0
1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 20115010015020025005,00010,00015,00020,00025,000May dissolved nitrite plus nitrate flux as N(thousands of metric tons)Area of mid-summer bottom water hypoxia (kmNitrite + nitrate flux^2 )
Low-oxygen areaNASA ImagesAdapted from USGS. Hypoxia in the Gulf of Mexico: Streamflow and
Nutrient Delivery to the Gulf of Mexico for October 2010 to May 2011 (2011).
a. The graph presents dissolved nitrite plus nitrate flux to the Gulf of
Mexico (bars), in the month of May 1985–2011, and (line) the mid-
summer area of the northern Gulf of Mexico in which bottom waters
exhibit very low oxygen levels (less than 2 mg/L), 1985–2010. This
area defines the size of the Gulf of Mexico dead zone (USGS 2011).b. Enhanced NASA satellite image illustrates summer phytoplankton
(algae) activity along the Gulf of Mexico coastline. Reds and oranges
indicate high concentrations of phytoplankton and river sediments,
and corresponding low-oxygen levels.Summary 293The Dead Zone in
the Gulf of Mexico
Nitrogen and phosphorus from the Mississippi River—
products of fertilizer and manure runoff from midwestern
fields and livestock operations—are deemed largely
responsible for a huge dead zone in the Gulf of
Mexico. Except for bacteria that thrive in oxygen-free
environments, no life exists in the dead zone because
the water there—where these nutrients have been
deposited—does not contain enough dissolved oxygen
to support fishes or other aquatic organisms (see
Figure a). Dead zones form seasonally worldwide; more
than 405 occur along global coastlines. The Gulf of Mexico
dead zone, one of the largest in the ocean, extends from the
seafloor up into the water column, sometimes to within a few
meters of the surface. In 2011 it covered about 21,000 km^2
(8000 mi^2 ), an area the size of New Jersey. It generally persists
from March to September. In March and April, snowmelt and
spring rains flow from the Mississippi River into the Gulf, and
the dead zone is most severe during June–August.
The low-oxygen condition in a dead zone, known
as hypoxia, occurs when algae (phytoplankton) grow
rapidly because of the presence of nutrients in the water
(see Figure b). Dead algae sink to the bottom and are
decomposed by bacteria, which deplete the water of
dissolved oxygen, leaving too little for other sea life.
Scientists are now seeing evidence that ocean warming
induced by global climate change may be exacerbating dead
zones. Dead zones, including the one in the Gulf of Mexico,
are expanding, they are emerging closer to shore than ever
before, and they are forming even in areas of the ocean that
don’t receive agricultural runoff.
Increased frequency and size of dead zones threaten
biodiversity and harm coastal fisheries. The EPA has taken
some measures to control nitrogen and phosphorus inputs
to the Mississippi River but recognizes that the dead zone
problem is immense in scope and will take billions of dollars
and decades of effort to fix.
CASE STUDY
Summary
1
The Global Ocean 274- The global ocean is a huge body of salt water that surrounds
the continents. It affects the hydrologic cycle and other cycles
of matter, influences climate and weather, and provides food
to millions.
✓✓THE PLANNER
- Prevailing winds over the ocean generate gyres, large, circular
ocean current systems that often encompass an entire ocean
basin. The Coriolis effect is a force resulting from Earth’s
rotation that influences the paths of surface ocean currents,
which move in a circular pattern, clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere.