Diurnal variation in the oxygen uptake of river sedimentsin vitro by use of continuous flow-through systems

Abstract

The diurnal exchange of oxygen, over the sediment-water interface, was measured on ‘undisturbed’ sediment cores incubated on the river bank of three reaches in the river Susaa, Denmark. The incubation chambers were equipped with a double pumping system, driving an internal and an external flow, respectively. The internal flow created a unidirectional flow over the sediment surface, with the same velocity as the natural river flow. River water was continuously pumped through the incubation chambers (external flow). Magnetic valves alternately shifted the inlet and outlet water through a registration chamber equipped with an oxygen probe and a temperature transducer (A/D 540, integrated circuit). The potential was amplified and registered every minute on a tape recorder.

The rate of exchange of oxygen in the chambers was described by the following equation:

$${\text{r - exch(t) = }}\frac{{{\text{flow(t)}}}}{{{\text{volume}}}}{\text{[inlet(t) - outlet(t)] - }}\frac{{{\text{d[outlet(t)}}}}{{{\text{dt}}}}$$

Various methods for the numerical solution of the differential equation were used. Due to a small amount of ‘noise’ on the probes, the cubic spline method gave an unrealistically high variation in the oxygen uptake. A better description was obtained, when a Fourier transformation of the inlet and outlet concentrations of oxygen was made. When 15 Fourier coefficients were used, the r-value of the model was better than 0.996.

On permanently dark sediment samples, a hysteresis effect between the oxygen exchange rate and the oxygen concentration and temperature appeared. The exchange rates were lower in periods with increasing oxygen concentration. A minimum in the exchange rate was obtained, before the minimum in the concentration of oxygen and temperature was found. This type of hysteresis could be explained by equilibration in connection with the transient state conditions. The hysteresis was probably enlarged by shifts in the size of the aerobic layer in the sediment. The importance of the hysteresis in oxygen balance models for rivers is discussed.