BEDFORM MOBILITY AND BENTHIC OXYGEN-UPTAKE

A non-consumptive probe was used to measure DO concentrations in mobile sand dunes of the Tarawera River, New Zealand, which receives pulp-mill effluent. Net oxygen depletion rates were low in the top 5-10 cm (occasionally 10-20 cm) of the sediments which indicates that turbulent dispersion transports oxygen to these depths. Below 10-20 cm the net depletion rate was independent of depth which implies that the oxygen resupply rate was negligible. Minimum DO concentrations were invariably found at the upstream (scouring) face of the dunes indicating that advection through the sediments was negligible. A simple analytical model showed that dune turnover explained 30% of the observed river deoxygenation rate. Benthic uptake arising from dune turnover was uniform down the river because an increase in dune height along the river was offset by a decrease in river DO. This helps explain why the river deoxygenation rate is uniform. A fragile layer of fluidized sediment covered the crests of each dune. It was inferred that oxygen uptake in this layer explained 60% of the river deoxygenation rate but an improved understanding of oxygen dynamics within the fluidized layer is required. In other mobile-bed rivers it is likely that dune turnover will be important in determining the benthic mass transfer rate.