Cold-water coral reefs and communities can be locally important calcium carbonate factories in continental shelf and slope environments, including submarine canyons. Here we present short-term and long-term estimates of coral carbonate production by colonial scleractinian coral communities in the 750-850 m depth range in Guilvinec Canyon, northern Bay of Biscay. Short-term (annual-decadal) estimates were calculated using local coral skeletal biomass, estimated as a product of coral size and abundance from ROV video surveys, a locally generated species-specific regression between coral colony size and wet weight, and published daily or annual percent growth rates for Lophelia pertusa and Madrepora oculata. A long-term (century-millennial) estimate of carbonate accretion for the same reef was derived from a piston core through the same coral community.

Average live colonial scleractinian skeletal biomass in the Guilvinec Canyon coral mounds was 153.9 ± 39.4 g CaCO3 m-2 . Applying published growth rates, the average annual gross carbonate production was 6.85 ± 1.79 g CaCO3 m-2 y-1, range 0-30.2 g CaCO3 m-2 y-1. This carbonate production rate was about one order of magnitude lower than previous estimates from the Norwegian shelf.

A 2011 piston core through the mound was analyzed by CT-scan and subsampled for coral abundance. An age model from previous 14C and U/Th ages of coral fragments in the core yielded a long-term average coral carbonate accretion rate of 78 g CaCO3 m-2 y-1 over the past ∼2150 y, range 40.8 (core-bottom) to 148.5 g CaCO3 m-2 y-1 in the upper half, about 1-2 orders of magnitude lower than previous estimates from other regions.

Low carbonate accretion rates observed in the Guilvinec Canyon mounds could be attributable to recent declines in live coral cover, indicated by low abundance of live corals in ROV surveys from this site, compared to other regions of the Northeast Atlantic.