Small stream ecosystems and irrigation : an ecological assessment of water abstraction impacts : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand

Abstract

Small streams are often used for small-scale water abstractions, but the effects of these water abstractions on the instream environment, invertebrate communities and ecosystem functioning of small permanent streams is poorly understood. This research extends current knowledge by surveying existing water abstractions and completing flow manipulation experiments in the field. Reduced discharge often decreases water velocity, water depth, and wetted channel width and can increase sedimentation, modify the thermal regime and alter water chemistry. In a survey of sites upstream and downstream of existing water abstractions, I found that downstream sites had higher densities of invertebrates, but fewer taxa sensitive to low water quality compared with upstream sites. There were greater differences in physicochemical characteristics such as velocity and conductivity and in invertebrate communities between upstream and downstream sites on streams where a larger proportion of total discharge was abstracted. Using before-after, control-impact (BACI) designed experiments, weirs and diversions were created to experimentally decrease discharge by over 85% in each ot three small streams, ranging from pristine to low water quality. The response of invertebrates to short-term (one-month) discharge reduction was to accumulate in the decreased available area, increasing local invertebrate density. After a year of reduced flow, the density of invertebrates and percentage of mayflies, stoneflies and caddisflies decreased at the pristine site, whereas only taxonomic richness decreased at the mildly polluted stream. Reduced discharge had no affect on the invertebrate community at the stream with the lowest water quality. Reduced discharge had little influence on leaf decomposition rates, but distances travelled by released coarse particulate organic matter (CPOM) increased with increasing discharge. The effects of reduced discharge on primary production were not consistent between streams. Overall, the severity (magnitude/duration) of flow reduction appeared to influence invertebrate responses to water abstraction although the outcomes of water abstraction were dependent on the invertebrate assemblage present in each stream.