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 leaf C:N and delta(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 NH4+ and PO43-, and T. testudinum leaf C:N:P and delta(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.