Environmental activists claim that synthetic fertilizer from farms is threatening the marine life in the Gulf of Mexico by causing the rampant spread of a “dead zone” of low-oxygen water.
New research, however, shows that the Gulf of Mexico had the seasonal “dead zones” of low-oxygen water as early as 1817—long before farmers even settled the Mississippi Valley.
Lisa Osterman of the U.S. Geological Survey says seabed sediment cores taken from the continental shelf off Louisiana show a series of low-oxygen—or hypoxic—episodes between 1817 and 1910. She says the shells of tiny microfauna, which don’t mind low oxygen levels and like the organic detritus in the “dead zones,” are more abundant in the sediments dated to the hypoxic surges.
Osterman’s sediment cores also confirm that high water flows in the Mississippi River are the biggest cause of the nutrient surges that produce low-oxygen conditions. Even as the surges nourish the Gulf marine ecosystem with nitrogen, phosphate, and organic matter, they also produce big algae blooms that sink to the bottom—absorbing local oxygen as they decay.
The Gulf of Mexico is one of the world’s richest marine ecosystems because of the nutrient-rich water coming down the Mississippi from the rich lands of the Corn Belt and Great Plains. More than 40 major rivers worldwide have similar seasonal low-oxygen zones where nutrient-rich fresh water meets salt water in enclosed bays. Much of the open ocean, by contrast, is a “marine desert” with little aquatic life.
In the Gulf of Mexico, a huge pod of 300 endangered sperm whales feeds on the outer fringes of the dead zone where the fish and squid they eat are most numerous.
Osterman says the Gulf of Mexico’s low-oxygen zones have gotten larger in recent history (post 1930s) when farmers began using synthetic fertilizers. Recent big floods in 1979, 1983, 1993, and 1998 have also sharply expanded the “dead zones,” sometimes for several years after the high water.
However, the hypoxic zones have never occupied even 1 percent of the Gulf water area, and even less of its volume because the hypoxia occurs in shallow water. Moreover, the mouth of the Gulf of Mexico is 400 miles wide. The Black Sea, by contrast, where hypoxic conditions are much more severe, is a nearly enclosed body of water drained only by the Hellespont, which is less than a mile wide.
Agriculturists note that there is little threat of any long-term damage to the Gulf from farm fertilizers. Even before farming, huge tonnages of nutrients came down the Mississippi from the millions of bison, antelope, and birds; the billions of grasshoppers that lived on the plains; and the organic detritus from the forests. That’s why there were hypoxic events in the early 1800s, and almost certainly long before.
Farm fertilizer use has not increased in the past 25 years, while yields in the corn fields that get most of the fertilizer have risen nearly 30 percent. Thus, 30 percent less nitrogen is probably being leached into the Mississippi’s tributary rivers today than in 1975.
