Environmental Engineering Reference
In-Depth Information
Figure 4.
Concentration of surface water nutrients (µM) and chlorophyll biomass (µgl
−
1
)on
the Louisiana/Texas shelf for multiple seasons in 1985-2002 (modified from [40]). The black
arrows indicate the large inputs of fresh water and nutrients from the Mississippi and Atchafalaya
Rivers, and minor contributions from the Calcasieu, Galveston, Nueces and other estuaries. The
shaded horizontal arrows indicate the typical distance alongshore for the mid-summer hypoxic
zone.
deltas (Fig. 6). The water between these two areas, however, is also depleted
in oxygen (less than 3 mg l
−
1
). Hypoxia more often forms a single continuous
zone (examples for 2001 and 2002 in Fig. 1). The 1998 hypoxic zone was
atypically concentrated in deeper water than usual on the eastern Louisiana
shelf. Although the bottom extent of hypoxia in 1998 was less than 1997 (Fig.
2), the volume of the hypoxic water mass in 1998 was greater than in 1997
[35]. The smaller area in 2003 resulted from a series of tropical storms that
moved through the area two weeks prior to the mapping cruise and disrupted
the stratification.
Most instances of hypoxia elsewhere in the northern Gulf of Mexico along
the shelf farther to the west and also east of the Mississippi River delta are
infrequent, short-lived, and limited in extent [30, 37]. Hypoxia on the upper
Texas coast is usually an extension of the hypoxic zone off Louisiana [18, 28].
Hypoxia east of the Mississippi River is also isolated and ephemeral, but occurs
