How drought and human pressures are changing our chalk rivers

England’s chalk streams provide vital habitats for biodiverse and bioabundant aquatic invertebrate communities that include many riverflies. But these iconic, groundwater-fed rivers are under pressure, from both human activities and climate extremes – including droughts, which are predicted to become more frequent and severe in our changing climate.

Following one of the driest springs on record, a UK-wide drought is currently unfolding, including below-average flows in rivers across much of south and east England. We need to better understand how river ecosystems respond to and recover from such extreme events, to inform evidence-based actions that promote their long-term resilience. Our research, conducted in collaboration with the Environment Agency, is revealing how droughts and human pressures are interacting to alter invertebrate biodiversity in chalk streams.

Here we share insights from this work and discuss how they could be applied to support chalk stream resilience as they adapt to an increasingly extreme climate.

Droughts in chalk rivers

Chalk streams naturally include perennial sections, which flow year round, and temporary sections such as winterbournes, which flow only when groundwater levels are high, typically in winter and spring. These rivers thus shrink and expand depending largely on aquifer levels, which are in turn determined by winter rainfall and can thus vary considerably among years (Figure 1; Sefton et al., 2019). During droughts, typically perennial sections can stop flowing or even dry out, and in temporary reaches, the extent of drying increases in both space and time (Figure 1).

Perennial and temporary sections harbour biodiverse, but distinct, aquatic invertebrate communities, each including mayfly (Ephemeroptera), stonefly (Plecoptera) and caddisfly (Trichoptera) larvae—the ‘EPT’ riverflies. In addition, winterbournes provide habitats for rare specialist insects such as the scarce purple dun (as featured in a recent WildFish blog post). Drought typically causes invertebrate biodiversity to decline, due to loss of fast-flowing habitats, low oxygen availability and elevated water temperatures in perennial reaches, and due to dry-phase durations and intensities that exceed species resistance capacity in temporary reaches (Sarremejane et al., 2022; Stubbington et al., 2024).

Figure 1: Changes in flow conditions during average (2015) and drought (2019) years in the chalk River Colne catchment. Reproduced from Sarremejane et al. (2022). The Environment Agency provided mapped data and photos.

Our research in the Chilterns chalk streams has used long-term (up to 30 years) aquatic invertebrate Environment Agency monitoring data to characterise how biodiversity changes depending on the duration and frequency of drying events – both during and beyond drought years. We showed that communities typically gain slow-flow adapted species, such as beetles and snails, but lose species that prefer fast-flowing water, such as riverflies, as dry-phase durations increase (Sarremejane et al., 2019). We also found that, aligning with previous research, communities at most sites recovered within 3 years of a drought ending – but some species need 10 years to recolonise.

Our research also demonstrates that drought-sensitive invertebrates with low recolonisation capacities, including limpets and some caddisfly families, become increasingly vulnerable to local extinction as drought duration and spatial extent increases (Sarremejane et al., 2021). Such findings highlight the importance of maintaining a diversity of habitats, including wet refuges, within river networks to allow species to recolonise post drought and thus maintain long-term population viability.

Droughts in interaction with human pressures

Flowing through farmland and some urban areas, chalk streams are impacted by multiple, interacting human pressures, including water pollution, and surface and groundwater abstraction. Abstraction can exacerbate drought-driven reductions in river flow, especially if water demand increases, for example during summer heatwaves. Low flows caused by both climatic drought and abstraction also reduce dilution of organic and inorganic pollutants including effluent released by water treatment works – which can come to comprise 100% of flow as water levels decline in temporary reaches. Decomposition of such organic matter in nutrient-rich, warm, slow-flowing waters can reduce oxygen availability for other organisms, potentially killing fish and sensitive invertebrates including EPT riverflies (Chadd et al., 2017).

Our research has shown that chalk stream invertebrate communities exposed to water pollution and abstraction experience greater declines in biodiversity during drought and take longer to recover after drought ends.

Our latest, soon to be published research – again done using Environment Agency monitoring data – suggests that, although most communities recover from individual droughts within 1–3 years, many communities experience occasional long-term shifts in their taxonomic composition (Figure 2), in particular after severe droughts. Unexpectedly, in some instances, competitive, pollution-tolerant species may be eliminated, apparently allowing more sensitive organisms such as EPT riverflies to recolonise after drought ends. Improvements in water quality in recent decades for example linked to the Water Framework Directive and the Urban Waste Water Treatment Directive may have created environments in which such shifts can occur.

