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Reviving Dead Zones

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Reviving Dead Zones C REDI T 78 SCIENTIFIC AMERICAN NOVEMBER 2006 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. Reviving Dead BY LAURENCE MEE Zones How can we restore coastal seas ravaged by runaway plant and algae growth caused by human activities? magine a beach crowded with vacationers en- the globe have emerged [see map on next page]. joying the hot summer sun. As children paddle Determining the causes of this destruction, I about in the shallows, foraging for shells and how it might be prevented and what to do to bring other treasures, dead and dying animals begin to affected areas back to life has been a prime focus wash ashore. First, a few struggling fish, then of my research since the early 1990s, when I pub- smelly masses of decaying crabs, clams, mussels lished my first paper on the ecological crisis in the and fish. Alerted by the kids’ shocked cries, anx- Black Sea. My work and that of others has now ious parents rush to the water to pull their children revealed important details of the events that de- away. Meanwhile, out on the horizon, frustrated grade coastal ecosystems in many parts of the earth commercial fishermen head for port on boats with and has turned up new information that could empty nets and holds. help establish pathways to recovery. This scene does not come from a B horror mov- ie. Incidents of this type actually occurred peri- odically at many Black Sea beach resorts in Roma- nia and Ukraine in the 1970s and 1980s. During that period an estimated 60 million tons of bot- tom-living (or benthic) life perished from hypox- ia—too little oxygen in the water for them to sur- vive—in a swath of sea so oxygen-deprived that it could no longer support nonbacterial life. At its extreme in 1990, the “dead zone,” lo- cated in the northwestern part of the sea off the mouth of the Danube River, extended over an area the size of Switzerland (40,000 square kilometers). On the other side of the world, in the Gulf of Mex- BLACK SEA DEAD ZONE became evident when dying sea life ico off the Mississippi River delta, another huge began washing ashore near the mouth of the Danube River dead zone appeared in the mid-1970s, which at its in the 1970s. Above, dead fish are left strewn across a Black Sea beach by the high tide. A satellite photograph of largest reached 21,000 square kilometers. In the the western Black Sea from 2000 (left) shows the huge past two decades, additional reports of dying or blooms of floating microscopic plants that resulted from depleted areas in coastal seas and estuaries across the river’s nutrient-rich discharge. C REDI T www.sciam.com SCIENTIFIC AMERICAN 79 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. Baltic Sea Black Sea Oregon coast Chesapeake Bay Seto Inland Sea Gulf of Mexico OXYGEN DEPLETION Year-round Periodic—usually annual Occasional DEAD ZONES—areas starved of oxygen (by bacterial decay of overabun- 1990. Polluted runoff from the land often triggers dead zone conditions, dant plant growth) and thus of most animal life—occur in coastal seas, often although some result naturally. The dead zone in the northwestern Black near developed countries. The number of regions affected has doubled since Sea is now much smaller than it was at its largest a few decades ago. Dead Zone Formation blooms and other unwanted effects. green or even brown as phytoplankton ocean researchers today link Plants enter the food chain when tiny populations burgeon, and the shade they the creation of most dead zones to a phe- seaborne animals (zooplankton), herbiv- cast deprives plants living below them of nomenon called eutrophication, the orous fish and filter-feeding bottom essential sunlight. Sea grasses in shallow overenrichment of the sea by nutrients dwellers such as mussels and oysters bays also become covered with small at- (principally compounds containing ni- graze them or when they die, decay, fall tached algae (epiphytes) and can ulti- trogen and phosphorus) that promote to the seabed, undergo bacterial decom- mately be smothered and die. Algae can ) plant growth. A certain amount of these position and are incorporated into the in addition envelop coral reefs, especial- “fertilizers” is essential to the health underlying sediments. This organic bot ly where heavy fishing pressure has - his page t ( of phytoplankton—floating algae and tom matter feeds the animals living there, thinned the ranks of resident grazers. ) ; other microscopic photosynthesizers including worms, shrimp and some fish. A major upsurge in the numbers of that form the base for most marine food Normally the numbers of phytoplank- phytoplankton and epiphytes immedi- chains—as well as for the well-being ton are limited by the availability of light ately causes difficulties for nearby sea of the sea grasses and algae that live and nutrients and by grazing. But large life, but an even worse situation arises preceding pages on the floors of shallow, well-lit