OXYGEN DEPLETION IN THE GULF OF MEXICO ADJACENT TO THE MISSISSIPPI RIVER - Past and Present Water Column Anoxia - page 233

rates are driven by river-derived nutrients stimulating in situ organic production
that sinks to the bottom layers.
Respiration in sediments is an additional oxygen sink for these waters. Dortch
et al. [13] suggested that this oxygen sink may sometimes equal respiration in
the overlying waters, but most results (field and modeling experiments) indicate
that the sediment consumption is seldom more than one-third of the total oxygen
uptake below the pycnocline. Current work by Z. Qui nones et al. (pers. comm.)
with oxygen isotopes will better define these processes.

3.3 Low Redox Conditions
In general, the relationships between bottom water oxygen concentration
and the concentration of dissolved inorganic nitrogen forms, phosphate and
silicate are inverse (Fig. 10).

Figure 10. Comparisons of bottom water dissolved oxygen concentration and bottom water
dissolved inorganic nutrient concentrations for a series of stations in 20 m depth on transect
C within 1 km of each other between 1985-2002 for all months. Data source: N.N. Rabalais,
LUMCON.

Most of the inorganic nitrogen is present as nitrate, but there are sometimes
significant amounts of ammonium. Denitrification rates in sediments from sta-
tions within the hypoxic zone ranged between 150 and 410 µmol N m − 2 hr − 1
[8, 7]. The highest rates were observed when bottom water oxygen concentra-
tions were between 1 and 3 mg l − 1 . Denitrification activity was significantly
lower at stations where dissolved oxygen was lower than 1 mg l − 1

or greater