Coring spot. A seasonal dead zone in the northern Gulf of Mexico developed occasionally in the 1800s, according to a new study. But its data suggest that the zone has become more intense in the last few decades as farmers cranked up fertilizer use. Coastal bottom waters off Louisiana now become depleted of oxygen almost every summer when nutrient-rich Mississippi River water causes populations of tiny marine plants called phytoplankton to explode. When they die, their decomposition sucks oxygen from the bottom waters. Fish and other animals then flee the area. Most scientists believe that chemical fertilizer is a major cause of the seasonal dead zone, but the fertilizer industry and a few scientists are skeptical (Science, 9 February 2001, p. 968). To probe past conditions, a team led by micropaleontologist Lisa Osterman of the U.S. Geological Survey in Reston, Virginia, took sediment cores from the consistently hypoxic zone. They dated cross sections and counted three species of tiny animals called foraminifers that tolerate low-oxygen waters. As far back as 1823, the hardy foraminifers thrived during Mississippi River flood years, suggesting that nutrients in floodwaters can trigger natural hypoxia. But the foraminifers were much more abundant after 1960, when Mississippi River Basin farmers began laying on commercial fertilizer. That has apparently driven low-oxygen episodes "very far off scale," says Osterman, whose study appears in the April issue of Geology. Although intrigued by the study, marine biologist Robert Diaz of the Virginia Institute of Marine Sciences in Gloucester Point cautions that the sediment-dating technique used can be off by a few years. CREDIT: LISA OSTERMAN/USGS