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Cold-water coral (CWC) reefs are distributed globally and form complex three-dimensional structures on the deep seafloor, providing habitat for numerous species. Here, we measured the community O2 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 non-invasive 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 O2 flux from a total reef area of up to 500 m2, with mean O2 consumption rates ranging from 11.6 ± 3.9 to 45.3 ± 11.7 mmol O2 m–2 d–1 (mean ± SE). CWC reef community O2 fluxes obtained from the BC incubations ranged from 5.7 ± 0.3 to 28.4 ± 2.4 mmol O2 m–2 d–1 (mean ± SD) while the O2 flux measured by BC incubations on the bare sediment reference site reported 1.9 ± 1.3 mmol O2 m–2 d–1 (mean ± SD). Overall, O2 fluxes measured with AEC and BC showed reasonable agreement, except for one station with high habitat heterogeneity. Our results suggest O2 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 NH4+. Our data indicate that the amount of living corals and dead coral framework largely contributes to the observed variability in O2 fluxes on CWC reefs. A conservative estimate, based on the measured O2 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.

During HERMES cruises in spring 2006 and 2007 to a cold-water coral (CWC) reef at the border between Norway and Sweden, we studied effects of the reef on the quality (C/N ratio) of suspended particles. The Tisler reef is situated on top of a southeast-northwest-oriented sill forming the connection between the Kosterfjord and the open Skagerrak. In spring 2006, two benthic landers equipped with sediment traps, current meters, and turbidity sensors were deployed at the northwest and southeast edges of Tisler reef. The unidirectional northwest currents during the deployment enabled comparison of the quality of particles before and after crossing the reef. Before the onset of the 2006 spring bloom, particles trapped at the northwest reef edge had a more degraded character than those at the southeast edge. When the spring bloom swamped the study area with fresh phytoplankton, cross-reef distinctions in particle quality disappeared. In spring 2007, near-bottom suspended particles were collected with submersible pumps during one part of a tidal cycle. Analysis revealed a difference in composition of suspended particulate matter along the reef, with highest quality at the northwest edge where particles were delivered, and lower quality at the middle and southeast edge of the reef. Results from both cruises show that particles passing over Tisler reef are subject to preferential removal of nitrogen, indicating that even a relatively small reef such as Tisler has an impact on the biochemistry of its environment. This evidence suggests that, globally, CWC reefs are hotspots of mineralization activity in the ocean.