Linking aquatic invertebrate δ15N and watershed N reduction processes across the US.
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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.