3Rs of drought: resistance, resilience and recovery - an opportunistic experiment

The summer of 2022 has seen the highest air temperatures ever recorded in England and the lowest July rainfall since 1935. This drought, the worst in Europe for 500 years, has resulted in the widespread drying of river channels over large parts of England and Wales. Whilst extreme in historic terms, the drought provides a foretaste of the conditions expected with climate change: such extreme hydroclimatic events are expected to become more frequent in the future. Our ability to predict the impact of such droughts on the biological communities living in rivers, and hence mitigate the most severe effects, is constrained by a limited understanding of the factors influencing their abilities to resist the effects of drought and to recovery once flow resumes. Here the substrate of the river bed is particularly important, as many animals will retreat into wet subsurface substrate once surface water has been lost. Without a clear understanding of how substrate characteristics affect the response of rivers to drought, river managers cannot prioritize the most vulnerable rivers for protection from drying.

We are in a unique position of having an experiment already set up in multiple replicated stream channels that will enable us to experimentally examine the effects of substrate/fine sediment on the response of river communities to drought and on their potential to recolonise and recover following the resumption of flow. The experiment, set up to look at the effects of substrate composition and fine sediment loading on the macroinvertebrate communities dried naturally as water levels declined during the drought, with the treatments left in situ. We will explore how substrate characteristics influence i) the ability of invertebrates to persist through drought in the river bed, ii) the ability of invertebrates to recolonise the river by emerging from the river bed once flow resumes, and iii) the relative importance of recolonisation from the river bed compared with other routes of colonization (aerial or drifting).