Coral Reefs Under Pressure

Coral reefs are often described as the rainforests of the sea, and the comparison is apt. Although they cover only a tiny fraction of the ocean floor, they shelter a startling proportion of all marine species, providing food and refuge for fish, molluscs, crustaceans and countless smaller creatures. They also protect coastlines from the force of storms and waves, and they support the livelihoods of millions of people through fishing and tourism. Yet these vast, colourful structures are built by animals so small and simple that early naturalists mistook them for plants or even stones. Understanding how reefs are made, and why they are now in danger, means looking closely at the tiny organisms at their heart.

A coral is not a single creature but a colony of many. Each individual, called a polyp, is a soft-bodied animal resembling a miniature sea anemone, with a ring of tentacles surrounding a central mouth. What makes reef-building corals special is their ability to draw dissolved minerals from seawater and lay them down as a hard skeleton of limestone beneath their bodies. As generation after generation of polyps grows, dies and is built upon, these skeletons accumulate over thousands of years into the massive structures we call reefs. The living coral forms only a thin layer over the surface of a vast foundation left by its ancestors.

The secret to a reef's success lies in a partnership. Living inside the tissues of each coral polyp are enormous numbers of microscopic algae. These algae capture sunlight and, through photosynthesis, produce sugars that supply the coral with much of the energy it needs to grow and build its skeleton. In return, the coral gives the algae a sheltered place to live and a supply of the raw materials they require. The algae also give the coral much of its brilliant colour. This close relationship explains why reef-building corals thrive only in clear, shallow, sunlit water, where the algae can gather enough light. It also explains why they are so vulnerable.

When the water around a reef becomes too warm, even by a small amount sustained over several weeks, the partnership breaks down. Stressed corals expel the algae living within them, losing both their main food supply and their colour. The result is a phenomenon known as coral bleaching, in which whole reefs turn a ghostly white. A bleached coral is not dead, and if cooler conditions return quickly enough the algae may recolonise it and the coral may recover. But if the heat persists, the starving coral will eventually die, and a reef that has taken centuries to grow can be devastated in a single season.

Warming is not the only threat. As human activities release ever greater quantities of carbon dioxide into the atmosphere, a portion of that gas dissolves into the sea. There it reacts with seawater to form a weak acid, gradually lowering the water's natural alkalinity in a process called ocean acidification. This change makes it harder for corals to obtain the minerals they need to build their limestone skeletons, slowing their growth and weakening the structures they have already made. Acidification acts quietly and over long periods, but its effect on reefs worldwide may prove just as serious as that of rising temperatures.

Reefs face a range of more local pressures as well. Sediment washed from cleared land can cloud the water and block the sunlight the algae depend on. Nutrients from fertilisers and sewage can trigger blooms of other organisms that smother corals or feed their predators. Careless fishing methods and the physical damage caused by anchors and divers add further strain. Because these local threats often act together with the global ones, a reef weakened by warming may be far less able to withstand pollution or storms than a healthy one would be.

The outlook is serious but not hopeless. Around the world, scientists and communities are working to give reefs a better chance. Some efforts focus on reducing local damage, by controlling pollution, managing fishing and setting aside protected areas where reefs can recover undisturbed. Others are more experimental, including attempts to grow corals in nurseries and transplant them onto damaged reefs, or to identify and breed varieties that can better tolerate warmer water. None of these measures can succeed on its own while the underlying causes of warming and acidification continue to grow. But they may buy time, and they reflect a growing recognition that the fate of coral reefs is bound up with decisions made far from the sea.