Alejandra Botero-Acosta , Gregory McIsaac, Richard Warner, Ellen Gilinsky, Jong Lee

Nutrient reduction strategies in the Mississippi and Atchafalaya River Basin (MARB) have been implemented since mid-2010’s to attenuate the impacts of non-point source pollution to the Gulf of Mexico. Of all nutrients, nitrate represents the largest threat due to its extended presence throughout the basin and its high solubility in water. To evaluate the performance of state reduction strategies, long-term changes of riverine nitrate should be identified. The objective of this study was to estimate the flow-normalized (FN) nitrate-N concentration and yield trends for the 2000-2020 period across the MARB. A harmonization and in-depth screening of paired nitrate-N and streamflow datasets resulted in a robust water quality monitoring network of 217 sites. Trends magnitude and likelihood were computed using the Weighted Regression on Time, Discharge, and Season (WRTDS) coupled to a bootstrap test, and trends results were correlated with basin features and initial values. Nutrient supply and flow components of trends were computed through the stationary and non-stationary flow normalization. Results indicated that 59.4% of the 217 sites had likely decreasing trends, while 27.6% likely increased, and the remaining 12.9% had no likely change detected. Reductions in riverine FN nitrate-N were clustered in the north central region of the MARB, with watersheds dominated by cultivated cropland having relatively high FN concentrations and yields in 2000 followed by likely downward trends. For the vast majority of sites, the streamflow component contributed to increased FN nutrients, but the nutrient supply component was more dominant than the streamflow component.