Relation between sediment-water fluxes of oxygen and silicate and faunal abundance at continental shelf, slope and deep-water stations in the northwest Mediterranean

Abstract

Measurements of sediment oxygen demand (SOD), silicate fluxes, benthic macroand meiofauna and phytopigments were made on continental shelf, slope and deep-water sediments in the northwest Meaiterranean in November-December 1991. The mean SOD (f SD), measured by onboard incubations of sediment cores with overlying water, was 301 + 83 pm01 m-' h-' for the shelf stations, 146 + 53 pm01 m-2 h-' for the slope stations, and 44 f 10 pm01 m-2 h-' for the deep-water stations. The SOD values for the slope and deep-water stations are comparable to earlier reported data from open oceanic sites. Considering the limited data set that we present, substantial downslope transport of organic matter at these stations cannot be concluded. Mean shcate effluxes from the sediment, measured in the same incubation experiments, amounted to 52 ? 14, 48 f 20 and 16 pm01 m-' h-' for shelf, slope and deep-water stations, respectively. Diffusive oxygen and silicate fluxes were calculated from pore-water profiles with Fick's first law of diffusion The irngatlon flux was defined as the difference between incubation and diffusive flux. The oxygen irngation flux contributed on average 39 ? 10%. 17 f 15% and 5 + 5% to the oxygen incubation flux for the shelf. slope and deep-water stahons. respectively, and showed a strong positive relation with macrofaunal density, illustrating the importance of macrofaunal activities on sediment-water solute exchanges. In contrast to the oxygen irrigation fluxes, the silicate irrigation fluxes showed less distinction between shelf and slope stations and contributed on average 60 + 6 %, 61 + 9 % and 19 % to the silicate incubation fluxes for the shelf, slope and deep-water stations, respectively. The relation between silicate irrigation flux and macrofaunal density was not significant. Another indication for the lack of organic matter transport across the slope was the monotonic decrease of faunal density, biomass and phytopigments w t h increasing depth. Only at the upper slope station were traces of a past deposition event found. Below this depth phytopigments were below detection level. The absence of phaeopigments in the near-bottom water and their high concentration in the shelf sediments indicated little resuspension of sediment.