A preliminary investigation on effects of fluctuating flows on invertebrates of the Hawea River, a large regulated river in New Zealand

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Abstract
An experiment with flow changing at 3, 9, and 18 m3 s‐1 h‐1 on consecutive days failed to produce any measurable effect on benthic invertebrates in the regulated Hawea River, New Zealand, but did result in increased numbers of drifting chironomid larvae, trichopteran larvae, and oligochaetes compared to days when flow was stable. Drift densities were at least as high on the day flow changed at 3 m3 s‐1 h‐1 (minimum flow 15 m3 s‐1, maximum flow 30 m3 s‐1) as on subsequent days when flow changed at 9 and 18 m3 s‐1 h‐1 (minimum flows both 15 m3 s‐1, maximum flows 60 and 105 m3 s‐1 respectively) indicating that rapid rates of change of flow may not necessarily cause more benthic invertebrates to enter the drift than slow rates of change. Fluctuating flows resulted in more animals entering the drift at a site 5 km below a control dam than at a site 0.8 km below the dam. Drift of non‐invertebrate organic matter (mainly periphyton) showed similar patterns to invertebrate drift. It is suggested that most of the invertebrates displaced by the flow fluctuations were associated with periphyton in the river, and as such, these taxa may have been more prone to being swept away during flow increases than other invertebrate taxa.