The dead zone is caused by an excess of
nitrogen flushing down the Mississippi, and the
primary source of that nitrogen is the farms of
the upper Midwest, mainly Minnesota, Iowa,
Illinois, and Indiana.
Freshwater flushing from the Mississippi
River spreads into the Gulf, trapping the
denser saltwater beneath it. Phytoplankton such
as algae growing in these shallow fecund waters
die and settle to the bottom. Bacteria degrade
the organic matter, consuming dissolved oxygen
from the water. The stratification of saltwater
and freshwater prevents wave action from mixing
and reoxygenating the bottom layer, and a dead
zone spreads along the seabed.