Dead Zones Spreading in World Oceans

A cold March wind blows across the Gulf of Mexico, throwing waves carrying red tide, a bloom of toxic algae, into Dan Leonard's sun-lined face. Leonard, who with his wife Carol owns Bull Bay Clam Farm, pushes his fishing boat to full power, skimming across Charlotte Harbor, Florida. He's heading toward a 1.6-hectare area on the gulf's muddy floor. There he seeded tiny clams the previous November, hoping they'd soon grow to market size. Leonard leases the bottom for his clam farm from the state of Florida, but he hasn't been able to harvest clams for weeks because of the red tide. Dead and dying mangrove trees lie like fallen soldiers all along the shore. The mangroves carry a warning, Leonard believes, one of ocean death. “Nutrients flowing into the gulf are killing life here, creating red tides and a huge dead zone where nothing can live. The water has next to no oxygen. Every summer, the dead zone grows, snuffing out more fish, crabs, and other animals. And we're the perpetrators of the crime, with our excess fertilizer and untreated sewage and other waste flowing into the gulf. The dead zone, once unheard of, is starting to kill everything in its path. In spring when I'm in the water tending my clam beds, I can almost feel it coming.” In late summer, pushed shoreward by a gentle east wind, low-oxygen water steals into Mobile Bay, Alabama, at night. Cries of “Jubilee!” carry along Mobile Bay's eastern shore, for ahead of this dead zone, known to locals as a jubilee, bottom-dwellers sense the low oxygen levels and scuttle away to avoid suffocating. Some are trapped as the hypoxic, or low-oxygen, waters approach shore. Crabs, eels, flounder, and other marine creatures wash in like so much flotsam. Fishers line docks and beaches, poised to grab this last line of life before the poisonous waters reach the shoreline. A jubilee for humans is hardly so for marine life, says Rick Wallace, director of the Auburn University Marine Extension and Research Center in Mobile, Alabama. “Some fish can easily swim over the top of the advancing low-oxygen water mass,” he says. “But bottom-dwelling fish and crustaceans can't escape. And once the oxygen-poor waters move into shore, the fish and shellfish swimming in front of them are also trapped. Previously occupying hundreds of acres of bay floor, these organisms now teem along the water's edge, trying to breathe.” Jubilee-affected blue crabs cling to pilings, blowing air bubbles in and out of their mouths. Flounder lie at the water's edge, gulping and trying to pass air over their gills. Eels burrow tail-first into moist sands, their heads in the air. Jubilees usually end at sunrise, when oxygen is produced by the photosynthetic activity of plant life in the water, says Wallace. “The affected fish and shellfish—those that are still living, that is—swim back to their habitat…until the next jubilee.” The phrase “dead zone”—coastal waters too low in oxygen to sustain life—is almost synonymous with the Gulf of Mexico. But a similar situation now exists in many other places, says Donald Boesch, president of the University of Maryland Center for Environmental Studies in Cambridge, Maryland. “There's a dead zone right outside my office window every summer in Chesapeake Bay,” says Boesch. “Since the 1970s, this lifeless zone has become a yearly phenomenon, sometimes affecting 40 percent or more of the bay.” Boesch says that the expanding dead zone could be changing the entire ecosystem of the Chesapeake. Animals tolerant of hypoxia are becoming more common in the bay's waters: jellyfish may be displacing oysters, crabs, and finfish like striped bass. Worldwide, there are now some 146 coastal dead zones. Since the 1960s, according to a report by the United Nations Environment Programme (UNEP), Global Environment Outlook Year Book 2003, the number of dead zones has doubled with each passing decade. Most are seasonal, but some persist year-round. Where did these killing fields for fish and other marine life begin? The complex chain of events begins not in the sea, but on land. Farmers often overfertilize their fields. The excess fertilizer, laden with nutrients like nitrogen, washes into creeks and rivers, where it's eventually carried into coastal bays and the open sea. “Humankind is engaged in a gigantic global [experiment] as a result of the inefficient [use] and often overuse of fertilizers, the discharge of untreated sewage, and rising emissions from vehicles and factories,” says Klaus Toepfer, UNEP's executive director. “Nitrogen and phosphorus from these sources is being discharged