Effects of flow and temperature alterations on benthic invertebrates in flume simulations

Rivers downstream of the water release point from hydropower plants often exhibit rapidly fluctuating flow regimes related to the switching on and off of turbines for power generation, i.e. hydropeaking (Gore and Petts [1]), and such daily alteration propagate for several km downstream. In the case of hydropower plants with high elevation storage reservoirs and hypolimnetic releases, hydropeaking waves are associated with abrupt temperature variations (i.e., thermopeaking, Zolezzi et al. [2]). During their downstream propagation, the hydrodynamic (HP) and thermal (TP) wave are slowly damped and propagate with different speeds (Toffolon et al. [3]), and the hydrodynamic wave always precedes the thermal one. As a consequence, the biota experience a first disturbance caused by the increase of bottom shear stress due to the HP wave (which induces a passive, catastrophic drift, see Carolli et al. [4], Bruno et al. [5] for references) followed by a second one caused by the sudden temperature change (which induces an active, behavioral drift, see Carolli et al. [4]), and the two types of drift can occur as distinct events. In the last few years (see 9th ISE), our research group has been conducting a set of experiments on the effects of hydropeaking and thermopeaking on benthic macroinvertebrates in a set of experimental outdoors flumes. In 2008-2009, we conducted two cold thermopeaking and two warm thermopeaking simulations, showing that some taxa responded to the abrupt thermal alterations by increasing in drift (Carolli et al. [4]). The drift induced was possibly behavioral, given the immediate drifting responses of invertebrates; this type of drift differs from catastrophic drift that usually occurs as a response to hydropeaking. In July 2009, we assessed the drift of benthic invertebrates induced by a HP wave followed by a cold TP wave, and compared it with the drift induced by only a cold TP wave (Bruno et al. [5]). Drift propensity increased during HP and TP simulations, with a synergic effect: drift was higher when the TP wave followed the HP one. We also recorded a selective effect: some taxa did not respond to the alterations, some taxa responded to the discharge variations and to the thermal variations, or to the thermal variations alone. In 2012, we added to theee simulation of a sequernce of HP and TP the effects of the mandatory flow, called Residual Flow, or Minimum Vital Flow (MVF), which are typical of the river reaches between the dam and the hydropeaking release point. MVFs are often of low, invariant volume when compared to the natural flow regime (e.g. Growns and Growns, [6]). Passive drift decreases in response to low water velocities during periods of low flow, but active drift increases (Dewson et al. [7]) because flow is insufficient to meet the nutritional and physiological requirements of the benthic taxa, or to maintain their preferred habitats (Dewson et al. [7]). The aim of this study was to assess if the different changes in MVF (baseflow) could help mitigating the negative effects (such as reduction of diversity) of the subsequent hydropeaking and thermopeaking waves