Gigantic submarine cold-water coral mounds

The stony corals Lophelia pertusa and Madrepora oculata on a coral mound off Mauretania. The reefs, built by these stony corals, are an important habitat for fish, crabs, mussels and other dwellers. Photo: Tomas Lundälv

With a length of around 400 kilometers, the world’s largest contiguous cold-water coral structure stretches along the seafloor off the coast of Mauretania. Dr. Claudia Wienberg of MARUM – Center for Marine Environmental Sciences at the University of Bremen and her colleagues have studied how the cold-water corals off the Mauretanian coast have developed over the past 120,000 years. Their results have been published in the journal Quaternary Science Reviews.

In contrast to tropical corals, which live in shallow waters bathed with light, cold-water corals are found in water depths of hundreds or even a thousand meters. More than half of the known coral species living today exist in complete darkness in the deep sea. They too are busy engineers that build impressive coral reefs. The cold-water coral species Lophelia pertusa is a key contributor to the reef formation.

It belongs to the group of stony corals and forms highly branched, bush-like colonies. Where many of these colonies coexist, reef-like structures form, providing a unique habitat for various other species such as soft corals, fish, crabs and sponges. For its entire life, an individual cold-water coral remains attached to the substrate onto which its larva first settled. Cold-water corals prefer to grow on their own kind, and thus create enormous structures on the seabed over periods of thousands to millions of years.

Alps off Mauretania

The world’s largest contiguous cold-water coral structure, with a length of around 400 kilometers, is situated along the coast of Mauretania. The coral mounds here can reach heights of 100 meters. “The height of the mounds and the length of this structure are really special. Actually, one could describe this as a cold-water coral underwater mountain range,” says Dr. Claudia Wienberg of MARUM – Center for Marine Environmental Sciences at the University of Bremen.

“Off Mauretania individual cold-water coral mounds have presumably grown together over time. There is nothing like this anywhere else in the world ocean.” Wienberg was one of an international team of researchers who intensively sampled this area, making use of the research vessel MARIA S. MERIAN, in order to learn more about the development of cold-water corals. She and her colleagues have now published their results in the professional journal Quaternary Science Reviews.

Oxygen deficiency puts coral growth on hold

Prof. Dr. Norbert Frank and his team at the University of Heidelberg analyzed coral fragments from the surface and from various depths within the seafloor and determined their ages. Using these with other analyses, the scientists were able to trace the development of the cold-water corals off Mauretania through the past 120,000 years. There have been repeated phases in the past when the growth rates reached peak values of 16 meters per 1000 years.

Not even the largest present-day cold-water coral reef off Norway is growing that fast. Almost 11,000 years ago the growth of the Mauretanian coral mound stagnated. At this time the living corals probably completely disappeared from the mounds. Only today are individual living cold-water corals beginning to appear again.

The coral growth depends on various environmental factors such as water temperature, oxygen content, food availability and the prevailing currents that transport food to the sessile cold-water corals. Among the influencing factors, the researchers identified the low oxygen content of about 1 milliliter per liter of water as being critical.

“That is extremely low. It has been assumed that the lowest limit for cold-water corals is 2.7 milliliters per liter, at which they could just survive, but no longer build reefs,” according to Wienberg. “The presence of individual cold-water corals now on the mounds indicates that, at times at least, they may survive in very low oxygen conditions, but they do not thrive.”

Their results suggest that the peak phases for the cold-water corals, when the mounds grew significantly in height, coincided with times when oxygen-rich water masses flowed into the area from the north. When the cold-water corals were flooded in the past by oxygen-poor water masses from the south, as is the case today, the mounds grew more slowly or not at all. Depending on the prevailing climate, the front between these water masses has shifted from north to south, and vice versa, and the corals were alternately enveloped by oxygen-rich or oxygen-poor waters.

According to Wienberg's theory, the cold-water corals found refuge at times of extremely low oxygen content in small ravines between the large mound structures. Today, many more cold-water corals can also be found in these canyons than on the mounds. The swimming coral larvae are mobile for a certain distance before they finally fall back to the seafloor. Migration could have occurred from the mounds into the canyons and – under the influence of the northern water masses – back again.

“Scientific prognoses suggest that zones of depleted oxygen in the world ocean will continue to expand,” says Wienberg. “Even though cold-water corals exhibit a high tolerance, this is definitely a deciding stress factor for these deep-sea ecosystems. Added to this are the rising water temperatures caused by climate change and increasing ocean acidity.”

Contact:
Dr. Claudia Wienberg
MARUM, Marine Sedimentology
Telephone: +49 421 218 65652
Email: cwberg@marum.de

Further information / Photo material:
Jana Stone
MARUM Public Relations
Telephone: +49 421 218 65541
Email: medien@marum.de

Original Publication:
The giant Mauritanian cold-water coral mound province: Oxygen control on coral mound formation
Claudia Wienberg, Jürgen Titschack, André Freiwald, Norbert Frank, Tomas Lundälv, Marco Taviani, Lydia Beuck, Andrea Schröder-Ritzrau, Thomas Krengel, Dierk Hebbeln
Quaternary Science Reviews 2018 doi.org/10.1016/j.quascirev.2018.02.012

Participating institutes:

Institute for Environmental Physics (IUP), University of Heidelberg
Institute of Marine Science (ISMAR-CNR), Italy
Laboratoire des Sciences du Climat et de l'Environnement (LSCE), France
MARUM – Center for Marine Environmental Sciences, University of Bremen
Senckenberg Research Institute, Wilhelmshaven
Stazione Zoologica Anton Dohrn, Italy
Swedish Institute for the Marine Environment, University of Gothenburg
Woods Hole Oceanographic Institution, USA

Using state-of-the-art methods and through participation in international projects, MARUM investigates the role of the ocean in the Earth’s system, particularly with respect to global change. It quantifies the interactions between geological and biological processes in the ocean and contributes to the sustainable use of the oceans. MARUM comprises the DFG Research Centre and the Excellence Cluster “The Oceans in the Earth System”.

https://www.marum.de/en/Discover/Coral-mound.html

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