Stream Ecosystem Response to Multiple Experimental Floods from a Reservoir

The effects of multiple experimental floods in the same year on the ecology of a river (River Spöl) below a large reservoir were investigated. In the flood year (2000), three floods of different magnitude (10 m/s on 15 June, 25 m/s on 5 July, and 10 m/s on 10 August) were implemented, each lasting 7–8 h. Regulated baseflow discharge was maintained at less than 2.5 m/s. Samples of macroinvertebrates, periphyton, water chemistry, and seston were collected for one year (1999) before and then immediately before and after each flood in the Spöl and in an adjacent reference tributary (Val da l’Aqua). Samples also were collected periodically between floods to evaluate temporal response patterns. During two of the floods, suspended sediments and seston were measured at 30 min to 1 h intervals. Suspended sediment and seston reached peak concentrations within the first hour of each flood, then decreased within about 2 h. Peak seston and suspended sediment concentrations during each flood decreased by an order of magnitude from the July flood to the August flood. Two-way ANOVA indicated significant flood effects on periphyton standing crops expressed as either chlorophyll a or ash-free dry mass (AFDM). Recovery of periphyton following each flood appeared to be progressively slower after the later (July and August) floods. Seston chlorophyll levels also were lower following each flood, being significantly correlated to periphyton biomass. Two-way ANOVA indicated that the floods significantly decreased macroinvertebrate taxonomic richness, density, and biomass. Taxa that decreased from the floods included the turbellarian Crenobia alpina and the gammarid Gammarus fossarum. Taxa that showed a fast recovery to the floods included the Chironomidae, Simuliidae, and Baetidae. Our data indicate a strong temporal effect of flood disturbance on benthic assemblages (periphyton and macroinvertebrates) below reservoirs that partly reflects species-specific life histories and traits, and the cumulative effects of earlier floods. Copyright # 2004 John Wiley & Sons, Ltd. key words: Switzerland; periphyton; flow regime; Gammarus; Fontinalis; temporal