July 19th US Global Change Seminar - "Origin, Impact, andImplications of the "Dead Zone" in the Gulf of Mexico"

Tony Socci tsocci at usgcrp.gov
Wed Jul 14 15:23:42 EDT 1999

                  U.S. Global Change Research Program Seminar Series

     Origin, Impact, and Implications of the "Dead Zone" in the Gulf of Mexico

What is the so-called "Dead Zone" in the Gulf of Mexico?  What are the
origins of the "Dead Zone"?  Is the "Dead Zone" a natural phenomenon or
is it a consequence of human activities, or both?  Are there similar "dead
zones" elsewhere?  How does the "dead zone" impact people and
ecosystems in the Gulf region and elsewhere?  Are there potential solutions
to the problem?

                                            Public Invited

                           Monday, July 19, 1999, 3:15-4:45 PM
                          Dirksen Senate Office Bldg., Room G-11
                                           Washington, DC

                                        Reception Following


Dr. Charles (Chip) Groat, Director, U.S. Geological Survey, Department
of the Interior, Reston, VA


Dr. Nancy Rabalais, Professor, Louisiana Universities Marine Consortium
(LUMCON), Chauvin, LA

Dr. Donald Scavia, Chief Scientist for the National Oceanic and Atmospheric
Administration's (NOAA) National Ocean Service, and the Director of the
National Centers for Coastal Ocean Science, Silver Spring, MD

Ms Tracy Huhns, R & K Fisheries, Lafitte, LA

                  Origin and Extent of the "Dead Zone"

Every spring the process begins and culminates in a vast region of
oxygen-starved ocean bottom that stretches along the Louisiana and Texas
coasts.  The phenomenon is known as hypoxia, but has been dubbed the "dead
zone" by environmentalists and fishermen. Hypoxia is defined by a dissolved
oxygen concentration in seawater of no more than 2 ppm (parts per million
in seawater).  At oxygen concentrations below this level fish trawlers are
unable to find or capture any live shrimp or bottom-dwelling fish in their
nets.  The low oxygen levels drive away fish, while bottom-dwellers such as
shrimp, crabs, snails, clams, starfish and worms eventually suffocate.
These features of an otherwise highly productive coastal ecosystem are
cause for concern since fisheries are a vital renewable resource of the
northern Gulf of Mexico.

Hypoxia results from a combination of natural and human-influenced factors.
The Mississippi River Basin is the third largest in the world, after the
Amazon and the Congo river basins, and drains about 41% of the conterminous
United States, a total of over 3,200,000 square kilometers, delivering
freshwater, sediments and nutrients to the Gulf of Mexico.  This drainage
basin includes all or part of 30 states, home to about 70 million people
and one of the most productive agricultural regions in the country with
over half (approximately 58%) of the total land area in the basin being
devoted to cropland.  The main stem of the Mississippi originates in
northern Minnesota and flows southward for more than 3700 kilometers to the
Gulf of Mexico; enroute the river is joined by the Missouri, Illinois,
Ohio, Arkansas and White rivers.

Upon entering the Gulf of Mexico, freshwater from the Mississippi
River drainage basin floats over the saltier, denser water of the Gulf,
resulting in a stratification of the water column.  This stratification
intensifies in the summer and prevents any oxygen housed within
the upper layers of the Gulf of Mexico from being introduced to the
bottom.  In addition, the Mississippi River discharge contains high
levels of nutrients, such as nitrogen, phosphorus and silica, some of
which is natural but much of which is derived from the widespread
application of fertilizers on farmlands that drain into the Mississippi
River and ultimately, into the Gulf of Mexico.  As in the case of
fertilizers applied to grasses or crops, these nutrients stimulate the
growth of phytoplankton (microscopic plants or algae) in the
surface waters of the Gulf.  These microscopic plants, in turn, support the
rest of the marine food web.  However, as these plants die and sink to the
bottom, the natural decomposition of this dead plant material depletes
the deeper waters of the Gulf of Mexico of what little oxygen it may contain.

Over the last four decades the amount of nitrogen delivered by the
Mississippi River basin has tripled.  More carbon is now being produced by
algae than was the case historically, and conditions of oxygen stress have
worsened.  This same process of hypoxia occurs elsewhere in the world where
humans have altered river chemistry.  Notable examples are the Black Sea,
Baltic Sea, Adriatic Sea, Chesapeake Bay, Long Island Sound, and the
Pamlico-Albemarle Sound.  The hypoxic zone in the northern Gulf of Mexico
is the third largest in the world and covers an area of ocean bottom 3,000
to 4,000 square miles in mid-summer, an area equal in size to the state of
New Jersey.

