EFFECT OF EXPERIMENTALLY INCREASING CONCENTRATIONS OF SUSPENDED SEDIMENT ON MACROINVERTEBRATE DRIFT

The effect of artificially elevating concentrations of suspended sediment on macroinvertebrate drift was studied in the Acheron River, 100 km north-east of Melbourne. Two experimental channels were established in the stream, and suspended sediment was introduced into one channel over a period of 1.5 h. The second channel was left undisturbed as a control. The concentration of suspended sediment was altered every 15 min, rising and falling to imitate concentrations reported during natural flood events. Drift was collected from two nets at the downstream end of each channel during each 15-min period. Collections were made for three 15-min periods before the introduction of the sediment and for three periods after the release. The addition of suspended sediment at a mean concentration of 133.4 mg L-1 over a 15-min period (compared with around 20 mg L-1 in the control channel) resulted in a sevenfold increase in the total number of drifting invertebrates. At lower concentrations (both before and after this peak concentration), drift densities were more similar to prerelease conditions. The number of drifting taxa also showed an increase during the period of high release. Although the experiment did not conform strictly to a full experimental design, the results indicated that there may be a threshold level of suspended sediment that initiates macroinvertebrate drift, and this experiment represents an appropriate starting point for future investigations.