Along the mid-Atlantic coast of the United States in numerous areas where the water is shallow enough for light to reach the bottom, heavy spring-summer growth of drift seaweed (macroalgae) covers sessile benthic fauna for periods of days to weeks. Coverage for such prolonged periods can exert sublethal effects and eventually lead to death. This study examines the effects of seaweed coverage on body weight, relative abundance, and mortality in an economically important bivalve Mercenaria mercenaria Linnaeus (hard clam) within the Delaware Inland Bays, a shallow estuary representative of systems in which the species occurs and where mortality events have been observed in recent years. Mean body weights (dry weight) were compared between five groups of clams (80–90 mm shell length), taken from two paired sites (each of which had adjacent areas of heavy and light seaweed), and one seaweed-free site during September of 1999. Heavy seaweed density persisted into mid-to-late summer and the groups of clams from these sites had the lowest mean weights. Both heavy seaweed areas also had low abundance of live clams and numerous recently dead clams, identified as such by their shells being still hinged together. Clams at heavy seaweed density sites were consistently out of the bottom, indicating stress. No hinged-dead were found at either of the light seaweed areas and clams were always buried as normal. These results confirm that prolonged coverage by heavy density mats of drift seaweed affects M. mercenaria detrimentally, and document a negative result of human-accelerated eutrophication in coastal waters. The findings emphasize the importance of good management of nutrient loads entering these and other similar shallow estuaries such that the ecological benefits of moderate seaweed growth are retained, whereas extremely dense growth that harms clams and other fauna, creating a public nuisance, is minimized.