Responses of benthic invertebrates to abrupt changes of temperature in flume simulations

Sudden instream releases of hypolimnetic water from hydropower plants (i.e. hydropeaking) can cause abrupt temperature variations (i.e. thermopeaking), typically on a daily basis. We investigated the thermopeaking effects on benthic invertebrate drift by simulating abrupt thermal shifts in experimental flumes. We conducted two cold thermopeaking and two warm thermopeaking simulations by quickly cooling the water by 3–4°C during the warm season and by warming the water by 2–3°C during the cold season at a rate of about 2.4 × 10−1°C min−1. This rate is very similar to those associated with hydropeaking waves in rivers in the same watershed as the experimental flumes. Although the achieved changes in temperature were within a tolerability range for benthic invertebrates, their drift increased threefold and fivefold, and twofold and fourfold in the two cold and two warm thermopeaking experiments, respectively. Assemblage composition of drift before and during the experiment differed, indicating that some taxa respond to abrupt thermal alterations. Larvae of Chironomidae, Simuliidae and Baetidae were the most abundant drifting taxa in all experiments during temperature alterations. The drift induced was probably behavioural, given the immediate responses of invertebrates that begun to drift within 2 min from the start of the simulations. This type of drift differs from catastrophic drift that usually occurs as a response to hydropeaking. The two types of drift can occur as distinct events in streams impacted by hydropower plants with high-elevation reservoirs and hypolimnetic releases because the propagation of the discharge and thermal waves are asynchronous, causing the benthic community to undergo two distinct but consecutive impacts. Our results suggest that the long-term effects of thermopeaking are mixed and synergical with those caused by hydropeaking and, on the long term, may alter the longitudinal distribution of benthic communities.