Over the past 25 years, off the Australian coasts, researchers and recreational divers alike have observed how marine heatwaves are putting enormous pressure on two of the most species-rich, productive and important ecosystems of the oceans. These are the tropical coral reefs and the kelp forests – also called the rainforests of the oceans.
Tropical coral reefs cover less than 0.1 per cent of the global sea floor. Nevertheless, they are the habitat for at least a quarter of all currently known marine species. This diversity is a result of the fact that, as they grow, corals construct enormous calcareous structures containing a myriad of caves, tunnels and niches, in which hundreds of thousands of other marine organisms find food and protection. But humans can also be included among the beneficiaries of the reefs. Worldwide, more than 500 million people in 90 countries benefit in some way from the ecosystem services provided by coral reefs. They fish on the reefs, do recreational diving, make a living from reef tourism, rely on the coral structures to break waves and protect the coasts, or attribute cultural and spiritual qualities to them.
Globally, however, the coral reefs are dying. At least half of them have already been lost due to a variety of regional factors. While they were already being adversely affected by improper fishing practices, eutrophication, water pollution and the clearing of mangrove forests (loss of filtering effect), corals are now suffering severely from the consequences of climate change. Carbon dioxide-rich water inhibits their ability to form skeletons, and threatens to dissolve their carbonate foundations. Because of reduced mixing in the surface waters, as well as the presence of near-coastal hypoxic zones, they lack sufficient oxygen for respiration at many locations.
But the greatest damage is being caused by heatwaves, even though tropical corals generally prefer warm water. They thrive in water with temperatures between 23 and 29 degrees Celsius. Some reef-building species can even tolerate temperatures up to 40 degrees Celsius, albeit only for short periods. If the animals are exposed to temperatures warmer than 29 degrees Celsius for an extended period of time (in the Red Sea the threshold value is higher), they suffer from heat stress and cast out their lodgers. These are symbiotic algae called zooxanthellae, which live within the tissues of the coral polyps, produce sugar through the process of photosynthesis, and supply a considerable amount of it to their hosts. Without the algae, the corals lose their most important source of food. They become susceptible to disease and, along with the algae, they also lose their colour, which is why this reaction to heat stress is also known as coral bleaching.
When a coral becomes bleached it does not die immediately. If the water cools down again within a relatively short time the algae can return and the colony might recover. But if the heatwave continues for a longer period, the corals will starve. Researchers are observing these deaths more and more frequently around the world as the number and intensity of marine heatwaves increases. In the summer of 2019/20, the Great Barrier Reef off the east coast of Australia was hit by a long-lasting heatwave, the third one within five years. It resulted in coral bleaching that, for the first time, extended across all three regions of the reef system and impacted more colonies than ever before. Australian coral experts agree that climate change has now arrived in the cooler, southern part of the reef. And because heatwaves are occurring more frequently around the world, corals now have less time to recover from heat stress and recolonize the dead regions. The best-known coral reef in the world is therefore one of the many tropical coral reefs worldwide that will continue to shrink as long as the greenhouse gas concentrations in the atmosphere are not reduced and water temperatures are not stabilized.
fig. 2.32 > Today, rising water temperatures are already forcing warm-water corals and kelp forests out of their traditional habitats. Australia’s Great Barrier Reef, for example, has lost around half of its corals in the past 20 years. If the sea continues to warm, the pressure on these two communities will become so great that they will have scarcely any prospect of survival.
Like coral reefs, kelp forests also form three-dimensional structures in the sea in which countless other species find protection and a home.
The dense underwater forests of brown large algae prefer cooler waters, however, and therefore grow predominantly in temperate and subpolar marine regions. They are found on about a quarter of all the world’s coasts, where they perform essential functions. During their growth, kelp forests extract large amounts of carbon dioxide from the sea. They buffer the shore areas against high waves, serve as spawning grounds for countless fish species, and play an important role in nutrient recycling in the sea. Their wellbeing depends on many factors, including sunlight, availability of nutrients, and the abundance of algae-feeding organisms (especially sea cucumbers and fish). For the most part, however, their distribution is determined by water temperature, which is why the world’s kelp forests in both the northern and southern hemispheres have been undergoing significant changes for decades.
On the coast of Japan, for example, the brown algae have been gradually retreating northward since the 1980s. This is because the Kuroshio Current is becoming warmer. At temperatures between 18 and 20 degrees Celsius, algae-feeding fish like the Japanese parrot fish Calotomus japonicus are especially active and eat kelp faster than the rate of new algal growth.
Similarly, a heatwave off the west coast of Australia in the summer of 2010/11 was intense enough to destroy 43 per cent of an original area of almost 1000 square kilometres of kelp forest. As a result, the northern boundary of its distributional area was pushed southward by around 100 kilometres. With its retreat, the former habitats of the giant algae are being colonized by tropical and subtropical marine species, including many grazing organisms that are preventing recovery of the kelp forests today by eating practically every sprout of the giant algae that appears.
This total regime change has serious consequences for the coastal ecosystems and everyone who benefits from them. Western Australia’s fishing and tourism industries, for example, depend heavily on the existence of kelp forests. If these continue to shift southward, there will be more serious negative consequences than just the millions in economic damages.
Many endemic species that can only live in the kelp forests will also die out locally. Where the kelp forests disappear, biodiversity declines, along with the amount of carbon fixed by the plants and the overall amount of animal and plant biomass, a trend that cannot be halted as long as ocean temperatures continue to rise.