The coast
wide extent of the Louisiana “dead zone” mapped this
week (July 24-29) is 11,840 square kilometers (or 4,564 square
miles), slightly smaller than the size of Connecticut, reported
Dr. Nancy Rabalais, Chief Scientist for Northern Gulf of Mexico
Hypoxia Studies. The low oxygen waters extended from near the
Mississippi River to the Louisiana/Texas border. The long-term
average since mapping began in 1985 is 12,700 km2 (or 4,800 square
miles).
The scientific
word for the commonly named Dead Zone is ‘hypoxia,’
or low oxygen, which results in the failure to capture fish, shrimp
and crabs in bottom-dragging trawls when the oxygen falls below
the critical level of 2 ppm. Higher in the water column and on
both inshore and offshore sides of the hypoxic area, there is
sufficient oxygen to support sizeable numbers of fish. The net
result, however, is a sizeable stretch of the Louisiana coast
unsuitable for supporting fish and shrimp.
The seasonal
formation and persistence of hypoxia are influenced by the discharges
and nutrient loads of the Mississippi and Atchafalaya rivers.
The fresher water forms a layer above the saltier Gulf waters.
Nitrogen and phosphorus in the river water stimulate the growth
of microscopic plants, the phytoplankton. These algae are either
transferred into the food web or end up as organic debris on the
sea floor. Their decomposition by bacteria depletes oxygen in
the lower waters until they no longer sustain the life of most
marine animals.
Water depths
affected were as shallow as 20 feet and as deep as 85 feet. The
hypoxic zone was smaller between the Mississippi River and the
Atchafalaya on the central Louisiana coast than between the Atchafalaya
River delta and Calcasieu estuary to the west.
Steve DiMarco
and colleagues at Texas A&M University and also the National
Marine Fisheries Service groundfish survey both conducted cruises
in the area Louisiana hypoxia in early July before the landfall
of Hurricane Dennis on the Florida coast. Their sampling grids
were limited to the southwestern Louisiana shelf, but they also
found hypoxia between the Atchafalaya River and the Calcasieu
estuary.
The size just
mapped was smaller than predicted using a model developed by Dr.
Eugene Turner of Louisiana State University, an investigator of
the research team, that relates the size with the May nitrate
load along with a term that adds the influence of the previous
year’s nitrate load. Turner predicted a size of 6,200 square
miles, which was larger than the measured size of 4,800 square
miles. The smaller than predicted size was expected because of
a tropical storm and hurricane that affected the area between
the Mississippi and the Atchafalaya rivers earlier in July. While
the two-layered system that supports the formation of hypoxia
was redeveloping at the time of the mapping cruise, the oxygen
level beneath that layer had not fallen below 2 ppm again. “I
would predict that a somewhat larger area of hypoxia would have
been mapped if the cruise had been conducted one week later than
planned and therefore closer to the size modeled by Turner,”
said Rabalais. Confirmation of this prediction may come from the
oxygen measurements taken by the NMFS groundfish survey that finished
their work on the southeast Louisiana coast on July 27 –
August 31.
Scientists
from the National Atmospheric and Oceanic Administration predicted
this summer’s dead zone to be less than 1,400 square miles
based on nutrient loads from the Mississippi and Atchafalaya rivers
in May and June. While high in late winter and early spring, the
nutrient loads were lower than average this year, probably due
to below average precipitation across much of the Mississippi
River Basin. The multiple models will help evaluate the influence
of the nitrogen load and variations in the physical structure
of the offshore coastal waters to improve assessments of hypoxia.
Additional
research conducted on the cruise was focused on the effects of
varying levels of hypoxia in controlling mercury methylation in
bottom sediments. These studies were conducted by Dr. David Senn,
Harvard School of Public Health, and funded by NOAA’s initiative
on Oceans and Human Health.
The scientific
party that mapped this year’s zone was from LUMCON, LSU,
Harvard School of Public Health, and Nicholls State University,
and was funded by NOAA’s Center for Sponsored Coastal Ocean
Research, part of the Ocean Service’s National Centers for
Coastal Ocean Science. The mapping was conducted from July 24-29
from aboard the research vessel, Pelican.
For further
information contact Dr. Nancy Rabalais, LUMCON, 985-851-2801,
nrabalais@lumcon.edu.
