Samoa Guide, 8. When corals turn white and die

Because most corals live only in warm tropical waters, it seems odd that corals will die when the water gets slightly warmer. They live close to the hottest temperature that they can tolerate, so it doesn't take much to push them over the limit. To explain what is happening, recall that corals are animals with colorful plant-like cells (zooxanthellae) living in their tissues.

As if that weren’t enough, we are also seeing more coral diseases on our reefs (who would have thought that corals can catch diseases?). Presumably the heatstressed condition of the corals makes them more vulnerable to diseases that had formerly been rare. In the photo at
right, a white band of death is sweeping across the coral, from left to right. Behind the white band (to the left), the coral polyps were killed about a week or two ago, giving time for greenish algae to start growing on the dead coral skeleton. The white band itself is a zone of freshly killed polyps and exposed white coral skeleton. In front of the white band (to the right) is the last remaining bit of live coral (purplish in color). The cause of this disease is not known yet. Although coral diseases have probably always been around, they spread rapidly on our reefs beginning in 2002.

To round-out this rather negative view of the problems coral face in our modern world, an even greater threat to them is a projected change in water chemistry in the ocean due to global warming. Just as carbon dioxide (the main greenhouse gas causing global warming) is increasing in the air, it also increases in seawater in its dissolved form. That will lower the pH of seawater which, in turn, may slow the rate at which corals build their calcium carbonate skeletons. The result is that coral growth would be slowed and there might even be an increased erosion of the reef itself.

Scientists predict that episodes of warm water temperatures will become more frequent due to a general warming of the earth. That's bad news for us. While it's unlikely that all of our corals will die off as the environment gets warmer, the number and/or abundance of corals may well decline here. That might impact American Samoa in two general ways. First, coral growth might not keep up with rising sea levels or the reef itself may begin to erode, thereby allowing more storm waves to reach our shorelines and cause damage to roads and houses. Second, a reduction in coral growth and number of species could reduce the diversity of habitats required by fish, so a downturn in reef catches could eventually occur. Both of these changes would probably occur at a slow but steady pace over the next 30 years.

What to do? Not too much, unfortunately, because American Samoa has little impact on the world's changing climate. On the other hand, it makes sense not to worsen the situation by further stressing our coral reefs with rubbish, sewage from piggeries, or dirt (sediment) from land-use activities that flows into streams and out onto the reefs. The brown water we see entering the ocean from streams after a heavy rainfall is harmful to the corals. Additionally, we should locate and protect any areas where corals appear to be naturally resilient to bleaching events. These hardy survivors could then help re-seed other areas where the corals had died.

When the coral is stressed by warmer than usual temperatures, the zooxanthellae are released from the coral, for reasons known only to them. What's left is a rather colorless coral animal overlying a bright white coral skeleton (see drawing). The animal portion of the coral may eventually recover its zooxanthellae and continue living, or it may die, depending on how stressed it gets. It's easy to tell when portions of the coral die because they become covered with fuzzy green algae.

A little bleaching now occurs here during most summers, but it was particularly bad in 1994, 2002 and 2003. Not all coral species were affected then, but those in shallow waters were hardest hit, although some bleaching down to the 130-foot depth was observed. Bleaching also occurred in 1998 when we experienced very low tides due to a strong El Nino event. The exposed corals turned white and died.

These cells use the sun's light to produce food which is also used by the coral animal. Many coral animals receive much of their food this way, so this relationship is quite important to the coral animal. The animal, in turn, provides the zooxanthellae with nutrients and a secure place to live. Both the coral and the zooxanthellae benefit from this arrangement.

Two very different kinds of stress cause corals to turn white: (1) clorox bleach, and (2) warm water temperatures. Clorox bleaching happens from time to time when a foolish fisherman dumps clorox onto the reef to kill fish. This is very short-sighted because it also kills everything else in the vicinity -- young fish, crabs, snails and corals -- and that harms the reef itself and reduces everyone else's catch.

Unusually warm water temperatures, due either to weather events or global warming, can also cause the coral to bleach. It only takes a slight increase above normal water temperature to bleach the coral. Bleaching can be caused by a short-term exposure (1-2 days) at temperature elevations of 3-4 degrees, or by long-term exposure (weeks) at elevations of only 1-2 degrees. To a diver, this may look like a pretty snowfall on the reef, but it indicates that the reef is seriously stressed.