Thought this might interest you lovers of the earth's waters.
DEAD ZONE - GULF OF MEXICO
A ProMED-mail post
Date: Mon, 2 Aug 1999 18:28:33 -0500
From: Martin Hugh-Jones
Source: The Advocate, Baton Rouge, 31 Jul 1999 [edited]
Scientists doing research in the Gulf of Mexico have found the largest ever
"dead zone," an area where oxygen in the water is so low it cannot support
As she has for most summers in the past decade, Louisiana Marine
Consortium's Nancy Rabalais cruised the gulf off the Louisiana coast
mapping the extent of low-oxygen water, a condition called "hypoxia."
Rabalais said this year's dead zone is the largest ever found. She said she
doesn't know why the dead zone is so big this year. The 1999 dead zone
measures 7,728 square miles -- about 12 times bigger than Lake
Pontchartrain and 16 times larger than East Baton Rouge Parish.
This year's size is much larger than the previously recorded maximum size
in 1995 by about 2000 square kilometers, or 700 square miles. Last summer,
the dead zone covered less seabed than in the past few years -- about 4,800
square miles -- but went far deeper than ever. This summer, the low oxygen
zone stretched west from the Mississippi River delta all the way to near
the Texas border, and from close to the shore along most of the Louisiana
coast out to 100-foot water depths. The low-level zone was also closer to
shore this year than last.
Rabalais said the term "dead zone" comes from the lack of sea life such as
shrimp, crabs and fish in its bottom waters. Trawling boats were noticeably
absent from the extensive area of low oxygen, with the exception of fleets
working off the mouth of the Sabine River and west of Atchafalaya Bay, she
said. When the bottom oxygen levels fall below 2 parts per million,
anything that can swim leaves the area, and trawlers catch nothing. The
displaced shrimp are concentrated on the periphery of the dead zone, where
trawlers focused their efforts. Animals that cannot escape, such as the
clams, snails and worms that live in the mud, eventually die if the oxygen
levels fall too low for too long.
The mixture of fresh and salt water and the nutrients carried by the
Mississippi River system are a large part of the reason Louisiana has the
largest fishery in the Lower 48 states. But it seems too many nutrients,
such as nitrogen, are flowing into the river system, causing algae blooms
that go well beyond what nature can handle, according to Rabalais. Possible
sources of that nitrogen include fertilizers used on farms in the
Mississippi River valley. Three studies, including one by the U.S.
Geological Survey, estimate that more than 50 percent of the nitrogen in
the river comes from farming activities.
Rabalais said water from the Mississippi and Atchafalaya rivers was easily
detectable in the gulf this year because of its much lower salinity and its
dense concentrations of phytoplankton, or microscopic algae or plants. The
fresh water lies on top of the saltier gulf waters, forming a two-layer
system. The phytoplankton flourish in the nutrient-enriched waters.
Eventually the phytoplankton die and sink to the bottom, or are consumed by
zooplankton, and waste from the zooplankton sinks to the bottom. Bacteria
decomposes all that organic matter on or near the bottom, using up the
oxygen in the lower waters.
Low winds and calm seas occur more commonly in the summer, creating
conditions for the persistence of the two-layer system and the development
of wide zones of hypoxia off the coast. The hypoxia zones usually last
until a tropical storm or cold front mixes the water in all layers.
[Byline: Mike Dunne, Advocate staff writer]
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