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Why can ocean acidification threaten cold-water coral communities at great depths?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

Ocean acidification threatens cold-water coral communities primarily by reducing the availability of carbonate ions, which these organisms require to build and maintain their calcium carbonate skeletons.12

Deeper ocean waters are naturally more acidic than surface layers because carbon dioxide dissolves into dense, cold water that descends through thermohaline circulation. These acidic lower layers are separated from the surface by a boundary known as the "saturation horizon," above which there are sufficient carbonate concentrations to support coral growth.1

Since 1800, increased ocean acidity has caused this saturation horizon to rise by 50 to 200 metres in midlatitude and polar waters. Because cold-water corals typically reside at depths between 40 and 1,000 metres, this upward shift places them at significant risk. If the boundary continues to approach the surface, it threatens to expose these communities to waters where they can no longer obtain the raw materials necessary for survival, potentially leading to a decline in reef building and the loss of habitat for dependent marine species.1