Growth Rates for Quahogs (Mercenaria mercenaria) in a Reduced Nitrogen Environment in Narragansett Bay, RI

Abstract

Nitrogen is essential in sustaining estuarine ecosystems, particularly lower trophic levels and filter-feeding organisms. However, excess nitrogen is an often-cited cause of coastal ecosystem decline. One of the largest sources of nitrogen in Narragansett Bay, RI, is effluent from wastewater treatment facilities discharging into the head of the Bay. Beginning in 2005, nitrogen reductions at wastewater treatment facilities were implemented to minimize eutrophication and improve water quality. These reductions in nitrogen have led to speculation as to how the Narragansett Bay ecosystem will respond. The filter-feeding clam Mercenaria mercenaria (Quahog) is a prominent component of the Narragansett Bay ecosystem and an economically important resource in Rhode Island. In the northeastern United States, studies of several estuaries have linked nitrogen increases with increased phytoplankton concentrations and faster growth of Quahogs. We evaluated Quahog growth rates to assess how this species has responded to nutrient reductions in the bay. In 2016 and 2017, we collected and aged Quahogs from 6 sites in Narragansett Bay along a gradient of historical nitrogen loading and primary production. We found that Quahog growth rates still vary through space, as previously documented, but despite the recent 50% reduction in nitrogen loading to Narragansett Bay, rates were not significantly different from rates reported for Quahogs collected from 1984 and 2005. Our findings suggest that the reduced nitrogen has not had adverse impacts to Quahog growth, and this information can be used to inform the population's dynamics. However, ecosystem responses to the nutrient reductions are ongoing and the ecology of Quahogs should continue to be monitored to ensure the sustainability of this highly valued commercial and recreational fishery in Narragansett Bay.