Spatial variability in oxygen and nutrient fluxes at the sediment-water interface on the continental shelf in the Gulf of Lions (NW Mediterranean)

The spatial variability of oxygen and dissolved nutrient exchanges across the sediment-water interface was studied on the continental shelf in the Gulf of Lions (NW Mediterranean Sea). Replicate sediment cores were sampled at nine stations (64-162 m depth) along two lines parallel to the coast during two cruises in March and June 1998. Sediment-water exchanges were measured using the core incubation technique. Surficial sediments, bottom water and interstitial water characteristics were also described. Fluxes of oxygen (3.72-8.83 mmol m-2 d-1), nitrate (0.026-0.283 mmol m-2 d-1), ammonium (-0.022 to 0.204 mmol m-2 d-1), nitrite (-0.034 to 0.002 mmol m-2 d-1), phosphate (-0.007 to 0.029 mmol m-2 d-1) and silicate (0.504-1.656 mmol m-2 d-1) were generally quite low. This has to be related to the oligotrophy of the Mediterranean Sea. Fluxes showed a weak spatial variability, and a significant correlation could be established between oxygen fluxes and the organic carbon content of surficial sediments. A general increase in ammonium, nitrate and phosphate release was also observed towards the coast and the mouth of the Rhône River. Nitrite uptake and silicate release showed high variability between cruises, and the change in silicate fluxes depended mainly on the location on the eastern or western part of the continental shelf. Over the whole continental shelf, calculated sediment mineralization rate represents 342 kt a-1 of organic carbon. The annual release from the sediments approximates to 14.1 kt dissolved inorganic nitrogen, 2.9 kt P, and 165 kt dissolved silica, which represent, respectively, an amount close to 5%, 7% and 28% of the nutrient requirements for primary production. When compared to nutrient inputs from the Rhône River, sediments appear to play a significant role in the biogeochemical cycles of the Gulf of Lions system, mainly for inorganic phosphorus and dissolved silica