16 Undergraduate Research (Posters)

Friday, 13 March 2009
8:30 AM - 12:30 PM

CONCORDANCE IN RANK-ORDER ABUNDANCE OF LIVING AND DEATH ASSEMBLAGES OF MOLLUSKS FROM A NORTH CAROLINA TIDAL FLAT

Paper number: 16-1
Program: Theme Sessions
Time: 8:30 AM – 12:30 PM

Authors

Abstract

Kidwell (2007) developed a method of assessing the degree of anthropogenic modification of a habitat by comparing the taxonomic composition of the local living community with that of local death assemblages. Discordance in rank-order abundance between living and death assemblages occurred in environments subjected to greater anthropogenic modification, particularly eutrophication. As part of a Research Experiences for Undergraduates study, we applied this technique to an intertidal, shelly, muddy-sand flat near Masonboro Sound, Wilmington, NC. Because this area has been subject to human influence (e.g., shellfishing, dredging of the adjacent Intracoastal Waterway, and nutrient input from nearby tidal creeks), we hypothesized that the live and death assemblages should differ significantly in taxonomic composition.

Sixteen samples were randomly taken from the upper 20 cm of sediment (~1 liter per sample) over ~ a 200 m2 area. In total, the pooled samples contained 83 live individuals, representing 7 bivalve genera and one gastropod genus. The same samples yielded nearly 3800 dead bivalve and gastropod individuals (3331 bivalve individuals in 37 genera, and 457 gastropods in 14 genera). Genera were ranked according to their abundance in both the living and death assemblage and Spearman's rank correlation was used to compare the abundance structure of the two assemblages. Contrary to our prediction, a significant rank correlation coefficient occurred between rank-order abundance of living and death assemblages for the bivalves and gastropods combined (rs = 0.5469, p<0.001), for bivalves (rs = 0.5496, p<0.001) and for gastropods (rs = 0.5473, p<0.05). This agreement is likely due to the fact that most species are rare within the death assemblage and absent from the live assemblage. Only one genus found in the live assemblage (Lithophaga) was not present in the death assemblage. The death assemblage was dominated by Crassostrea, of which only one live individual was found. Ostreola, Chione, and Mercenaria were common in both the live and death assemblage. Rarefaction analysis indicates that the death assemblage is slightly enriched over the live assemblage compared to what would be expected based on sample size alone (13.8 genera expected compared to 8 live; 95% confidence interval = 9.3 – 18.4 genera expected).