During
Louisiana's long coastal summers, May through September, a
phenomenon known as hypoxia, or oxygen depletion, occurs in
the waters of the state's continental shelf. Hypoxic areas,
sometimes called "dead zones,” are sections of
the sea floor where dissolved oxygen is too low to sustain
typical sealife. The number of tiny bottom-dwellers –
polycheate worms, mud crabs, and snails, for example –
is sharply reduced. Larger animals such as fish and shrimp
leave, swimming to the surface where oxygen is adequate or
to other parts of the Gulf of Mexico.
Oxygen
Sustains Life
Dissolved
oxygen is essential in water's ability to support life. Oxygen
enters the water through the respiration of aquatic plants,
which produce it during photosynthesis, and through the contact
of surface water with air. A dissolved oxygen level of 4 milligrams
per liter is enough to sustain most aquatic animals, but levels
below 2 milligrams per liter cause varying degrees of stress
and, sometimes, death. The absence of dissolved oxygen is
called anoxia and most animals die if caught in anoxic water
for any length of time.
Causes
of Hypoxia
Water
stratification, or layering, is one contributor to hypoxia.
In the summer, when the sun is hot and winds are normally
mild, water from the Mississippi and Atchafalaya rivers flowing
into the gulf does not mix well with the saltwater but, instead,
floats above it. The heavier saltwater stays close to the
bottom. Dissolved oxygen remains in the lighter surface water,
but oxygen in the lower layer of water and at the bottom is
continuously depleted through the decay of organic matter
and the respiration of bottom-dwelling animals. Bottom waters
lose oxygen faster than surface waters can replace it. If
stratification lasts longer than two or three days, hypoxia
develops.
Occasionally,
storms are responsible for widespread hypoxia. Most recently,
Hurricane Andrew pushed an anoxic water mass ashore at Point
au Fer, trapping and killing about 80 million fish. Louisiana's
fishkill mounted to 187 million when, in the Atchafalaya basin,
the hurricane first churned up organic material that robbed
the water of oxygen as it decomposed and then flushed more
hypoxic water out of the swamps.
The foremost
cause of hypoxia, however, is the load of nutrients –
nitrates, phosphates, and silicates – brought to the
Gulf of Mexico by the Mississippi River. The river's watershed
is the largest in the United States, draining 41 percent of
the nation. On its southward journey past city sewage treatment
plants, agricultural fields, industrial operations, and residential
gardens, the river collects enormous amounts of these nutrients
and ultimately empties them onto the continental shelf.
The river's
discharge spreads in a thin layer above the heavier seawater,
and its load of nutrients stimulates the growth of phytoplankton,
masses of microscopic algae that die and fall to the bottom
if uneaten by fish and zooplankton. Animal fecal pellets also
sink, adding to the accumulation of waste. It's the subsequent
decomposition of all this organic material by microorganisms,
especially bacteria, that quickly strips the bottom waters
of oxygen.
Summer
water stratification is normal in the Gulf of Mexico and so
is the decay of dead animals and plants by oxygen-consuming
bacteria. But over-enrichment of the water by nutrients causes
an abnormal production of phytoplankton and a resulting increase
in decay.
Impacts
on Sea Life
The basic
threat to commercially important fish and shrimp species is
the impact of hypoxia on their food supply. Research has found
that, in hypoxic areas, the numbers and species of benthic
animals – the tiny organisms that live on or beneath
the water bottom and form a major source of food for fish
– are far fewer than in bottom waters with normal oxygen
levels. Thus, fish and shellfish crowd into oxygenated areas
where they compete for food in smaller habitats and more easily
fall prey to enemies.
Hypoxia
can also affect shrimp spawning and migration. For example,
white shrimp are bottom spawners; thus, the timing of egg
release is critical if coincident with hypoxia. Brown shrimp
spawn earlier and farther offshore than white shrimp, and
their larvae migrate to inshore nursery areas before oxygen
depletion is at its worst. But juvenile brown shrimp return
to offshore waters during the height of hypoxic conditions,
during which a normally risky journey becomes even more life-threatening.
Can
Hypoxia Be Reduced?
Hypoxia
has multiple sources and managing water quality in the Mississippi
River to reduce it in the Gulf of Mexico would require a coordinated
multistate effort. The states drained by the Mississippi River
watershed would have to agree on methods to regulate nonpoint
source pollution, upgrade sewer systems, and standardize agricultural
technology and then to apply them on a massive scale. The
Mississippi watershed – the fifth largest in the world
– may be too large and hypoxic processes too biologically
complex for such a mass effort to be workable, even if the
states could come to agreement. Controlling hypoxia may be
more feasibly accomplished through a proliferation of smaller,
local projects – for example, the regulation of industrial
waste –water discharges or the improvement of water
quality in local upriver freshwater systems that feed into
the Mississippi.
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