A sea change in Smuggler's Cove? Detection of decadal-scale compositional transitions in the subfossil record

Molluscan death assemblages in coastal marine settings are compositionally dynamic in time and space. Here, we describe a significant temporal faunal transition recorded over some two decades in the upper sedimentary veneer of a currently accumulating shell bed in Smuggler's Cove, St. Croix, US Virgin Islands. It had been demonstrated previously that molluscan death assemblages in the study area reflect the living assemblages from which they were derived. Further, a gradient among these assemblages reflects an environmental gradient of decreasing seagrass cover and increasing bioturbation, transitioning from lucinid bivalves and grazing gastropods inhabiting dense seagrass to other bivalves and predatory gastropods in bioturbated areas. More than two decades after initial sampling, the study area was resampled to investigate whether the environment and assemblages had changed relative to those reported in the earlier investigation. Results demonstrate that, while the general faunal gradient has remained intact, a notable area-wide biotic transition occurred among the living molluscan assemblages, during the intervening period, and is reflected in the accumulated death assemblage. This change is characterized by a major decline in the previously prolific grazing gastropod, Cerithium litteratum, and a similarly dramatic increase in the abundance of another grazing gastropod, Eulithidium affine. While the cause of this transition is not yet known, it demonstrates the compositional dynamism of this time-averaged assemblage on a decadal time-scale.