Fluxes and budget of organic matter in the benthic boundary layer over the northwestern Mediterranean margin

Abstract

Sediment traps were deployed at depths of 26 and 645 m at two stations on the continental margin of the Gulf of Lions (northwestern Mediterranean). During the same period, surficial sediments were sampled by box corer. The material collected by bottom sediment traps and in corresponding surface sediments was analysed for total organic carbon, hydrolysable organic carbon, nitrogen, sugars, amino acids and lignin-derived compounds. Seasonal variations in organic inputs and the difference between particles from bottom layers and sediment were compared. For the continental shelf station, the annual averages of organic compound fluxes were found to be: 552 mg m −2 d −1 (orgC), 183 mg m −2 d −1 (N), 283 mg m −2 d −1 (hydrolysable orgC), 181 mg m −2 d −1 (Ceq. glucose) and 478 mg m −2 d −1 (amino acids). These values would have to be reduced by half if the large fluxes of autumn, due to resuspension during storm events, were excluded. For the slope, the average annual fluxes were evaluated as: 92.7 mg m −2 d −1 (orgC); 9.4 mg m −2 d −1 (N); 74.1 mg m −2 d −1 (hydrolysable orgC); 11.8 mg m −2 d −1 (Ceq.glucose); and 68.2 mg m −2 d −1 (amino acids). The values obtained for material trapped over the shelf are 4–7 times (orgC and amino acids) and 15–19 times (sugars and nitrogen) higher than for the slope. In contrast, the content in organic compounds of surficial sediments on the slope is 2–3 times higher than that of the shelf deposits. Budgets of orgC transformation at the sediment-water interface were based on calculations which include bottom orgC fluxes, sedimentation rates and orgC content for the first centimetre of deposits. For the continental shelf area, 5.3 g m −2 y −1 have accumulated and 16.7 g m −2 y −1 are mineralized. For the canyon and adjacent slope, the figures are 0.4 and 0.6 g m −2 y −1, respectively. Over the upper adjacent slope, the major part of organic matter is transported by advective processes, which contribute to the sediment interface, especially by chanelling through the canyon axis. Advective inputs of fresh organic material are maximized in late winter-early spring. Nearly 50% of the continental shelf organic matter is assumed to be exported from the shelf, to the adjacent slope.