OXYGEN DEPLETION IN THE GULF OF MEXICO ADJACENT TO THE MISSISSIPPI RIVER - Past and Present Water Column Anoxia - page 220

processes of increased phytoplankton biomass, carbon accumulation, and oxy-
gen depletion are more likely to occur in systems with long water residence
times and where the water column is stratified by salinity, temperature, or both.
The amount of suspended sediment delivered to a coastal system also may
influence whether enhanced production will result from the changing nutrient
inputs.
Nutrient load increases result from increasing human populations and their
activities - application of nitrogen and phosphorus fertilizers, planting of legu-
minous crops, atmospheric deposition of nutrients, and municipal and industrial
wastewater [1, 50]. The consequences can be desirable or undesirable, as per-
ceived by humans. Increased nutrients stimulate increased primary production
and increased secondary production, and often the yield of commercially im-
portant fisheries. The negative effects may include increased noxious or harmful
algal blooms, diminished water clarity, shifts in trophic interactions that do not
result in the diatom-zooplankton-fish food web, loss of essential habitat and
oxygen depletion. Over the last half of the 20 th century, the impacts of eutroph-
ication, including oxygen depletion, increased in frequency and severity and
expanded geographically [3, 10, 27]. This trend will continue in the future with
increasing nutrient loads [43].
The continental shelf of the northwestern Gulf of Mexico is representative
of systems in which nutrient flux to the coastal ocean has resulted in eutroph-
ication and subsequently hypoxia. One other shelf environment with a similar
scenario of changing nutrient loads and worsening hypoxia is presented for the
northwestern shelf of the Black Sea that receives discharges from the Danube,
Dniepr, and Dniestr rivers (Zaitsev, present volume).

2. DISTRIBUTION AND DYNAMICS OF HYPOXIA
We define hypoxia as dissolved oxygen less than 2 mg l − 1 , or ppm, because
bottom-dragging trawls usually do not capture any shrimp or demersal fish
below this level [40]. (Dissolved oxygen of 2 mg l − 1 equates to 1.4 ml l − 1
or 63 µM, and approximates 20% oxygen saturation in the 25 ◦ C and 35 psu
bottom water of the hypoxic zone.) Sharks will modify their behavior to escape
oxygen concentrations that fall below 3 mg l − 1 . When dissolved oxygen values
are below 2 mg l − 1 , they are often less than 1 mg l − 1 , a level that is stressful or
lethal to benthic macroinfauna.
The second largest zone of oxygen-depleted coastal waters in the world is
found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf,
an area that is influenced by the freshwater discharge and nutrient load of the
Mississippi River system [33] (Fig. 1). The mid-summer areal extent of hypoxic
waters averaged 13,000 km 2 over the period 1985-2004, with the largest size
mapped in 2002 at 22,000 km 2 (Fig. 2). The seasonal cycle of freshwater