HAWAI'I'S ENVIRONMENT
Reef algae as significant in Hawai'i as coral
By Jan TenBruggencate
Advertiser Columnist
There's a lot of talk these days about the importance of Hawai'i's coral reefs.
But there is a class of scientists who will argue that the concept of a coral reef around Hawai'i can be misleading.
Although they've got plenty of coral, most of the reefs around the Islands are made up primarily of algae.
"We have more algal reefs than coral reefs," said University of Hawai'i botany professor Celia Smith.
When most of us think of algae, we think of green and red limu or seaweed. But the reef-building algae are hard as rock. They're called crustose coralline algae.
Renowned limu expert Isabella Abbott said that most are pink to purple in color. They grow on rocks and on dead corals, and in many places, are the cement that sticks the reef together.
If you dive on many shorelines around Hawai'i, you may find rocks and boulders that are rough and pink in color. They might have coral or lava rock inside, but that hard pink shell is coralline algae.
Abbott said that more than half of the reefs around islands from Kaua'i north to Kure Atoll are made up of coralline algae.
Geologist Chuck Blay found that many of the beaches on Kaua'i have sand that is from 40 percent to 60 percent coralline algae particles and only 20 percent to 30 percent coral. The rest is mostly bits of rock and shell.
He said that once dead, coralline algae lose their color and the particles are very white — whiter than the creamy color of coral sand. The beaches of Midway Atoll in the Northwestern Hawaiian Islands are mainly coralline algae and are blindingly white, he said.
Around O'ahu, Moloka'i and South Maui, corals have the edge. Perhaps 60 percent of the reefs there are corals, Abbott said.
On newer shores, including the Big Island and the windward coast of Maui, the corals haven't had much time to get settled on the young volcanic rocks, and algae again outrank corals on the reefs.
Corals are limited by temperature; they don't like the water too cold or too hot.
Scientists refer to the latitude of Kure Atoll as the Darwin Point, the distance from the equator at which the water is too cold to support coral growth.
There are still crusty reefs in cold waters poleward of the Darwin Point. They're just not coral.
"Corallines occur from the tropics to polar regions, and from the intertidal down to more than 200 meters water (600 feet) depth," writes Michael Rasser at his Web site.
"The Arctic is just packed with coralline algae," Abbott said.
Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Contact him at (808) 245-3074 or e-mail jant@honoluluadvertiser.com.
