Shellfish Restoration and Aquaculture Projects as a Means to Mitigate Coastal Nutrient Pollution
Michael Rice, University of Rhode Island
Abstract
In many estuaries around the country, the current populations of shellfish are greatly reduced in comparison to the early part of the 20th Century. For example, in 1910 Narragansett Bay of Rhode Island supported oyster aquaculture operations producing about 25,000 metric tons of oyster meats annually. Last year in 2006, the yield of oysters was about 2.35 million pieces, or about 45 metric tons. Bivalve populations in estuaries can filter a substantial fraction of the water volume on a daily basis: for example the hard clams of the Providence River section of Narragansett Bay can filter, on average, a volume equivalent to 20% of the water that is physically exchanged owing to the tides. The filtration of phytoplankton food from estuarine waters is also a mechanism for removing nutrients N and P from the water and incorporating these nutrients into harvestable shellfish biomass. The volume of oysters harvested in Narragansett Bay in 1910 is estimated to compensate for the nitrogen wastes produced by about 440,000 humans living in the watershed. In addition to incorporating N and P into their tissues, filter-feeding bivalves deposit waste that stimulates decomposition and nitrogen removal processes in the sediments and, through their filter-feeding, they act to moderate the boom and bust cycles of phytoplankton blooms that can create night-time hypoxic (low oxygen) events. If properly sited and managed, shellfish restoration projects and commercial shellfish aquaculture operations can be valuable allies in efforts to foster and maintain good water quality in public waters.