Contemporary witnesses in the sea: In order to be able to better assess the effects of climate change on the coral reefs in our oceans, scientists take a look at the stony remains of past reefs. For cold-water corals, a study of reef remains from the last 20,000 years reveals: Adequate food and oxygen can help these corals survive warm periods, too.
Corals are not only found in shallow, light-filled waters, but also spread along the slopes of deeper sea areas around the world. They are home to countless marine creatures and play an important role in maintaining food webs and nutrient cycles. However, the architects of these complex reef structures, which provide shelter and food for many organisms, face many dangers. In addition to the trawls of fishing boats, which cause mechanical destruction of the reefs, a change in environmental conditions is particularly dangerous for the corals. These include, for example, the warming and acidification of the oceans, the decreasing oxygen content and also the varying nutrient inflow. If one of these parameters changes, for example due to climate change, this can affect the health of the entire coral reef.
Coral, tell me about the past
Since some coral reefs have existed for thousands of years and continue to grow, they are perfect witnesses to historical changes in the ocean caused by climate variability. Rodrigo da Costa Portilho-Ramos and his colleagues from the Center for Marine Environmental Sciences at the University of Bremen (MARUM) also used this information to draw conclusions about the future reactions of cold-water corals to environmental changes. Unlike tropical corals, cold-water corals thrive at greater depths, with less light, and in colder water.
To do this, the researchers examined the occurrence of calcareous skeletons of the cold-water coral Lophelia pertusa in the sediments of six locations in the Atlantic Ocean and the Mediterranean. The team determined how these corals have responded to changes in key environmental factors over the past 20,000 years. “This time span also includes the last major global warming event, which heralded the transition from the glacial period to the current interglacial period, causing a sea level rise of about 120 meters and a major restructuring of the ocean-atmosphere system,” explain da Costa Portilho-Ramos and his colleagues.
Food and oxygen as key elements
The results show that the effects of different environmental parameters on the cold-water corals vary greatly. For example, the researchers could not find any direct effect on the growth and occurrence of coral reefs for changes in temperature or salinity of the water. However, the situation was completely different when it came to the food supply and the oxygen content of the water: as soon as these two parameters changed, Lophelia pertusa also disappeared or returned to a region. “Therefore, we assume that food supply and the availability of oxygen will be the decisive factors when it comes to the life and death of cold-water corals,” says da Costa Portilho-Ramos.
Corals still seem to be able to cope well with a decrease in oxygen levels as long as they have a high-quality supply of plankton and other microorganisms that they can filter out of the seawater. The researchers are therefore slightly optimistic about the future and believe that cold-water corals will be able to withstand the changes in the ocean as long as there is sufficient food. However, if global climate change continues to advance at a rapid rate, these corals, too, may be unable to adapt quickly enough and eventually disappear from the oceans in large numbers.
Source: Center for Marine Environmental Sciences at the University of Bremen; Article: PLOS Biology, doi: 10.1371/journal.pbio.3001628