Effects of bioturbation in oxic and hypoxic conditions: a microcosm experiment with a North Sea sediment community

Sediment cores of 20 cm diameter contammg the natural benthic fauna were subjected to low oxygen conditions in a laboratory microcosm system. After several days of oxic conditions ('oxic stage') the oxygen content of the water was reduced to 25% saturation for 15 d ('hypoxic stage'), followed by a 'reoxygenation stage'. Effective solute transport rates were calculated using measurements with the conservative tracer ion bromide. Profiles of oxygen and SC02 were measured and molecular diffusive as well as effective fluxes, accountmg for effective solute exchange, were calculated. The overall response of the benthic community was to compensate for low oxygen content of the overlying water by increased pumping activity. On average, effective diffusion coefficients (Den} were 3 times higher in hypoxia than under oxic conditions. Den reached 1.5 x cm2 ssl, a value 30 times that of molecular diffusion. During hypoxia we observed low molecular diffusive O2 flux, higher effective O2 flux, as well as an increase in SC02 within the sediment. We interpret this as a shift of transport away from diffusion within the bulk sediment interstices (oxic conditions) to the advective transport pathways along burrows during hypoxia. This facilitates fast transport of oxygen and bromide along burrows and contrasts with the slower transport of C 0 2 from the interstices governed by molecular diffusion. In this transient situation calulations based on gradients result in an unrealistic molar ratio of fluxes (C02 /02) as high as 11.