MOLLUSCAN LIVE-DEAD AGREEMENT IN BARNEGAT BAY, NJ

Barnegat Bay, located in southern New Jersey, is an estuary under political and environmental scrutiny. It is particularly prone to eutrophication as it is shallow, with a highly developed watershed, and a long flush rate (54-74 days). Whereas benthic indices, such as AMBI, have been used to assess anthropogenic impact in Barnegat Bay, a lack of temporal context in these indices have left them potentially misleading. A baseline is important when analyzing degree of impact, particularly in systems that have been impacted for longer than monitoring records extend. A live-dead fidelity approach allows anthropogenic impact to be assessed by using the death assemblage as a baseline. Recent work by Kidwell has shown that live and dead assemblages in “pristine” systems should be concordant in terms of taxonomic similarity and rank-order abundance. When the live and dead assemblages differ, this can indicate ecological change. Applying this method to Barnegat Bay, live and dead molluscan assemblages were collected in three locations along a north-south gradient. Fourteen samples were collected from holes 1 m² by 30 cm deep, washed through a 5mm sieve, and sorted for all mollusks. Only right valves of bivalves with intact hinges and gastropod shells with the spire present were counted. Eighteen species of mollusks were identified in the death assemblages whereas only eight were identified in the live assemblages. Taxonomic similarity (Jaccard-Chao) values ranged from 0.586- 0.663 and rank-order abundance (Spearman rho) values ranged from 0.281-0.439. Evenness (PIE) was on average higher in death assemblages (0.81) than in live assemblages (0.72). Taken together, these values indicate that the assemblages are relatively concordant; the assemblages collected in Barnegat Bay display lower taxonomic similarity and rank abundance values than pristine estuaries typically do, but do not indicate high levels of impact. Evenness values also do not indicate impact. A possible explanation for such concordance is that disturbance has been happening for such an extended amount of time that the death assemblage has equilibrated to the impacted live assemblage. Results from dating of shells will help to assess this hypothesis. Additionally, future sampling along the north-south gradient will improve sample size and strengthen results.