Abstract. The cold-water coral Lophelia pertusa is one of the few species able to build reef-like structures and a 3-dimensional coral framework in the deep oceans. Furthermore, deep cold-water coral bioherms may be among the first marine ecosystems to be affected by ocean acidification. Colonies of L. pertusa were collected during a cruise in 2006 to cold-water coral bioherms of the Mingulay reef complex (Hebrides, North Atlantic). Shortly after sample collection onboard these corals were labelled with calcium-45. The same experimental approach was used to assess calcification rates and how those changed due to reduced pH during a cruise to the Skagerrak (North Sea) in 2007. The highest calcification rates were found in youngest polyps with up to 1% d −1 new skeletal growth and average rates of 0.11±0.02% d −1 (±S.E.). Lowering pH by 0.15 and 0.3 units relative to the ambient level resulted in calcification being reduced by 30 and 56%. Lower pH reduced calcification more in fast growing, young polyps (59% reduction) than in older polyps (40% reduction). Thus skeletal growth of young and fast calcifying corallites suffered more from ocean acidification. Nevertheless, L. pertusa exhibited positive net calcification (as measured by 45 Ca incorporation) even at an aragonite saturation state ( a ) below 1.

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We studied dissolved organic and inorganic carbon fixation in 2 sponge species from deep water coral mounds, viz. the demosponge Higginsia thielei and the hexactinellid sponge Rossella nodastrella. Sponges were collected between 500 and 700 m depth on coral mounds in the Rockall Trough (NE Atlantic). Prokaryote densities in sponge associations were on average 2.0 × 10 8

Abstract. The cold-water coral Lophelia pertusa is one of the few species able to build reef-like structures and a 3-dimensional coral framework in the deep oceans. Furthermore, deep cold-water coral bioherms are likely among the first marine ecosystems to be affected by ocean acidification. Colonies of L. pertusa were collected during a cruise in 2006 to cold-water coral bioherms of the Mingulay reef complex (Hebrides, North Atlantic). Calcium-45 labelling was conducted shortly after sample collection onboard. After this method proved to deliver reliable data, the same experimental approach was used to assess calcification rates and the effect of lowered pH during a~cruise to the Skagerrak (North Sea) in 2007. The highest calcification rates were found in youngest polyps with up to 1% d−1 new skeletal growth and average values of 0.11±0.02% d−1(±S.E.). Lowering the pH by 0.15 and 0.3 units relative to ambient pH resulted in a strong decrease in calcification by 30 and 56%, respectively. The effect of changes in pH on calcification was stronger for fast growing, young polyps (59% reduction) than for older polyps (40% reduction) which implies that skeletal growth of young and fast calcifying corallites will be influenced more negatively by ocean acidification. Nevertheless, L. pertusa revealed a positive net calcification (as indicated by 45Ca incorporation) at an aragonite saturation state (Ωa) below 1, which may indicate some adaptation to an environment that is already relatively low in Ωa compared to tropical or temperate coral bioherms.

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