Cold-water coral (CWC) reefs are distributed globally and form complex threedimensional structures on the deep seafloor, providing habitat for numerous species. Here, we measured the community O 2 and dissolved inorganic nitrogen (DIN) flux of CWC reef habitats with different coral cover and bare sediment (acting as reference site) in the Logachev mound area (NE Atlantic). Two methodologies were applied: the noninvasive in situ aquatic eddy co-variance (AEC) technique, and ex situ whole box core (BC) incubations. The AEC system was deployed twice per coral mound (69 h in total), providing an integral estimate of the O 2 flux from a total reef area of up to 500 m 2 , with mean O 2 consumption rates ranging from 11.6 ± 3.9 to 45.3 ± 11.7 mmol O 2 m −2 d −1 (mean ± SE). CWC reef community O 2 fluxes obtained from the BC incubations ranged from 5.7 ± 0.3 to 28.4 ± 2.4 mmol O 2 m −2 d −1 (mean ± SD) while the O 2 flux measured by BC incubations on the bare sediment reference site reported 1.9 ± 1.3 mmol O 2 m −2 d −1 (mean ± SD). Overall, O 2 fluxes measured with AEC and BC showed reasonable agreement, except for one station with high habitat heterogeneity. Our results suggest O 2 fluxes of CWC reef communities in the North East Atlantic are around five times higher than of sediments from comparable depths and living CWCs are driving the increased metabolism. DIN flux measurements by the BC incubations also revealed around two times higher DIN fluxes at the CWC reef (1.17 ± 0.87 mmol DIN m −2 d −1 ), compared to the bare sediment reference site (0.49 ± 0.32 mmol DIN m −2 d −1 ), due to intensified benthic release of NH 4 + . Our data indicate that the amount of living corals and dead coral framework largely contributes to the observed variability in O 2 fluxes on CWC reefs. A conservative estimate, based on the measured O 2 and DIN fluxes, indicates that CWC reefs process 20 to 35% of the total benthic respiration on the southeasterly Rockall Bank area, which demonstrates that CWC reefs are important to carbon and nitrogen mineralization at the habitat scale.

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This study used a novel approach combining biological, environmental, and ecosystem function data of the Logachev cold-water coral carbonate mound province to predictively map coral framework (bio)mass. A more accurate representation and quantification of cold-water coral reef ecosystem functions such as Carbon and Nitrogen stock and turnover were given by accounting for the spatial heterogeneity. Our results indicate that 45% is covered by dead and only 3% by live coral framework. The remaining 51%, is covered by fine sediments. It is estimated that 75,034–93,534 tons (T) of live coral framework is present in the area, of which ∼10% (7,747–9,316 T) consists of Cinorg and ∼1% (411–1,061 T) of Corg. A much larger amount of 3,485,828–4,357,435 T (60:1 dead:live ratio) dead coral framework contained ∼11% (418,299–522,892 T) Cinorg and <1% (0–16 T) Corg. The nutrient turnover by dead coral framework is the largest, contributing 45–51% (2,596–3,626 T) C year–1 and 30–62% (290–1,989 T) N year–1 to the total turnover in the area. Live coral framework turns over 1,656–2,828 T C year–1 and 53–286 T N year–1. Sediments contribute between 1,216–1,512 T C year–1 and 629–919 T N year–1 to the area’s benthic organic matter mineralization. However, this amount is likely higher as sediments baffled by coral framework might play a much more critical role in reefs CN cycling than previously assumed. Our calculations showed that the area overturns 1–3.4 times the C compared to a soft-sediment area at a similar depth. With only 5–9% of the primary productivity reaching the corals via natural deposition, this study indicated that the supply of food largely depends on local hydrodynamical food supply mechanisms and the reefs ability to retain and recycle nutrients. Climate-induced changes in primary production, local hydrodynamical food supply and the dissolution of particle-baffling coral framework could have severe implications for the survival and functioning of cold-water coral reefs.

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