Abstract
Water abstraction for irrigation during seasonal low discharge periods alters the natural flow regime of a river. The response of macroinvertebrates to a significantly altered flow regime due to summer water abstraction for irrigation was assessed. Macroinvertebrate communities upstream and downstream of three major water abstraction points along the Manuherikia River were sampled during the irrigation season, and then immediately after and three weeks after water abstraction had ceased and a natural flow regime had been restored. The process of macroinvertebrate community recovery from water abstraction was assessed longitudinally along the river, and also spatially within each site by examining re colonisation of re-wetted substrates along the edge of the stream channel. Benthic invertebrates were identified to the lowest possible taxonomic level to ensure a sensitive assessment of the impacts of water abstraction. Following the cessation of abstraction and the resumption of a natural higher discharge flow regime, the densities of macroinvertebrates across all sites decreased, with the densities of macroinvertebrates being lowest downstream of the three water abstraction points. Densities remained the highest at the most upstream abstraction point and were relatively lower at the two most downstream abstraction points along the river. Recovery from water abstraction in the permanently wetted channel did not occur within the time period of the study, though the densities at the most upstream site showed more resilience than the downstream sites along the river. The densities of univoltine taxa were relatively lower downstream of all takes relative to macroinvertebrate densities upstream of abstraction points. Immediately following the conclusion of the summer water abstraction season, the benthic invertebrate densities in newly wetted channel edge downstream of the water takes were relatively lower than channel edges upstream of the abstraction points, although three weeks later, macroinvertebrate densities were similar. However, the edge communities downstream were dominated by multivoltine taxa rather than longer-lived univoltine taxa. The lag in the recovery of stream communities following the resumption of a natural discharge regime appears to be caused by the slower recovery of univoltine species with time-restricted reproductive seasons that are unable to numerically respond to the increase in available habitat. The information gained from this study will contribute to knowledge on the river ecosystems in New Zealand and elsewhere, and provide information for future river management in New Zealand.