seas. increases in nitrogen and phosphorus when oxygen levels in bottom waters de- ) ( But too much of these nutrients in illu- concentrations enable these minute photo- cline. Lower oxygen concentrations ap- UNEP GEO YEARBOOK 2003 minated waters greatly accelerates plant synthetic organisms to multiply in great pear when bacteria consume oxygen to dead f ish ( growth, leading to disruptive algal profusion. The water eventually turns break down the masses of organic matter that result from animal wastes and the dead bodies of organisms that multiply Overview/Coastal Seas in Trouble SOURCE: BASED ON ); RADU MIHNEA during eutrophication. Much of this ma- ) ; ■ River-borne plant nutrients from the land are killing off life in parts of shallow terial accumulates on the seafloor, where seas around the globe, resulting in so-called dead zones. oxygen is relatively scarce to begin with. ■ Fertilizing chemicals cause microscopic plants floating near the surface Oxygen finds its way into the water satellite image to overgrow, depriving any plants living on the bottom of light and leading to from either photosynthesis or physical satellite image large increases in the amount of decaying organic material falling to the diffusion from the air at the sea surface. seafloor. The bacteria living off the dead organisms use up seafloor oxygen, Should an area whose bottom is covered thereby causing the loss of most animal life there. with dead plants also feature a strong ■ Significant reductions in agricultural and sewage runoff, as well as controls density gradient that prevents mixing on overfishing, can restore these key marine ecosystems. with the overlying water column, the THE VISIBLE EARTH/NASA ( oxygen at the bottom can soon become THE VISIBLE EARTH/NASA ( 80 SCIENTIFIC AMERICAN NOVEMBER 2006 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. exhausted, leading to the die-off of entire minuscule zooplanktonic crustaceans. chain engenders more organic matter on animal communities. (Such gradients These animals, in turn, serve as food for the seafloor, which enhances the risk can stem from temperature or salinity fish. Increased nutrient concentrations that a dead zone will follow. differences in the water at various depths.) affect the mix of phytoplankton species Ecosystems altered by eutrophica- This basic sequence— eutrophication such that diatoms often become out- tion also become more vulnerable to in- leading to phytoplankton blooms, excess numbered by smaller or less digestible vasion by foreign species, which can ar- bacterial activity at the bottom, oxygen types. When eutrophication produces rive, for example, in the ballast discharge depletion, and the death of existing plants massive phytoplankton blooms, cope- from transoceanic ships. In the 1980s and animals—has occurred in almost ev- pods often are unable to graze on the the comb jellyfish Mnemiopsis leidyi, ery dead zone examined by researchers. new, abundant phytoplankton species as which probably originated off the east- The details do vary, however, ac- well as the large quantities of organic de- ern coast of the U.S., took up residence cording to the local biological and phys- tritus that result from the disruption of in the Black Sea. By 1990 this voracious ical conditions as well as the rate of sup- the natural ecosystem. This change fa- dead-end predator dominated the eco- ply of plant nutrients from the land. vors the growth of highly tolerant gelat- system completely, at its maximum at- Poorly flushed estuaries, for example, inous organisms such as Noctiluca (re- taining an astounding density of up to are particularly vulnerable to the effects sponsible for nighttime phosphorescence five kilograms per square meter. of eutrophication, because low water that occurs when the water surface is Sometimes shellfish reefs can help flows lead to slow replenishment of oxy- disturbed). Biologists sometimes call stave off degradation of an ecosystem. In gen in bottom waters. This reduction in these jellyfishlike fauna “dead-end spe- many estuaries on the eastern seaboard oxygen has been a persistent problem cies” because higher-level predators have of the U.S., oysters act as ecosystem en- along the eastern seaboard of the U.S., difficulty living off them. Their presence gineers by accumulating into huge reefs where large estuaries, such as the Chesa- reduces the efficiency of the food chain, rising several meters from the seabed; peake Bay, have been affected. leading fish stocks to wane. these reefs support a diverse assemblage The excess of nitrogen and phospho- Such an imbalance in the food chain of organisms, including flounder, snap- rus arriving in coastal seas results in large can be worsened by intense commercial per, silver perch and blue crabs. measure from the changing habits of peo- fishing, particularly where these efforts Hunter Lenihan of the University of ple living in the areas draining into the target high-value “top predator” species California, Santa Barbara, and Charles sea.
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