The Subduction Crew

WHAT DOES “BLEACHING” MEAN?

by Homer Montgomery

0105 Z - R/V Melville, North of Saipan

Bleaching is an ugly word to coral reef biologists. The word is especially horrible in this part of the world. Coral reefs abound. Most are sick. Some are dead. Bleaching is a condition caused when water temperature gets too high. Precisely what occurs is that symbiont algae called zooxanthellae are expelled from the coral host in these high temperature conditions.

Zooxanthellae perform a vital role in the life of corals. They provide photosynthetic products (amino acids and sugars) to the coral. Corals supply plant nutrients (ammonia and phosphates) to the zooxanthellae. Zooxanthellae also give corals their color. Thus, corals that have expelled zooxanthellae are bleached. They literally are white. They are dying or dead.

How warm is too warm in terms of ocean temperature? Almost all coral reefs today exist within 1-2° C of their maximum temperature range. Evidence from isotope studies indicates that tropical ocean temperatures have varied by only 2° C during the past 18,000 years. Lately the world’s oceans have been heating at the phenomenal rate of approximately 1° C per hundred years.

Special heating events such as El Niño can produce water temperatures well above maximums. The 1998 El Niño event was disastrous for coral reefs. Damage was tremendous. The complete loss of entire reefs occurred. Given these considerations, the future for coral reefs on planet earth is not good. An international coral reef conference on Bali this past summer documented the disaster. The complete loss of coral reefs on a world scale is predicted.

Coral reefs are considered the tropical rain forests of the sea. They cover <0.2% of the ocean floor, but coral reefs host >25% of all marine species. Coral reefs are home to approximately 25% of the fish caught in developing countries and to 10% of the total amount of fish caught for human consumption. Reefs are huge tourist attractions keeping many tropical nations out of abject poverty. One hundred million tourists visit the Caribbean each year. SCUBA diving in the Caribbean has a projected $1.5 billion dollar value for the year 2005.

Is this predicted fate of coral reefs avoidable? Almost certainly not. The first major mass bleaching event occurred in 1979. Today the condition is critical. This pace of destruction is staggering. Visit the Florida Keys or the Caribbean with someone who has been diving the area for a number of years. No one is optimistic about the future. Coral reefs will not become extinct, but they will be in severe decline for hundreds of years.

The idea of global warming is a hot political issue. There are those who still hold that this heating cycle is a “natural event.” Essentially no scientists remain in this camp. Even if you remove human impact on global warming from the equation, 50% to 70% of all coral reefs on earth are still under direct threat from human activities, threats such as pollution and wanton destruction.

Ever wanted to go snorkel or dive on a coral reef? Do it soon.

Communications | Humankind | Ecology | Geology