Effects of bioirrigation on the spatial and temporal dynamics of oxygen above the sediment–water interface

Burrow ventilation by tube-dwelling benthic animals affects solute exchange between sediments and water by 2 means. Drawing of O-2-rich water into the burrow increases O-2 availability in the sediment and stimulates biogeochemical and microbial processes, whereas flushing of the burrow creates a 3-dimensional flow field above the burrow, which induces mixing. Previous studies have revealed the role of the diffusive boundary layer (DBL) thickness on the exchange of solutes between the sediment and overlying water. Mapping the O-2 gradient within the DBL is a challenging task in the presence of benthic faunal activities. We used a novel lifetime-based laser induced fluorescence (LIF) technique that enables unobstructed observations of spatial and temporal O-2 dynamics above burrows inhabited by midge larvae (Chironomus plumosus). We observed instantaneous plumes of O-2-depleted water released from the outlet of the burrows and drawdown of O-2-rich water above the inlet caused by peristaltic pumping of C. plumosus larvae. Vertical O-2 gradients changed dynamically during burrow ventilation relative to in a control tank without animals. The advective transport of O-2 above the opening caused by burrow ventilation degraded the O-2-concentration gradient. For a range of larvae densities that is frequently observed in ponds and lakes, the advective transport caused by burrow ventilation was the dominant transport mechanism.