TAPHONOMIC BIAS IN CHESAPEAKE BAY MOLLUSKS: EFFECTS ON SPECIES COMPOSITION, DIVERSITY, AND ABUNDANCE

The Chesapeake Bay faces a myriad of anthropogenic problems, including over-fishing, nutrient pollution, and increased turbidity. Restoration of benthic communities relies on accurate estimates of Holocene diversity and abundance; however, studies of Bay molluscan paleoecology and taphonomy have yet to be attempted. The purpose of this study is to examine preservational bias and to explore its effects on species composition, diversity, and abundance in Chesapeake Bay benthic mollusks. We applied a live-dead assemblage comparison approach to ten sites distributed throughout the main channel and major tributaries of the Bay. Data on death assemblages were obtained from replicate samples, collected over a two-year period from each site. These samples were collected using a Young’s modified grab, sieved into four fractions (4mm, 2mm, 1mm, and 0.5mm), and stored temporarily in formalin. Both whole specimens and fragments were sorted and identified to species level whenever possible. Data on live communities for the same sites, sampled biannually over the past 20 years, were provided by the benthic monitoring division of the Chesapeake Bay program. This study focused specifically on comparisons of species composition, abundance, diversity, and evenness in the live community versus the death assemblage. Species composition comparisons were assessed using: (1) the percentage of species in the live community found in the death assemblage, (2) the percentage of species in the death assemblage found in the live community, and (3) the percentage of individuals of species found in the death assemblage that are also found in the live community. Abundance was quantified using three metrics (raw, rank, and proportional) and the Simpson index, Shannon-Wiener index, and Pielou’s J’ were used to quantify diversity and evenness. Preliminary results indicate that the percentage of live molluscan species recorded in the death assemblage of the Chesapeake Bay (33-85% within each site) varies considerably, but does fall within the range documented in other estuarine environments.