Approximately 400 coastal estuaries around the world contain so little oxygen in their waters that they are nearly devoid of life. They are chronic dead zones and there are about twice as many now as there were just a couple of years ago. One of the largest occurs in the Mississippi River estuary and is approximately the size of the state of New Jersey. Another occurs in the estuary of the Yangtze River. These dead zones occur when fertilizer run off from agricultural fields enters rivers and is deposited in estuaries. The nutrients feed great blooms of alga, which, in turn, become food for massive amounts of bacteria. It is the growth of these bacteria that depletes the estuarine oxygen. The dead zones enlarge when rainfall is high and run off increases. There is an irony in this pattern. As we strive to increase food production in agricultural heartlands by applying large amounts of relatively cheap fertilizer, we decrease marine food production in the estuaries affected by dead zones.
The growth of dead zones is an ominous sign that we must transform agriculture. We need to use less fertilizer on our crops and implement practices, such as no-till agriculture, that keep nutrients in the soil where they belong. Climate change models suggest that the agricultural heartlands of south central China and the United States mid-west will receive more rainfall and more storms in the future, thereby increasing the size, severity and number of dead zones. The Mississippi River flow, for example, is expected to increase 20%. Additionally, global warming will likely produce warmer and more stratified coastal waters, further increasing dead zones. Transforming agriculture is an urgent global priority and the U.S. and China could greatly benefit by addressing this priority through collaborative research and dialogue.
Diaz, R.J. et al. 2008. Spreading Dead Zones and Consequences for Marine Ecosystems. Science 321: 926-929.