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Stable Isotope Identification of Nitrogen Sources for United States (U.S.) Pacific Coast Estuaries
Citation:
Brown, C., Jim Kaldy, P. Fong, TChris MochonCollura, AND Pat Clinton. Stable Isotope Identification of Nitrogen Sources for United States (U.S.) Pacific Coast Estuaries. 2016 Ocean Sciences, New Orleans, LA, February 21 - 26, 2016.
Impact/Purpose:
Nutrients are the leading cause of water quality impairments in the United States, and as a result tools are needed to identify the sources of nutrients. We assembled a regional scale dataset to evaluate the utility of using stable isotopes to identify nitrogen sources to estuaries along the Pacific Coast of North America (spanning from Alaska to Mexico). Gradients in isotope data were compared to nitrogen sources estimated by the USGS using the SPARROW model. In California estuaries, the elevation of isotope data appeared to be related to anthropogenic nitrogen sources. In Oregon estuaries, the nitrogen levels of streams flowing into the estuaries are related to forest cover, rather than to developed land classes. The N isotope ratio of macroalgae suggested that the ocean and nitrogen-fixing trees in the watersheds were the dominant nitrogen sources. In some Oregon estuaries, there was an elevation of δ15N above marine end members in the vicinity of wastewater treatment facility discharge locations, suggesting isotopes may be useful for distinguishing inputs along an estuarine gradient.
Description:
We used natural abundance stable isotope data to evaluate nitrogen sources to U.S. west coast estuaries. We collected δ15N of macroalgae data and supplemented this with available data from the literature for estuaries from Mexico to Alaska. Stable isotope ratios of green macroalgae were compared to δ15N of dissolved inorganic nitrogen of oceanic and watershed end members. There was a latitudinal gradient in δ15N of macroalgae with southern estuaries being 7 per mil heavier than northern estuaries. Gradients in isotope data were compared to nitrogen sources estimated by the USGS using the SPARROW model. In California estuaries, the elevation of isotope data appeared to be related to anthropogenic nitrogen sources. In Oregon systems, the nitrogen levels of streams flowing into the estuaries are related to forest cover, rather than to developed land classes. In addition, the δ15N of macroalgae suggested that the ocean and nitrogen-fixing trees in the watersheds were the dominant nitrogen sources. There was also a strong gradient in δ15N of macroalgae with heavier sites located near the estuary mouth. In some Oregon estuaries, there was an elevation an elevation of δ15N above marine end members in the vicinity of wastewater treatment facility discharge locations, suggesting isotopes may be useful for distinguishing inputs along an estuarine gradient.