The deep ocean, often thought to be barren, harbors vibrant ecosystems. Cold water coral reefs are intricate structures built by marine invertebrates in environments far removed from sunlight and warmth. These deep-sea formations represent a significant component of global marine biodiversity and ocean health.
What Are Cold Water Coral Reefs?
Cold water coral reefs are underwater structures composed of calcium carbonate skeletons, built by corals that do not rely on sunlight. Unlike tropical corals, which depend on symbiotic algae, cold water corals obtain nutrition by filtering organic matter and zooplankton from ocean currents. These reefs thrive in deep, dark waters, from 40 meters to over 1,000 meters deep. Temperatures in these habitats are consistently low, between 4°C and 13°C.
The primary types of cold water corals that form reefs include stony corals, such as Desmophyllum pertusum (formerly Lophelia pertusa), and various gorgonians, known as sea fans. Desmophyllum pertusum is a widespread reef-building species, dominant in the North Atlantic, found at depths from 39 to 3,383 meters. These corals grow slowly, at 4 to 25 millimeters per year, forming massive structures up to 300 meters high and several kilometers in diameter over thousands to millions of years.
Cold water coral reefs are globally distributed, found in all oceans from polar to temperate regions, including fjords, continental shelves, slopes, and seamounts. For example, the Norwegian continental shelf contains one of the most extensive cold water coral habitats known. Their formation and survival depend on specific environmental conditions, including stable temperatures, hard substrates for attachment, and strong currents that deliver a steady supply of food and prevent sediment accumulation.
Life in the Deep Sea
Cold water coral reefs serve as significant biodiversity hotspots in the deep ocean, providing complex three-dimensional habitats in an otherwise often flat and muddy seafloor. These intricate structures offer shelter, feeding grounds, and nursery areas for a diverse array of marine life. Over 1,300 species have been recorded living on Lophelia pertusa reefs in the Northeast Atlantic alone.
The diverse communities supported by these reefs include invertebrates such as sponges, anemones, starfish, brittle stars, and crustaceans. Many fish species, including commercially important ones like shrimp, crabs, groupers, rockfish, and snappers, also rely on these habitats. The branches of corals provide refuge, while smaller animals may burrow into the coral skeletons. Organisms in these deep-sea environments exhibit adaptations to low light and cold conditions.
Why Cold Water Reefs Matter
Cold water coral reefs contribute significantly to ocean health and global ecological processes beyond their role as biodiversity havens. They play a part in nutrient cycling, trapping organic material and processing it within the reef system. These reefs contribute to carbon sequestration, fixing dissolved inorganic carbon as calcium carbonate in their skeletons and burying organic compounds in sediments. This process contributes to the ocean’s ability to store carbon in these deep-water systems.
The longevity and slow growth rates of cold water corals make their skeletons valuable natural archives, recording past climate conditions over millennia. By studying the chemical composition of their skeletons, scientists can reconstruct historical ocean temperatures and chemistry, providing insights into long-term climate change. These reefs also hold potential for scientific discovery, with new species frequently found in these underexplored environments. Some organisms associated with deep-sea corals have yielded compounds with potential biomedical applications, such as anti-inflammatory, anti-viral, and anti-tumor properties.
Protecting These Fragile Ecosystems
Cold water coral reefs face numerous threats, primarily from human activities that disturb the seafloor. Deep-sea bottom trawling is a major destructive fishing practice that can cause irreversible physical damage to these slow-growing and fragile ecosystems. The heavy nets and rollers used in trawling can destroy coral structures that have taken thousands of years to form. Oil and gas exploration and production, along with the placement of cables and pipelines, also pose risks through physical disturbance and potential pollution.
Climate change presents additional challenges, particularly through ocean acidification. As the ocean absorbs increasing amounts of carbon dioxide from the atmosphere, its chemistry changes, reducing the concentration of carbonate ions. This makes it more difficult for corals to build and maintain their calcium carbonate skeletons, potentially leading to dissolution of their structures. While cold-water corals are less directly affected by ocean warming than tropical species, shifts in ocean currents and food supply due to warming can also impact their survival. Pollution, including plastic debris and chemical contaminants, further stresses these remote ecosystems.
Conservation efforts include establishing Marine Protected Areas (MPAs) to provide safe havens from destructive human activities. Improved fishing regulations, such as bans on deep-sea trawling in specific areas, are also being implemented. International agreements, like the United Nations Convention on the Law of the Sea (UNCLOS) and the Convention on Biological Diversity (CBD), provide frameworks for protecting marine biodiversity, including cold water corals. Continued research and technological advancements, such as the use of remotely operated vehicles (ROVs) for mapping and monitoring, are helping scientists better understand and protect these deep-sea environments.