Page 429 - Environment: The Science Behind the Stories

tHe SCIeNCe BeHINd tHe StoRy

Hypoxia and the ing into the Gulf were polluted with agri-
gulf of Mexico’s cultural runoff, and the nutrient pollution
from fertilizers spurred algal blooms
“dead Zone” whose decomposition by bacteria
snuffed out oxygen in wide stretches
She was prone to seasickness, but of ocean water (pp. 126–127). This
Nancy Rabalais cared too much work had clearly demonstrated the
about the Gulf of Mexico to let that dr. nancy rabalais, LuMcon interconnections between freshwater
stop her. Leaning over the side of an aquatic systems and the Gulf, and how
open boat idling miles from shore, pollutants from farm fields in the upper
she hauled a water sample aboard— In 1991, Rabalais made that map Midwest could exert effects far away at
and helped launch efforts to breathe public, earning immediate headlines. the mouth of the Mississippi River.
life back into the Gulf’s “dead zone.” That year, her group mapped the size Many Midwestern farming
Since that first expedition in 1985, of the zone at more than 10,000 km advocates and some scientists,
2
Rabalais, her colleague and husband (about 4000 mi ). Bottom-dwelling such as Derek Winstanley, chief of
2
Eugene Turner, and fellow scientists shrimp were stretching out of their bur- the Illinois State Water Survey, chal-
at the Louisiana Universities Marine rows, straining for oxygen. Many fish lenged the findings. They argued that
Consortium (LUMCON) and Louisiana had fled. The bottom waters, infused the Mississippi naturally carries high
State University have made great pro- with sulfur from bacterial decomposi- loads of nitrogen from runoff and that
gress in unraveling the mysteries of the tion, smelled of rotten eggs. Rabalais’s team had not ruled out
region’s hypoxia—and in getting it on The group’s years of monitor- upwelling in the Gulf as a source of
the political radar screen. ing also enabled them to explain and nutrients.
Rabalais and other researchers predict the dead zone’s emergence. As But sediment analyses showed
began by tracking oxygen levels at rivers rose each spring (and as fertilizers that Mississippi River mud contained
nine sites in the Gulf every month and were applied in the Midwestern farm many fewer nitrates early in the century,
continued those measurements for states), oxygen would start to disap- and Rabalais and Turner found that
five years. At dozens of other spots pear in the northern Gulf. The hypoxia silica residue from phytoplankton
near the shore and in deep water they would last through the summer or fall, blooms increased in Gulf sediments
took less frequent oxygen readings. until seasonal storms mixed oxygen between 1970 and 1989, paralleling
Sensors, as they are lowered into the into hypoxic areas. rising nitrogen levels. In 2000, a federal
water, measure oxygen levels and send Over time, monitoring linked the integrative assessment team of dozens
continuous readings back to a ship- dead zone’s size to the volume of river of scientists laid the blame for the dead
board computer. Further data come flow and its nutrient load. The 1993 zone on nutrients from fertilizers and
from fixed, submerged oxygen meters flooding of the Mississippi created a other sources in the fresh waters emp-
that continuously measure dissolved zone much larger than the year before, tying into the Gulf.
oxygen and store the data. whereas a drought in 2000 brought low Then in 2004, while repre-
The team also collected hundreds river flows, low nutrient loads, and a sentatives of farmers and fishermen
of water samples, using lab tests to small dead zone (Figure 1). Similar rela- debated political fixes, Environmental
measure levels of nitrogen, salt, bacteria, tionships between river flow and dead Protection Agency water quality scien-
and phytoplankton. LUMCON scientists zone size have been seen ever since. tist Howard Marshall suggested that to
logged hundreds of miles in their ships, In 2005, the dead zone was predicted alleviate the dead zone we’d be best
regularly monitoring more than 70 sites in to be large, but Hurricanes Katrina and off reducing phosphorus pollution from
the Gulf. They also donned scuba gear Rita stirred oxygenated surface water industry and sewage treatment. His
to view firsthand the condition of shrimp, into the depths, decreasing the dead reasoning: Phytoplankton need both
fish, and other sea life. Such a range of zone that year. nitrogen and phosphorus, but there is
long-term data allowed the researchers The source of the problem, now so much nitrogen in the Gulf that
to build a “map” of the dead zone, track- Rabalais said, lay back on land. The phosphorus has become the limiting
ing its location and its consequences. Mississippi and Atchafalaya rivers drain- factor on phytoplankton growth.

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