There's no mystery as to what triggers this annual hypoxic zone,
as the oxygen-starved region is formally termed. Into the Gulf of
Mexico, the Mississippi River deposits water that is heavily enriched
with plant nutrients, principally nitrate. This pollutant fertilizes
the abundant growth of tiny, floating algae. As blooms of the algae go
through their natural life cycles and die, they fall to the bottom and
create a feast for bacteria. Growing in unnatural abundance, the
bacteria use up most of the oxygen from the bottom water.

Dead zones tend to develop in quiet, deep water a few km offshore.
Typically, they appear where a river spews rich plumes of nutrients
into water that's stratified because of either temperature or salinity
differences between the bottom and the top of the water column. If the
water doesn't mix, oxygen isn't replenished in the lower half.

…Sixty-eight large, persistent, and recurring dead zones spanning the
world's seas were reported for the first time during the 1990s. . .
.  On March 29, [2005], the United Nations Environment Program . .
. concluded that there are [now] some 150 recurring and permanent dead
zones in seas worldwide.

…Fully oxygenated waters contain as much as 10 parts per million of
oxygen. Once oxygen falls to 5 ppm, fish and other aquatic animals have
trouble breathing. Sharks begin vacating areas with 3 ppm of oxygen,
while most other fish can hold out until about 2 ppm. Sediment dwellers
that can't leave a hypoxic zone begin dying at around 1.5 ppm.

…When spring rains scour farm fields as far upstream as
Minnesota and [Iowa,] spilling huge quantities of nitrogen
into the Mississippi, it's only a matter of weeks before the oxygen
concentrations in the Gulf begin to respond. “Once a decline starts, it
goes from about 5 [ppm] to close to 0 in about 7 to 10 days,” [says
Nancy Rabalais, an aquatic ecologist with the Louisiana Universities
Marine Consortium in Chauvin].