GEOGRAPHIC VARIATION IN MOLLUSK ASSEMBLAGES AND ITS RELATIONSHIP TO GEOCHEMISTRY: ST. CROIX, U.S. VIRGIN ISLANDS

A combination of physical, chemical, and biological parameters may influence the compositions of marine molluscan assemblages. Here, we examine the association of geochemical variation with the distributions of molluscan life and death assemblages in shallow seagrass beds around St. Croix, U.S.V.I. Seven localities, chosen primarily because they are adjacent to unique sites of anthropogenic activity along the coastline, were sampled: three on the south shore, and four on the north. At each locality, sampling was conducted every 10 m along 30-m transects; bulk sediments, which were collected down to 40 cm below the sediment-water interface, were extracted using an airlift and sieved in situ through 2 mm mesh. These samples were collected in three discrete depth intervals of ~13 cm each to assess stratigraphic variation in molluscan composition; a corresponding set of sedimentological samples was collected from the wall of the hole left behind after airlifting. At each station, a fourth sediment sample was taken adjacent to the sediment-water interface. In the lab, all mollusks larger than 2 mm were picked from the bulk sediments and identified to species level; live mollusks were counted separately. Sediment samples were ground to a fine powder in a mixer mill, pressed into pellets, and subjected to x-ray fluorescence to quantify the concentrations of major and trace elements.

Nonparametric multidimensional scaling (NMDS) revealed that the large majority of samples exhibited more among than within-locality variation in molluscan and geochemical compositions; although all localities were characterized by dense seagrass cover, each exhibited compositional distinctiveness. For some trace elements, there were clear differences in composition between localities on the north shore and those on the south shore. Furthermore, canonical correspondence analysis (CCA) of the combined taxonomic and geochemical data showed that biotic composition was strongly correlated from locality to locality with compositional variations in several elements. While all of these elements occur naturally in seawater, the concentrations of some are enriched beyond natural levels, suggesting anthropogenic influence both on geochemistry and indigenous molluscan communities.