The Ohio and Mississippi River levels were falling Wednesday at the site where engineers blasted holes in a Missouri levee to relieve pressure. But unleashing torrents of water across 35 miles of farmland in what has already been a terrible flooding season could carry other consequences.
One risk, scientists cautioned, is fertilizer runoff from the flooded farm country along the Mississippi. As it moves downstream, they predicted it would contribute to the largest-ever summertime depletion of oxygen in the Gulf of Mexico, posing a substantial risk to marine life.
The concern is that the water is likely pulling up components of fertilizers-notably nitrogen and phosphorus-and washing them downstream toward the Gulf, helping slash oxygen to levels marine life can't survive, said Nancy Rabalais, a marine scientist who is executive director of the Louisiana Universities Marine Consortium on the Gulf coast.
Those chemicals act as nutrients in the Gulf, intensifying the growth of microscopic plants. Microbes eat away at those plants. In the process, they consume oxygen, reducing it to levels that kill marine life.