Benthic Oxygen and Nitrogen Exchange on a Cold-Water Coral Reef in the North-East Atlantic Ocean

Cold-water coral (CWC) reefs are distributed globally and form complex three-dimensional structures on the deep seafloor, providing habitat for numerous species. Biogeochemical cycling and community metabolism of CWC reefs have mostly been studied by targeting sub-sampled organisms (e.g. corals, sponges). 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) for a total of 69 h, 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). Six box cores from two coral mounds were incubated onboard for ~24h to quantify O₂ and DIN fluxes. The 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⁺.Parallel measurements of the DIN flux in the BC incubations also revealed intensified benthic release of NH₄⁺, which gets partly nitrified, 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). 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.