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) we re determined in August 1991 and February 1992 for a total of 16 stati ons located along the northeastward transport path of organic matter i n the North Sea. The shape of Si(OH)4 profiles indicated that at sever al stations mass transport is controlled by turbulent diffusion induce d by wave and current mixing in the upper, perturbated centimetres of the sedimentary column. The spatial distribution of silica effluxes cl early reflected the depositional environment, with highest exchange ra tes linked to areas of recent deposition. Outside the main deposition areas, transient deposition of fresh planktonic material is a key proc ess in explaining observed silica effluxes. The temporal variability o f silica effluxes followed the annual cycle of pelagic primary product ion. During August 1991, measured fluxes ranged from 0.18 to 8.90 mmol es Si m(-2) day(-1). Fluxes obtained during February 1992 were decreas ed by a factor between 2 and 8. Fluxes measured before and after inact ivation of fauna with N-2-flushing permitted an estimation of the bioi rrigation to be made. The latter accounted for an enhancement of solut e exchange ranging from 1.1 to 3.4.