Rare cold water coral ecosystem discovered off coast of Mauritania

The coral wall on the continental shelf off the coast of Mauritania measures 50 to 60 m high and is 190 km long. When the Swedish robot pilot Tomas Lundälv of the Sven Lovén Centre of the University of Gothenburg set down a diving robot on the seafloor at a depth of 615 m, the scientist on board the research ship found himself via a video link in the middle of a flourishing coral ecosystem. André Freiwald reports on a heavily calcified Lophelia coral with orange-red polyps and gorgonias, which, beside the reef-building stony corals, formed imposing octocoral gardens in the dark and otherwise inaccessible habitat. According to the excited expedition report, giant clams also hang on the coral galleries, in exactly the same way as is found elsewhere in Norwegian reef systems.

Such impressive ecosystems were previously only known above all from regions of the sea located much further to the north, around Scandinavia and in the Irish Sea. Unlike their tropical relatives, found by snorkellers and scuba divers in the illuminated and significantly warmer surface waters, cold water corals live at a cold 13° in the dark and nutrient-rich deep sea region below 200 m. André Freiwald was aware of a loose cold water coral reef which extends to southern regions. Until now, however, scientists had only found fossil coral reef structures on the seafloor off the coast of Gibraltar and Morocco.

While the Maria S. Merian, equipped with a dynamic positioning system, accompanied the diving robot step by step, the on-board coral team followed the exploratory dive, around 60 km west of Cape Tamirist, which took them into absolutely uncharted waters. Meter by meter, the device worked its way up the slope following a navigation chart drawn up by the Senckenberg scientist, Dr. Lydia Beuck, when at a depth of approximately 500 m, the coral group discovered further Lophelia colonies in a bizarre rock formation which nevertheless have a significantly more fragile development of calcification. In his report, André Freiwald writes that at the same time, the diversity of sponges and large crustaceans at the location increased significantly. Among other things, the scientists found the powerful carrier crab Paromola here, and on diving through the rocky landscape, also found the giant deep sea oyster Neopycnodonte, also never before observed so far to the south. These giant oysters form thick populations and can be described as Methuselahs among animals, with some individuals living for over 500 years.

For the scientists on board the Maria S. Merian, the discovery of the ecosystem with living cold water corals came as a surprise. André Freiwald sees one reason for the southern occurrence of the anthozoans which are adapted to cold temperatures in the upwelling ocean cells steered by the Passat wind. The offshore winds push the surface waters from the Mauritanian cliffs out into the open ocean and thereby permit a following flow of cold and nutrient-rich water from the depths. This evidently not only ensures that the Mauritanian waters are among the richest of any in fish but also presumably also provides the cold water corals with appropriate feed. According to statements by coral experts, the marine creatures feed on the nutrients released by plankton organisms.

The 16^th research voyage with the Maria S. Merian, under the leadership of Professor Hildegard Westphal from the Leibniz Centre for Tropical Marine Ecology in Bremen, ends on 20 November in Mindelo (Cape Verde). Until then, it will stop at and chart further parts of the coral system in the canyon of the continental shelf off the coast of Mauritania. André Freiwald expects the next dives on this expedition to provide information as to whether the newly discovered ecosystem represents a single structure or whether a spatially extended living reef province exists in the southern waters.

The samples documented and permanently fixed during the voyage by Dr. Claudia Wienberg (MARUM, University of Bremen) and the Italian expert on corals, Dr. Marco Taviani (CNR-ISMAR, Bologna) will then be further investigated in the home laboratories.