Published: August 4th, 2008
Every year for the past couple of decades, scientists have tried to estimate the size of the dead zone that forms where the Mississippi River enters the Gulf of Mexico. Some years it is several times as large as Lake Pontchartrain. Last year it was the size of New Jersey. This year, it may well be as large as Massachusetts, possibly even exceeding the size it was in 2002 — nearly 8,500 square miles where almost nothing lives.
The dead zone is technically an area of hypoxia, or low oxygen content, first detected in the early 1970s. A decade later scientists realized it was caused largely by agricultural nitrogen — and some urban effluent — washed downstream from farms throughout the Mississippi watershed. That feeds algae, which consume the oxygen in the water as they decompose and lower oxygen levels to a point where life cannot be sustained.
The mechanisms that create the dead zone are entirely natural — algae feeding and dying — but there is nothing natural about the zone itself. It is almost entirely an artifact of modern agriculture, accompanied by treated and untreated sewage and industrial runoff. Most years the hypoxic zone dissipates in winter and re-forms in spring. At depth, the water is simply vacant of life and has the characteristic rotten-egg smell of hydrogen sulfide. The size of the zone depends on many different conditions. But this spring was extraordinary. There were widespread floods across the Midwest, mostly after the fields had already been fertilized. The result is a plume of fertility washing out into the Gulf of Mexico, where it fertilizes only death.
The dead zone at the mouth of the Mississippi is not the only one. There are dozens of smaller hypoxic zones around the American coastline where rivers spill into the sea. The same is true at river deltas around the globe, where the nitrogen load from fertilizer may be lower but the load of urban runoff — including partially treated and untreated sewage — is much higher. There could be no starker reminder of the tragic human tendency to treat the oceans as dumping grounds. And there is no better symbol of the paradox of American agriculture — the very richness applied to the fields is the source of ecological death hundreds of miles away.