Influences of the biotic and structural components of Crassostrea virginica on the oyster reef community.

Abstract

The hard, complex reef structure created by the eastern oyster, Crassostrea virginica, provides refuge and habitat that protects many organisms, allowing them to settle, survive, and spawn. In addition, oysters create copious amounts of biodeposits, which potentially serve as a basal nutrient resource for the reef ecosystem. I investigated the influence of oyster reef structure and oyster biodeposits on the reef community through a series of field experiments and mesocosm studies. Initially, the communities that colonized live oyster reefs were compared to communities that colonized empty oyster shell reefs, to evaluate the potential influence of live oysters that were actively feeding and creating biodeposits. Community assemblages on the two reef types were similar and no differences were seen with species level comparisons of abundance or biomass between the two treatments. The impact of oyster shell structure on energy transfer up the food chain from the basal resource of oyster biodeposits to the predator, Gobiosoma bosc, through the amphipod, Melita nitida was then investigated. Oyster shell structure effectively provided protection to amphipods, with reduction of predation impacts in high complexity habitats when a predator was present. Next, stable isotope signatures (d13C and d15N) of dominant reef species and basal resources, including oyster biodeposits, were measured seasonally to evaluate the major resource contributors to the reef. Overall, most carbon sources appeared to be pelagic in nature and an additional unidentified carbon source from outside of the oyster reef was incorporated into the food web. Finally, a d15N tracer study, utilizing biodeposits labeled with elevated d15N values, indicated that both Melita nitida and Neanthes succinea could incorporate Crassostrea virginica biodeposits and pass these nutrients to higher trophic levels. These studies suggested that oyster structure played a prominent role in defining the oyster reef community by providing habitat and protection for reef organisms. Mesocosm studies and isotopic analysis indicated that while some deposit feeders could consume oyster biodeposits, biodeposits were likely not a large component of their diet. Overall, these results suggest that structure was the dominant factor driving community organization on the reef, with minimal influence from oyster biodeposits.