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Changes in growth rates in Mercenaria mercenaria in Narragansett Bay
Citation:
Robinson, S. AND A. Oczkowski. Changes in growth rates in Mercenaria mercenaria in Narragansett Bay. Coastal and Estuarine Federation (CERF) 24th Biennial Conference, Providence, Rhode Island, November 05 - 09, 2017.
Impact/Purpose:
Mercenaria mercenaria, or quahog, is a commercially important species harvested in Rhode Island waters. Improvements made to wastewater treatment facilities have reduced the nitrogen load into Narragansett Bay substantially in recent years. Because the quahog is a filter-feeding organism requiring plankton for growth and nitrogen concentrations can affect plankton composition and concentration, the concentration of nitrogen can potentially affect food quality and growth. The growth rate of commercial species such as the quahog is important information for state environmental agencies to help assess stock management.
Description:
The state of Rhode Island has taken several measures to reduce nitrogen (N) added to Narragansett Bay and waters leading to the Bay. These include upgrading wastewater treatment facilities to tertiary treatment and increasing the holding capacity of storm water to prevent storm runoff from overwhelming treatment facilities. These improvements have reduced inorganic N loads to the Bay by more than half in the last decade. As the region becomes more successful in limiting N loads to Narragansett Bay it is anticipated that plankton composition and concentration could change, altering the food supply for the filter-feeding bivalves. Long-term declines in surface water chlorophyll concentrations are well documented in Narragansett Bay. The impact of N reductions in this N-limited system are unknown. Upper Bay water quality is anticipated to improve while impacts on the food-limited lower Bay are unknown. We hypothesized that the N reductions may impact the growth rate of Mercenaria mercenaria, or northern quahog. This study documents how the growth rates of quahogs, the most economically important shellfish species in Narragansett Bay, are responding to the recent changes. We calculated age and compared growth rates, calculated using the von Bertalanffy growth model, of M. mercenaria from 9 sites in Narragansett Bay and one site from a nearby salt pond west of the Bay and unimpacted by the wastewater N reductions. Current growth rates were compared to growth rates of M. mercenaria in Narragansett Bay reported in studies from the 1980s and 2005 using an existing R-programmed likelihood ratio test to evaluate von Bertalanffy growth curves. Additionally, the rate of growth was assessed relative to changes in dissolved inorganic nitrogen concentration and relative levels of production in the water over the same time period as the ages of the quahogs.