Citation:

Brown, C., Jim Kaldy, D. Wise, W. Rugh, K. Willard, P. Fong, TChris MochonCollura, AND C. Fong. Latitudinal gradients in oceanic and watershed nitrogen sources to Pacific coast estuaries of North America. Coastal and Estuarine Research Federation, Providence, RI, November 05 - 09, 2017.

Impact/Purpose:

We have demonstrated that stable isotopes are a useful tool for identifying nutrient sources at a regional scale and elevated stable isotope values indicate degree of urbanization and the presence of anthropogenic nutrient sources. 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). In California estuaries, the elevation of isotope data of macroalgae are similar to nitrate isotope from the watershed and appeared to be related to anthropogenic nitrogen sources. In contrast, nitrate coming from the watersheds for Oregon systems has low isotope ratio reflective of the presence of red alder trees in the watershed which have symbiotic nitrogen fixing communities. Three estuaries (Willapa, Grays and Necanicum) have elevated macroalgae isotope ratios which may be related to the Columbia River plume. The average estuarine macroalgae isotope ratio are related to degree of urbanization, forest cover, and a combination of wastewater, concentration agriculture, manure, and developed land nitrogen sources. This abstract contributes to SSWR 4.02B.

Description:

To assess the relative importance of terrestrial versus oceanic nutrient sources, we assembled natural abundance nitrogen stable isotope (δ15N) data for nitrate, green macroalgae, seagrass (Zostera marina) and mussels in the nearshore and in estuaries along the west coast of North America. We found a latitudinal gradient in nearshore nitrate isotope (δ15N-NO3) of -0.2 ‰ per degree latitude with more depleted isotope ratio to the north. Primary producers (green macroalgae, Zostera marina) located in the nearshore and the marine dominated portion of Pacific Coast estuaries exhibited a similar latitudinal gradient in δ15N of -0.3 ‰ per degree latitude. This latitudinal gradient is similar to the pattern of δ15N previously published for intertidal mussels (Mytilus californianus), which reflect the isotope ratio of the phytoplankton they consume. The consistent latitudinal gradients for two primary producers and a consumer, and the similarity to the gradient in nearshore δ15N-NO3, suggests that the pattern is a result of differences in oceanic source waters. On the watershed side, there is a gradient in the δ15N-NO3with southern California systems receiving nitrate with a δ15N-NO3 of about +12 ‰, while for Oregon estuaries the δ15N-NO3 is less than +1 ‰ for most systems reflecting the presence of nitrogen fixing red alder trees in Oregon coastal watersheds. In southern California estuaries, estuarine δ15N of macroalgae are elevated above the marine end member; while in Oregon estuaries they are depleted. Three estuaries (Willapa, Grays and Necanicum) have elevated macroalgal δ15N which may be related to the Columbia River plume. Estuarine macroalgal δ15N (estuarine average) is related to degree of urbanization, forest cover, and a combination of nitrogen sources including wastewater, concentrated agriculture, manure, and developed land, thus demonstrating that stable isotopes are a useful tool to identify nitrogen drivers at a regional scale.

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