Corals – their mention evokes mental images of a colourful diversity in bright, clear blue water. But there are also species that hide their beauty: cold-water corals live in depths of up thousands of metres and often at single-digit water temperatures.
Most widespread is the stony coral Lophelia pertusa. From Northern Norway, a band of reefs draws along the continental plate margins to West Africa, another one between Nova Scotia (Canada) and the Gulf of Mexico. They can even be found in the Mediterranean and in the Caribbean.
Most Lophelia reefs are located at a depth between 200 and 400 metres. However, a reef at only 39 meters in the inner Trondheim Fjord is known as well as a site in the New England Seamount Chain at more than 3000 metres.
Daylight does not reach those depths – and unlike most tropical corals, cold-water corals are not fed by photosynthesizing zooxanthellae symbionts. They filter plankton from the water and therefore thrive in regions where strong currents serve them food. With water temperatures between four and almost 14 degrees Celsius, Lophelia pertusa tolerates a relatively wide span, but most of them can be found at temperatures between six and eight degrees.
Another important stony coral inhabiting of reefs founded by Lophelia pertusa, is Madrepora oculata. It is a cosmopolitan as well and lives at water depths between 55 and just under 2,000 metres. But their zigzag branches are too fragile to form extensive subsea thickets.
Two soft coral species are often found along with Lophelia: The red, pink or cream-colored Paragorgia arborea, also called “bubblegum-coral” because of their bulbous branches. Countless polyps live in their soft outer layer. When they extend their tentacles to feed, the branches look as if they were dotted with flowers. The up to three metres high Paragorgia is even overtopped by the orange Primnoa resedaeformis. It grows up to six meters in height and has been found at depths up to 3200 meters. Basket stars often climb the Primnoa branches to catch plankton food from there.
Stony corals like Lophelia pertusa build their skeletons from aragonite, a particularly soluble form of calcium carbonate. Therefore, scientists assumed that their growth will be impaired if less carbonate ions are available due to increasing ocean acidification and a decreasing seawater pH.
Therefore they investigated Lophelia pertusa‘s reactions to various aspects of climate change in the laboratories at GEOMAR: ocean acidification, rising water temperatures and a change in food supply.
A long-term experiment revealed that growth declined and individual branches were damaged when the water was undersaturated with aragonite (Ω <1) – a condition that could be achieved in 2100, according to model calculations of the IPCC in case emissions continue to develop at current rates. In warmer water, however, the corals grew better. When both factors were combined, the positive temperature effect compensated for the negative effect of acidification – regardless of how much food the animals received.
Can we be optimistic about Lophelia‘s future because of this observation – since ocean acidification is likely to occur side by side with a rise in water temperatures? From the perspective of science, there are still many questions to answer before Lophelia pertusa‘s fate becomes clearer. Unlike at the laboratory, in nature, Lophelia is integrated into a complex reef community, that is determined by many interdependencies. In addition to rising water temperatures and acidification, other impacts can come into play as well. For this reason, scientists try hard to take their experiments into the field and include other organisms in them.