Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 299

50ºN

North America

Gulf of
Mexico

10ºN

120ºE 60ºE

(a)

30ºN

Atchafalaya R.
Mississippi R.

1

29ºN

1

3

50 km

93ºE 91ºE 89ºE

Longitude

(b)

0

1

-10

-20

-30

0 10 20 30 40 50

Distance (km)

Figure 9.19 Distributions of dissolved oxygen (mg l 1 ) in the 'dead zone' off the coast of
Louisiana and Texas. (a) Bottom water dissolved oxygen in mid-summer 1993. (b) A section
across the shelf (dotted line in (a)) showing the low dissolved oxygen limited to the deeper
water. This section was taken in August 1990 during upwelling favourable winds, which have
brought low oxygen water shoreward into shallower areas. Adapted from Rabalais et al., 2002 ,
courtesy of Annual Reviews.

sediments become stressed at 1 mg l 1 , and die when oxygen falls below about 0.5 mg
l 1 . The scale of this problem is such that a task force, led by the U.S. Environmental
Protection Agency, has been set up to plan and implement the reduction of nitrogen
loading on the Mississippi; considering the size of the Mississippi catchment, which