Aside from the obvious ravages of fishermen who blast the reefs and pour cyanide on them, the reefs are also threatened by rising ocean temperatures that many marine biologists attribute to global warming and short-term climate events such as El Nino episodes in the Pacific Ocean. Most corals live very close to the upper limits of their heat
A coral reef (Jeff Dixon)
tolerance. Temperature rises of only a few degrees over a sustained period cause death of living organisms within coral reefs.
The scope of corals' devastation from climate change and other human impacts rivals the losses endured by the flora and fauna of the world's great rainforests. According to a number of estimates, half of the world's coral reefs may be lost by 2025 unless urgent action is taken to save them from the ravages of pollution, fishing with dynamite and other explosives, and warming waters. Many of the coral reefs that are falling prey to human-induced destruction are among the largest living structures on Earth. Many are more than 100 million years old.
Coral that has lost its ability to sustain plant and animal life turns white, as if it has been doused in bleach. Afterwards, the dead coral often becomes cloaked in a choking shroud of gray algae. Because coral polyps and their calcium carbonate skeletons "are the foundation of the entire ecosystem, fish, mollusks, and countless other species, unable to survive in this colorless graveyard, rapidly disappear, too" (Lynas, 2004, 107).
Warnings of coral reefs' destruction have been widespread in the scientific literature. The journal Science, for example, devoted a cover story to the subject in its August 15, 2003, issue, wherein T. P. Hughes and colleagues concluded that "[t]he diversity, frequency, and scale of human impacts on coral reefs are increasing to the extent that reefs are threatened globally" (Hughes et al., 2003, 929). Prominent among these impacts is anthropogenic warming of the atmosphere and oceans that may exceed limits under which corals have flourished for a half-million years. Some types of corals are more vulnerable to warming than others, however, so "reefs will change rather than disappear entirely" (Hughes et al., 2003, 929). Rising ocean temperatures, however, will certainly reduce biological diversity among corals.
Callum M. Roberts and colleagues, writing in Science, sketched the scope of possible extinctions faced by the world's coral reefs:
Analyses of the geographic ranges of 3,235 species of reef fish, corals, snails, and lobsters revealed that between 7.2 percent and 53.6 percent of each taxon [type of coral] have highly restricted ranges, rendering them vulnerable to extinction.. .. The 10 richest centers of endemism cover 15.8 per cent of the world's coral reefs (0.012 per cent of the oceans) but include between 44.8 and 54.2 per cent of the restricted-range species. Many occur in regions where reefs are being severely affected by people, potentially leading to numerous extinctions. (Roberts etal., 2002, 1280)
Traveling in the Indian Ocean, Mark Spalding, lead author of the World Atlas of Coral Reefs (Spalding et al., 2001) wrote, in the London Guardian,
Over the next six weeks we watched the corals of the Seychelles die. Corals are to reefs what trees are to forests. They build the structure around which other communities exist. As the corals died they remained in situ [in place] and the reefs became, to us, graveyards. Fine algae grows over a dead coral within days, and so the reefs took on a brownish hue, cob-webbed. In fact, the fish still teemed and in many ways it still appeared to be business as usual, but as we traveled—over 1,500 kilometers across the Seychelles—the scale of this disaster began to sink in. Everywhere we went was the same, and virtually all the coral was dying or already dead... . What I witnessed in the Seychelles was repeated in the Maldives and the Chagos Archipelago. In these Indian Ocean islands alone, 80 to 90 per cent of all the coral died. (Spalding, 2001,
Warming Waters Choke Life out of Lake Tanganyika
Two independent teams of scientists studying central Africa's Lake Tanganyika, Africa's second-largest body of freshwater, have found that warming at the lake's surface has reduced mixing of nutrients, reducing the lake's population of fish. These reductions affected the local economy as fishing yields fell by a third or more during 30 years, with more declines expected. When its waters were cooler, Lake Tanganyika's fish supplied 25 to
40 percent of the protein consumed by neighboring peoples in parts of Burundi, Tanzania, Zambia, and the Democratic Republic of Congo.
Lake Tanganyika is a tropical body of water that experiences relatively high temperatures year-round, so the scientists were surprised to discover that further warming affected its nutrient balance to such a great degree. Like other deepwater lakes, however, Tanganyika relies on temperature differences at various depths to mix water and nutrients. Such mixing is very critical in tropical lakes with sharp temperature gradients that stratify layers of water, with warm, less-dense layers on top of nutrient-rich waters below.
"Climate warming is diminishing productivity in Lake Tanganyika," Catherine M. O'Reilly and colleagues wrote. "In parallel with regional warming patterns since the beginning of the twentieth century," they continued, "a rise in surface-water temperature has increased the stability of the water column" (O'Reilly et al., 2003, 766). A regional decrease in average wind speed over the lake also contributed to reduced mixing of the 1,470-meter-deep lake, "decreasing deep-water nutrient upwelling and entrainment into surface waters" (O'Reilly et al., 2003, 766). Fish yields have declined roughly 30 percent, the scientists wrote, in an example "that the impact of regional effects of global climate change on aquatic ecosystem functions can be larger than that of local anthropogenic activity or over-fishing" (O'Reilly et al., 2003, 766). Lake Tanganyika is especially vulnerable because year-round tropical temperatures accelerate biological processes, "and new nutrient inputs from the atmosphere or rock weathering cannot keep up with the high rates of algal photosynthesis and decomposition" (Verschuren, 2003,731-732).
