Everything Science Forum

A marine dead zone is an area of ocean or sea where no marine animals can live.  The water is hypoxic because microrganisms (algeal blooms) use up all the oxygen.

I've heard recent discussions of this and I think it is a critical issue that needs to be addressed urgently because it's been ignored.

"Dead Zone" threatens shrimp fishery
http://marketplace.publicradio.org/shows/2006/07/13/PM200607139.html

SAM EATON: A strange thing happens every so often along the Louisiana coast. Something known as the dead zone creeps into the shallow bays and bayous like an invisible monster from the deep. At the shrimp docks in Lafitte it's the stuff of legend. Fourth-generation shrimp fisherman A.J. Fabre has seen it first-hand.
 
A.J. FABRE: What happens is it just depletes all oxygen. So there's nothing, nothing that can live without the oxygen.

There is a scientific explanation. Spring runoff from the Mississippi is loaded with nitrogen-based fertilizers from farms. The fertilizer has the same effect in the Gulf as it does on the Midwest fields it came from. But instead of giving corn a growth spurt, the nitrogen fuels massive algae blooms that then die and suck all of the oxygen out of the water as they decompose. Fabre says the science makes perfect sense, but watch the sea life's reaction to the dead zone and it's more the stuff of science fiction.
 
FABRE: Crabs get on top the water and wanta jump out of the water. Shrimp is jumping, fish is jumping. I mean, it's . . . they're smothering. For a human being to understand it, just — if you'd be smothering, what would you do? — and they panic. The seafood panics.

. . . .

DOUG DAIGLE: The trend for this hypoxic or low-oxygen zone that forms every summer has been that it's roughly doubled in size over the last decade. And it's getting up to a point now where it's . . . For a number of years it's been the size of several small New England states.

. . . .

The Gulf Coast dead zone has been growing in size ever since synthetic nitrogen fertilizers were first introduced to farms post World War II. The latest figures peg the annual nitrogen runoff into the Gulf in excess of 2.2 billion pounds. A study by the National Marine Fisheries Service found that the annual shrimp catch has dropped nearly a quarter in recent years.

Shrimper A.J. Fabre, who's also the head of the Louisiana Shrimp Association, says it's easy to see why. He says before the dead zone became a problem, shrimp boats used to fish a much larger area.

. . . .

Fabre . . . . says the government has known about the dead zone for more than 30 years. It just lacks the political will to address the cause. Fabre says in the grand scheme of Washington politics the Louisiana fisherman is expendable.

The Gulf of Mexico has one of the largest marine dead zones in the world.

Dead zones are hypoxic (low-oxygen) areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. The term could as well apply to the identical phenomenon in large lakes. In March 2004, when the recently-established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003) it reported 146 dead zones in the world oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometer, but the largest dead zone covered 70,000 square kilometers.

http://en.wikipedia.org/wiki/Dead_zone_%28ecology%29

Aquatic and marine dead zones can be caused by the process of eutrophication, triggered by an excess of plant nutrients (nitrogen and phosphorus) from fertilizers, sewage, combustion emissions from vehicles, power generators, and factories. In a cascade of effects, the nutrients trigger a bloom of phytoplankton at the bottom of the marine food chain, allowing zooplankton to proliferate. As phytoplankton and zooplankton die and sink below the photic zone where photosynthesis can occur, a bloom of natural bacterial degradation exhausts the water's dissolved oxygen.

Dead zones can also be produced by the natural event of river flooding. Large amounts of fresh water empty into the ocean forming a thick layer of fresh water atop the denser salt water, effectively forming a barrier between the ocean water and oxygen in the atmosphere. (Osterman, 2004)

The process can be reversed, but that requires effort.

 :koala