Given the natural conditions of the northern Gulf of Mexico as the
recipient of large quantities of freshwater and nutrients, it is logical to
assume that hypoxia has always occurred in the Gulf.  However, the first
documented hypoxic condition along the Louisiana coast was in 1972.
Thereafter, systematic sampling of the waters began in 1985.  Thus, the
modern observational database from which to determine long-term trends is
minimal. However, the accumulation of centuries of sediment that now make
up the Mississippi River delta can be used to reconstruct a reliable suite
of indirect measures of historical environmental changes in the Gulf

Analyses of these sediments indicate that carbon production has increased,
the productivity of phytoplankton, and in particular diatoms (microscopic
marine plants whose skeletons are composed of natural glass), has
increased, and oxygen conditions have worsened.  Some of the changes date
back to the turn of the century, but the problems have noticeably worsened
and accelerated since the late 1940s and early 1950s.

This scenario of worsening oxygen conditions in coastal waters of the Gulf
of Mexico adjacent to the nitrogen-enriched effluent of the Mississippi
River has obvious consequences to living resources and humans.  Similar
situations now exist throughout the world's coastal ocean regions where
environmental disruption caused by a planetary overload of nitrogen is
emerging as a new global concern.  Smaller ecosystems with nutrient
enrichment problems have been able to rebound when management interventions
have resulted in a reduction in the amount of nitrogen entering the systems
in question.  The scale of the Mississippi River watershed and the size of
the hypoxia zone in the Gulf however, are daunting barriers to success
which is likely to come slowly.

                  Human Impact and Possible Remedies

Since the early 1900s the hydrology of the Mississippi River system has
been altered by levies, locks, dams and reservoirs that have in turn led to
dramatic changes in the transport of water, sediments and nutrients from
throughout the basin into the Gulf.  Changes in agricultural practices over
time, such as the use of tile-drains agricultural lands, ditches and other
means to lower the water table and increase the efficiency of farming
methods, have hastened the transport of water from the landscape to the
river system and subsequently to the Gulf.  Nitrate, which is the most
soluble and mobile form of nitrogen, is easily leached from the soils into
these efficient drainage systems and is subsequently delivered much more
rapidly from the land to the Mississippi River system, which has led to a
larger nutrient flux to rivers in the basin and ultimately into the Gulf of
Mexico over time.  While long term historical data on nitrogen
concentrations in the basin are spotty, recent data suggest that average
influx of nitrate to the Gulf of Mexico has nearly tripled in the last four
decades resulting in a mean annual influx of 1.6 million metric tons of
nitrogen per year.  Models suggest that fertilizer and the soil inorganic
nitrogen pool are the largest source of this nitrogen, contributing
approximately half of the annual total nitrogen flux from the basin to the

Coincident with the change in nitrate flux has been changes in both the
average streamflow and the interannual variability of streamflow throughout
the Mississippi River basin. Over the last 100 years, annual precipitation
in most sites in the Mississippi River basin has increased by 5 to 20%,
coincident with a nationwide average increase in precipitation of 10 to
20%.  Similarly, US Geological Survey data indicates that streamflow for
the Mississippi was 30% higher during 1980-1996 than between 1955-1970.
Streamflow also appears to have been more variable during the last 15 to 20
years, and this variability has been shown to be strongly correlated with
nitrate flux.  In general, during dry years there is little rainfall to
transport nitrogen from the soil and unsaturated zones to streams and
nitrogen flux (particularly nitrate) is low.  Nitrate levels have been
demonstrated to build up in soils during dry years, largely as a result of
reduced uptake by crops.  By contrast, during periods of heavy
precipitation nitrate that has accumulated in the soil can be flushed into
streams via agricultural drains, ground water discharge and overland flow
at much higher rates than usual.  Thus wet years which follow dry years
tend to produce the largest influx of nitrate from the basin to the Gulf.
For example, data indicate that only a small area of hypoxic waters
developed in the Gulf during the 1988 drought, but the massive amount of
nitrogen introduced during the flood of 1993 caused the hypoxic zone to
more than double in size.  In fact, the drought of 1988 and the flooding of
1993 suggest that abrupt and short-term climate events such as the El Nino
Southern Oscillation (ENSO) can greatly influence the development and
extent of hypoxia in the Gulf.

