OXYGEN DEPLETION IN THE GULF OF MEXICO ADJACENT TO THE MISSISSIPPI RIVER - Past and Present Water Column Anoxia - page 231

Figure 9. Continuous bottom water dissolved oxygen in 20-m depth on the continental shelf
west of the Mississippi River (April-November 1993) (mooring site in Fig. 1). The horizontal
dashed line defines hypoxia. Data source: N.N. Rabalais, LUMCON.
of Louisiana shelf are similar to other areas of the world ocean. However, the
suboxic conditions for the northern 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.
3.1 Hydrogen Sulfide
The occurrence of anoxia and production of H 2 S in bottom waters on this
shelf are limited even though the continental shelf is seasonally hypoxic over a
large area and oxygen concentrations are often below 0.5 mg l − 1 . While instru-
mentation may limit accuracy at levels below 0.1 mg l − 1 , the presence of H 2 Sin
bottom waters is a definitive indicator that the dissolved oxygen concentration
is 0.0 mg l − 1 .H 2 S concentrations up to 50 µM have been measured in bottom
water samples that emitted a strong odor of H 2 S (N.N. Rabalais et al. unpubl.