Long-term ecological responses of the River Spöl to experimental floods

Abstract

The river Spöl flows from Livigno reservoir on the Swiss–Italian border in the central Alps. Flow regulation since 1970 has decreased and stabilized the mean annual discharge of 8.6 to 1.0 m3/s (winter: 0.55 m3/s, summer: 2.5 m3/s). Thirty experimental floods were released between 2000 and 2016 to improve the ecology of the river. A 100-m reach ~2.3 km downstream of the dam was used to monitor changes in water physicochemistry, periphyton, transported and benthic organic matter, and macroinvertebrates. Fish redds (native Brown Trout) were counted each autumn along the same stretch (~2.6 km) of river. Most physicochemical variables displayed strong seasonal cycles unrelated to the flow program, but N, P, and water temperature increased significantly over the 18-y study. Seston decreased after each high flow, but was seasonal and increased over the study. Periphyton biomass decreased after each high flow to low levels and increased between high flows. Benthic organic matter decreased 2× over the study but increased between floods. Macroinvertebrate densities decreased over the study. High flows reduced macroinvertebrate densities to <10,000 individuals (ind)/m2, with densities usually increasing to ~10,000 to 20,000 ind/m2 between floods. Floods reduced taxonomic richness with increases between floods. Ordination showed temporal shifts in macroinvertebrate assemblages over the study. Mayflies and stoneflies responded positively and chironomids and gammarids negatively to the floods over the study. Fish redds increased from 58 in 1999 to >200 by 2003. Redd counts decreased to <30 after an accidental sediment spill in 2013 but rebounded to 153 redds in 2016. Macroinvertebrate assemblages recovered within months after the spill. Long-term monitoring was essential for elucidating ecosystem dynamics and evaluating biological responses to pulse disturbances, such as the sediment spill, during the study.