Effects of fine suspended sediment releases on benthic communities in artificial flumes

The Italian Alps feed a large number of reservoirs for hydropower production, which are losing storage capacity due to natural inflow of sediment of different origin (alluvial, glacial). Local government and local environmental agencies authorize periodical sediment flushes with a mandatory release regime when such measure is technically feasible. Management of reservoirs often includes fine sediment pulses, which cause several ecological impacts on downstream water bodies. We conducted a set of simulations in five semi artificial flumes naturally fed by an unimpacted Alpine stream (Trentino region, NE Italy), to: i) identify possible thresholds of concentration of fine suspended sediment inducing drift in the benthic community and, ii) assess the dynamics and intensity of the drift responses in the dominant taxa. The results can help to identify the least impacting release management practices. Sediment pulses were simulated by adding fine material of known concentration to the upstream end of the flumes. The benthic organisms drifting from the whole flume were collected by filtering the whole outflow for consecutive short time intervals. We tested four different concentration values, i.e. 10x,100x,250x,500x the base concentration of 4 NTU, and we repeated the simulations in two periods: July, when the community is composed mainly of young larval instars and the sediment wave lasted 10 minutes, and October, when later larval stages are dominant and the wave lasted 20 minutes. In July, the maximum concentration induced a significantly higher drift response than the three lower ones. In October, even if the sediment wave was twice as long as July one, drift responses where lower, and only the responses to the highest and lowest concentrations differed significantly. In our simulation, the only possible cause for the observed increase in drift was the sediment in the suspended phase, as the deposition of sediment was negligible, and discharge did not increase, thus allowing disentangling the effects of increasing suspended sediment from those of increasing discharge. Our results, although restricted to Alpine streams, and with a seasonal bias which needs further investigations, allowed to assess the dynamics of benthic community and taxa, rather than assessing the resulting effects on the benthic community (as it occurs with the studies based on a before‑after sampling design), and will possibly provide indications towards the future management of sediment releases in Alpine streams.