Senckenberg Research

Unknown Giants : Cold-Water Reefs

 Lophelia colonies from the continental margin off Mauritania with squat lobsters (Eumunida bella).
Lophelia colonies from the continental margin off Mauritania with squat lobsters (Eumunida bella).

 

 

Coral reefs exist not only in the shallow, warm waters of the South Seas, but also in the deep, cold waters of Norway's fjords. And elsewhere, too. In recent years, researchers have been able to show that many parts of the sea bed, previously shown only as blank areas on the charts, are made up of so-called cold water reefs. A unique cold water reef database is being built up at the 'Senckenberg  am Meer' facility in Wilhelmshaven.

When we hear the term ‘coral reef, we tend to associate it with subtropical or tropical temperatures, clear waters and spectacular submarine landscapes teeming with thousands of organisms. Until only a few years ago, even scientific treatises covering the phenomenon of reef growth concerned themselves exclusively with recent or fossilized reefs in shallow-water regions in the tropics.

Coral habitats are surveyed with remotely-operated vehicles such as the QUEST from MARUM.
Coral habitats are surveyed with remotely-operated vehicles
such as the QUEST from MARUM.

In nautical terminology, the word ‘reef’ simply refers to shallows that can represent a hazard to shipping. Within just ten years, our traditional understanding of reef-building process es could be extended dramatically. Modern coral reef maps show a loose belt that stretches from the Norwegian Barents Sea north of the Arctic Circle to Mauritania along the north-east Atlantic continental margin. There is also a sparse belt of reefs along the east coast of Florida that reaches into the Gulf of Mexico, and recently scientists have reported the existence of a huge reef system along the south Brazilian-U ruguayan continental margin in the South Atlantic. How could such large structures on the sea floor remain undetected for such a long time? What is so special about these reefs, for which the term ‘cold-water reef’ has been coined? Furthermore, what role does Senckenberg have to play in researching these cold-water reefs? 

How the cold-water reefs were discovered

The fact that there are local, large collections of stony corals in the deep and cold waters of the Norwegian fjords as well as off Ireland and in the Gulf of Biscay has been known to high-seas fishermen for 200 years. The scientific community only concerned itself sporadically with this phenomenon, and it was only with the advent of suitable technology that it became possible to obtain accurate cartographic information. Now, research ships routinely use multi-beam echosounder systems to make detailed maps of the sea bed, and both manned and unmanned underwater vehicles now allow investigation of deeply fissured continental shelves, deep-sea canyons and the flanks of steep underwater mountains. Until only a few years ago, these regions remained mare incognitum – unknown seas – on sea-floor maps, and there was a high risk of losing expensive equipment there.

Lophelia colonies from the continental margin off Mauritania with the brachyuran Chaceon maritae.
Lophelia colonies from the continental margin off Mauritania with
the brachyuran Chaceon maritae.

Darwin’s slimeheads and file clams are characteristic coral associates of Westafrican cold-water coral reefs.
Darwin’s slimeheads and file clams are characteristic coral associates
of Westafrican cold-water coral reefs.

Only 17 stony corals that have the potential to build up reefs are known throughout the world, and in the known cold-water reefs only one or two species dominate the scenery. In the North Atlantic, these are mainly the colonial species Lophelia pertusa and Madrepora oculata. In combination with hydrodynamic processes, their biological characteristics allow for the generation of a reef-like relief within a few hundred years, creating a multiplicity of hard and soft substrates in different levels on the sea floor. The heterogeneity of the resulting habitats is considered to be a significant contributing factor concerning the biodiversity and the large biomass concentrations that are recorded in cold-water reefs.

The other factor has to do with the special trophic ecology and the material cycles that cause sustained changes in the biochemical constitution of the seawater that flows through the reefs. Cold-water reefs only grow where there is a sufficiency of suspended nutrients and food that is supplied on account of the bottom current. Also, the oscillating currents caused by the tidal cycles ensure that sedimentation rates are low, increasing the chance that colonizable hard substrates are available for the larvae of reef-building corals. When larvae have successfully established themselves and undergone metamorphosis, the corals start to monopolize their terrain by rapidly secreting a calcareous skeleton. They then start growing up into the water column at rates of 10 to 25 millimetres per year.

