NOAA announced yesterday that the “dead zone” at the outlet of the Mississippi River will likely be larger than usual this summer, due to a combination of a rainy spring with increased stream flows and an increased fertilizer load from farms in the Mississippi basin. They forecast between 7,450 and 8,456 square miles of hypoxic water—about the size of New Jersey. Watch the following video for a nice visualization.
(Video available in large size here)
Nitrogen and phosphorus are the active ingredients in synthetic fertilizer. These elements, critical to the making of proteins, DNA, and other important life materials, are typically the limiting nutrients in soils, so adding synthetic fertilizer increases crop yields.
Nitrogen and Phosphorus, however, are also limiting nutrients in large areas of the ocean. This means that when fertilizer runs of from farms and enters the ocean via rivers, it tends to fertilize phytoplankton, the single-celled algae that form the base of the oceanic food chain. Huge algal blooms result, creating a sudden food bonanza for zooplankton and fish.
Soon, however, the zooplankton and fish start pooping, the phytoplankton start dying and sinking, and the bacteria go to town decomposing it all. Decomposition uses oxygen. Since the Gulf is stratified and stable during the summer, with warm fresh Mississippi water on top of the less-warm salty Gulf water and few big storms to mix it up, there is no way for new oxygen to get mixed down into the water column. As the bacteria feast on dead phytoplankton and fish poop, they draw down on the dissolved oxygen in the water, eventually leaving it too dilute for fish and most animals to filter out with their gills. When fish get caught in the hypoxic water, they go belly-up.
The NOAA press release may be found here.