First in a two part series on dead zones in coastal waters. Part II: “Limiting Dead Zones” is available to subscribers at Limiting Dead Zones.
Summer tourists cruising the waters off Louisiana or Texas in the Gulf of Mexico take in gorgeous vistas as they pull in red snappers and blue marlins. Few realize that the lower half of the water column below them may lack fish, despite the piscine bounty near the surface. For many years now, an annual dead zone has developed in the Gulf, beginning as early as February and sometimes lasting until mid-fall. This zone—water where the oxygen content is so low that denizens can’t survive—tends to leave no surface clue.
Although the precise timing and size of the Gulf’s dead zone varies with the weather, in many years it encompasses 22,000 square kilometers, a parcel of underwater real estate roughly the size of New Jersey. Fish that can evacuate as oxygen drops do so—although abandoning their home habitat may render them vulnerable to predators. Crustaceans and other seafloor life that can’t leave fast enough simply die.
There’s no mystery as to what triggers this annual hypoxic zone, as the oxygen-starved region is formally termed. Into the Gulf of Mexico, the Mississippi River deposits water that is heavily enriched with plant nutrients, principally nitrate. This pollutant fertilizes the abundant growth of tiny, floating algae. As blooms of the algae go through their natural life cycles and die, they fall to the bottom and create a feast for bacteria. Growing in unnatural abundance, the bacteria use up most of the oxygen from the bottom water.
Dead zones tend to develop in quiet, deep water a few km offshore. Typically, they appear where a river spews rich plumes of nutrients into water that’s stratified because of either temperature or salinity differences between the bottom and the top of the water column. If the water doesn’t mix, oxygen isn’t replenished in the lower half.
The good news is that the Gulf’s dead zone disappears each winter, observes Fred Wulff of the University of Stockholm. In the eastern Baltic Sea, where he works, a permanent dead zone covers up to 100,000 square km. Nasty blooms of blue-green algae in the Baltic also lead to regular beach closures and fish kills.
Caused almost exclusively by human activities, coastal dead zones are becoming increasingly common and recurrent, observes Robert J. Diaz of the Virginia Institute of Marine Sciences in Gloucester Point. His group finds that the number of major dead zones has been roughly doubling every decade since the 1960s.
On March 29, the United Nations Environment Program issued its first Global Environment Outlook Year Book, a volume highlighting issues requiring urgent attention. The report drew notice to the increase in major coastal dead zones. After examining unpublished data by Diaz’ team, the U.N. body concluded that there are some 150 recurring and permanent dead zones in seas worldwide.
Over the past century, “overfishing was the leading environmental issue affecting our seas,” Diaz says. “In the new millennium, it’s going to be oxygen.”
Fully oxygenated waters contain as much as 10 parts per million of oxygen. Once oxygen falls to 5 ppm, fish and other aquatic animals have trouble breathing. Sharks begin vacating areas with 3 ppm of oxygen, while most other fish can hold out until about 2 ppm. Sediment dwellers that can’t leave a hypoxic zone begin dying at around 1.5 ppm.
In some dead zones, oxygen hovers at 0.5 ppm or lower for months.
Marine ecologists have documented both large and small dead zones in U.S. coastal waters throughout the past decade. Diaz and his coworkers wanted to extend the findings worldwide. During the past several years, they scoured many years of marine-science reports for indications of large dead zones.
Sixty-eight large, persistent, and recurring dead zones spanning the world’s seas were reported for the first time during the 1990s. Most, Diaz says, appear to be ecosystems that had at that time just reached their breaking point. The problem of dead zones is escalating rapidly and globally, he concludes.
His team is now investigating whether recurring dead zones are mushrooming in size or impact. Making such assessments won’t be easy, he concedes, because even for the best-studied sites, the dead zones in the Gulf of Mexico and the Mid-Atlantic region’s Chesapeake Bay, quantitative data remain meager.
Nancy N. Rabalais, an aquatic ecologist with the Louisiana Universities Marine Consortium in Chauvin, has been trying to fill in some gaps. She’s been mapping the Gulf of Mexico’s dead zone for roughly 20 years.
When spring rains scour farm fields as far upstream as Ohio, Minnesota, and Montana, spilling huge quantities of nitrogen into the Mississippi, it’s only a matter of weeks before the oxygen concentrations in the Gulf begin to respond. “Once a decline starts, it goes from about 5 [ppm] to close to 0 in about 7 to 10 days,” Rabalais says.
