrogen-loading and elevated temperatures. Characterizing the mussel populations is integral to understanding the effects of global change on the Narragansett Bay ecosystem.

The density, biomass, condition index, and growth rates of ribbed mussels were all positively correlated with the nitrogen-loading gradient in Narragansett Bay. Fecundity did not follow the same pattern but was significantly greater in June at one of the marshes and was negatively correlated with shell length and condition index. These results suggest that although mussels reproduce continuously, gametogenesis and spawning may have occurred prior to sampling. Another unexpected outcome is that with greater food availability resulting from nitrogen-loading, there are both costs and benefits. At the marsh with the highest nitrogen levels, Apponaug, the recruitment, growth, and condition index of the mussels were all significantly greater than at the other two marshes, but the high density may have limited the size of the mussels or even contributed to higher rates of mortality through intraspecific competition. At the opposite end of the nitrogen-loading gradient, mussels at Fox Hill had the greatest average shell length, but the density, biomass, condition index, recruitment, and growth rates were significantly lower than the other two marshes, suggesting that this less perturbed marsh may still be nitrogen-limited despite the historical nitrogen-loading in Narragansett Bay.

The results from this study are critical for documenting the variation among the ribbed mussel populations within differing nitrogen-loaded marshes, but will also be used as a benchmark for a longer-term study analyzing the historical responses of this species to changes in nutrient loads into Narragansett Bay.