Lake Tanganyika is the second-deepest lake in the world and the second-richest in terms of biological diversity; it has at least 350 species of fish, with new ones being discovered regularly. Nutrient mixing has been vital for its biodiversity (Connor, 2003). Piet Verburg, of the University of Waterloo in Canada, and O'Reilly, of the University of Arizona, who led the studies, found that warmer temperatures and less windy weather in the region has been starving the lake's life of essential salts that contain nitrogen and sulfur (Verburg et al., 2003, 505-507). Verburg and colleagues utilized profiles of temperature changes in the lake between 110 and 800 meters deep and found that degree of temperature stratification had tripled. In other words, the various levels of the lake had mixed less, depriving fish of food since 1913.
O'Reilly and colleagues, writing in Nature, suggested that the lake's productivity, measured by the amount of photosynthesis, has fallen by 20 percent, which could easily account for the 30 percent decline in fish yields. The scientists said that climate change, rather than overfishing, was mainly responsible for the collapse in Tanganyika's fish stocks. With additional warming, fish populations in the lake are expected by the scientists to decline further (Connor, 2003).
"The human implications of such subtle, but progressive, environmental changes are potentially dire in this densely populated region of the world, where large lakes are essential natural resources for regional economies," the scientists said. Dirk Verschuren, a freshwater biologist at Ghent University in Belgium, said that both studies could explain why sardine catches in Lake Tanganyika have declined between 30 and 50 percent since the late 1970s (Verschuren, 2003, 731-732). "Since overexploitation is at most a local problem on some fishing grounds, the principal cause of this decline has remained unknown," Verschuren said. "Taken together... the data in the two papers provide strong evidence that the effect of global climate change on regional temperature has had a greater impact on Lake Tanganyika than have local human activities. Their combined evidence covers all the important links in the chain of cause and effect between climate warming and the declining fishery" (Connor, 2003). --
Some sea species thrive on conditions that kill others. For example, consider jellyfish, which seem to increase their size and populations (as well as potency of their stings) in warmer, polluted waters. In some areas the increase also appears to be part of a natural cycle (jellyfish populations are also declining in a few other areas) (Pohl, 2002, F-3). By the summer of 2004, reports indicated that jellyfish populations were on the rise in Puget Sound, the Bering Strait, and the harbors of Tokyo and Boston. "Smacks" or swarms of jellyfish shut down fisheries in Narragansett Bay, parts of the Gulf of Alaska, and sections of the Black Sea. In the Philippines, 50 tons of jellyfish shut down a power plant, causing blackouts, when they were sucked into its cooling system (Carpenter, 2004, 68). In late July 2003, thousands of barrel jellyfish and moon jellyfish washed up on the coast of southern Wales.
"Jellies are a pretty good group of animals to track coastal ecosystems," said Monty Graham, a scientist at the University of South Alabama. "When you start to see jellyfish numbers grow and grow, that usually indicates a stressed system" (Pohl, 2002, F-3). Those stresses include increased water temperature, a rise in nutrients (from fertilizers and sewage), and depleted stocks of other fish, often caused by overfishing, which removes thejellyfish's competitors. All of these changes are usually human-caused, according to Graham.
In Australia, regarding jellyfish stings, Jamie Seymour, a jellyfish expert at James Cook University, said in 2002, "This year [was] incredibly abnormal" (Pohl, 2002, F-3). Seymour believes that strong, unusual wind patterns helped to blow the jellyfish toward the shore, where they flourished in
Ajellyfish (scyphomedusa or sea nettle) (Jeff Dixon)
unseasonably warm waters. Seymour, who has analyzed the venom from each sting that receives hospital treatment in the Barrier Reef region for years, had never seen the type of venom that killed two tourists in 2002. In the Gulf of Mexico, according to a report in The New York Times, shrimp fishermen are struggling with a rising numbers of jellyfish that fill their nets with slimy gelatin, ruining their catch (Pohl, 2002, F-3).
At about the same time when increasingly potent jellyfish were being found in the South Pacific, a report appeared in The Boston Globe describing a massive infestation of jellyfish in Narragansett Bay and Long Island Sound. A group of fishermen who expected "an array of marine life in their nets... got jellyfish, nothing but jellyfish; jellyfish so plentiful that the gelatinous organisms came up dangling through the net like slimy icicles. And with each haul came more" (Arnold, 2002, C-1).
"Eventually it seemed that our deck was coated with Vaseline," said Captain Eric Pfirrmann, who works for Save The Bay, a group whose members engage in environmental issues related to Rhode Island's Narragansett Bay. He piloted a research vessel that had taken several high school teachers on a marine field trip. "I've seen blooms like this before," Pfirrmann said, "but never so early in the summer." The culprit is a nonstinging invertebrate about the size and shape of a tulip blossom and commonly known as the combjelly. These jellyfish, along with sea squirts (an entirely different organism), were taking over Long Island Sound, thriving in large part because water temperatures have risen about 3°F over the past two decades, according to scientists (Arnold, 2002, C-1). - -
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