Hawai'i coral suffers bleaching
| • | Map: The Hawaiian Islands |
By Jan TenBruggencate
Advertiser Science Writer
Reefs on the far northwestern end of the Hawaiian chain are showing significant damage this summer from a coral bleaching event a year ago.
NOAA
It is the result of very warm conditions in those waters last year, and may be linked to global warming, ocean scientists said.
Scientist say the atolls of Kure, Midway, and Pearl and Hermes were most affected by bleaching last year.
The corals most severely affected are montipora, often called rice corals, which are commonly found on the back reefs just inside the waves that break on the face of the fringing reef of Hawai'i's northernmost islands and atolls.
Cauliflower corals, or pocillopora, are also significantly affected. But these appear to be recovering better than the rice corals, said marine ecologist Jean Kenyon, who works with the National Oceanic and Atmospheric Administration Fisheries' Coral Reef Ecosystem Investigation Program.
The death of corals can change a reef's character, affecting food supplies for coral-eating creatures and shelter for many species. That can affect fish populations and change the food chain for predators such as ulua, sharks and seals.
Last summer's major bleaching event did not extend to the main Hawaiian Islands, although reefs around those islands have suffered from bleaching in the past.
The Northwestern Hawaiian Islands bleaching primarily affected the three atolls most distant from the main islands: Kure, Midway, and Pearl and Hermes, with lesser effects on the next island, Lisianski, which has an extensive adjacent reef region called Neva Shoal.
Kenyon participated in a portion of a survey of the Northwestern Hawaiian Islands reefs this summer.
"We saw it everywhere," she said of the bleaching's effects.
Bleaching can occur for a variety of reasons, but Kenyon said the probable cause in this case was several weeks of calm weather with unusually warm waters that drove corals into conditions of severe stress.
Reef-building corals are made up of small animals called polyps that contain single-celled plants called zooxanthellae. When stressed, the polyps lose their zooxanthellae, and because they depend on the plants for some of their food, begin to starve. If conditions don't improve, the coral polyps can die. Often, seaweeds move in to grow over the dead coral, changing the character of the reef.
"If corals die, almost immediately, turf algae move in. Every inch of habitat gets covered," said Rusty Brainard, chief of the Coral Reef Ecosystem Investigation Program.
He agreed that calm weather was a major factor.
"The bleaching in August and September of 2002 was widespread and massive. Our belief is that it was a combination of the very warm water for a six- to 10-week period caused by very light winds for a sustained period of time." That prevented mixing of cooler deep water with warm surface water, Brainard said.
He said scientists also believe the flat surface conditions allowed the sun's ultraviolet radiation to penetrate the waters, where some of its strength would normally have been dissipated by reflection and refraction on waves. The effect: coral sunburn.
"One of the things we saw is that the tops of the coral were more bleached, even though the water temperatures were the same" along the sides and bottoms, he said.
On Pearl and Hermes, Midway, and Kure atolls, the bleaching-susceptible rice and cauliflower corals form a substantial part of the life on the back reef. That's why those areas were most affected, Kenyon said.
"The recovery or lack thereof was very much a matter of where the species were," she said.
Kenyon said she is increasingly convinced that the damage to Northwestern Hawaiian Islands reefs is directly related to a worldwide climate warming trend.
"This is basically a manifestation of global warming. A year ago I would have waffled on that, but in the last several weeks, key articles" by respected scientists in scientific journals have supported linking warming to coral bleaching, she said.
"There is a key threshold. When reefs sustain for several weeks consecutively a 1 degree (celsius or 1.8 degrees fahrenheit) temperature above the mean average maximum," corals lose their zooxanthellae, she said.
Last summer, both satellite data and temperature sensors moored in the Northwestern Hawaiian Islands confirmed the high temperatures that corresponded to scientific reports of bleaching, she said.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
