Effects of community structure on the seagrass Thalassia testudinum

Abstract

The influence of community structure on the seagrass Thalassia testudinum was studled in 3 distinct communities-low and high density monocultures, and patches intermixed with Halodule wrightii, in Cockroach Bay, Tampa, Florida. T. testudlnum shoot-specific leaf mass and growth were significantly higher in low density n~onocultures, and both variables were negatively correlated with short-shoot density. Intraspeclfic, competition-density effects in high-density seagrass beds may be respons~ble for the relatively lower shoot-speclfic leaf mass and growth rates, possibly d u e to the reduction of available light from dense leaf canopies. These observations are supported by significantly higher T testudinum leaf C . N and 6I3C 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 7 testudinum shoot-specific leaf mass and growth in mixed species patches may b e 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,' and PO," and T testudinum leaf C:N:P and 6151\' 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 intraspeclflc competition, and to a lesser degree, interspecific interactions with H. wr~ghtii, are mportant determinants of T testudlnum productivity and, potentially, seayrass cornnlunlty structure in Cockroach Bay.