Shrimp and bottom-dwelling fish tend to evacuate into a halo around the periphery of the hypoxic zone, she notes. This hasn’t escaped the notice of fishing fleets, which sometimes fill their landing quotas of commercially valuable catch by trawling the edges of a dead zone.
However, such fishing success can mask a pending catastrophe, Diaz warns. In Europe, he recalls, “fishermen were laughing at scientists in the mid-’70s,” when the latter cautioned that hypoxia was threatening bottom-dwelling aquatic life in the eastern end of the North Sea separating Norway, Denmark, and Sweden. Harvests of Norwegian lobsters, for instance, remained robust through 1978.
The next year, however, these shellfish and the area’s many bottom-dwelling fish were gone. The earlier bumper crops had reflected landings of oxygen-stressed animals that had left their burrows and other familiar turf to breathe easier, Diaz explains.
Fishing for indicators
Although scientists haven’t observed fish dying in the Gulf of Mexico, J. Kevin Craig of Duke University in Beaufort, N.C., may be seeing harbingers of an impending crisis in brown shrimp (Farfantepenaeus aztecus), the Gulf’s highest-valued species. He has investigated two parameters of the animals’ health: size and lipid content.
Over the past 3 decades, the average size and therefore price of Gulf shrimp has been falling, Craig notes. His data also show that the concentration of lipids in a shrimp’s body—representing the energy stores these animals carry—tends to be 20 to 25 percent lower in animals caught in low-oxygen areas than in those caught in fully oxygenated water. The combination of factors suggests that hypoxia slows the animals’ growth, the aquatic ecologist says.
By contrast, Craig’s team found “no obvious negative effects of hypoxia on growth or lipid content of the Atlantic croaker [Micropogonias undulatus].” Although this bottom-dwelling finned fish, as shrimp do, migrates just beyond the dead zone when oxygen concentrations plummet, its lipid concentration doesn’t suffer. Also, its average size hasn’t diminished over the years during which the Gulf of Mexico dead zone has grown.
In fact, Craig says, since the displaced fish normally hovers at the edge of hypoxic zones—where many other evacuees also hang out—croakers may actually benefit from the oxygen crisis. To a predatory croaker, he speculates, the edge of the dead zone is “like a smorgasbord.”
Denise Breitberg of the Smithsonian Environmental Research Center in Edgewater, Md., has witnessed a similar dichotomy of dead-zone winners and losers in the Chesapeake. Anchovies (Anchoa mitchilli), for instance, spawn in surface waters, releasing eggs that sink to the sediment. If the eggs land in a hypoxic area, they’ll die.
On the other hand, Breitberg has found that the Bay’s gelatinous species—its comb jellies (Mnemiopsis leidyi) and stinging sea nettles (Chrysaora quinquecirrha)—are quite tolerant of hypoxia. “Both can survive for several days at 0.5 [ppm oxygen], a habitat from which finfish are excluded,” she reports.
Breitberg worries that a growing dead zone in the bay each summer is creating a habitat that favors jellyfish over the commercially valuable finfish, crabs, and oysters. Despite the nation’s most aggressive state and local efforts to curtail nutrient releases into local waters, last year’s dead zone in the Chesapeake was the largest ever measured.
Accounts describing occasional bouts of hypoxia in the Gulf of Mexico date back to 1884, when a Mobile, Ala., newspaper reported a “jubilee”—a prolonged, anomalous run of fish and crabs into the shallows at Mobile. According to Diaz, although the reporter recommended that local citizens avail themselves of this “gift from God,” it and subsequent jubilees almost certainly stemmed from the runoff of plant nutrients from farms and towns, which led to marine organisms’ fleeing a new dead zone.
For U.S. ecologists, a nagging question today is how much reduction in nutrient inputs to the Gulf of Mexico must occur for its dead zone to shrink substantially. Over the past few years, Don Scavia of the National Ocean Service in Silver Spring, Md., has developed a computer model of the annual Gulf dead zone. By correlating river inputs with the dead zones that Rabalais has mapped since 1985, Scavia’s team calculated relationships between freshwater flow, the Mississippi’s nitrate content, and the Gulf’s oxygen concentrations.
