In recent years, coral reefs in American Samoa have turned pure white on several occasions. They look freshly bleached, quite pretty, but that's a clear sign that things are bad for them.

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.

Underwater photo showing several kinds of coral on our reefs that turned white, or “bleached”, due to heat stress in 1994.

Nature can also cause the coral to bleach with extra warm water temperatures. 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.

It seems odd that corals will die when the water gets slightly warmer, because most corals live only in warm tropical waters. Nonetheless, 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. These cells use the sun's light to produce food, which is shared with 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.

Normal coral polyp with plant-like cells inside the coral's tissues.	Heat-stress causes the coral polyp to release its plant-like cells.

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 occurs here during most summers, but it was particularly bad in 1994. Not all coral species were affected then, but those in shallow waters were hardest hit, and some bleaching down to the 130-foot depth was observed. Bleaching occurred again in 1998 when we experienced very low tides due to a strong El Nino event. The exposed corals turned white and died. In more recent years, we have narrowly escaped major bleaching incidents that hit nearby Fiji and western Samoa, but fortunately the area of warm ocean waters that caused the bleaching did not extend as far northeast as American Samoa.

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 of corals here may well decline. That could cause two problems in American Samoa. First, a reduction in coral growth and number of species could reduce the diversity of habitats used by fish, so a downturn in fish catches from the reef could occur. Second, the reef itself could begin to erode, allowing more storm waves to reach our shorelines and cause damage to roads and houses. Both of these changes would probably occur at a slow but steady pace over the next 50 years.

What to do? Not 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. Additionally, we should also identify 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.

P. Craig, NPS