Effects of community structure on the seagrass Thalassia testudinum

The influence of community structure on the seagrass Thalassia testudinum w as studied in 3 distinct communities: low and high density monocultures, an d patches intermixed with Halodule wrightii, in Cockroach Bay, Tampa, Flori da. T. testudinum shoot-specific leaf mass and growth were significantly hi gher in low density monocultures, and both variables were negatively correl ated with short-shoot density. Intraspecific, competition-density effects i n high-density seagrass beds may be responsible for the relatively lower sh oot-specific leaf mass and growth rates, possibly due to the reduction of a vailable light from dense leaf canopies. These observations are supported b y 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 hi gh C and N demands and reduced discrimination of C. Lower T. testudinum sho ot-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. Difference s 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 compet ition, 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.