Environmental Engineering Reference
In-Depth Information
OXYGEN DEPLETION IN THE GULF OF MEXICO
ADJACENT TO THE MISSISSIPPI RIVER
Nancy N. Rabalais
1
and R. Eugene Turner
2
1
Louisiana Universities Marine Consortium, Chauvin, Louisiana 70344, USA
2
Coastal Ecology Institute, Louisiana State University, Baton Rouge, Louisiana 70803, USA
Abstract
The seasonal formation of a bottom water layer severely depleted in dissolved
oxygen has become a perennial occurrence on the Louisiana continental shelf
adjacent to the Mississippi River system. Dramatic changes have occurred in this
coastal ecosystem in the last half of the 20
th
century as the loads of dissolved
inorganic nitrogen tripled. There are increases in primary production, shifts
in phytoplankton community composition, changes in trophic interactions, and
worsening severity of hypoxia. The hypoxic conditions (dissolved oxygen less
than 2 mg l
−
1
) cover up to 22,000 km
2
of the seabed in mid-summer. Dissolved
oxygen concentrations seldom decrease to anoxia, but are often below 1 mg
l
−
1
and down to 0.5 mg l
−
1
. The continental shelf of the northwestern Gulf of
Mexico is representative of systems in which nutrient flux to the coastal ocean
has resulted in eutrophication and subsequently hypoxia. The Mississippi River
influenced continental shelf is similar to systems, such as deep basins and fjords,
with regard to biogeochemical processes of oxic versus suboxic conditions in
the water column and sediments. However, the suboxic conditions for the Gulf
of Mexico are less persistent in time and space due to the dynamic nature of the
open continental shelf system. Also, anoxia at the seabed is not as common or
long lasting.
Keywords:
continental shelf, Gulf of Mexico, Mississippi River, hypoxia, anoxia, eutrophi-
cation, nitrogen, phosphorus, silica
1. INTRODUCTION
Anoxic and suboxic conditions exist naturally in the world's oceans in fjords,
deep basins, and oxygen minimum zones [19, 24]. Similar conditions frequently
occur where upwelling systems associated with western boundary currents im-
pinge on the continental shelf. Oxygen depletion in coastal waters not subject to
upwelled nutrients results from eutrophication usually initiated and maintained
by increased nutrient loads under stratified conditions. Not all coastal systems
with elevated nutrient loads undergo eutrophication or develop hypoxia. The
225
