The influence of community structure on the seagrass Thalassia testudinum was studied in 3 distinct communities low and high density monocultures, and patches intermixed with Halodule wrightii, in Cockroach Bay, Tampa, Florida. T. testudinum shoot-specific leaf mass and growth were significantly higher in low density monocultures, and both variables were negatively correlated with short-shoot density, Intraspecific, competition-density effects in high-density seagrass beds may be responsible for the relatively lower shoot-specific leaf mass and growth rates, possibly due to the reduction of available light from dense leaf canopies. These observations are supported by significantly higher T. testudinum leat C:N and δ 13 C in low density monocultures, which suggests that high rates of growth are coupled with high C and N demands and reduced discrimination of C. Lower T testudinum shoot-specific leaf mass and growth in mixed species patches may be partially explained by interspecific competition with H. wrightii, however, the exact mechanisms by which these species interact were not discovered. Differences among communities in sediment NH 4 + and PO 4 3- , and T. testudinum leaf C:N:P and δ 15 N suggest that community structure in seagrass meadows plays an important role in sediment nutrient dynamics and, potentially, nutrient availability. However, low leaf C:N and C:P ratios suggest that nutrients are not limiting in this system. This study shows that intraspecific competition, and to a lesser degree, interspecific interactions with H. wrightii, are important determinants of T testudinum productivity and potentially, seagrass community structure in Cockroach Bay.