After the Reef Turns White
1A coral reef can change from a patchwork of colors to a field of pale shapes during unusually warm ocean conditions. Many reef-building corals contain tiny algae within their tissues. These algae provide much of the corals' food through photosynthesis and contribute to their color. When heat stresses the corals, they can lose these partners. Their white skeletons then show through their mostly clear tissues, a change called bleaching.
2A bleached coral is not necessarily dead. If stressful conditions ease soon enough, surviving corals can regain algae and rebuild their energy supplies. During bleaching, however, they have less food available. If the stress continues, they may die. Even corals that survive may grow more slowly or produce fewer young for a time. A return of color, therefore, does not mean that every part of reef life has returned to its earlier condition.
3Recovery also involves the next generation. Corals produce young called larvae, which travel in the water before settling. Some larvae come from surviving corals nearby; others arrive from more distant reefs. Larvae that settle on suitable hard surfaces can begin growing into new colonies. This process takes time, and the arrival of larvae alone does not guarantee that they will survive.
4Conditions on the reef affect those young corals. Large seaweeds can take over space where coral might settle. Plant-eating fish help by grazing on these seaweeds. Too much sediment can cover settling surfaces or smother young corals. Protecting grazing fish and reducing sediment from land can improve conditions for recovery, although these actions do not directly cool the ocean.
5Researchers look beyond color when checking a reef's progress. They may record how much living coral covers an area, count small coral colonies, and observe which kinds of coral are present. Comparing these observations over time helps them distinguish a brief improvement from a broader recovery. Different reefs can follow different paths, depending on damage, surviving coral, and local conditions.
6Another period of extreme heat can interrupt that progress before young colonies mature. Local protection cannot remove the danger of repeated ocean warming. On a recovering reef, new coral colonies may be growing even while other areas remain bare.