Environmental Engineering Reference
In-Depth Information
data). H
2
S concentrations of 2-5µM were chemically detected, when there was
still a faint H
2
S odor. Thus, the 'odor indicator' has been used to determine the
occurrence of anoxia. In a mid-summer survey of 80-90 stations, there are at
most 10 stations in which the bottom water collections smelled of H
2
S (N.N.
Rabalais et al. unpubl. data). For the nine-station transect C, up to two stations
per month in June through September can have the H
2
S smell. The presence
of sulfur-oxidizing bacteria at the sediment-water interface on many occasions,
observed both by divers and video surveillance from remotely operated vehicles
[31], indicates that extremely low oxygen concentrations, though not anoxic,
commonly allow for the presence of these bacteria on the sediment surface.
3.2 Respiration and Oxygen Production
The Louisiana shelf hypoxic area, as represented by a 20-m station (mooring
site, transect C, Fig. 1), is predominantly heterotrophic throughout the year [21].
The difference between bottom water oxygen deficit (measured oxygen content
below the oxygen content at 100% saturation) and surface water oxygen surplus
(measured oxygen content above the oxygen content at 100% saturation) is
greatest in April-September.
Light conditions partially influence where hypoxic water masses are lo-
cated and their severity. Extinction coefficients may be sufficiently improved
at the edge of the hypoxia water mass to support benthic oxygen production.
The Bierman et al. [2] model indicated that deeper light penetration might be
more important with regard to hypoxia distribution in the western portion of
the Louisiana shelf compared to the eastern area where the water clarity is
lower (either due to suspended sediments or shading from high algal biomass).
With sufficient light, photosynthesis at or near the sediment-water interface
will occur and offset oxygen uptake processes to the point that anoxia does not
frequently occur. The low oxygen concentration observed in respiration exper-
iments (average 3.4 mg l
−
1
,
n
= 40), however, suggests that benthic oxygen
production was relatively low [13].
The oxygen consumption rates in near-bottom waters were measured during
several spring and summer cruises of multiple years [34, 44, 45]. Rates varied
between 0.0008 to 0.29 mg O
2
l
−
1
hr
−
1
, and were sufficient to reduce the
in situ
oxygen concentration to zero in less than four weeks. The rates were inversely
related to depth and decreased westward of the Mississippi River delta, consis-
tent with the decrease in nutrients and chlorophyll
a
concentrations in surface
waters. Respiration rates per unit chlorophyll
a
were highest in the spring, in
shallower waters, and also closest to the Mississippi River delta. These results
(1) indicate a strong vertical, rather than horizontal, coupling between oxygen
consumption in bottom waters and organic loading from surface waters, and
(2) are consistent with the hypothesis that the higher water column respiration
