RELATION BETWEEN SEDIMENT-WATER FLUXES OF OXYGEN AND SILICATE AND FAUNAL ABUNDANCE AT CONTINENTAL-SHELF, SLOPE AND DEEP-WATER STATIONS IN THE NORTHWEST MEDITERRANEAN

Measurements of sediment oxygen demand (SOD), silicate fluxes, benthic macro- and meiofauna and phytopigments were made on continental shelf , slope and deep-water sediments in the northwest Mediterranean in Nov ember-December 1991. The mean SOD (+/- SD), measured by onboard incuba tions of sediment cores with overlying water, was 301 +/- 83 mu mol m( -2) h(-1) for the shelf stations, 146 +/- 53 mu mol m(-2) h(-1) for th e slope stations, and 44 +/- 10 mu mol m(-2) h(-1) for the deep-water stations. The SOD values for the slope and deep-water stations are com parable 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 silicate effluxes from the sediment, measured in the same incubation experimen ts, amounted to 52 +/- 14, 48 +/- 20 and 16 mu mol m(-2) h(-1) for she lf, slope and deep-water stations, respectively. Diffusive oxygen and silicate fluxes were calculated from pore-water profiles with Fick's f irst law of diffusion. The irrigation flux was defined as the differen ce between incubation and diffusive flux. The oxygen irrigation flux c ontributed on average 39 +/- 10%, 17 +/- 15% and 5 +/- 5% to the oxyge n incubation flux for the shelf, slope and deep-water stations, respec tively, and showed a strong positive relation with macrofaunal density , illustrating the importance of macrofaunal activities on sediment-wa ter 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 t he lack of organic matter transport across the slope was the monotonic decrease of faunal density, biomass and phytopigments with increasing depth. Only at the upper slope station were traces of a past depositi on event found. Below this depth phytopigments were below detection le vel. The absence of phaeopigments in the near-bottom water and their h igh concentration in the shelf sediments indicated little resuspension of sediment.