into rivers and the coastal environment or being deposited from the atmosphere, triggering alarming and sometimes irreversible effects.” The environmental significance of nitrogen has been studied for more than 150 years, but scientists have only recently begun to understand how nitrogen cascades through ecosystems, creating environmental problems along the way. Worldwide, humans create about 160 million metric tons of nitrogen each year, compared to natural rates of terrestrial biological nitrogen fixation, which are between 90 and 120 million metric tons annually. Nitrogen, says the UNEP report, is fast accumulating in the environment. “Nitrogen is necessary to increase crop yields,” states the report, “but plants are inefficient at taking it up, and often more fertilizers and animal wastes are added than the plants need. As a result, only a fraction of the nitrogen applied to soils actually ends up in crops; in some regions, it is less than 20 percent.” The rest is on the loose. The recently released United Nations Millennium Ecosystem Assessment Synthesis Report (March 2005), compiled by 1300 scientists in 95 countries, states that more land has been converted to agriculture since 1945 than in the 18th and 19th centuries combined. And more than half of all synthetic nitrogen fertilizers ever used on the planet have been spread across fields since 1985. Another major source of excess nitrogen is in nitrogen oxides created from burning coal, oil, and natural gas. Untreated or partially treated human and animal wastes also add nitrogen to aquatic ecosystems. There the nitrogen fertilizes microscopic plant life—phytoplankton—just as it encourages plants on land to grow. In the presence of excessive concentrations of nitrogen and phosphorus, phytoplankton proliferate, reaching massive blooms. When the plankton die, they fall to the seafloor and are digested by microorganisms. The process removes oxygen from the bottom water and creates low-oxygen zones, says Donald Scavia, director of Michigan's Sea Grant Program in Ann Arbor. Sometimes these zones are anoxic, with no oxygen. Most marine life cannot survive in hypoxic conditions. Fish and other animals that can swim away do, says Nancy Rabalais, a marine ecologist at the Louisiana Universities Marine Consortium in Chauvin. Other animals that can't leave the scene, such as bottom-dwelling starfish, suffocate. “At minimal concentrations just above anoxia,” states Rabalais, “bacteria form white mats on the sea floor.” When oxygen levels reach zero, there is no sign of aerobic life, she says, “just anoxic black sediments.” Fully oxygenated waters may have as much as 10 parts per million (ppm) of oxygen. Once oxygen falls to 5 ppm or below, fish have trouble breathing. Sharks leave oxygen-deprived waters at about 3 ppm, but some fish remain until the level reaches 2 ppm or below. Animals that are sediment dwellers, and so can't leave a hypoxic zone, die at 1.5 ppm. In some of the world's dead zones, oxygen drops below 0.5 ppm and remains there for months, leaving a wide swath of death in its wake. Dead zones range in size, says Boesch, from small areas of coastal bays and estuaries to huge areas of the open sea, where they can reach tens of thousands of square kilometers. Most are found in temperate waters, off the eastern coast of the United States and in the seas of Europe. Some are brewing in the waters off China, Japan, Brazil, Australia, and New Zealand. The world's largest dead zone is in the Baltic Sea, where nutrient-enriched runoff from farms has combined with nitrogen deposition from the burning of fossil fuels and human waste discharged directly into the sea's waters. In the summers of 2002 and 2004, a new dead zone joined the worldwide ranks: waters off Newport, Oregon, suddenly had oxygen levels well below the minimum needed to support most marine life. Large numbers of crabs, rockfish, and other organisms that couldn't flee the area fast enough were smothered. In this case, the dead zone may have been caused not by excess nutrients but by an intermittent upwelling of oxygen-poor deep waters common off the US West Coast. If that is so, scientists believe, the Oregon dead zone won't recur frequently. But a possible change in Pacific Ocean circulation patterns could make this newest dead zone an annual event. At last count, 43 of the world's known coastal dead zones occur in US waters. The Gulf of Mexico dead zone is now the planet's second largest. Scientists say that low oxygen levels are appearing this year much earlier than in the past. Last year, no hypoxia was found until June; this