MEASURING THE FLUX OF OXYGEN TO A MUDDY SEDIMENT WITH A CYLINDRICAL MICROCOSM

We have examined the effects of changing the flow velocity and the oxy gen concentration in the water overlying a muddy sediment on the flux of oxygen across the sediment-water interface and on the distribution of oxygen within the pore water. The experiment was carried out on an intertidal sediment from the western Wadden Sea, using a cylindrical m icrocosm with a calibrated flow regime. Steady-state and transient-sta te models were used to estimate the values of the effective diffusion coefficient for oxygen in the pore water. Increasing the flow velocity caused a significant though small increase in the oxygen concentratio n in the pore water, but had little effect on the concentration gradie nt at the sediment-water interface. The concentration gradient in the boundary layer was too small at any of the flow velocities to account for the oxygen flux into the sediment via molecular diffusion. This is ascribed to a pressure gradient which exists in rotating flows, disru pting the diffusive boundary layer and augmenting the flux via advecti on. Model calculations indicate that about 25% of the flux can be attr ibuted to irrigation by burrowing organisms, but in contrast to previo us results with sandy sediments, irrigation of the pore water caused b y the radial pressure gradient can be considered to be negligible. The effective diffusion coefficient ranged from 4.10(-9) m2.s-1 at a dept h of 1 mm below the sediment-water interface to 1.10(-9) M2.s-1 deeper in the sediment. These-estimates are within a factor of 1 to 3 of the molecular diffusion coefficient for oxygen, which is surprisingly clo se in view of the high numerical densities of meiofauna and macrofauna in this sediment.