Effects of substratum restoration on salmonid habitat quality in a subalpine stream

Stream substratum restoration is a widely applied tool to improve spawning habitat quality for salmonid fishes. However, there is a lack of studies which comprehensively assess effects of the restoration on site, as well as on downstream habitats. Our study addressed effects at both locations and compared abiotic (analyses of texture, penetration resistance, oxygen concentration, redox, nitrite, nitrate, ammonium, pH, electric conductivity, temperature) with biotic (depth-specific macroinvertrebrate abundance and diversity, brown trout hatching success) indicators before and after excavation of the substratum in a highly colmated brown trout spawning site. Strong improvements of hyporheic water conditions (increased oxygen supply and redox potential, reduced concentrations of nitrite and ammonium) as well as ~50 % reductions of substratum compaction and fine sediment content were observed 1 day after the restoration measure. Improvements of habitat quality were still detectable 3 months after treatment. Consequently, the hatching success of Salmo trutta eggs increased from 0 % to 77 % after the restoration. Short-term decrease of macroinvertebrate abundance (from 13.1 to 3.9 macroinvertebrates/kg substratum) was observed within the hyporheic zone of the restoration site, but after 3 months, the number of taxa increased from 13 to 22 taxa and abundance reached 17.9 macroinvertebrates/kg. Significantly increased fine sediment deposition was detected within 1 km downstream of the restoration site and may negatively affect these habitats. Trade-offs between positive effects at restored sites and negative effects in downstream habitats need to be considered for a comprehensive evaluation of stream substratum restoration.