Cities Beneath the Waves

From a satellite, a coral reef is barely perceptible: a pale smudge of turquoise on the edge of a tropical coast, easily mistaken for shallow sand. Together, reefs occupy well under one percent of the ocean floor. Yet they are home to roughly a quarter of all marine species, a concentration of life so disproportionate that biologists frequently compare reefs to cities, the crowded, noisy places where an unlikely number of creatures have found ways to live alongside one another. The foundation of this metropolis is, curiously, a partnership. Corals are animals, not plants or rocks, and each reef is built from the accumulated limestone skeletons of countless tiny polyps. But a polyp can hardly be called self-sufficient. Embedded in its tissue live microscopic algae that capture sunlight and pass on much of the energy they produce, in return for shelter and a steady supply of nutrients. It is this arrangement that gives corals their colour, and it explains why reefs thrive only in clear, sunlit, shallow water, where the algae can do their work. The arrangement is also precarious. When the sea becomes unusually warm, the algae begin to malfunction and the coral expels them, leaving its tissue transparent and its white skeleton visible, a phenomenon known as bleaching. A bleached coral has not given up; if temperatures fall quickly enough, the algae can return and the colony recover. If the heat persists, however, the coral loses its principal source of food and cannot long survive. Marine heatwaves of the kind that have affected stretches of the Great Barrier Reef, which extends for some two thousand three hundred kilometres along the Australian coast, have therefore caused widespread alarm. Reefs matter well beyond the aquarium-friendly appeal of their inhabitants. They shelter fish that millions of people rely on for food, they attract tourists whose spending supports coastal economies, and their solid structures soften the force of waves before they reach the shore. To lose a reef is therefore to lose a kind of natural infrastructure. Responses are, accordingly, becoming more imaginative. In so-called coral gardening, fragments are grown in underwater nurseries and attached to damaged reefs, while researchers examine why some corals tolerate heat better than others, in the hope of propagating the more resilient varieties. Such work is encouraging but cannot, by itself, change the fundamental problem. Most specialists agree that the most effective intervention is also the least glamorous: reducing the emissions that warm the ocean, and curbing the pollution and overfishing that leave reefs less able to cope. Restoration, in other words, can buy time; it cannot replace a change in habits.