Source: Mother Jones
Why such massive annual dead zones? It's a matter of geography and concentration and intensification of fertilizer-dependent agriculture. Note that an enormous swath of the US landmass—41 percent of it—drains into the Mississippi River basin, as shown below. It's true that even under natural conditions, a river that captures as much drainage as the Mississippi is going to deliver some level of nutrients to the sea, which in turn will generate at least some algae. But when US Geological Survey researchers looked at the fossil record in 2006, they found that major hypoxia events (the technical name for dead zones) were relatively rare until around 1950—and have been increasingly common ever since. The mid-20th century is also when farmers turned to large-scale use of synthetic fertilizers. Now as much a part of Mississippi Delta life as crawfish boils, the Gulf dead zone wasn't even documented as a phenomenon until 1972, according to NOAA.