Field experiments were conducted from 1980 to 1982 to test how shell growth of the suspension—feeding bivalve Mercenaria mercenaria within a single estuarine system around Cape Lookout, North Carolina, varied with (1) year, (2) density within enclosed 1—m2 plots, (3) the presence of enclosure walls, (4) the source of the clams, (5) site, and (6) various two—way interactions between these factors. Growth rates generally did not change between years, although occasional interactions between site and year were caused by specific sites exhibiting significant temporal changes in growth rate. Mercenaria at densities of 80 individuals/m2 tended to grow less than at densities of 10 individuals/m2, with a significant average reduction of 18% for 6—cm clams and a nearly significant 10% reduction for 3—cm clams, presumably because of enhanced local food depletion in benthic boundary layers at higher densities. The presence of enclosure walls enhanced Mercenaria growth by 15—21%, perhaps because hydrodynamics roughness of projecting walls increased mixing into the otherwise depleted bottom waters. Clams taken from a single common source grew an average of 43—86% faster than locally collected clams at each of five other sites scattered throughout the estuarine system. This effect of clam origin did not diminish in the 2nd yr, suggesting that the difference was more likely to have a genetic than a physiological basis. Mercenaria growth differed by as much as threefold among sites within the estuarine system and even among habitats within a single localized water mass. Despite this substantial growth variation among sites and occasional site x year interactions, the effects of manipulating local density, clam origin, and presence/absence of enclosure walls did not vary significantly with site. A simple additive model serves to describe how site effects combined with experimental effects to explain Mercenaria growth rate (defined as ln[final size/initial size]).

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