IMPACT OF BOUNDARY-LAYER FLOW VELOCITY ON OXYGEN UTILIZATION IN COASTAL SEDIMENTS

Small pressure gradients generated by boundary flow-topography interac tions cause advective pore water flows in permeable sediments. Advecti ve pore water exchange enhances the flux of solutes between the sedime nt and the overlying water, thus generating conditions for an increase d utilisation of oxygen. We compared a less permeable (k = 5 x 10(-12) m(2)) with a permeable sediment (k = 5 x 10(-11) m(2)) typical for co astal and shelf sediments. Total oxygen utilisation (TOU) in incubated sediment cores was measured in 10 laboratory experiments using recirc ulating flow tanks (33 runs). TOU was a function of now velocity in pe rmeable sediment where advective pore water now occurred. TOU increase d with the increasing volume of sediment flushed with oxygenated water . We found that TOU increased by 91 +/- 23% in coarse sand when now in creased from 3 to 14 cm s(-1) (38 mounds m(-2) height 10 to 30 mm, now measured 8 cm above the sediment). Addition of fresh algal material c aused a stronger stimulation of TOU in the coarse sand than in the fin e sand (4 additional flume runs). After the addition, intensive oxygen consumption reduced the oxygen penetration depth in the advectively f lushed zone of the coarse sediment. However, counteracting this proces s, advective flow maintained an oxic sediment volume still larger than that in the less permeable sediment. Flow-enhanced oxygen utilisation is potentially effective in permeable beds of coastal and shelf regio ns, in contrast to the situation in cohesive sediments limited by pred ominantly diffusive oxygen supply.