SPATIAL AND TEMPORAL VARIABILITY OF BENTHIC SILICA FLUXES IN THE SOUTHEASTERN NORTH-SEA

Sediment-water exchange fluxes and pore water profiles of Si(OH)(4) were determined in August 1991 and February 1992 for a total of 16 stations located along the northeastward transport path of organic matter in the North Sea. The shape of Si(OH)4 profiles indicated that at several stations mass transport is controlled by turbulent diffusion induced by wave and current mixing in the upper, perturbated centimetres of the sedimentary column. The spatial distribution of silica effluxes clearly reflected the depositional environment, with highest exchange rates linked to areas of recent deposition. Outside the main deposition areas, transient deposition of fresh planktonic material is a key process in explaining observed silica effluxes. The temporal variability of silica effluxes followed the annual cycle of pelagic primary production. During August 1991, measured fluxes ranged from 0.18 to 8.90 mmoles Si m(-2) day(-1). Fluxes obtained during February 1992 were decreased by a factor between 2 and 8. Fluxes measured before and after inactivation of fauna with N-2-flushing permitted an estimation of the bioirrigation to be made. The latter accounted for an enhancement of solute exchange ranging from 1.1 to 3.4.