The carbon cycle at the ocean margin in the northern Gulf of Biscay

The aim of this paper is to provide a synthesis of several multidisciplinary studies performed during the OMEX I project mostly presented in this volume. Our final goal is to construct a carbon cycle for the shelf break of the northern Gulf of Biscay. This margin is characterized by a steep slope adjacent to a broad continental shelf, under limited influence of terrestrial input. Remote sensing images indicate the occurrence of cold water masses at the shelf break induced by complex physical processes related to the presence of the bathymetric discontinuity and the existence of frequent heavy storms. The primary production at the ocean margin remains however moderately high, around 200 g C m−2 a−1, with a f-ratio of 0.5. Most of the resulting new production is respired either in the water column below the euphotic zone (∼50 g C m−2 a−1) or at the sediment-water interface (∼20 g C m−2 a−1). According to sediment trap measurements, the difference between new production and respiration (∼30 g C m−2 a−1) may be exported to the slope and the open ocean. OMEX I benthic studies have shown that organic carbon (∼0.1 g C m−2 a−1) is not accumulated significantly in sediments and that no depocenter can be identified in this margin area. As expected, the primary production and thus the fluxes of organic carbon decrease with distance away from the shelf break and do not exceed ∼140 g C m−2 a−1 at a distance of 150 km from the shelf break. Despite some lateral inputs, there is again no significant accumulation of organic carbon in the sediments of the adjacent slope or abyssal plain. A nitrogen cycle is also described, based on the carbon cycle, demonstrating the importance of vertical mixing at the margin.