Impact of drought on stream ecosystem structure and functioning

Climate change is projected to increase the frequency and severity of extreme events, adding to the plethora of existing pressures that streams and rivers already face. Compound events such as drought may comprise numerous stressors that occur in concert to elicit ecological change. However the causal mechanisms of such impacts remain unknown, and research attempting to disentangle impacts of compound events, or link effects across levels of ecological organisation, remains in its infancy. This research investigates impacts of key drought stressors –sedimentation, dewatering and warming – across multiple ecological, hierarchical levels. At the individual level, macroinvertebrates displayed differential thermal sensitivity to warming which may explain idiosyncratic ecological responses reported elsewhere, whilst sedimentation intensified predator-prey interactions. Mesocosms were effective tools for studying drought stressors independently and in combination at the community and functional level. Dewatering main effects reduced the density of a common taxon and functional feeding group biomass, whilst all three stressors sometimes interacted together in complex ways. Stressors also had quantifiable effects at the whole-system level, e.g. stream metabolism. This study provides initial findings pertaining to drought impact causative mechanisms across multiple levels of ecological complexity, highlighting the importance of an experimental approach to predict future effects of compound events