Effects of a multi-year experimental flood regime on macroinvertebrates downstream of a reservoir

We examined the response of stream macroinvertebrates to a multiple-year experimental flood regime downstream of a large reservoir. Benthic samples were collected from the River Spol prior to the initial flood (1999) and at periodic intervals before and after eight floods from 2000 through 2002. Three artificial floods occurred each in 2000 and 200 1, and two floods were implemented in 2002. We also sampled macroinvertebrates in an adjacent tributary (Val da l'Aqua) on the same dates as in the Spol to assess the natural temporal variability in assemblage structure. The regulated baseflow discharge in the Spol was < 2.5 m(3)/s, whereas the floods ranged from 10 m(3)/s to over 25 m(3)/s with some flood peaks reaching > 40 m(3)/s for a short period. Repeated measures ANOVA indicated that the floods significantly reduced macroinvertebrate densities in the Spol, although recovery to pre-flood densities occurred within a matter of weeks to densities found in 1999. A principal components analysis revealed that assemblage composition shifted in response to the recurring floods, first from 1999 to 2000 and then from 2000 to 2001/2. Taxa that decreased in abundance due to the floods included the Gammaridae (Gammarus fossarum) and Turbellaria (Crenobia alpina). Taxa that increased in abundance included Baetidae, Chironomidae, and Simuliidae. Some Plecoptera, Trichoptera, and Heptageniidae that were negatively impacted by the floods in 2000, subsequently increased in abundance. Our data suggest that the response of macroinvertebrates to experimental floods occurs over a period of years rather than months, as species composition adjusts to the new and more variable habitat template. Future changes are expected as additional species begin to colonize the river from adjacent sources. The results clearly show that the experimental flood regime should be maintained if resource managers wish to sustain the development of a more natural macroinvertebrate assemblage.