Figure 2: A persistent drought-related, site-specific shift in invertebrate community composition in 2001–2016 in the River Colne, a relatively unimpacted perennial chalk stream. Points represent community composition as summarised by a multivariate ordination method. Adapted from Stubbington et al. (2024).

Towards improving chalk stream resilience to drought

Supporting the priorities in Catchment-Based Approach’s (CaBA) Chalk Stream Restoration Strategy and its Implementation Plan, our results indicate that actions taken to support natural flow regimes, habitat diversity and water quality could support the drought resilience of biodiverse chalk stream communities (Stubbington et al. 2022). Such actions need to be supported by evidence: understanding of hydrological and ecological variability in space and time, across perennial and temporary sections, and across a hydrological continuum from drought to flooding. Such knowledge could pinpoint when and where to prioritise actions that maximise ecological resilience, thus minimising drought impacts and promoting post-drought recovery. Such actions include:

• Restoring healthy flow regimes, by reducing water abstraction and restoring physical habitats, to promote water retention in wet refuges such as groundwater-fed pools and subsurface sediments, in which organisms can survive drought and from which they can subsequently recolonise.
• Reconnecting upstream to downstream river habitats, to enable motile species to move to and from refuges as flow declines and recovers.
• Continuing improvements in water quality, including by addressing pollution from point sources such as sewage treatment works.

Monitoring drought impacts on both habitat quality and biological communities is vital to underpin such actions, informing us when and where to take actions that support river flows and ecosystem resilience. Citizen science initiatives such as SmartRivers have a role to play in this and give local communities the knowledge and tools to document drought impacts in their local watercourses. But protecting chalk streams will require coordinated action involving stakeholders including farmers, water companies, regulators and local communities.

Chalk rivers are not just biologically important – they’re part of our national cultural and natural heritage. They inspire poets and painters, support livelihoods, and offer places for peace and recreation. But as droughts intensify in our changing climate and as human pressures persist, we risk losing not just their biodiversity, but the many benefits they provide to people.

Our research helped better understand how these pressures interact – and how we might build more resilient rivers. We call for ongoing collaboration between researchers, managers and citizen scientists to enable rapid translation of such understanding into action on the ground.

Acknowledgements

This research was part funded by the Environment Agency under the project reference numbers: SC220011 and SC220024. Most data analysed for this research was provided by the Environment Agency and is openly available here.

References

Chadd, R. P., England, J. A., Constable, D., Dunbar, M. J., Extence, C. A., Leeming, D. J., Murray-Bligh, J. A., & Wood, P. J. (2017). An index to track the ecological effects of drought development and recovery on riverine invertebrate communities. Ecological Indicators, 82, 344–356. https://doi.org/10.1016/j.ecolind.2017.06.058

Sarremejane, R., England, J., Dunbar, M., Brown, R., Naura, M., & Stubbington, R. (2024). Human impacts mediate freshwater invertebrate community responses to and recovery from drought. Journal of Applied Ecology, 61, 2616–2627. https://doi.org/10.1111/1365-2664.14771

Sarremejane, R., Messager, M. L., & Datry, T. (2022). Drought in intermittent river and ephemeral stream networks. Ecohydrology, 15, e2390. https://doi.org/10.1002/eco.2390

Sarremejane, R., Stubbington, R., Dunbar, M. J., Westwood, C. G., & England, J. (2019). Biological indices to characterize community responses to drying in streams with contrasting flow permanence regimes. Ecological Indicators, 107. https://doi.org/10.1016/j.ecolind.2019.105620

Sarremejane, R., Stubbington, R., England, J., Sefton, C. E. M., Eastman, M., Parry, S., & Ruhi, A. (2021). Drought effects on invertebrate metapopulation dynamics and quasi‐extinction risk in an intermittent river network. Global Change Biology, 27, 4024–4039. https://doi.org/10.1111/gcb.15720

Sefton, C. E. M., Parry, S., England, J., & Angell, G. (2019). Visualising and quantifying the variability of hydrological state in intermittent rivers. Fundamental and Applied Limnology, 193, 21–38. https://doi.org/10.1127/fal/2019/1149

Stubbington, R., Dimon, J., England, J., & Watts G. (2022). Chalk streams of the future: the effects of climate change on biodiversity in England’s iconic river ecosystems [online]. https://doi.org/10.6084/m9.figshare.21663269.v1

Stubbington, R., England, J., Sarremejane, R., Watts, G., & Wood, P. J. (2024). The effects of drought on biodiversity in UK river ecosystems: Drying rivers in a wet country. WIREs Water, 11, e1745. https://doi.org/https://doi.org/10.1002/wat2.1745