A seasonal dead zone in the northern Gulf of Mexico developed occasionally in the 1800s, but it's become more intense in the last few decades as farmers cranked up fertilizer use, according to a new study of sediment samples from the Gulf. The results suggest that while low-oxygen waters happen naturally, modern farming practices have made the condition much more common.
Coastal bottom waters in the northern Gulf of Mexico now become depleted of oxygen almost every summer. This happens when nutrient-rich Mississippi River water spreads on top of saltier seawater, causing populations of tiny plants called phytoplankton to explode. The phytoplankton die, sink, and decompose, sucking oxygen from the bottom waters. Fish and other animals then flee the area, and most creatures that remain suffocate.
Most scientists believe that commercial fertilizer is a major cause of the seasonal dead zone, but the fertilizer industry and a few scientific skeptics claim instead that the zone occurs naturally (Science, 9 February 2001, p. 968). Since reliable oxygen measurements in the Gulf date back only to 1985, micropaleontologist Lisa Osterman of the U.S. Geological Survey in Reston, Virginia, and her colleagues developed an indirect measurement of historical low-oxygen conditions. They took sediment cores from an offshore region that frequently goes hypoxic, sliced the cores, and determined the age of each slice by measuring the decay of radioactive lead washed in with the sediments. They then counted three species of tiny animals called foraminifers that tolerate low-oxygen waters.
The hardy foraminifers thrived during Mississippi River flood years measured back to 1823, suggesting that floodwaters carried enough nutrient-rich sediment to trigger hypoxia. But the three foraminifers have grown much more abundant in core sediments since 1960, when farmers began using commercial fertilizer widely on the vast farm lands of the Mississippi River basin. The results mean that "low oxygen is a naturally occurring process," but that fertilizer use has driven it "very far off scale," says Osterman, whose study appears in the April issue of Geology.
Marine biologist Robert Diaz of the Virginia Institute of Marine Sciences in Gloucester Point says that the authors "may be on to something," but because the sediment-dating technique they used can be off by a few years, they're "overreaching the data." But micropaleontologist Pat Blackwelder of the University of Miami in Florida differs, saying that "the cores show very nicely that [dead zone] intensity is much greater over the last 50 years."