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Thursday, April 5, 2001

NATURAL SELECTIONS

Climate change blamed for Okinawa coral death


Scientists at the University of the Ryukyus in Okinawa have published evidence showing that global climate changes in 1998 devastated coral reefs around Sesoko Island. The report, published in the April edition of the journal Ecology Letters, comes on the heels of George W. Bush's unilateral abandonment of the 1997 Kyoto treaty on climate change.

Sea temperatures in Okinawa were warmer in 1998 than at any time in the previous 10 years. That year also recorded the strongest El Nino on record. A mass bleaching of coral in Okinawa reduced coral species richness by 61 percent and reduced coral cover by 85 percent. The scientists surveyed the reef in 1997, 1998 and 1999, and so were able to make direct comparisons between years.

Coral reefs are made by invertebrate animals (class Anthozoa) in symbiosis with algae called zooxanthellae. The algae provide essential photosynthetic products to the animals, and the animals provide the algae with a safe place to live. This is the central reason why species-rich reefs can exist in the nutrient-poor waters of the tropics. Like trees in a tropical rain forest, reefs are the basic producers, and if they disappear, so does the rest of the community.

Bleaching is caused when coral's symbiotic algae die. Coral tissue then loses its color, revealing the white calcite skeleton. Mass bleaching events have been reported regularly since 1979, and after 1998's El Nino, bleaching was reported from all the major tropical oceans of the world.

The destruction shows (and it seems that some politicians still need telling) how global climate change can negatively impact some of the most diverse ecological communities on the planet. Knock-on effects are already apparent in Okinawa.

"Fish behavioral ecologists at Sesoko now struggle to find species, like damselfish, that have a close relationship with branching corals," said Kazuhiko Sakai of the Tropical Biosphere Research Center at the University of the Ryukyus.

The branching corals, popular with snorklers and scuba divers, are the species that support diverse communities, and these corals were hard-hit by the bleaching. In addition, soft coral cover was reduced by 99 percent.

Although coral diversity decreased overall, some species profited by the increase in sea temperature. These were the hard, thick corals that form massive, encrusting colonies supporting relatively few other organisms.

Zooxanthellae algae live in cells of the animals that form coral. Thick corals are more tolerant to harsh conditions, and their zooxanthellae are better sheltered than in the more delicate branching corals. They can endure the warmer sea temperatures that kill the algae in branching and soft corals.

Another special feature of branching corals is that they are "larval brooders" -- when they reproduce, they retain their larvae within the reef. Other types of coral are "broadcast spawners," releasing their larvae into the open sea, where they disperse over a wide area.

This means that branching corals form relatively genetically isolated reefs, and this leads to high genetic diversity between reef sites. If a reef dies, that diversity is lost forever.

"If global warming continues, the corals cannot recover," said Sakai, one of the authors of the new study. "1998 was not a freak year: There will be more like that unless global warming is decreased by humans."

There will be more like that, especially now that the United States -- the world's most polluting nation -- has decided to go it alone, rejecting the Kyoto Protocol's call to cut CO2 emissions by 2012.

The Okinawa study was headed by Yossi Loya, a marine biologist at Tel Aviv University, when he was foreign visiting professor of the Tropical Biosphere Research Center.

E-mail Rowan Hooper at rowhoop@nies.go.jp


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