The effects of eutrophication on Mya arenaria and Mercenaria mercenaria: Growth, survival, and physiological responses to changes in food supply and habitat across estuaries receiving different nitrogen loads

Increased land-derived nitrogen loads have eutrophied many estuaries, potentially changing estuarine features important to commercially harvested bivalves. To define how land-derived nitrogen loads affect bivalves, I compared growth, survival, and physiological responses of quahogs, Mercenaria mercenaria, and softshell clams, Mya arenaria, across estuaries receiving different N loads.; First, to determine how nitrogen enrichment affected estuarine features and relate those changes to food supply and habitat for clams, I measured isotopic composition, chlorophyll a, carbon, and nitrogen in seston and sediment, and other environmental variables at the sediment-water interface where clams live and feed. Major effects of nitrogen enrichment on near-bottom seston and surface sediment were to (1) increase microalgal concentrations and reduce carbon to nitrogen ratios, increasing quantity and quality of available foods, and (2) reduce oxygen content in sediments, potentially reducing habitat quality.; Second, to determine how N loads affected bivalve growth and survival, I measured shell growth and survival directly in transplanted clams and estimated growth throughout life indirectly in native clams by the von Bertalanffy growth model. Although growth was limited by salinity in some estuaries, and low oxygen concentrations may have reduced survival, the major effect of N enrichment on clams was increased growth through increased food supply.; Third, to link bivalve responses to land-derived N loads and determine which food sources were most important to clams, I compared nitrogen and carbon isotopic signatures in foods to those in clam tissues. Clams selected particles primarily from seston, and nitrogen loading did not change the type of particles assimilated.; Fourth, to assess nitrogen and carbon assimilation and turnover in clam tissues in response to changes in food supply, I measured changes in nitrogen and carbon isotopic composition of seston and sediment and in tissues and egesta of transplanted clams. Isotopic signatures in clam tissue and egesta responded rapidly to changes in food supply, with most change due to growth. Nitrogen turnover rates were 0.6% per day in both species.; These results are biologically relevant to ecology and stock management of clams, and suggest that the major effect of N enrichment on clams was increased secondary production.