Then, the team ran the model backward, plugging in annual measurements for the past half-century of nitrate concentrations, the annual cycle of the Mississippi’s flow, and weather data. The calculations indicate that dead zones didn’t become large, annual phenomena until the mid-1970s, says Scavia, who is currently the director of the Michigan Sea Grant program in Ann Arbor. But now that it’s perennial, the hypoxia phenomenon will be hard to vanquish, the model also indicates.
By running the model forward in time, Scavia’s team analyzed how much farmers and other polluters in the Gulf watershed—an area covering 41 percent of the lower 48 states’ area—would have to scale back their nitrogen releases to limit the zone to an annual average of just 5,000 square km., a target set by the federal government 3 years ago. The researchers’ conclusion: a 40 to 45 percent annual cutback in the nutrient releases.
That nitrogen reduction is daunting, says Robert W. Howarth of Cornell University. “Over the past 20 years, nitrogen pollution in coastal waters has increased pretty steadily, about 1 percent per year,” he notes. A biogeochemist, Howarth chaired a National Academy of Sciences committee that studied nutrient pollution in coastal waters and 4 years ago issued a report finding that the problem, affecting almost all U.S. coastal waters to some degree, was so serious that urgent national action was imperative.
To date, Howarth tells Science News, because the federal government currently seeks only voluntary controls on nutrient runoff, there hasn’t been much action. In fact, budget cuts are reducing the already-scheduled monitoring.
Instead of getting better, the Gulf’s dead zone could quickly get a lot worse, says Scavia. “There comes a time when the fisheries collapse,” he says. Not only will commercial harvests plummet, but fish and shrimp reproduction will also drop off. In some cases, a commercially popular fish might completely disappear.
Unfortunately, he says, no one knows how close the Gulf is to that point. It might take a year, or it could take 2 decades. The problem, Scavia notes, is that once a hypoxia-fostered collapse starts, “it happens fast” and can be devilishly hard to reverse.
Laurence Mee of the University of Plymouth in England knows the problem well, having studied just such a transformation in the Black Sea. There, a recurring summer dead zone emerged in 1973, fueled by heavy fertilizer use in Eastern Europe. Mee says that from the beginning, the huge dead zone—at times much bigger than the Gulf’s—fostered a change in the Black Sea’s ecosystem. Commercially valuable and heavily harvested fish such as turbot declined, while “junk fish” such as gelatinous species began to dominate, Mee says.
Every summer, algal blooms darkened much of the Sea’s water, shading sea grasses and seafloor algae, which died. As this important food source and habitat for fish disappeared during the 1980s, a “huge [seafloor] ecosystem, which was certainly the size of Belgium or the Netherlands, disappeared in the space of about 4 or 5 years,” Mee says.
An alien species—comb jellies hitchhiking on ships from the Chesapeake Bay—then took over (SN: 7/4/98, p. 8: https://www.sciencenews.org/pages/sn_arc98/7_4_98/bob1.htm). By 1991, Mee notes, “There were about 1 billion tons, wet weight, of comb jellies in the Black Sea.” This mass of inedible invertebrates exceeded the weight of the entire world’s commercial fish catch, he says.
A short time later, a bigger alien comb jelly—also from the east coast of North America—invaded the Black Sea and began dining on the out-of-control smaller jellies. This improved the environment because the resulting biomass of big jellies was smaller than that of the initial invaders.
In an odd twist, the Black Sea’s ecology is now showing signs of recovery. For instance, new recruits are reviving some dead-mussel beds. Moreover, Mee points out that in the Black Sea, “hypoxia events are very rare now.”
What happened, he explains, was that with the fall of Communism, economic strains in Russia, Ukraine, Moldova, Romania, and Bulgaria sharply reduced agricultural spending on fertilizer. Therefore, nitrate runoff into the Black Sea plummeted in the 1990s.
But economic collapse or reduced farming is a poor strategy for controlling dead zones. Instead, Mee argues, “We should learn to be a little more clever about how we do our agriculture, so that we limit the runoff of those nutrients.”
Part II: “Limiting Dead Zones.” Available to subscribers at Limiting Dead Zones.
Correction: According to aquatic ecologist Nancy N. Rabalais, the crustacean in the third photo is not a “brown shrimp,” as stated. Its identity is uncertain, however, because the original photo, from the National Oceanic and Atmospheric Administration, labeled the animal a brown shrimp.