The Earth works in mysterious ways. Meteorologists tell us a butterfly flapping its wings in Asia can potentially give rise to a storm in the Midwest. Now it seems that intense storms in the Midwest can lead to fish dying in the Gulf of Mexico.
Ever hear of a "dead zone"? I don't mean the book by Stephen King or David Cronenberg's movie adaptation of it. I'm referring to the scientific term dead zone, which refers to a typically large swath of ocean that is so depleted of oxygen (or hypoxic), that most aquatic life caught in one -- including crabs, shrimp, and fish -- either suffocate to death or escape the region. In either case dead zones are bad news for commercial and recreational fishing.
Evidence suggests that the same hypoxic conditions that lead to dead zones may also result in an even more deadlier phenomenon -- red (or brown) tide, though this connection is not quite as well established. In addition to giving the ocean an eerie tint, the algae blooms that cause red tides release toxins into the water that kill fish and even threaten human health.
You Say Fertilizers, I Say Pollution
So what causes dead zones? Too much fertilizer. Fertilizers contain nutrients like nitrates and phosphates that help grow the crops we eat. But a lot of the nutrients used by farmers never get to our kitchen table; instead they are washed by rain into rivers and eventually carried out into the ocean.
Under certain conditions, nutrient-laden river waters flowing into the ocean trigger huge algal bloom. Eventually these algae die, and as they decay, oxygen dissolved in the water is consumed. If too much algae die, virtually all the oxygen is depleted, creating a hypoxia and a dead zone that may lead to red tide conditions as well. Because dead zones are usually caused by excess nutrients from river runoff, they are almost always found near deltas or estuaries.
Dead zones are a growing worldwide problem. Researchers have identified 415 areas that may be hypoxic. Of these, 169 are documented dead zones, while 233 are regions of concern, and 13 are systems in recovery.
Gulf of Mexico's Summertime Death Valley
America's worst dead zone is in the Gulf of Mexico at the mouth of the Mississippi River. In a good year the dead zone may extend only 2,000 or 3,000 square miles. But in other years it can extend over 5,000 square miles or more. (This chart shows the different sizes of dead zones from 1985 to 2007.) The largest dead zone seen so far occurred in 2002 and covered an area of almost 8,500 square miles -- the size of New Jersey.
The Nutrient-Flood Connection
Scientists have established a clear link between nutrients in the Mississippi and the Gulf dead zone. But other factors play a role as well. For example, algal blooms require sunlight and warm stable waters, so dead zones generally peak in the summer. Tropical storms can dissipate a dead zone by churning up oxygen-rich water from the depths to the surface.
Floods and droughts are also important. In the summer of 1988, when a fierce drought gripped the Midwest, there was little runoff, nutrient levels were low, and almost no hypoxia occurred. By contrast in 1993, a year that experienced record flooding along the Mississippi, the dead zone grew some 40 percent to about 7,000 square miles. In 2000, spring drought conditions limited the dead zone to less than 2,000 square miles.
Dead zones exact an economic as well as an environmental toll. Estimated costs of this year's record flooding in the Midwest are at $6 billion, but are expected to rise. And the estimates don't include the impact to the Gulf fisheries. Maybe they should. Scientists at the Louisiana Universities Marine Consortium predict that this year's dead zone will be the largest on record: about 8,800 square miles. The reason? All the additional nutrients flowing into the Gulf as a result of Midwestern flooding. That means a lot less fish landings for Gulf fishermen.
It Could Get Worse...
It's hard to predict the future, but climate change could very well exacerbate the dead zone problem because a warmer world could trigger more intense flooding.
But It is Not Hopeless...
To make the Gulf waters less hypoxic, nutrient concentrations in the Mississippi River need to come down. That won't be easy. About 74 percent of the nutrients reaching the Gulf are from fertilizers. In 1950 we used only about 4 million tons of fertilizer per year; since the late 1970s that amount has ballooned to about 20 million tons annually. Is it realistic to expect farmers to use less fertilizer? Actually yes -- we can lower fertilizer use without lowering food availability, for example, by not using corn for ethanol. Eating less meat can make a difference. In fact, Dr. Simon Donner of Princeton University estimates that if we were all vegetarians, there would be no dead zone in the Gulf at all.
Another approach is to restore forests and wetlands along riverbanks. These natural buffers trap nutrients. The Conservation Reserve Program (CRP) does just that. Unfortunately, CRP has come under attack as farmers lobby to pull land out of the program -- a move that will not help shrink the Gulf dead zone. Technological solutions on the horizon also offer some hope. Time-release fertilizers allow farmers to apply nutrients more efficiently so they use less and there is less runoff. Innovative cropping systems show promise too.
Strange connections -- the world is full of them. They're something to think about the next time you sit down to a meal. That hamburger you're eating may have helped wipe out schools of fish in the Gulf.
Dr. Bill Chameides is the dean of Duke University's Nicholas School of the Environment. He blogs regularly at www.thegreengrok.com.
Record Gulf Dead Zone Is Expected - New York Times (July 16, 2008)
Flood Waters to Widen 'Dead Zone' in Gulf of Mexico - USA Today (June 20, 2008)
Hypoxia in the Gulf Of Mexico - NOAA Coastal Services Center
U.S. Floods to Create Record Dead Zone - New Scientist (July 15, 2008)