year, it made its appearance in March. The gulf 's dead zone might therefore cover a much larger area this year than in previous years, says Steve DiMarco, an oceanographer at Texas A&M University in College Station. DiMarco's team plans to take samples of northern Gulf of Mexico waters throughout the summer and into the fall, to track the extent of this year's dead zone. “It's summer, and everything's blooming, but this isn't good news for the Gulf of Mexico,” says Robert Howarth, a scientist at Cornell University in Ithaca, New York. During the warm months of the year, oxygen levels in the northern Gulf of Mexico fall well into the hypoxic zone. Annual surveys begun in 1985, says Howarth, demonstrated that hypoxia was most often found west of the outfalls of the Mississippi and Atchafalaya Rivers. This low-oxygen zone averaged some 7800 to 9100 square kilometers (km2)—until 1993, the year of the Great Mississippi River Flood. After the flood, the Gulf of Mexico dead zone doubled in size, to more than 18,100 km2. Flood waters carried tremendous loads of excess nutrients. From 1994 through 1998, the gulf's dead zone was estimated at 10,000 km2; in 1999, it grew to 20,000 km2; then, in 2000, it was measured at about 4400 km2, according to the US Environmental Protection Agency (EPA) report Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico. The Action Plan calls for the reduction of the Gulf of Mexico dead zone to a five-year running average of less than 5000 km2 by the year 2015; this goal, states the report, will be accomplished by limiting discharges of nitrogen and other nutrients to the gulf. Last summer, Rabalais and colleagues mapped a Gulf of Mexico dead zone slightly larger than average, about 15,000 km2. “The dead zone extended from the Mississippi River delta almost to the Texas coast,” says Rabalais. “The hypoxic waters were very close to shore, a result of north winds and onshore currents that pushed the zone towards the beach.” Water depths affected were as shallow as four meters. The close proximity of the dead zone to the coast precluded shrimp trawling there, says Rabalais. Shrimpers and fishers had to move beyond the dead zone to find their catch. Landings of brown shrimp, perhaps the most important seafood product from the Gulf of Mexico, have fallen from a record high in 1990 to annual lows corresponding to years when the dead zone is largest. Scientists believe that the gulf's dead zone is the result of excess runoff of nutrients from agriculture in the large Mississippi River drainage basin, which extends northward through midwestern states as far as Minnesota. The Mississippi drains 31 states and 40 percent of the contiguous United States. More than 55 percent of agricultural lands and 33 percent of US farm-related jobs are found in the river's drainage area. The Mississippi is responsible for more than $98 billion annually in agriculture revenues. But the success of farming in the Mississippi River drainage area has come at a cost, says Howarth, chair of a committee responsible for the National Academy of Sciences (NAS) report Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution (2000). Since 1980, the Mississippi and Atchafalaya Rivers have discharged at least 1.6 million metric tons of nitrogen, 100,000 metric tons of phosphorus, and 200,000 metric tons of silica into the Gulf of Mexico each year. The waters of these two major river systems provide 60 percent of the gulf 's fresh water. When they reach the Mississippi delta, they form a broad, nutrient-rich plume, says Scavia, that extends from Louisiana to eastern Texas. According to results of a study by Lisa Osterman of the US Geological Survey in Reston, Virginia, published in the April 2005 issue of the journal Geology, seasonal dead zones in the northern Gulf of Mexico are nothing new. They existed as long ago as the 1800s. But the dead zone's modern-day extent is what's astounding, Osterman and other scientists say. To look at past dead zones, Osterman's team took sediment cores from areas where the gulf's more recent dead zone occurred. The scientists dated the sediments and counted three species of foraminiferans (“forams”) in the cores; these species tolerate hypoxic waters. As far back as 1823, the forams thrived during Mississippi River flood years, suggesting that nutrients in floodwaters can indeed trigger hypoxia. But the forams in Osterman's core samples were most abundant after 1950, when farmers began using more fertilizer. “Our data show,” wrote Osterman and coauthors, “that the low-oxygen events since 1950 were more extreme than any that had