Environmental Engineering Reference
In-Depth Information
Figure 5.
Distribution of frequency of bottom water hypoxia in mid-summer on the
Louisiana/Texas shelf for 1985-2002 (modified from [14]).
Figure 6.
Isopleths of bottom water dissolved oxygen concentrations (mg l
−
1
for 7-17 July
1986 with values less than 2 mg l
−
1
stippled. Note the extent of values less than 3 mg l
−
1
, a level
below which some nekton are excluded. Data source: N.N. Rabalais, LUMCON.
more frequently during flood stages of the Mississippi River when summer
currents move more water to the east of the birdfoot delta. From limited data
where both sides of the delta were surveyed ( [44], National Marine Fisheries
Service unpubl. data), there is no evidence that the area of low oxygen forms a
continuous band around the delta.
Seasonal Variability.
The broad spatial coverage of hypoxia has been rou-
tinely measured only once per year in mid-summer, and the spatial extent over
the whole shelf during other parts of the year is less well known. More fre-
quent sampling along a transect C on the southeastern Louisiana coast (labeled
in Fig. 1 and 5) indicates that critically low dissolved oxygen concentrations
occur from as early as late February through early October and nearly contin-
uously from mid-May through mid-September [37]. Data from trawl surveys
in the Mississippi River bight indicate that hypoxia occurs in that area in
