In 1997 and 1998, and again in the early part of 2000, large areas of coral reefs in tropical oceans around the world suffered devastating, and in many cases deadly, bleaching events. Scientists attribute the first round of major bleaching to the especially severe El Niño Southern Oscillation that occurred. The heat associated with that ENSO heated coastal waters to the point that the corals lost the microscopic algae, or zooxanthellae, that give them their color and provide them with nutrients.
Coral reefs around the Main Hawaiian Islands and in the Northwestern Hawaiian Islands largely escaped damage even as reefs elsewhere suffered – including reefs in U.S. waters around Palmyra, Kingman Reef, and the Line Islands, and Howland and Baker islands.
A recent report, “Status of Coral Reefs of the World: 2000,” issued by the Global Coral Reef Monitoring Network (GCRMN) suggests Hawai`i may be one of the last holdouts of healthy coral reefs if the climate is warmed along the lines predicted by scientists studying the effects of increasing levels of carbon dioxide in the atmosphere. “If predicted rates of greenhouse gas emissions continue and these are confirmed as the trigger for global warming,” the report states, “shifts in global climate like the El Niño-La Niña phenomenon of 1997-98 will recur with increased severity and frequency. Coral reefs which are recovering will be set back by recurring bouts of coral bleaching and mortality. An additional climate problem is looming as corals will probably grow more slowly with more fragile skeletons due to increases in CO2 concentrations in sea water.”
However, the report goes on to state in the chapter discussing Hawaiian reefs, “because the location of the Hawaiian chain is favored by oceanic gyres and deep water surrounding all the islands, these are predicted to be among the last reefs to experience major bleaching events.” Still, they are vulnerable. Another recent report by a team of Hawai`i scientists led by Dave Gulko of the Department of Land and Natural Resources’ Division of Aquatic Resources, notes that “many of the corals live close to their maximum temperature limits and elevated seawater temperatures will result in some level of bleaching and/or mortality.”
At a recent meeting of the Western Pacific Regional Fishery Management Council, that special “favored” position of reefs in the Northern Pacific was underscored by reports from scientists who in the last year had conducted rapid ecological assessments of reefs in U.S. waters in both the Northwestern Hawaiian Islands and, closer to the equator, in the Phoenix and Line Islands.
Corals in the more southerly areas showed signs of repeated bleaching – “many events over many years,” said Rusty Brainard, one of the scientists participating in the studies. Because there is no human habitation of these areas, the bleaching was probably caused by the same phenomena responsible for coral bleaching elsewhere, said Brainard, who works with the National Marine Fisheries Service. In the colder waters surrounding the Northwestern Hawaiian Islands, reef-building corals seemed to have weathered the 1997-98 El Niño event without apparent damage, reported Alan Friedlander of the Honolulu-based Oceanic Institute. Friedlander was one of the scientists participating last fall in the month-long rapid assessment of the Northwestern Hawaiian Islands called NOW-RAMP (a somewhat selective acronym standing for Northwestern Hawaiian Islands Rapid Assessment and Monitoring Program).
— Patricia Tummons
Volume 11, Number 7 January 2001