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HYPOXIA
The Gulf of Mexico’s Summertime Foe

Each summer the richly productive bottom waters off Louisiana’s coast are transformed from a region of teeming marine life into a vast area known as the “Dead Zone,” which can encompass up to 8,500 square miles. Within this zone, the water near the bottom is hypoxic—nearly depleted of oxygen. The organisms living there must either escape or die of suffocation. Its effects are not yet fully defined, but the cause of this phenomenon is clear. Simply stated, the Gulf of Mexico is suffering from too much of a good thing.


Grey Snappers, like all other Gulf species, prefer water with abundant oxygen and food. The Dead Zone provides neither.
Photo by: NOAA

The Gulf owes its great productivity to abundant nutrients such as nitrogen, phosphorus and silica, which enhance the growth of marine life. Nutrients encourage the growth of algae, a basic element of the Gulf food chain. More algae mean more plankton, more shrimp and more fish. But algal production is a good thing only up to a point. The volume of nutrients now delivered to the Gulf is causing algae to grow too fast, with deadly consequences for these waters.

Algal Bloom or Red Tide?


Chaetoceros, a red-tide organism

A “bloom” occurs when algae begin to rapidly reproduce, forming billions of algal cells in surface waters, sometimes turning the waters a distinctive green. However, many types of algae, such as dinoflagellates, can produce toxins that poison the waters and kill almost all nearby organisms, producing a “red tide.” Even fish cannot always escape fast enough to survive this deadly menace.

Hypoxic Waters

Algal cells flourish in the well-lit surface waters of the Gulf each spring and summer, drifting to the bottom after death. Also, the algae are eaten by marine animals, which then produce fecal matter that also drops to the bottom. There, the accumulated organic matter begins to decompose.

This slow decay uses oxygen, depleting the oxygen living organisms at the bottom need. Eventually, unless the oxygen is replaced, the bottom waters become hypoxic—having less than two milligrams of dissolved oxygen per liter.

In a marine system with two distinct water layers, over-production by algae may cause hypoxia, disrupting bottom communities and causing mobile organisms to leave the area. Modified from National Center for Appropriate Technology, www.ncat.org/nutrients/hypoxia/hypoxia1.htm.

Summertime Stratification

During most of the year, oxygen-rich water reaches the bottom and decomposition rates are normal. However, in spring and summer, when the algae bloom, the surface and bottom waters of the Gulf stratify into two layers. Warm, fresh and less dense water from the Mississippi and Atchafalaya rivers spreads out on the surface of the deeper and colder Gulf water. Calm winds and warm sunshine prevent the layers from mixing thoroughly. The hypoxia begins at the bottom, climbs toward the surface and spreads throughout the summer months. The lower waters may not receive fresh oxygen from May until September.

The coming of autumn brings tropical storms, hurricanes, cold fronts and cooler water flowing into the Gulf. These mix the Gulf waters and replenish oxygen at the bottom once more.

Measuring and Monitoring the Dead Zone

The Dead Zone may reach from the Gulf floor to the surface, but most often stretches upward from five to 30 meters. It occurs over a changing area and set a record in 2002 when it covered 8,500 square miles. The zone forms primarily in the summer when the Gulf waters are calmer and the waters stratify easily. From spring to early fall, scientists monitor its movement with oxygen sampling equipment at various depths. Transects of the coastal waters map the extent of the zone.

Life in the Dead Zone

During the summer months of low oxygen, while fish and other mobile animals can escape the developing Dead Zone, bottom dwellers that are attached to the ocean floor are doomed as the normal bottom ecosystem collapses. Significantly, when the autumn mixing brings oxygen levels back to normal, the bottom communities do not have the chance to revive. Instead, the area tends to be re-colonized by only a few short-lived species (such as polychaete worms) that can establish themselves in the aftermath of the Dead Zone. Larger, longer-lived species (such as gastropods, bivalves, starfish, brittle stars and sea anemones) vanished from this part of the Gulf 30-40 years ago. These new bottom communities fall far short of the normal ecosystems in diversity, abundance or biomass. Ken Teague of the U.S. Environmental Protection Agency describes it as “the seasonal elimination of a vast area of productive habitat. In the Dead Zone, the original bottom communities were lost a long time ago.”

Graph
As nutrients in an ecosystem increase so does productivity. But when too many nutrients cause over-production, decomposing organic matter may begin to use up the available oxygen. In stratified waters, this can result in hypoxia.

Often, the fish and shrimp that escape the Dead Zone during the spring and summer congregate at its edges. These concentrations have caused many Louisiana shrimpers to leave their normal fishing areas to make a good catch. Other fishermen who take their catches from the uppermost waters may not be affected unless the Dead Zone reaches the very top of the water column. In the rare cases when this happens, the water does not have a different appearance; it’s merely devoid of life.

Dead zone frequency map
The location of mid-summer hypoxic waters off the Louisiana coast can be predicted using data collected in prior years (1985–1999). Courtesy Nancy Rabalais, Louisiana Universities Marine Consortium.

Is This So Bad?

Large hypoxic zones that occur at the mouths of rivers are not unusual. They can be found in Long Island Sound, Chesapeake Bay, the Baltic Sea and the Black Sea. However, the Gulf hypoxic zone is among the largest, in some years covering an area comparable to New Jersey.

Because research on the Dead Zone is still relatively recent, scientists studying the Gulf know only the most fundamental facts about it. They can prove it exists and know when and why it will form. But they understand only some of its effects on ecosystems. For example, while the catch of brown shrimp declined during the years of greatest hypoxia (1992-1997), scientists are not certain that the Dead Zone caused it.

Does the Dead Zone Mean Dead Fish?

Massive fish kills in the Gulf are usually associated with red tides, not the Dead Zone, as fish can usually avoid the hypoxic zone and survive. Only when the oxygen-depleted waters are pushed onshore into bays and lagoons do fish become trapped. At its present size, the Dead Zone does not commonly reach shore.

Still, although little is known at present, the Gulf ’s fish and shrimp ecosystems show signs of stress. The Dead Zone is a prime suspect for:

No one yet knows if the Dead Zone is a serious threat to coastal fishery and recreation industries.Will there be a point at which the enormous load of nutrients carried to the Gulf causes the ecosystem to collapse? Only time will tell.