IDENTIFYING TERRESTRIAL CONTROLS ON SPATIAL VARIATION IN MOLLUSCAN DEATH ASSEMBLAGES, ST. CROIX, USVI

Diagnoses of the degree to which biotic assemblages vary in space as consequences of environmental agents provide fundamental baselines for understanding how they respond to environmental change. Occurrences of spatial variation in marine assemblages are typically ascribed to local variation in the marine environment, but there has been less consideration of potential influences from proximal onshore environments. Here, we analyzed terrestrial impacts on shallow marine communities from seven localities around St. Croix, USVI, within nearshore, seagrass-associated molluscan death assemblages that show significant site-to-site variation in molluscan composition. The marine localities are similar with respect to several primary environmental attributes, including salinity, temperature, depth, and substrate (dense seagrass); this allowed us to isolate the influence of terrestrial factors on molluscan death assemblages. Terrestrial variables were extracted from watersheds on the island that drain into the offshore sample localities, and include bedrock lithology, topographic indices, average annual precipitation, and 13 land-use categories. Among several compelling relationships, we found that diagnostic bedrock types in watersheds correlated with molluscan composition in the adjacent marine localities. Precipitation also correlated with marine composition, particularly for Salt River Bay, which receives two to three times more rainfall over the montane regions of the watershed annually than any other locality. Of the anthropogenic variables, agriculture showed its strongest influence on molluscan composition in Ha’Penny Bay, an area along the south shore whose catchment area is characterized by a significant amount of agricultural activity. These preliminary results provide compelling evidence that local terrestrial factors, even those several kilometers upstream from the marine samples, may play important roles in controlling the spatial variation of marine mollusks in St. Croix’s nearshore environments.