Average
Size "Dead Zone," In Anything But An Average Year
Posted:
7/26/04
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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