The effect of nitrogen loading on the growth rates of quahogs ( Mercenaria mercenaria) and soft-shell clams ( Mya arenaria) through changes in food supply

Abstract

Development of coastal areas in Cape Cod has increased nitrogen (N) loading to the region's estuaries. Nitrogen supply limits phytoplankton, the major food source of many filter-feeding bivalves living in the estuaries, including quahogs ( Mercenaria mercenaria) and soft-shell clams ( Mya arenaria). N enrichment may therefore affect growth rates of these shellfish by altering their food supply in the water column. In this study, food quantity, as measured by chlorophyll a as a proxy for phytoplankton, was found to be higher in estuaries subject to higher N load. The food quality in these estuaries, measured by the ratio of C to N, remained constant. Results of growth rate experiments on quahogs and soft-shell clams planted in three estuaries of different N load revealed that the shellfish grew fastest in estuaries where phytoplankton abundance was highest. Nitrogen stable isotope analysis confirmed that shellfish were consuming food from within their own estuaries, rather than food from an outside source. Therefore, it appears N load affected the growth rates of the shellfish primarily through causing changes in the quantity, rather than the quality, of the food supply.