On a sweltering night in late July, a team of scientists gathered onboard the R.V. Pelican in Cocodrie, Louisiana, a 116-foot blue-and-white vessel resting in a bed of water hyacinths under a stormy sunset. As the clock ticked toward their midnight castoff, the scientists raced to set up the last of their equipment in the ship's small laboratories. For the next week, they would be working 24-hour days in tight quarters to measure what little oxygen was left in the Gulf of Mexico's infamous dead zone. The dead zone results from excess nutrients, mostly from fertilizers, running off farmland into the Mississippi River and finally into the Gulf. The size of the oxygen-starved zone changes every year, growing as more nutrients make their way into the Mississippi and shrinking when tropical storms or strong currents mix the water. Nearly every year since 1985, teams from Louisiana State University and the Louisiana Universities Marine Consortium have spent a week at sea mapping the zone. At each of the dozens of sites dotting the coastal Louisiana, these teams don hard hats and steel-toe boots and then cast into the sea the box cores, sensors and water sampling bottles that make their work possible. The route the team takes is the same every year, allowing each crew to contribute to a decades-long catalog of changes in these waters. It's an invaluable way to see how policies and industry changes have altered biodiversity in the Gulf and nutrient flow through the Mississippi. This year's crew of 12 -- all students, staff members and friends of Dr. R. Eugene Turner, the principal investigator and an L.S.U. marine ecologist -- were eager to join the storied tradition of the cruise and worked frenetically in the hours before castoff. "The first day is always so exciting," said Dr. Nancy Rabalais, by far the calmest presence onboard. Waiting next to a plastic kiddie pool, where a large metal apparatus lined with seawater sampling bottles was partly submerged for a final test of its sensors, she correctly predicted that they would find a smaller-than-forecasted dead zone this year. Unfortunately, that smaller size was not the result of improved conditions in the Gulf. Rather, it happened that the cruise set sail just after the tumultuous passing of Tropical Storm Bertha, which churned oxygen into the waters. So this year's cruise focused not just on generating a map of the dead zone, but also on determining how quickly the process of deoxygenation happens. "It was re-forming basically as we moved through," Dr. Rabalais said. "We were surprised that it happened in a matter of minutes."

Discovering the Dead Zone The mapping project started with Dr. Nancy Rabalais, a marine ecologist at Louisiana State University, and her quest to understand what was happening in the ocean alongside her native Louisiana. A love of crabs led to her early work in ecology, studying fiddler crabs in Cocodrie, but she soon started thinking about the larger problems plaguing local marine life, including one issue whose effects loomed large but were still unmeasured in the early 1980s: the dead zone. "Hypoxia was right in my backyard," she said. Relics from the early years of the hypoxia cruise filled her office: small comics shakily drawn while onboard the ship, a portrait of her sitting in the deck hammock that research crews used to fight over, a bejeweled gold crown with colorful letters reading "Hypoxia Queen." Dr. Rabalais wonder who's going to want all this junk when I clear out the lab," said Dr. Rabalais, now in her 70s. Pulling a faded collection of papers from a binder, she unfurled the hypoxia cruise's original handwritten data tables -- hundreds of pale-green rows long since digitized by students. She had a record of every organism found in the early water samples, most identified down to the species. Soon after joining the marine consortium in 1983, Dr. Rabalais received funding from National Oceanic and Atmospheric Administration to measure the hypoxic area off the coast of Louisiana. It was unexpectedly large -- now known to be...