Competition between the seagrasses Thalassia testudinum and Syringodium filiforme in a Caribbean lagoon

In typical Caribbean seagrass beds, several seagrass species CO-occur and may compete for resources. The decline in Syringodium filiforme as Thalassia testudinum becomes abundant during successional development of Caribbean seagrass beds suggests that competition occurs between these seagrasses. The hypothesis that T. testudinum outcompetes S. filiforme for Light and sediment nutrients was tested in situ within a shallow seagrass bed in St. Crolu, U.S. Virgin Islands. Exploitative conlpetition with T testudinum for primarily sediment nutrients but also light limited the leaf shoot density of S. filiforme but had little effect on leaf growth. Removal of the T testudinurn leaf canopy increased hght above the saturation level of photosynthes~s by S. filiforme and leaf shoot density and belowground blomass of S. filiforme doubled within 9 mo compared to control areas. The T. testudinunl canopy protected S. f ~f o r m e leaves from breaking. Fertilization of the sediments with nitrogen and phosphorus increased leaf shoot density and belowground biomass of S. fibforme, an effect enhanced by T. testudinum canopy removal. Belowground competition for nutrients limited the area1 aboveground productivity of S. filiforme more than exploitation of light by T testudinum. Increased shoot density of S. filiforme resulting from greater rhizome density, rhizome branching, and leaf pairs per node contributed more to enhanced aboveground productivity with additional light or nutrients than increases in leaf growth.