From the Guardian:
Scientists are confronting growing evidence that BP’s ruptured well in the Gulf of Mexico is creating oxygen-depleted “dead zones” where fish and other marine life cannot survive.
In two separate research voyages, independent scientists have detected what were described as “astonishingly high” levels of methane, or natural gas, bubbling from the well site, setting off a chain of reactions that suck the oxygen out of the water. In some cases, methane concentrations are 100,000 times normal levels.
Other scientists as well as sport fishermen are reporting unusual movements of fish, shrimp, crab and other marine life, including increased shark sightings closer to the Alabama coast.
Larry Crowder, a marine biologist at Duke University, said there were already signs that fish were being driven from their habitat.
Dead zones and it’s causes are a phenomenon already quite well-known in the Gulf of Mexico. From a Carlton College site:
The Gulf of Mexico dead zone is an area of hypoxic (link to USGS definition) (less than 2 ppm dissolved oxygen) waters at the mouth of the Mississippi River. Its area varies in size, but can cover up to 6,000-7,000 square miles. The zone occurs between the inner and mid-continental shelf in the northern Gulf of Mexico, beginning at the Mississippi River delta and extending westward to the upper Texas coast.
The dead zone is caused by nutrient enrichment from the Mississippi River, particularly nitrogen and phosphorous. Watersheds within the Mississippi River Basin drain much of the United States, from Montana to Pennsylvania and extending southward along the Mississippi River. Most of the nitrogen input comes from major farming states in the Mississippi River Valley, including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana. Nitrogen and phosphorous enter the river through upstream runoff of fertilizers, soil erosion, animal wastes, and sewage. In a natural system, these nutrients aren’t significant factors in algae growth because they are depleted in the soil by plants. However, with anthropogenically increased nitrogen and phosphorus input, algae growth is no longer limited. Consequently, algal blooms develop, the food chain is altered, and dissolved oxygen in the area is depleted. The size of the dead zone fluctuates seasonally, as it is exacerbated by farming practices. It is also affected by weather events such as flooding (more info) and hurricanes.
Hypoxia, by the way is defined as ‘ the condition in which dissolved oxygen is below the level necessary to sustain most animal life – generally defined by dissolved oxygen levels below 2mg/l [miligrams/liter] (or ppm [parts per million])’. The following map shows the extent of the Gulf of Mexico dead zone.
Size of dead zone in Gulf of Mexico
In 2010 researchers predicted one of the largest dead zones ever, covering an area of around 6,500 and 7,800 square miles, an area roughly the size of Lake Ontario, this without factoring in the effect of the Gulf oil gusher.
Note that in 2009 the same researchers predicted a dead zone covering the area of 7,450 and 8,456 square miles, which is about the size of New Jersey. It turned out to be much smaller than forecasted, covering roughly around 3,000 square miles. However, the researchers think that this is attribuable to short-term effects and not a sign of improvements in conditions.
BP seems to be aware of that the oil gusher could create such dead zones, in fact, the whole Gulf of Mexico and the East Coast of Florida could become dead zones. According to OpEdNews:
A BP insider, providing information to OpEdNEws.com, reports that scientists and engineers, fearing the worst, have envisioned a worst case scenario, “It could very well be that the entire Gulf and the East coast of Florida could become dead zones, with no aquatic life at all.”
The Gulf of Mexico is not the only dead zone on Earth. There are about 150 of them, says NASA. As NASA points out, not all areas of ‘anoxia’, which is an extreme form of ‘hypoxia’ are caused by pollution. The largest such area is the Black Sea, which is entirely anoxic below a depth of 150m. This is due to the fact that lower waters do not mix with upper waters which leads to a stratification of the Black Sea, i.e. a layering, that normally remains in place if not disturbed by external events.
It would be interesting to learn – and I haven’t found that information yet – what the chemical environment and reactions in such dead zones are and if there could be any additional health risk in that – as if one would need any additional, it’s already bad enough as it is.