The Dead Zone isn't just a 1979 Stephen King novel, it's also a very large hypoxic zone in the Gulf of Mexico. Hypoxia means that there is very little oxygen present in the water, and so very little marine life can survive there. This is mostly caused by agricultural runoffs that go down the Mississippi river and end up fertilizing massive algae blooms that deplete oxygen levels.
Scientists have been tracking this dead zone for over 25 years, and this year it's among the smallest that its been since that time. Why? Are we doing something better this year, or is there another natural cause?
Unfortunately, it looks like it's the latter.
The smaller area reflects the drought conditions across the US in that the freshwater discharge and associated nutrients delivered to the Gulf of Mexico was mostly below average in spring and approached the 80-year minimum discharge as the mapping cruise neared its end. Chief Scientist, Dr. Nancy Rabalais, reported that “the smaller area was expected this July 2012, but the distribution across the shelf differed from any other documented to date.” [...]
Although the size of the hypoxic area was small this July 2012, this is not because sufficient actions have been taken within the watershed’s landscape to reduce nutrient pollution. The issue of nutrient overload, of both nitrogen and phosphorus, remains a critical issue for the health of water bodies within the Mississippi River Basin and in the northern Gulf of Mexico. Efforts should continue in full force to reduce nutrient loads. (source)
For more on the effects of hypoxia, check out this page at Wikipedia. It's truly a big deal for marine ecosystems, and the sooner we can stop dumping so much fertilizer in the sea - and conversely, the less we waste on our crops - the better.
This isn't just a problem in the Gulf of Mexico. Here's a map of marine dead zones around the world:
Here's a high-resolution version of the map.