MEASURING ANTHROPOGENIC INFLUENCE ON MOLLUSCAN POPULATIONS OF MASONBORO INLET, NORTH CAROLINA, USING LIVE-DEAD AND LIVE-LIVE ASSEMBLAGES

For centuries humans have been impacting the environment. Human activities alter habitats and affect species distribution and abundance. Kidwell (2007) proposed that taxonomic similarity between live and dead assemblages decreases with increasing anthropogenic eutrophication, and that rank-order agreement in species abundance drops where anthropogenic eutrophication is suspected for data sets that include only adult individuals. Kidwell (2008) proposed that there are more similarities between live and dead assemblages collected at the same time from a single locality than between live assemblages collected in successive years.

To test Kidwell’s hypotheses, we collected 16 random samples of ~ 2 liters from the upper 20 cm of sediment over a ~200 m² area in an intertidal shelly muddy-sand flat near Masonboro Sound, Wilmington, NC. We compared these samples with those of a similar study conducted in 2008 (Dietl et al. 2009). In total we found 231 live specimens in the samples, including 12 bivalve and 3 gastropod genera. All samples in total contained 3,878 dead individuals (32 bivalve and 11 gastropod genera). Genus richness was compared using rarefaction. Results show the dead assemblage was not significantly enriched over the live assemblage. Comparison of samples collected this year vs 2008 indicated no significant difference in richness between the live samples, but the dead gastropod assemblage was richer in 2009 than 2008, and dead bivalve richness was greater in 2008. We used Spearman‘s rank correlations to compare the rank-order abundance of genera in the live and dead assemblages. Rank correlations for the 2009 comparisons were: live-dead bivalves, r_s= 0.7988 (p<0.01); live-dead gastropods, r_s= 0.2909 (ns). The positive correlation for live-dead bivalves may suggest a lack of eutrophication or may occur because most taxa are rare in the dead assemblage and absent from the live assemblage. The rank correlations between samples collected in 2008 and 2009 were: live-live bivalves, r_s= 0.2420 (ns); dead-dead bivalves, r_s= 0.7820 (p<0.001); live-live gastropods, r_s=-0.3500 (ns); and dead-dead gastropods, r_s=0.7483 (p<0.02). These results confirm that there are more similarities between live and dead assemblages collected at the same time than between live assemblages collected in successive years.