Invertebrate and periphyton response to flow variability and managed flushing-flows in the Opuha and Opihi Rivers, New Zealand

We compared the influence of managed and unmanaged flow regimes on biota in two adjacent rivers of similar size. First, we monitored hydrology, sediment stability, periphyton biomass and benthic and drifting invertebrates at single sites on the Opuha (dam-controlled flow) and Opihi (natural flow) rivers in New Zealand for 13 months. Second, we monitored the same variables before, during, and after seven flushing-flows (small artificial floods of magnitude 2.5-10x baseflow) at 4 sites below the Opuha Dam (0.3 – 25 km from dam). Periphyton biomass and invertebrate densities at both sites were inversely correlated with flow variability and magnitude. Invertebrate richness was not correlated with flow variability. Flushing-flows reduced periphyton biomass by 47-73% near the dam and 0-40% downstream. Efficiency of periphyton removal was positively related to pre-flush periphyton biomass and negatively related to distance from the dam. Invertebrate density, richness, and composition were not significantly altered by flushes. However, invertebrate community structure varied longitudinally (i.e., proximity to dam was important). Flushing-flows are a cost-effective method for reducing nuisance periphyton below dams. However, flushing-flows without large magnitude events or sediment augmentation may deplete fine sediment and eventually limit the effectiveness of periphyton removal.