Even this initial carpet of coral represents an efficient ‘feeding machine’ which consists of a wall of coral polyps that filters planktonic organisms and dissolved substances out of the bottom current. Under ideal conditions, the coral carpet develops into a broad coral thicket that reaches 20 - 50 centimetres in height within two decades. Extremely fine particles (detritus) in the suspension are deposited at its base, sealing the coral interstices with a soft substrate. Further away from the coral habitats, the suspended matter remains in the bottom current and, if anything, only deposits a few millimetres in 1,000 years.

Our working group was able to confirm sediment accumulation rates within coral habitats of over six metres per 1,000 years. The geological consequence of this effective ‘sediment trapping’ is a reef deposit with a distinctly bimodal constitution, namely pelagic or hemipelagic (fine grained) sludge and coarse coral fragments including the fossilizable remains of reef inhabitants (molluscs, echinoderms, etc.).

Today, over 6,000 Lophelia reefs are now known to exist on the Norwegian Shelf. Over the last 11,000 years, immediately following the melting of the Fennoscandian ice shield on account of rising temperatures after the last Ice Age, these organisms have created reef structures up to 30 metres high. The largest of these reefs consist of contingent coral walls extending to over 35 kilometres. It is clear that these ecosystems are dependent on nutrient-rich masses of water to maintain their metabolic turnover.

By means of experiments on material cycles of Lophelia reefs conducted over a period of years, it could be shown that on the Norwegian Shelf up to 25 % of the carbon export from the photic zone (into which light penetrates) is extracted from the deep water and metabolized by the reef community or deposited in the sediments. This reef effect also explains the dominance of filter feeders and suspension feeders in the biotic community. Generally, benthic communities (communities of organisms that each have specific roles to play) on the continental slope tend to be characterized by detritus feeders.

The first database on the role of cold-water reefs

At Senckenberg am Meer in Wilhelmshaven, the world’s first database is being established with the aim of shedding light on the species composition and the role of organisms in the complex world of a cold-water reef. The Marine Geology Section in Wilhelmshaven is currently researching the biological interactions between reef corals and accompanying organisms using state-of-the-art imaging techniques (e. g. RE M / μCT). Through increasingly closer cooperation with the marine zoology taxonomists in Frankfurt and the German Centre for Marine Biodiversity Research (DZMB), it is discovering a large number of symbiotic (including parasitic) relationships between the reef inhabitants. A highlight of this research has been the discovery of the oldest known non-clonal animal, a deep-sea oyster that can live to over 500 years.

Madrepora oculata-corals
The framework-building Madrepora oculata provides a microhabitat for
a group od solitary corals (Desmophyllum dianthus).

Over the last two years, scientists from Professor Freiwald’s working group have been studying cold-water reefs on the Mauritanian continental slope and in the Porcupine Seabight. With a view to investigating interrelationships concerning connectivity and species spectra, the Wilhelmshaven team investigated the deep reefs on the edge of the Mexican Campeche Bank and in the Florida Straits. The researchers have a unique sample and underwater video archive at their disposal in Wilhelmshaven. The Marine Geology Section and the University of Bremen are linked by means of a kooperative professorship. Together with the geologists and engineers at the MARU M Centre for Marine Environmental Sciences, questions with geological relevance are addressed through the analysis of sediment cores, and scientific aspects of technological developments in deep-sea infrastructure are covered. This has enabled the clarification of important findings concerning the effects of abrupt environmental change on cold-water reefs within the framework of a European research consortium. The findings constitute part of the mapping of biological resources within the EU ’s exclusive economic zone (200 nautical mile zone) and have already been used in the reform of fishing policies, as many reef areas on the continental edges of Europe have been designated as prohibited zones for bottom trawling.

 

Author

 

Prof. Dr. André Freiwald

Prof. Dr. André Freiwald has been Department Head for Marine Research since 2010, and since 2012 he has been the Executive Director of Senckenberg am Meer in Wilhelmshaven. Within the framework of a cooperation with the University of Bremen, he holds a W3 professorship for Marine Geology. His field of research involves aspects of bio-sedimentation (especially non-tropical carbonate deposits). At present, his main focus is on polar calcareous algae and deep-sea coral reefs. He has studied these ecosystems intensively, using manned and unmanned underwater vehicles in over 30 expeditions.

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