NITROGEN ISOTOPES IN MARINE MOLLUSKS AS INDICATORS OF NITROGEN LOADING IN COASTAL NORTH CAROLINA

Nitrogen loading to coastal ecosystems is a growing concern. Elevated N can lead to dysoxic dead-zones, which in addition to negatively affecting the ecosystem, can lead to financial loss for shellfish aquaculturists. Significant sources of coastal eutrophication include both sewage effluent and agricultural fertilizer. Nitrogen isotope signatures (δ¹⁵N) serve as a valuable resource for detecting nitrogen pollution in a given environment, and can indicate the source of the pollution. Wastewater is typically enriched in ¹⁵N, while inorganic fertilizer is depleted in ¹⁵N. Marine mollusks are reliable recorders of environmental nitrogen isotope signatures, as they are typically primary consumers feeding on phytoplankton which incorporate N from the dissolved inorganic nitrogen. Mollusks are also locally common, have a global distribution, and generally withstand pollution. Mollusk tissues also integrate long time periods as opposed to point sampling water, providing time averaged δ¹⁵N values. In this project we aim to capitalize on the utility of several marine mollusk species to investigate pollution sources along the North Carolina coast in the Cape Lookout region. Specifically, we sampled the bivalve species Mercenaria mercenaria, Chione cancellata, Argopecten irradians, Noetia ponderosa, Donax variabilis, Crassostrea virginica and Atrina sp., and the gastropod species (welks) Busycon carica and Sinistrofulgur perversum during the summer of 2018. We sampled shells from regions of high and low housing density to examine the potential of elevated nitrogen pollution rates due to runoff from leach fields implemented in septic tank systems. We also sampled Mercenaria mercenaria tissues over time, with samples sporadically ranging back to 2012. We hypothesize shells sampled in regions of greater housing density will exhibit higher δ¹⁵N values as compared to uninhabited regions due to ¹⁵N enriched wastewaters leaching from septic tanks. However point-sampled suspended particulate δ¹⁵N values collected in the fall and spring show the opposite trend with lower δ¹⁵N values in creeks and lagoons (about +2.2‰) versus the ocean outside the barrier island complex (about +5.6‰).