Stable Isotope Identification of Nitrogen Sources for United States (U.S.) Pacific Coast Estuaries

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 used natural abundance stable isotope data to evaluate nitrogen sources to U.S. west coast estuaries. We collected macroalgae and analyzed these samples for natural abundance of stable isotopes (δ¹⁵N) 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 Oregon estuaries, the δ¹⁵N of macroalgae suggested that the ocean and nitrogen-fixing trees in the watersheds were the dominant nitrogen sources with heavier sites located near the estuary mouth. In California estuaries, the gradient was reversed with heavier sites located upriver. 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.