Anyone who has ever seen a coral reef live may know this feeling of being a guest in another world. Everywhere you look, fish and other creatures are searching for food, mating, building nests, lurking for prey or carrying out other important activities. Most people aren’t bothered, but some also look back curiously.
Such a reef is only a tiny part of the underwater world that lies beneath the surface of the oceans. Just like on land, there are mountains and valleys, different landscapes and ecosystems. And like forests, moors and meadows on land, most underwater ecosystems are also severely damaged by human influence. Many are so badly affected that it is no longer enough to provide them with protection. In order to save them, they first have to be painstakingly renatured. Four examples.
Tropical coral reefs
The year 2023 was catastrophic for tropical coral reefs. “Extreme marine heatwaves have hit large parts of the eastern tropical Pacific and the Caribbean,” Write scientists led by climate researcher Ove Hoegh-Guldberg in the specialist journal Science. Since May 2023, coral reefs have been bleached in many countries, including Mexico, Costa Rica, El Salvador, Colombia and Panama. During bleaching, the corals shed so-called zooxanthellae, small algae that live inside and supply them with important nutrients. If the water cools down quickly, the corals can recover. But if the heat wave lasts longer, as it will in many places in 2023, the corals will die. With ocean temperatures expected to continue rising due to climate change and marine heat waves becoming more frequent, the World Biodiversity Council (IPBES) estimates that 99 percent of all coral reefs could be gone by the end of the century. The countless animals and plants that live in the reefs would also disappear with them. The loss would also have consequences for around a billion people for whom the reefs are currently a source of food, protection from storm surges or a source of tourist income.
How to restore coral reefs? The most effective way to save tropical coral reefs would be to stop climate change and thereby reduce the frequency and duration of marine heat waves. But since it is questionable whether this will be possible quickly enough or at all, the researchers led by Ove Hoegh-Guldberg are proposing a kind of triage for coral reefs. In their view, priority should be given to reefs that are likely to suffer the least from global warming and therefore have the best chance of survival. Nevertheless, there are numerous projects around the world attempting to revive dead coral reefs. A common method is to cut off parts of intact coral and reintroduce them to the destroyed reefs. In other projects, the corals for reef transplantation are bred in the laboratory – and sometimes equipped with genes that are supposed to make them tolerant to heat. In some places, reef restoration works surprisingly well: A team led by Ines Lange from the British University of Exeter recently published in the scientific journal Current Biology reported, that there is a reef off the Indonesian island of Sulawesi that was destroyed in the course of the “Mars Coral Reef Restoration Program” restored, fully recovered within four years. The reef had been shattered into countless pieces by dynamite fishing. The reef rescuers therefore first had to create a suitable substrate again before they could settle new corals. “We found that restored coral reefs can grow just as quickly as intact coral reefs just four years after coral transplantation,” says Ines Lange in a press release. “The speed at which the reef recovered was incredible.”
Kelp forests
Kelp forests are the underwater counterpart to rainforests on land. Like these, they are considered to be particularly rich in species. Echinoderms, cnidarians, molluscs, armopods, arthropods, worms, sponges and of course many different fish live in kelp forests; also countless still undiscovered animal and plant species. And like forests on land, in which different types of trees grow, kelp forests are not made up of a single species, but of many different species of so-called macroalgae. Some of these species grow up to 40 meters long, others grow less than a meter high. Since the large algae have no roots but rather an adhesive disk, kelp forests require a rocky substrate to which the plants can hold on. They also need a low water temperature compared to corals. One of the largest kelp forests is the 8,000 kilometer long Great Southern Reef on the southern coast of Australia. “In many areas of Australia and around the world, kelp forests are declining,” writes a team led by marine ecologist Cayne Layton from the University of Tasmania in a review article in the science journal Frontiers in Marine Science. The causes for this are diverse and include “both physical and biological factors”. In the west of Australia, warming seas in particular are affecting kelp, while in the south, seawater pollution is destroying it. “A big threat to kelp forests are diadem sea urchins,” says Bernadette Pogoda. “The animals eat away at the macroalgae’s adhesive discs, causing them to tear off.” Instead of species-rich kelp forests, there are now sea urchin deserts off Tasmania, where the animals cover the entire seabed.
