Water abstraction impacts stream ecosystem functioning via wetted-channel contraction

Water abstraction is rapidly increasing worldwide in order to respond to escalating demands for water, food and energy. Abstraction can alter the hydrological regime of streams and rivers, reduce in-stream habitats, change water quality and affect fluvial communities. It can also impair ecosystem functioning, although this aspect has been seldom assessed. We experimentally tested the effect of water abstraction on stream functioning in a headwater mountain stream using a before and after control-impact (BACI) design. We quantified changes in water physicochemical properties, biofilm biomass and activity (exo-enzymes and metabolism), and ecosystem processes (nutrient cycling, organic matter retention and breakdown). Water abstraction did not affect water physicochemical properties, but reduced the biomass and the alkaline phosphatase activity of biofilm per surface unit, the areal uptake of nutrients, and the travel distance of organic matter. It did not affect the biofilm metabolism and breakdown of submerged leaf litter, but decreased the total breakdown. The impacts were larger when analysed per unit of channel length. All variables, except benthic chlorophyll-a, were significantly reduced by abstraction, due to the contraction of the wetted channel. The decrease in the biomass and activity of biofilm, as well as in nutrient uptake and total organic matter breakdown observed in this study could increase nutrient concentration in downstream reaches and reduce the energy transfer to higher trophic levels. Ecosystem services of permanent headwater streams are likely to be compromised with the reduction of wetted-channel width.