2003
NEWS ARCHIVE
Tropical
Storms Put the Oxygen Back Into the “Dead Zone”
July 29, 2003
Two tropical
storms in as many weeks have mixed up the coastal waters of the
northern Gulf of Mexico and disrupted the usual widespread extent
of summertime waters severely depleted in oxygen, reports Dr.
Nancy Rabalais of LUMCON who just completed this summer’s
mapping. The result was a half again smaller “dead zone”
this summer than the average size for the last 10 years. The size
just mapped on a six-day cruise was 8,560 square kilometers (=3,300
square miles).
Larger patches of water
with low oxygen were located near the Mississippi River delta
off Terrebonne Bay, off Atchafalaya Bay, and along the southwestern
Louisiana coast near Calcasieu estuary. Smaller patches were located
farther offshore. The Texas coast was spared from low oxygen waters,
but also received the brunt of Hurricane Claudette on July 7-8.
Tropical Storm Bill crossed the Louisiana coast near Morgan City
on June 30. Both storms created 10 to 15 foot seas along the Louisiana
coast. The waves mixed well-oxygenated waters from the surface
down through the water column. Eventually hypoxia will reform.
The scientific word
for the commonly named Dead Zone is “hypoxia,” or
low oxygen. Dead Zone refers to the failure to capture fish, shrimp,
and crabs in bottom-dragging trawls when the oxygen concentration
falls below a critical level in bottom waters. Higher in the water
column, however, there is sufficient oxygen to support sizeable
numbers of fish and swimming crabs.
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, and the resulting
two-layer system inhibits the oxygen in the surface waters from
penetrating to depth. Nutrients stimulate the growth of microscopic
plants, the phytoplankton. These single celled plants either end
up in the food web, which off Louisiana supports valuable commercial
fisheries, or end up as organic debris on the sea floor. The decomposition
of this organic matter by bacteria uses up the oxygen to the point
that it becomes depleted and lower than what is necessary to sustain
the life of most marine animals.
High river discharge
in spring 2003 and another peak of fresh water to the Gulf in
June, along with the nutrients carried in the flow, started the
annual progression of hypoxia. Hypoxia was well established and
widespread along a line of stations off Terrebonne Bay sampled
in mid-June. At that time, eight of nine stations out to 100-ft
water depth were severely depleted in oxygen, said Dr. Nancy Rabalais,
who studies hypoxia in the region. She’s chief scientist
on the project based at LUMCON.
Several mathematical
models based on the amount of river discharge and nutrients loaded
into the Gulf of Mexico in spring and early summer 2003 predicted
that the size of this year’s low oxygen area would be in
the range of 15,000 to 17,000 square kilometers (=6,000 to 7,000
square miles). The models, however, do not take into account the
mixing that resulted from the two tropical storms that passed
through the hypoxic zone two to three weeks before the mapping
cruise, she added
“It was obvious
that the water column was more mixed than in most summers,”
said Rabalais. Even with a very strong signature of Mississippi
and Atchafalaya river water far out into the study area, there
was not the usual strong difference in temperature and salinity
from the surface to the bottom. That physical structure is necessary
for the formation of hypoxia along with the nutrient-enhancement
of the phytoplankton. “If we were to collect these data
two weeks from now, I would predict a much larger size,”
said Rabalais.
The scientific
party from the Louisiana Universities Marine Consortium (LUMCON)
and Louisiana State University is funded by the National Oceanic
and Atmospheric Administration, Coastal Ocean Program. For further
information contact Nancy Rabalais, LUMCON, 985-851-2836.
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