Total Phosphorus trends in Mississippi and Atchafalaya River Basin Watersheds: Exploring the roles of streamflow and watershed features 2000-2020

A consistent method of analyzing riverine phosphorus concentrations and load trends across subbasins of the Mississippi Atchafalaya River Basin (MARB) can indicate whether and where nutrient loads and concentrations have increased or decreased over the long-term. This can help evaluate the success of federal and state nutrient reduction plans being implemented across the landscape and inform adaptive management of load reduction practices. In this study, water quality and streamflow monitoring stations meeting common criteria within sub-watersheds of the MARB were identified and flow normalized (FN) total phosphorus (TP) yield and concentration trends for the period of 2000-2020 were computed through the Weighted Regression on Time, Discharge, and Season (WRTDS), which allowed estimation of streamflow and non-streamflow components of trends. Of the 132 TP sites meeting screening criteria for load calculations and trend analysis, 33.3% and 50.8% had likely increases and decreases of FN concentrations, respectively, while FN yield showed a nearly opposite distribution of trends: 54.5% likely increases and 28.8% likely decreases. Watersheds dominated by urban and cultivated cropland tended to have high FN TP concentrations and yields, and many of the urban dominated watersheds had decreasing yield trends. Factors other than streamflow were dominant for 74.2% and 39.4% of sites for concentration and yield trends, respectively. Trends were weakly correlated with land cover and other watershed variables. Identifying causal factors for trends probably requires finer scale analysis of individual watersheds.