2004
NEWS ARCHIVE
Average
Size "Dead Zone," In Anything But An Average Year
July 26, 2004
The coast wide extent of the Louisiana "dead
zone" mapped this week is slightly larger than average at
15,040 km2 (or 5,800 square miles), according to officials at
the Louisiana Universities Marine Consortium. Through it, scientists
have been studying the dead zone annually for many years. The
“dead zone” is a low oxygen area in the Gulf of Mexico
along the Louisiana coast. The long-term average since mapping
began in 1985 is 13,000 km2 (or 5,000 square miles). The river
flow and the offshore conditions prior to the mapping cruise this
year, however, were abnormal, reminiscent of the Great Mississippi
River Flood of 1993. The river in 2004 peaked in discharge several
times in January, February, March, again in May, then persisted
in a prolonged, above average flow in June and into July.
This year's low oxygen area extended from the
Mississippi River delta almost to the Texas coast. The low oxygen
bottom waters were very close to shore during this summer's mapping,
as a result of north winds and onshore currents that pushed the
zone towards the beach. Water depths affected were as shallow
12 feet and as deep as 100 feet, but mostly within the 70-foot
contour. The close proximity of the low oxygen close to shore
precluded the presence of shrimp trawlers from that area.
Freshwater from the Mississippi and Atchafalaya
rivers was distributed far across the Louisiana coast, accompanied
by massive algal blooms. These massive algal blooms are not toxic,
but noxious in that they create scummy water at the surface and
the organic matter that sinks to the Gulf bottom leads to the
depletion of oxygen there. The algal blooms were not the same
as the noxious ones in May along the southeastern Louisiana coast
that created problems for fishers.
The scientific word for the commonly named “dead
zone” is “hypoxia” or low oxygen. It was coined
by fishers to describe the failure to capture fish, shrimp, and
crabs with bottom-dragging trawls when the oxygen falls below
the critical level of 2 parts per million (ppm) in bottom waters.
Higher in the water column, however, there is sufficient oxygen
to support sizeable numbers of fish, and they often seek refuge
there from the low oxygen.
The seasonal formation and persistence of hypoxia
are influenced by the discharges of the Mississippi and Atchafalaya
rivers. The fresh water forms a fresher layer above the saltier
Gulf waters. Nutrients in the water stimulate the growth of microscopic
plants, phytoplankton. These are either transferred up the food
web, which supports valuable commercial fisheries, or end up as
organic debris on the sea floor. The decomposition of the organic
debris depletes oxygen in the lower waters until the conditions
no longer sustain the life of most marine animals there.
This year's
mapping of the dead zone is the 20th anniversary of the systematic
survey of the low oxygen that began in 1985 under the direction
of Dr. Don Boesch, then director of the Louisiana Universities
Marine Consortium (LUMCON), with initial funding from the National
Oceanic and Atmospheric Administration (NOAA). Since then, LUMCON
and LSU collaborators under the direction of Dr. Nancy Rabalais
have maintained the mapping of the low oxygen each summer with
funding primarily from NOAA. The scientific party that mapped
this year's hypoxic zone was from LUMCON, Louisiana State University,
Texas A&M University Galveston, and the University of Scranton
and was funded by NOAA’s National Ocean Service, Coastal
Ocean Program. The mapping was conducted from July 21-25, 2004,
from aboard the research vessel, Pelican. For further information
contact Nancy Rabalais, LUMCON, 985-851-2836, nrabalais@lumcon.edu.
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