Water abstraction in small lowland streams: Unforeseen hypoxia and anoxia effects.

Flow reduction generated by water abstraction can alter abiotic and biotic properties of stream ecosystems. We hypothesized that reducing stream flow will reduce oxygen levels affecting sensitive invertebrates. We experimentally suppressed flow with longitudinal barriers in two lowland streams of mesotrophic and eutrophic status In each stream we fixed an upstream free flowing control and two downstream disturbed stretches without flow: an initial stagnation stretch and a final drought stretch separated from the stagnation by sand bags to force a greater lowering of the water level. Invertebrates were sampled in control and disturbed stretches before and after the experimental setup for 10weeks, and temperature and oxygen were recorded with data loggers. Flow reduction caused a significant decrease in oxygen, resulting in hypoxia (<4mg O2/L) in the stagnation stretches and anoxia (0mg O2/L) in the drought stretches mainly at night, without influencing water temperature. Invertebrate responded with differential sensitivity to flow and oxygen reduction, some indicator taxa declined at 7.3mg O2/L, others at 6.3mg O2/L, while at 5.3mg O2/L many taxa were severely reduced. Flow reduction generated oxygen depletion, reducing rheophilous and oxygen dependent taxa, while favouring tolerant limnophilous taxa with atmospheric respiration. Passive filterers and scrapers were significantly reduced. Our results indicate that flow reduction can cause hypoxia and anoxia in lowland streams and is an unforeseen effect not addressed in the assessment of flow reduction impacts to streams. Further research is required to evaluate if spatially extensive flow reductions and hypoxia result in long-term impairment of stream biodiversity and function.