DIAGNOSING ENVIRONMENTAL CHANGE IN SEAGRASS BEDS BASED ON STRATIGRAPHIC TRANSITIONS IN MOLLUSCAN DEATH ASSEMBLAGES

For the diagnosis of temporal ecological changes in shallow marine communities, long-term census data of life assemblages are generally not available to provide direct evidence of compositional changes through time. In the absence of these data, live/dead fidelity analyses are utilized because they provide points of comparison between the past and the present. The tens of cm of sediment typically sampled, however, can represent anywhere from a few hundred to several thousand years-worth of accumulation in most shallow settings, limiting opportunities to assess historical changes in the run-up to the present day. Analyzing, instead, discrete depth intervals may permit assessments of changes through time in settings like seagrass beds, where bioturbation is inhibited and there is the potential for original stratigraphy to be maintained.

We used this approach in seven seagrass-covered localities around St. Croix, USVI in 2011 and 2012. At each sampling station, three sequential samples, each ~13 cm thick, were airlifted with the aid of a cylindrical steel template 30 cm in diameter and 40 cm in length, driven into the substrate prior to airlifting. Multiple stations were sampled at 10 m intervals along the transects at each locality. In the lab, live and dead individuals were identified and counted, and samples were compared to diagnose compositional changes, if any, among the three intervals, and to determine whether there was any tendency for shallower—and presumably younger—intervals to exhibit greater compositional fidelity to the life assemblages. We found that, indeed, most sites display systematic compositional changes with depth and the greatest fidelity was found between the life assemblage and the uppermost layer of the death assemblage. Clearly, the upper layer is sufficiently dynamic to incorporate changes from the life assemblage rather quickly, as suggested by our previous work. Furthermore, we identified taxa that contributed noticeably to compositional transitions at some sites. These included the tellinid Scissula, whose abundance is anomalously high in the deepest intervals from Smuggler’s Cove, and the mussel Brachidontes, whose abundance at a locality adjacent to an extensive landfill increases toward the surface and closer to shore and is also unusually abundant in the life assemblage.