How to restore kelp forests? There are basically two methods, write the researchers led by Cayne Layton. With “assisted recovery” you try to support the underwater forest so that it can recover itself – for example by removing the sea urchins that gnaw on the macroalgae. Or by installing artificial reefs on the ocean floor in the hope that kelp will colonize there again. During “active restoration”, the macroalgae are replanted. One of the most successful reintroduction projects is the Operation Crayweed. Over the course of eight years, scientists have succeeded in establishing a new kelp forest off the coast of Sydney. The researchers anchored adult kelp plants to various locations on the deforested rocks using degradable nets. At some point the kelp began to reproduce on its own. The researchers led by Cayne Layton write that the underwater forest is now just as large as it was before deforestation.
Oyster reefs
We know that we have lost 85 percent of the world’s oyster reefs, says Bernadette Pogoda, marine biologist at the Alfred Wegener Institute in Bremerhaven, who is also involved in the international and European Society for Ecological Restoration. “The reason for this is massive overfishing.” Like coral reefs, oyster reefs protect coastlines from flooding and soil erosion. They are also considered hotspots of biodiversity because many other creatures find food and shelter in them. “Hundreds of other organisms grow on the oysters,” Pogoda says. “Small sponges and hydrozoans, fish lay their eggs there.” From an ecological point of view, oyster reefs in temperate areas have the same importance as coral reefs in the tropics.
How to restore oyster reefs? “There are efforts around the world to do this,” says Pogoda. Also in Germany: goal of the project Restore for example, it is to re-establish an oyster reef in the North Sea protected area Borkum Riffgrund. To do this, scientists first sunk stones to create a reef base on the sandy seabed. Divers then secured specially bred European oysters (Ostrea edulis) using biodegradable hemp nets. There are already initial successes: “We are observing an increase in fish species and all sorts of sessile organisms on the oyster shells,” says Pogoda. The plan is to establish various reefs measuring several hectares in the North Sea.
Seagrass meadows
“Seagrass meadows are one of the strongest CO₂ stores of all,” says Gregory Fuchs from the Ecologic Institute in Berlin. And in the long term. Because when plants die and decay, the carbon stored in them is not released again. “The carbon gradually passes into the sediments” and is stored there, says Fuchs. Preserving and restoring seagrass meadows is therefore not only a contribution to species protection, but also to climate protection. “Seagrass meadows are found on the coasts of all continents with the exception of Antarctica,” writes a team led by Stephanie Valdez from the American Duke University in Beaufort in a review article on seagrass meadow restoration in Frontiers in Marine Science. In Germany, almost 150 square kilometers of sandy seabeds along the coast are covered with seagrass. Globally, the area covered by seagrass is estimated to have declined by 30 to 60 percent, the scientists write in Frontiers in Marine Science. “In some places it has completely disappeared.” The causes for this are deforestation, for example for the construction of ports, trawling, in which the plants are torn out as collateral damage, so to speak, water pollution and climate change.
Seagrass can live for several hundred years and plays a central role in the marine ecosystem. Among other things, it is a kind of nursery for many fish species. “Also for economically important fish species such as cod or herring,” says Fuchs. Seagrass also improves water quality because the plants absorb nitrogen and phosphorus and release oxygen. The plants also stabilize the sediment and prevent coastal erosion.
How can you restore seagrass meadows? It is difficult to bring back seaweed once it has disappeared. goal of the project Seastore, in which five other research institutes are involved in addition to the Geomar Helmholtz Center for Ocean Research in Kiel, is to regreen the Baltic Sea. To do this, divers plant individual plants laboriously by hand. The organization Seaforester is looking for more effective methods to enable restoration of larger areas. A promising approach is to “inoculate” small stones with seaweed seeds and then let the plants grow into small plants on land. “The stones with the seaweed plants can then be scattered from a boat, without divers or technical equipment,” the organization writes on its homepage.
The special thing about all of these underwater landscapes is that they offer marine organisms something that is in short supply in the oceans: three-dimensional structures and a solid surface to hold on to. They are all so-called biodiversity hotspots, i.e. particularly rich in species. Preserving and restoring coral reefs, kelp forests and other ecosystems is important for more than just species conservation reasons. “It’s about securing the marine ecosystems so that they can meet our growing demands,” says Sina Löschke, who is, among other things, responsible for the project Sea4Society committed to making the sea and its ecosystems an ally against climate change. By protecting and restoring underwater landscapes, two global crises can be combated at once: climate change and species extinction.