Environmental Engineering Reference
In-Depth Information
from zooplankton grazing settle through the water column and concentrate
in fine layers at pycnoclines. Respiration in these interfaces often exceeds
oxygen production via photosynthesis so that oxygen depleted layers form
at or immediately below the concentration of organic matter. Examples are
provided from within areas of hypoxia in July 2004 following a period of
unseasonably high discharge of the Mississippi River (Fig. 8). Mid water
oxygen minima, however, are not limited to areas of oxygen-depleted bot-
tom water. These features are common near the Mississippi River plume
in spring and diminish in frequency and strength with distance from the
discharge.
High Frequency Oxygen Measurements.
Continuously recording (15-min
interval) oxygen meters have been deployed near the bottom at a 20-m station on
transect C since 1990. There is variability within the year and between years, but
the pattern (Fig. 9) that usually develops at this location includes: (i) gradual
declines of bottom oxygen concentrations through respiration and more rapid
reoxygenation from mixing events, (ii) persistent hypoxia and often anoxia
for extended periods in May-September,
(iii) isolated intrusions of higher
oxygen content water from depth during upwelling-favorable wind conditions
followed by a movement of the low oxygen water mass back offshore, and (iv)
tropical storms, hurricanes or cold fronts in the late summer and fall that mix
the water column sufficiently to prevent prolonged instances of low oxygen
concentrations [33].
Another recording oxygen meter was deployed in a similar depth of 20-m
but 77 km to the east and closer to the Mississippi River delta where the depth
gradient of the shelf is much steeper [39]. At that station hypoxia occurred for
only 44 percent of the record from mid-June through mid-October (compared to
75 percent at the station on transect C). There was a strong diurnal pattern in the
oxygen time-series for the former and not for the latter. The dominant coherence
of the diurnal peaks of oxygen concentration from the site in the Mississippi
River bight with bottom pressure records suggests that the dissolved oxygen
signal was due principally to advection of the interface between hypoxic and
normoxic water by tidal currents.
3. BIOGEOCHEMICAL PROCESSES IN HYPOXIA
Most of the biogeochemical investigations of the highly productive plume
of the Mississippi River have focused on surface waters with regard to bacterial
production, respiration, carbon remineralization, and nutrient uptake and regen-
eration, and less often with similar processes in sediments overlain by hypoxic
waters. Little is known about nutrient and carbon transformations within the
mid-water hypoxic layers. The biogeochemical processes of oxic versus hy-
poxic conditions and oxic/anoxic interfaces in the water column and sediments
