ASSESSMENT OF ANTHROPOGENIC IMPACT ON CRASSOSTREA VIRGINICA REEFS IN SOUTHEASTERN NORTH CAROLINA USING LIVE-DEAD ANALYSIS

Along the southeastern North Carolina coast, Crassostrea virginica reefs are an ecologically and economically important component of the estuarine ecosystem. Discordance between the taxonomic composition of dead and live assemblages of mollusks has been suggested to indicate recent anthropogenic change (Kidwell 2009).

Previously, Lambert et al. (2009) conducted live-dead analyses to determine the magnitude of anthropogenic impact at three sites on oyster reefs at each of five locations near Wilmington, NC: Howe Creek, Pages Creek, Hewletts Creek, Masonboro Island 1 and 2. They used Kidwell’s (2009) live-dead method to assess the fidelity of the dead assemblages of mollusks (taken from the top 30cm of the oyster reef matrix) to the living community in terms of rank order abundance and species richness. Despite an independent water quality assessment illustrating varying levels of human impact, all Lambert et al. (2009) samples yielded concordance in live/dead assemblage composition. It is possible, however, that the dead assemblages used by Lambert et al. (2009) were not old enough to be used as a baseline; i.e., the dead assemblages accumulated after anthropogenic impact.

In this study, we resampled the same reefs as Lambert et al. (2009) to a greater depth (60cm). We compared previous results with current samples in which oysters were dominantly found at all depths. Comparisons were made between the dead assemblage at 30-60cm and the live assemblage (0-30cm) using Spearman’s rho (which compares rank order abundance of species in the two assemblages) and the Chao-Jaccard test, which determines similarity of species composition and abundances of the two assemblages. Preliminary results from Howe Creek Sites 2 and 3 suggest concordance between the live and dead assemblage; values for the Spearman’s rho and Chao-Jaccard coefficients were statistically significant. Spearman’s rho-values calculated for Howe Creek Site 2 and 3 were 0.691 and 0.461. The Chao-Jaccard values calculated for Howe Creek Site 2 and 3 were 0.956 and 0.928. The concordance may indicate that deeper samples are also post-impact. Alternatively, the methods employed may not be sensitive enough to use with low-diversity samples. Radiocarbon dating of oysters will be performed to determine if the deeper samples employed in this study are still post-impact.