The influx of nitrate to the Gulf of Mexico is likely to respond quickly
and dramatically to future changes in precipitation patterns and the timing
of precipitation.  Currently, most general circulation models (GCMs)
project that increases in temperature are likely to result in a more
vigorous hydrological cycle.  Recent observations on water vapor appear to
validate this projection.  However, the most significant projected impacts
from GCMs, for both the Mississippi River Basin and the Gulf of Mexico, are
likely to result from changes in the frequency and intensity of extreme and
short-lived events such as droughts, floods hurricanes, and El Nino/La Nina

In considering an array of potential options for mitigating or alleviating
altogether, the hypoxic condition in the Gulf, one of the most attractive
and potentially effective options for reducing the amount of nitrogen and
other nutrients coming into the Gulf might be to create and restore
strategically-placed wetlands and riparian zones where they can maximally
intercept agricultural drainage and thus optimize nitrogen removal through
plant uptake and denitrification.  The construction and restoration of
strategically-placed wetlands would not only contribute to the reduction of
nitrogen coming into the Gulf, thus decreasing the hypoxia, but would also
serve as sinks for carbon dioxide.  Other benefits would include
improvements in stream and river water quality and drinking water
protection, enhancing terrestrial wildlife in river corridors, and
providing increased flood protection.  The latter is especially
significant.  If future climate change increases the vulnerability of the
Mississippi River Basin to flood events, the combined effects of reducing
nutrient loading while increasing flood protection would clearly benefit
beleaguered ecosystems and people alike.


Dr. Nancy Rabalais is a Professor at the Louisiana Universities Marine
Consortium (LUMCON) where she has resided since 1983.  She teaches marine
science courses at LUMCON and in the Department of Oceanography and Coastal
Sciences at Louisiana State University.  Dr.  Rabalais' research interests
include the dynamics of hypoxic (oxygen deficient) environments,
interactions of large rivers with the coastal ocean, estuarine and coastal
eutrophication, benthic ecology, and environmental effects of habitat
alterations and contaminants.  Dr. Rabalais is a Fellow of the American
Association for the Advancement of Science, President of the Estuarine
Research Federation, and an Aldo Leopold Leadership Program Fellow.  She
has also recently been named as the recipient of the Blasker Award for
Science and Engineering for her outstanding scientific work on identifying
and understanding the linkages between the Mississippi River drainage basin
and the Gulf of Mexico.  Dr. Rabalais earned her Ph.D. in Zoology from the
University of Texas at Austin in 1983, and her BS and MS degrees in Biology
from Texas A&I University, Kingsville, in 1972 and 1975, respectively.

Dr. Donald Scavia is the Chief Scientist for the National Oceanic and
Atmospheric Administration's (NOAA) National Ocean Service and the Director
of  the  National  Centers  for  Coastal Ocean Science.  Prior to taking on
these new roles, Dr. Scavia was Director of  NOAA's Coastal Ocean Program
(COP).  Before  coming  to  the  COP,  Dr.  Scavia was a research scientist
at  NOAA's  Great Lakes Environmental Research Laboratory (GLERL) in Ann
Arbor, Michigan.  While at  GLERL  he  carried  out  a broad range of field
and laboratory  research and modeling studies on ecosystems of the Great
Lakes, with  particular  emphasis  on  food-web  dynamics  and nutrient
cycling.   He  has  served  on the Board of Directors of the International
Association for Great Lakes Research and the American Society of Limnology
and Oceanography, and is currently an Associate Editor for the journal
Estuaries.  He  has  served  in the following capacities under the
President's  Science  Advisor: Chair of the Subcommittee on U.S. Coastal
Ocean Science; Executive Secretary for the Subcommittee on Water Resources,
Coastal  and  Marine Environments; co-chair of the Ecosystem Working Group;
and co-chair of the Subcommittee on Ecological Systems.  Dr. Scavia  holds
a Ph.D. in Water Resources Engineering from the University of Michigan, and
BS and MS degrees in Environmental Engineering from Rensselaer Polytechnic

Ms Tracy Kuhns - No biography was available at the time of the printing.

The Next Seminar is scheduled for September 22, 1999

Tentative Topic: Drivers of Climate Change

For more information please contact:

Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office, 400
Virginia Ave. SW, Suite 750, Washington, DC 20024; Telephone: (202)
314-2235; Fax: (202) 488-8681 E-Mail: TSOCCI at USGCRP.GOV.

Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov.  A complete archive of seminar summaries can also be
found at this site.  Normally these seminars are held on the second Monday
of each month.

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