Page 430 - Environment: The Science Behind the Stories

Sabine lake Lake Calcasieu Atchafalaya River Figure 1 The map in (a) shows
Mississippi River dissolved oxygen concentrations in
bottom waters of the gulf of Mexico off
Dissolved the Louisiana coast in 2011 and 2012.
oxygen
(mg/L) Areas in red indicate the lowest oxygen
7.0 levels. Regions considered hypoxic
(< 2 mg/L) are encircled with a black
line. The dead zone forms to the west
1 2011 Gulf of Mexico 50 km of the mouth of the Mississippi River
because prevailing currents carry
nutrients in that direction. The graph in
Sabine lake Lake Calcasieu Atchafalaya River (b) shows that the size of the hypoxic
Mississippi River
zone (shown by bars) is correlated
with the amount of nitrogen pollution
Dissolved entering from the Mississippi River
oxygen
(mg/L) (shown by line). In addition, floods
7.0 increase the size of the hypoxic zone by
bringing additional runoff (as in 2011),
whereas droughts decrease its size by
1 2012 Gulf of Mexico 50 km reducing nutrient runoff (as in 2012).
Tropical storms decrease the size of
(a) Dissolved oxygen at bottom hypoxic water by mixing oxygen-rich
water into the dead zone (as in 2003).
25,000 250 Scientists and policymakers aim to
Hypoxic area 5-year reduce the size of the dead zone to
Flux average 5000 km (1930 mi ). Data from Nancy
2
2
20,000 200 Rabalais, Louisiana Universities Marine
Consortium (LUMCON).
Area (square kilometers) 15,000 150 Nitrogen flux in May (thousands of metric tons) square kilometers was it? In which year
In which year was the hypoxic
zone largest, and how many
was nitrogen flux the highest, and what
10,000
100
was its approximate value that year?
5000 goal 50
No data

0 0
1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

(b) Area of hypoxic zone in the northern Gulf of Mexico

Since then, research has sup- adequate to eliminate the dead zone. All this research is guiding a fed- CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
ported this contention, and scien- Scientists also maintain that large-scale eral plan to reduce farm runoff, clean
tists now propose that nitrogen and restoration of wetlands along the river up the Mississippi, restore coastal
phosphorus should be managed jointly. and at the river’s delta, like those pro- wetlands, and shrink the Gulf’s
Moreover, recent research indicates filed in the Central Case Study, would dead zone. It has also led to a bet-
that a federally mandated 30% reduc- best filter pollutants before they reach ter understanding of hypoxic zones
tion in nitrogen in the river will not be the Gulf. around the world.

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