Image: NASA, public domain.
In 2008, 415 Dead Zones Were Identified
NASA has just released a new map of oceans showing aquatic dead zones around the world (places where water is so low in dissolved oxygen that sea creatures can't survive, something we've written about a lot in the past), and it's not a pretty picture. You can see the high-resolution version of the map here. Notice how most dead zones are concentrated near densely populated areas (dark brown on the land parts of the map).
Photo: Wikipedia, CC/GFDL
Wikipedia has a good description of the main causes of dead zones:
Aquatic and marine dead zones can be caused by an increase in chemical nutrients (particularly nitrogen and phosphorus) in the water, known as eutrophication. These chemicals are the fundamental building blocks of single-celled, plant-like organisms that live in the water column, and whose growth is limited in part by the availability of these materials. Eutrophication can lead to rapid increases in the density of certain types of these phytoplankton, a phenomenon known as an algal bloom. Although these algae produce oxygen in the daytime via photosynthesis, during the night hours they continue to undergo cellular respiration and can therefore deplete the water column of available oxygen. In addition, when algal blooms die off, oxygen is used up further during bacterial decomposition of the dead algal cells. Both of these processes can result in a significant depletion of dissolved oxygen in the water, creating hypoxic conditions. Dead zones can be caused by natural and by anthropogenic factors. Use of chemical fertilizers is considered the major human-related cause of dead zones around the world. Natural causes include coastal upwelling and changes in wind and water circulation patterns. Runoff from sewage, urban land use, and fertilizers can also contribute to eutrophication. (source)
The good news is that dead zones are reversible. Fertilizer is often used in much greater quantities than required because farmers have no way to know exactly what the right quantity is, so they prefer to put more than not enough, leading to a lot of the extra finding its way to the sea. Better ways to monitor crop fields with sensors and satellites can help farmers reduce their fertilizer use and thus reduce runoffs.
For more on water issues, check out our special feature Blue August!