Isotopic Composition Reveals the Impact of Oyster Aquaculture on Pelagic Nitrogen Cycling in a Subtropical Estuary

Abstract

To offset estuarine eutrophication, interest is increasing in restoring oyster reefs and expanding oyster aquaculture. However, ecosystem-scale evidence is lacking on oyster assemblages’ impacts on estuarine pelagic nitrogen (N) cycling. Using a multiple-isotope approach and isotope-mixing model, we examined the sources, transformations, and influence of intensive oyster aquaculture on N pollution in a subtropical estuary. The salinity-dependent NO3– and NH4+ concentrations and their correlations with isotopic signals (δ15N-NO3–, δ18O-NO3–, δ15N-NH4+) indicated the nutrient spatial distribution in low-salinity areas was largely regulated by mixing between freshwater and seawater. However, the intensive oyster aquaculture greatly increased nitrification in the estuary. In high-salinity areas where oyster assemblages were absent, the assimilation of NO3– by phytoplankton became dominant and sharply increased the δ15N-NO3– and δ18O-NO3–. Soil organic nitrogen and fertilizer, domestic sewage, and wastewater treatment plants were the major NO3– sources in the estuary, while internal nitrification contributed 20.6% to the NO3– pool. Oyster biodeposits comprised up to one-third of the particulate organic matter in the water column, and as much as 47.3% of the NH4+ pool could be from the oysters. Our study shows that oysters significantly contribute to the pelagic nutrient pools and N transformations, adding an important dimension to our understanding of oyster assemblages’ impacts on estuarine N cycling.