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Application of Stable Isotopes to Understand Environmental Processes in Multiple Habitats
Citation:
Brooks, R., J. Compton, A. Nahlik, Catrina Nowakowski, J. Lin, R. Sabo, W. Rugh, L. Trine, AND M. Weber. Application of Stable Isotopes to Understand Environmental Processes in Multiple Habitats. HABs, Hypoxia, and Nutrients Research Webinar, Webinar -virtual, OR, March 26, 2025.
Impact/Purpose:
Total nitrogen (N) concentration is one of the most significant stressors to biological condition of US waterways. Aquatic N concentrations are strongly related to the amount of N added to a watershed, but concentrations can vary by over an order of magnitude for the same rate of inputs. Here we develop a tool to help understand why some watersheds have lower aquatic N concentrations than others when the amount of N applied to the landscape is the same. This tool uses data from EPA’s National Aquatic Resource Surveys (NARS) and National Nutrient Inventory (NNI), and stable isotopes of N measured in aquatic insects. We find that when N inputs to the watershed are high, N stable isotopes can identify watersheds that are efficient at denitrification, significantly lowering stream N concentrations. Results from this analysis will help managers promote effective ways to reduce the amount of excess N from entering surface waters.
Description:
Total nitrogen (N) concentration is one of the most significant stressors to biological condition of US waterways. Anthropogenic input of N added to a watershed is a strong determinant of aquatic N concentrations, but concentrations can vary by over an order of magnitude for the same rate of inputs. Here, we explore the variation in watersheds to regulate N concentrations in US rivers and streams using data from EPA’s National Aquatic Resource Surveys (NARS) and National Nutrient Inventory. Specifically, we identify which watershed properties are associated with lower aquatic N concentrations within the watershed when N inputs are high. Previous research illustrated that d15N of Chironomidae represents fractionating N-removal processes like denitrification at high levels of watershed N inputs within the NARS surveys. Nitrogen removal, as indicated by higher values of d15N, was strongly associated with lower stream N concentrations when watershed N inputs were above 100 kg N/ha. Increasing % forest and wetland landcover were also associated with decreasing N concentrations when inputs were high, but to a lesser degree than d15N. These variables did not correlate with d15N and are thus not responsible for the N removal represented by d15N, but likely represent N uptake and retention within watersheds. Climate variables explained ~12% of d15N variation. Greater precipitation was associated with lower d15N values indicating lower N removal, while higher temperature and relative humidity were associated with higher d15N indicating higher N removal. Other watershed predictors of N removal processes remain elusive, thus isotopic indicators such as d15N of aquatic insects are extremely helpful to integrate the relative importance of watershed-scale N removal processes that lower aquatic N concentrations. Based on this analysis, we classified NARS into those where N inputs have the largest impact on aquatic N concentrations, those where N removal lowers N concentrations, and those where N retention and uptake lower N concentrations. Understanding how watersheds function in terms of regulating aquatic N concentrations and loads given the amount of watershed N input will be critical to helping managers reduce stress on US waters from excess N. (2000 characters max, now 1931).