FIDELITY OF MOLLUSCAN LIFE AND DEATH ASSEMBLAGES: A FOUR-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. As part of a four-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 72 live individuals, representing 10 bivalve genera and 3 gastropod genera. The same samples contained 2,624 dead individuals, including 2,462 bivalve individuals from 32 genera and 162 gastropods from 12 genera. Spearman’s rank correlation was used to compare the rank-order abundance of species within the living and death assemblages. Significant rank correlations appear between living and death assemblages for bivalves (r_s=0.5417, p<0.008) and for gastropods (r_s=0.7674, p<0.003). This fidelity is generally consistent with results from previous years. For both bivalves and gastropods, successive living communities yield lower, nonsignificant correlations with the exception of the 2010-2011 comparison (bivalves: r_s =0.5162, p<0.000; gastropods: r_s =0.5290, p<0.0002). Rarefaction analysis indicates that the death assemblage is greatly enriched in comparison to the living assemblage: 25.2 genera expected dead versus 10 live (95% confidence interval). Corresponding with trends from previous years, both bivalve and gastropod death assemblages are enriched over the live assemblages.