A MULTI-SPECIES SURVEY OF NITROGEN ISOTOPES IN MARINE MOLLUSK SOFT TISSUES FROM A NORTH CAROLINA COASTAL SYSTEM: AN INDICATOR OF ANTHROPOGENIC NITROGEN LOADING?

Anthropogenic 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 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 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 mollusks 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. Measured δ¹⁵N values range from 3.13‰ to 9.49‰, indicating low-level wastewater pollution. It is possible, however, that additional loading of nitrogen from fertilizer runoff with low δ¹⁵N values counteracts the high nitrogen isotope signature of wastewater in this region. Furthermore, a large range in δ¹⁵N values within species, as well as between individuals of each species at a given site suggests little correlation between housing density and measured δ¹⁵N values. Little variation among Mercenaria mercenariaspecies between 2012 and 2018 indicates that there has been no significant change in nitrogen loading during this time.