Bivalves filter a variety of particles from the water column, including phytoplankton, detritus and other organic matter. However, little is known about how bivalve diets vary across species, location, and season. The purpose of this project was to establish a baseline from which to measure trophic dynamics, organic matter sources, and general nutrition of shellfish across aquaculture farm sites to understand the impact of aquaculture on marine coastal food webs. Stable isotopes, in particular carbon (13C) and nitrogen (15N), relate and identify different species dynamics to increase knowledge of trophic level interactions within a food web. Carbon indicates different primary producers. The rate of convergence over time of δ13C signatures between phytoplankton and shellfish signifies how quickly shellfish respond to changes in organic matter as a nutrition source. Additionally, δ15N provides a measure of trophic fractionation between the organic matter food source and predators. Mussels (Mytilus edulis) and oysters (Crassostrea virginica) were fed the same species of phytoplankton (Nannochloropsis sp) over a 60 day period. Samples collected at seven different intervals underwent treatment from an elemental analyzer, mass spectrometer, and a reduction reactor to identify the ratios of the carbon and nitrogen isotopes present in the sample.
Results/Conclusions
The average fractionation δ13C for oysters was -0.8 ‰ and 0.2 ‰ for mussels; both are close to the literature value of 0-1‰. A separation occurred between day 15 and day 30 in the fractionation δ15N for both bivalves. The literature value for δ15N is 3.4‰ and the average fractionation δ15N for mussels prior to day 15 was 15. 9‰ (SD 1.73) and is 2.4‰ (SD 0.7) for the average after day 30. A similar trend is shown in oysters where the δ15N average was 18.9‰ (SD 1.79) prior to day 15 and the average after day 30 was 5.8‰ (SD 1.97). This indicates that it takes 30 days for organic matter to be incorporated into its consumers tissues. These results are comparable to those of the Great Skua (Catharacta skua) which has a 14.4 day assimilation rate of δ15N in blood and feathers. This provides a baseline between the bivalves and their diet which corresponds to trophic level interactions surrounding aquaculture farms that will impact future farm placement in coastal waters.