occurred in the previous 180 years, and support the interpretation that increased use of fertilizer has amplified this process.” The proof, says Rabalais, lies in Europe's Black Sea. As a result of the economic collapse of the former Soviet Union and subsequent declines in subsidies for fertilizers, the dead zone in the Black Sea has largely disappeared. “In the 1990s there was lower input of nutrients to the Danube River, which empties into the Black Sea,” she says. “Signs of recovery soon started to appear. There should be little doubt about the strong relationships among human activities, nutrient loads and eutrophication, and the demise of a coastal ecosystem.” A report issued in August 2004 by the EPA regional office in Atlanta, Evaluation of the Role of Nitrogen and Phosphorus in Causing or Contributing to Hypoxia in the Northern Gulf, states that excess phosphorus, as well as nitrogen, flowing into the Mississippi River is to blame for the Gulf of Mexico's dead zone. Scientists and others are concerned that farmers may jump on the bandwagon of the EPA's 2004 Action Plan Report, currently in draft form and under peer review, as a way to avoid reducing their use of nitrogen-containing fertilizers. “The report has some serious deficiencies, however,” says Boesch. He, Scavia, and Howarth sent EPA officials a letter criticizing the report, believing that nitrogen is clearly the problem in the marine waters of the Gulf of Mexico. According to the NAS Clean Coastal Waters report, phosphorus is the nutrient that usually controls freshwater over-enrichment, or eutrophication. “In that report in most coastal marine is by The EPA Action Plan had the many “The of this the extent of the Gulf of Mexico dead is on reducing nitrogen loads to the northern of the through this will also by reducing phosphorus as A is what's according to Dan the in Atlanta, and but out on the gulf are the dead zone is Reducing phosphorus but nitrogen from fertilizer like the to who and not off track in plans to nitrogen in the Mississippi River, or the Gulf of Mexico will end up being the Dead Dead zones are also found in the Great A dead zone in now every year in the shallow area off to hypoxic zone are low water levels and fertilizer runoff from large The dead zone has for the past years and now the oxygen in the waters of the the of the Scientists that it's affecting the of the fish and along the of years ago had their dead zone with which to according to scientist of the University of in Ann Arbor. In a published in the 2004 issue of the journal Geology, in nutrient input to the perhaps from to fields for and other caused and the a in the out in the But the remains of a dead zone still in to should excess nutrients be added to its waters. States “The from the of the remains in for nutrient input the hundreds of years that have that be the of the Great the Gulf of Mexico, the Baltic Sea, and the Black Sea, hundreds of years after we're will use the to change the The Mississippi River some million metric tons of sediment into the Gulf of Mexico each year. This from the the brown water of the Mississippi with the blue water of the gulf two after a The river enough sediment from its to the coast of Louisiana each year. of and University of and the phytoplankton can create dead zones, areas of water so of oxygen that sea life cannot If phytoplankton is by fertilizers or other more is produced at the of the The to the where it and on excessive and the oxygen that fish, crabs, and other sea creatures on for life. how ocean from to summer in the Gulf of Mexico. of ocean waters, which may large of from the of the Mississippi River all the way to the Texas and high concentrations of phytoplankton and river how ocean from to summer in the Gulf of Mexico. of ocean waters, which may large of from the of the Mississippi River all the way to the Texas and high concentrations of phytoplankton and river The National and measured low-oxygen water in the as the water in the on the previous In this and low oxygen swim the water's by an off the of Louisiana in the Gulf of Mexico. The oxygen in these waters was high at the the was parts per million When the oxygen falls below 2 ppm, as it in a dead zone, fish leave the area. by Nancy Rabalais, Louisiana Universities Marine water in the summer of 1990 fish and other marine life the at The fish were bottom like red black sea and shrimp, which also on the were along with the many fish National this summer, scientists in the United States and will one of the most to on dead zone. The will use 10 and to data on the and of oxygen in the and University of