Drift behavior of blackfly larvae in controlled conditions

Drift, i.e. the downstream transport of benthic organisms, is a primary driver in structuring benthic communities and ensuring distribution and colonization along the river continuity. Stream invertebrates may actively enter the water column to move away from unsuitable habitats, to avoid predators an competitors, or as a life stage requirement (i.e., active or behavioural drift), or being passively dislodged as a consequence of increased activity and/or changed positions on the bottom, or by the effect of the shear stress acting on the river bad which dislodges animals; during floods or high flows, (ie., passive or catastrophic drift). While an extensive literature has explored many aspects of extrinsic and intrinsic drivers of drift, of its diurnal and seasonal timing and of the ecological meaning of this phenomenon, less specific attention has been posed specifically to drift in blackflies, though they frequently represent a major component of the drift. In one of the most relevant studies, by Adler et al. (1983) found rather homogeneous drift behaviour in five blackfly species from North America, with higher drift rates at dusk and lowest in the afternoon, when temperature reaches its daily maximum. To fill some of these gaps, we used simulations in flumes were we manipulated the physical-chemical parameters, and assess the responses of blacklies to each alteration. e conducted three different experiments from 2008 to 2013 in a set of five artificial flumes (20 m long, section 0.30 x 0.30 m) fed by a near-to-pristine Alpine stream. Discharge in the flumes can be regulated by operating sluice gates, and temperature can be modified for short periods of time by releasinge cold or warm water from a tank into the flumes. A first experiment was conducted in 2008-2009, where we simulated the sudden and repeated changes in water temperature that affect rivers downstream of hydropower plants fed by high elevation reservoirs with hypolimnetic release. When turbines are operating and releasing water, the temperature of the receiving river reach increases of 3-4 C° in winter, and decreases of 3-6 C° in summer. These temperature changes occur suddenly, and both conditions (warming and cooling) were tested in the flumes. Blackflies reacted by behavioural drift in all cases, with some differences between decrease and increase in temperature. A second experiment was conducted in September 2013, aimed to assess the drift behaviour of blackfly larvae during repeated hydropeaking events (one five hour hudropeaking event repeated daily for five consecutive days), simulating the normal operation scheme of a storage hydropower plant. In this case, blackflies drifted at the very beginning of the event but then, differently from other taxa, resisted the higher water velocity and tended to increase in the drift after the return to baseflow. A third experiment was conducted in October 2013, and focused on the distance travelled by blackflies larvae following a physical disturbance of the substrate. In each of the five flumes we disturbed the substrate at increasing distance from the downstream end, were the drift nets were positioned. Blackfly larvae exhibited a good capacity to re-attach to the substrate, when compared to other benthic invertebrates. Again a behavioural drift was observed when conditions returned to normality, reacting to the series of disturbances. Overall, our simulations allow providing information on the intensity and mechanisms of blackflies drift in response to discharge and temperature alterations