THE DIAGNOSIS AND SIGNIFICANCE OF STRATIGRAPHY IN SUBFOSSIL MOLLUSCAN ASSEMBLAGES PRESERVED IN SEAGRASS BEDS: ST CROIX, US VIRGIN ISLANDS

Death assemblages that occupy the upper 40 cm of sediment in shallow marine settings around St. Croix have previously been shown to equilibrate to recent, decadal scale change observed in life assemblages. This calls into question the ability of death assemblages to serve as pre-disturbance baselines, particularly in areas where there is evidence of bioturbation in at least the upper half meter of sediment. Here, we test the hypothesis that seagrass beds, because their dense root-rhizome mats inhibit bioturbation, may prevent mixing and, with it, a record of biotic transitions leading up to the present day. We sampled seven localities around St. Croix, collecting three separate sediment depth intervals of ~13 cm each to a total depth of ~40 cm below the sediment-water interface, and we found that stratigraphy was indeed preserved, to varying degrees, at all seven of the localities. We determined that stratigraphic changes in composition were primarily mediated by shifting abundances of a few common species. An evaluation of substrate variations implied by the changing abundances of species with different substrate preferences, suggested that substrates remain relatively constant through time and are dominated throughout by Thalassia-rich environments, with the most variation between the shallowest death assemblage and the living community. Calculating fidelity for the life assemblage compared to the death assemblage in each of the depth intervals, the shallowest death assemblages were typically more similar to the life assemblage than were deeper samples, suggesting that that deeper intervals may, indeed, provide an indication of different, past community states.