FIDELITY OF MOLLUSCAN LIFE AND DEATH ASSEMBLAGES: A FIVE-YEAR STUDY TO ASSESS THE IMPACT OF ANTHROPOGENIC PRESSURES ON A NORTH CAROLINA TIDAL FLAT

To determine the degree of human modifications to marine ecosystems, molluscan live-dead fidelity studies compare the taxonomic composition of a local living community to the local death assemblage. Following Kidwell’s (2007) hypothesis, environments that suffer from anthropogenic stresses yield low fidelity (discordance) in rank-order abundance between living and death assemblages. In addition, Kidwell (2008) proposed that fidelity will be greater between concurrent living and death assemblages than between living communities sampled in successive years. For a five-year study, this live-dead approach has been applied to an intertidal, shelly, muddy-sand flat near Masonboro Sound, Wilmington, NC that is subjected to anthropogenic pressures, e.g. residential eutrophication, shellfishing, and dredging of the nearby Intracoastal Waterway.

As in previous years, sixteen bulk samples were collected from the upper 20 cm of sediment over a 200m² area. Overall, the pooled samples yielded 133 live individuals, representing 9 bivalve genera and 2 gastropod genera. The same samples contained 10,936 dead specimens, including 10,325 bivalve specimens from 31 genera and 611 gastropods from 14 genera. Spearman’s rank correlation was used to compare the rank-order abundance of species within the living and death assemblages. A significant rank correlation appears between living and dead bivalve assemblages (r_s=0.4402, p<0.0028). This fidelity is consistent with results from previous years. A nonsignificant rank correlation for living and dead gastropods (r_s=0.039, p<0.147) indicates mixed results when compared to previous years. For both bivalves and gastropods, successive living communities yield significant correlations for the 2011-2012 comparison (bivalves: r_s =0.3984, p<<0.001; gastropods: r_s =0.397, p<0.0004) and the 2010-2011 comparison (bivalves: r_s =0.5162, p<<0.001; gastropods: r_s =0.5290, p<0.0002); live-live comparisons were not significant for earlier years. Rarefaction analysis indicates that the death assemblage is greatly enriched in comparison to the living assemblage: 45.13 genera expected dead versus 11.26 live (95% confidence interval). As in previous years, both bivalve and gastropod death assemblages are enriched over the live assemblages.