TEMPORAL AND GEOGRAPHIC VARIATION IN MOLLUSCAN ASSEMBLAGES FROM SEAGRASS BEDS AROUND ST. CROIX, US VIRGIN ISLANDS: TOWARDS THE DELINEATION OF NATURAL VERSUS ANTHROPOGENIC DRIVERS

Coastal development has had dramatic effects on shallow marine molluscan communities. The island of St. Croix provides a prime opportunity to assess anthropogenic effects on the compositions of molluscan assemblages because it accommodates a range of environmental modifications around its perimeter in association with major industries, dense populations, and agriculture. More pristine stretches of coastline are also available to establish baselines for comparison with heavily modified areas. Our long-term goal is to determine the effects of various types of anthropogenic modification around St. Croix on molluscs associated with dense seagrass, where root-rhizome mats inhibit bioturbation by Callianassa, thereby permitting assessment of stratigraphic changes in death-assemblage composition, and in the sedimentological and geochemical properties of associated sediments. Here, we focus on an initial set of samples collected in June, 2011 at several seagrass-covered locations around the island.

For all samples, the 2-mm-and-greater size fraction was collected from the sea floor in 30 cm-diameter cylindrical columns, with penetration to 40 cm below the sediment-water interface. Efforts were made to maintain the stratigraphic integrity of each column by separately airlifting the top, middle, and bottom thirds. Samples collected previously from two locations, Salt River Bay (1989) and Smuggler’s Cove (1980 and 2002) were also available for direct comparison with those collected from both venues in 2011.

Analyses of samples reveal stratigraphic/temporal trends in species composition within columns at several localities. A temporal pattern was also observed when comparing the 2011 samples to those collected previously at Smuggler’s Cove, where an earlier analysis demonstrated significant compositional changes between 1980 and 2002. Compositions have not reverted back to their 1980 states, and exhibit greater similarity to 2002 than to 1980. Geographic variation in composition is also apparent among several localities. As our investigation unfolds, geochemical and sedimentological analyses will be used to help diagnose specific signatures of anthropogenic modification and, therefore, to help determine whether spatial and temporal variation resulted from natural or anthropogenic processes.