Citation:

Brown, C., Jim Kaldy, A. Zimmer Faust, D. Wise, W. Rugh, TChris MochonCollura, J. Stecher, Pat Clinton, AND C. White. Contributions to the Nutrient Toolbox: Identifying Drivers, Nutrient Sources, and Attribution of Exceedances. Western Numeric Nutrient Criteria Workshop, San Francisco, CA, May 01 - 03, 2018.

Impact/Purpose:

Nutrients are a leading cause of impairments in the United States, and as a result tools are needed to identify nutrient sources and identify causes of exceedances of water quality standards. To further the development of numeric nutrient criteria, the US EPA is hosting a series of workshops to highlight approaches for developing nutrient criteria and tools for use in nutrient management. We will be presenting tools and approaches developed at the EPA which can be used to identify drivers, nutrient sources, and identify causes of exceedances.

Description:

Nutrients are a leading cause of impairments in the United States, and as a result tools are needed to identify drivers of nutrients and response variables (such as chlorophyll a), nutrient sources, and identify causes of exceedances of water quality thresholds. This presentation focuses on development of indicators and tools for water quality management in estuaries located along the Pacific coast of the United States. We explore how factors in the watershed and nearshore ocean condition influence nutrient and chlorophyll a levels in Oregon estuaries. Estuaries along the west coast of the United States periodically have high nitrogen and phosphorous, high chlorophyll a, and low dissolved oxygen levels due to the intrusion of oceanic water into the estuaries. This import of oceanic water often results in estuarine water quality conditions which exceed water quality standards and indicators of eutrophication status. Logistic regression models were developed to identify exceedances associated with ocean input. In addition, to receiving nutrient inputs from the coastal ocean, estuaries also receive nutrients from the watershed, including both point and non-point inputs. To assess the relative importance of terrestrial versus oceanic nutrient sources, we assembled natural abundance stable isotope (δ15N) data for nitrate, green macroalgae, and seagrass (Zostera marina) in the nearshore and in estuaries along the west coast of North America. At a regional scale, the isotope ratio (δ15N) of green macroalgae is related to degree of urbanization and inversely related to forest cover in the watersheds. After normalizing for gradient in ocean δ15N-NO3, the nitrogen contributions from concentrated agriculture and developed land estimated using SPARROW explain about 42% of variance in macroalgae (δ15N), thus demonstrating that stable isotopes are a useful tool to identify nitrogen drivers at a regional scale. One limitation of nitrogen stable isotopes is that they cannot distinguish human and animal waste. To overcome this limitation, we have been using nitrogen stable isotopes combined with microbial source tracking to more clearly identify watershed inputs.

Record Details: