Abstract
The Mississippi River runs 2,350 miles to reach the Gulf of Mexico, receiving input from hundreds of tributaries. At its terminus, the Mississippi and Atchafalaya Rivers (Mississippi/Atchafalaya River Basin) interconnect, discharging freshwater and nutrients into the Gulf of Mexico. With the right physical conditions over the shelf, these nutrients stimulate high algal growth, in turn driving development of the hypoxic zone found in that area. To assess the variability of nutrient transport to the northern Gulf by these rivers, estimates of the average annual flow and nutrient delivery from the total Mississippi-Atchafalaya River Basin were taken from the United States Geological Survey, a federal agency. The nutrients considered are ammonia, nitrate and nitrite, organic nitrogen, organic phosphorus and orthophosphate. There was an increase in total water flow (10.17%), a decrease in total nitrogen (-12.13%), and an increase in total phosphorus (22.73%) over the period of 1969 to 2016. Reasons for the decreases seen in organic nitrogen flux can be attributed to the construction of reservoirs along the river, which remove particulate matter, including nitrogen, from delivery downstream. Increase in organic phosphorus and orthophosphate fluxes can be explained by increased land cultivation and associated use of phosphate fertilizer. There is a strong potential that any improvements being made in water quality through management changes upstream are not significant enough to change nutrient discharge to the Gulf of Mexico.