K. Booij et al., MEASURING THE FLUX OF OXYGEN TO A MUDDY SEDIMENT WITH A CYLINDRICAL MICROCOSM, Netherlands journal of sea research, 32(1), 1994, pp. 1-11
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.