Microbial transect of the Mississippi River reveals distinct geographic regimes driven by nitrogen and phosphate inputs

At 2,320 miles the Mississippi River (MSR) is the 4th longest on earth, with a watershed draining 31 U.S. states and two Canadian provinces. The MSR is an important ecological and economic resource for much of the country, connecting the Gulf of Mexico (GoM) with 40% of the land mass of the continental United States. However, in 2015, the American Watershed Initiative gave the Mississippi delta a D+ for its failing infrastructure, ecosystem health, and clean water. One consequence of water quality issues can be found in the northern GoM “dead zone.” The 2015 GoM hypoxic zone was the largest since 2002, attributed to the large flux of nutrients into the MSR from heavy rains. As an important connector of the headwaters of the north and the GoM, understanding how changes in nutrient and sediment loading impact the microorganisms driving biogeochemistry of the river is more important then ever. To do this, we chose 38 sampling sites spanning the entire MSR based on their proximity to major U.S. cities and important tributaries. At each site, water was filtered sequentially through 2.7 μm and 0.22 μm filters, DNA was isolated for 16S rRNA gene community sequencing, and flow-through was collected for biogeochemical (NO3, NH4, SiO3, NO2, and PO4) analysis.