Long-Term Mississippi River Trends Expose Shifts in the River Load Response to Watershed Nutrient Balances Between 1975 and 2017
Excess nutrients delivered by the Mississippi River (MR) have contributed to hypoxia in the Gulf of Mexico. Here, we quantified annual changes in nitrogen (N) and phosphorus (P) river loads and nutrient balances at the MR Outlet to determine drivers in river load trends from 1975 to 2017. N and P loads increased by 18% and 9%, respectively, but trends were asynchronous and non-monotonic. River N loads increased from 1975 to 1985, decreased from 1985 to 1995, then remained relatively stable through 2017. P loads decreased from 1975 to 1985, were stable from 1985 to 1995, increased from 1995 to 2005, then were stable through 2017. Nutrient balances have been considered a key driver to river nutrient loads and represent inputs (fertilizer, manure, deposition, wastewater, N-fixation, weathering) minus outputs (nutrient uptake and removal in harvest, and soil and river surface N emissions). A large shift in the river load response to watershed nutrient balances occurred between 1975 and 2017; annual nutrient balances and river loads were positively correlated between 1975 to 1985, but after, a disconnect between both the N and P balances and river loads emerged, and the subsequent river load patterns were different for N versus P. Time lagged nutrient balances representing legacy inputs were key drivers, but other factors for which data were not available also affected changing river nutrient loads over time. These other factors —potentially including changes in tile drainage, a variety of agricultural best management practices, and(or) increased precipitation contributed to increased N and decreased P retention in the watershed over time. Furthermore, our analysis showed that in the case of N, these factors were potentially just as important in explaining changes in river nutrient loads over time as N balances, and in the case of P, they were even more important. Additional data and modelling efforts are required to better identify these specific factors and quantify their individual impacts at the scale of the Mississippi River Basin.