LIVE-DEAD FIDELITY OF MOLLUSCAN ASSEMBLAGES IN ANTHROPOGENICALLY IMPACTED SEAGRASS HABITATS, NORTH CAROLINA

Molluscan live-dead fidelity studies investigate the influences of anthropogenic activities on marine ecosystems by comparing the taxonomic composition of a local living community to the local death assemblage. Kidwell (2007) hypothesized that environments subjected to anthropogenic stresses yield low fidelity (discordance) in rank-order abundance between living and death assemblages. This live-dead approach was applied to three intertidal, siliciclastic seagrass environments of North Carolina impacted by different human activities: 1) Oyster Creek (OC), agricultural/terrestrial runoff and trash; 2) Bogue Sound (BS), freshwater runoff and clam harvesting; and 3) Chadwick Bay (CB), boat traffic and dredging of the Intracoastal Waterway.

At each site, bulk sediment samples were collected from the upper 40 cm of the substrate at 10m intervals along three 30m transects. Spearman’s rank correlation was used to compare the rank-order abundance of genera within the living and death assemblages at and among sites. When bivalve and gastropod assemblages are pooled, there are significant rank correlations between living and dead assemblages at all sites (OC: r_s=0.3492, p<0.0004; BS: r_s=0.4288, p<0.0008; CB: r_s=0.2801, p<0.0006). Death assemblages for each site are also significantly correlated to one another (OC vs. BS: r_s=0.6962, p<<0.0001; OC vs. CB: r_s=0.4438, p<<0.0001; BS vs. CB: r_s=0.3212, p<0.0014). Living assemblages are not significantly correlated among sites. Rarefaction analyses indicate that living and dead assemblages at Chadwick Bay are enriched compared to the other sites and that Oyster Creek yields the lowest diversity and abundance (95% confidence interval). Death assemblages at all sites are enriched in comparison to corresponding living assemblages. Significant live-dead fidelity contradicts our hypothesis. However, similarity of death assemblages and disparity of living communities among the three sites suggest that different anthropogenic pressures may yield specific ecologic responses by and consequences for molluscan assemblages. Field observations suggested that Chadwick Bay is the most stable, least impacted environment and Oyster Creek is the most stressed. Significant rank correlations and rarefaction results support these inferences.