Gi­gantic sub­mar­ine cold-wa­ter coral mounds off Mauretania

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.”