Nitrogen isotopes in marine mollusks as indicators of nitrogen loading in coastal North Carolina

Nitrogen loading to coastal ecosystems is a growing concern. Elevated nitrogen can lead to hypoxic dead-zones, which in addition to negatively affecting the ecosystem, can lead to financial loss for marine fisheries and 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 pollution source, as wastewater is generally 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 (DIN). Further, mollusks are 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 <em>Mercenaria mercenaria, Chione cancellata, Argopectin irradians, Noetia ponderosa, Crassostrea virginica, Donax variabilis</em>, and <em>Atrina sp.</em>, and the gastropod species (whelks) <em>Busycon carica </em>and <em>Sinistrofulgur perversum </em>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 <em>Mercenaria mercenaria </em>tissues over time, with samples sporadically ranging back to 2012 in order to analyze potential long-term trends in nitrogen loading in this region. Further, we measured shell length to assess a possible correlation between mollusk age and nitrogen isotope signature. Measured δ¹⁵N data indicate that creeks exhibit possible low-level wastewater pollution, as notable differences in nitrogen isotope signature exist between creeks and outer banks with outer banks exhibiting lower δ¹⁵N values. Lack of overall trend in δ¹⁵N values between 2012 and 2018 suggests that there has been no significant change in nitrogen loading in this region throughout recent time.

Union College Schaffer Library Digital Projects

Union College Schaffer Library Special Collections, 807 Union St., Schenectady, NY 12308; 518-388-6620; https://www.union.edu/schaffer-library