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).

Given that SARS-CoV-2 RNA is detectable in faeces for prolonged periods (even for otherwise asymptomatic individuals), efforts have so far concentrated on trying to map its prevalence using sewage samples, e.g. via our partners at Bangor University (NERC Urgency Grant NE/V004883/1). Because live viruses have also been detected in the stools of patients affected by COVID19, there is growing concern about the risks of faecal-oral transmission to humans and/or wildlife (where the virus first originated) via sewage outflows and overspill. This is particularly worrying as, for example, hundreds of tonnes of raw sewage enter the Thames each year when sewers overflow during rainstorms, effectively bypassing sewage treatment works (STWs) when they exceed capacity. We combine expertise from Life Sciences and Mathematics at Imperial College, corona virology at Nottingham University, and a network of collaborators to fill this gap and to complement ongoing work in related (but not overlapping) areas. We have also already secured £49K of internal funding from Imperial College to prime the lab work, as a direct in-kind contribution. First, the potential for sewage (via effluent discharge, storm overflows, and other forms of run-off) to contribute to transmission to humans and wildlife will be measured by assessing RNA concentration and viral infectivity from environmental samples, from sewage outflows down to rivers, estuaries, and faeces from wildlife. Second, using data on concentrations of SARS-CoV-2 RNA in sewage and in the environment, we will provide models of population-level prevalence of COVID19 and elucidate key environmental transmission routes for management.

In our changing world there is an increasing urgency to understand interactions among multiple environmental stressors, such as pollution and warming. Much of the concern surrounding multiple stressors is due to their potential to interact, creating more severe impacts than they would do independently. Freshwater ecosystems are particularly vulnerable and freshwater biodiversity is the most threatened across the globe: a recent report estimated average population declines of >80% among freshwater vertebrate species compared to <40% in terrestrial and marine species (since 1970; WWF Living Planet Report, 2018). Although the combined impacts of multiple stressors has started to receive more attention, our knowledge on their interactive effects still remains almost non-existent. In reality, stressors are unlikely to occur in the same space at exactly the same time, yet studies that measure the combined effects of multiple stressors often assume this to be the case. In other words, they lack temporal realism. Most of these studies also lack biological realism by quantifying the effects of stressors on model species at lower levels of organisation (e.g. range shifts, survival, abundance) and ignoring feeding interactions. Here, we will consider how the order, or sequence, of stressor events alters individual-to-ecosystem responses of freshwaters, with a focus on food web interactions. Ecosystems will have multiple responses to the multiple stressors they face, including changes in diversity, abundance, body size and feeding behaviour. Even minor alterations to any of these can shift food web structure, with implications for the effects of future stressors, yet these critically important interactions have been largely ignored to date. This leaves us with little or no predictive ability about the consequences of future change in natural systems. Therefore, here we will use mesocosm experiments to quantify the combined effects of staggered nutrient pollution and warming events (i.e. previous exposure) on freshwater ecosystems, and scale our results up to the catchment level by adapting a suite of dynamic water quality models. Our experimental results will be used to parameterize temperature and nutrient controlled population sizes and growth rates, and to simulate how these changed rates will alter food web structure at the larger river system scale. This interdisciplinary study will generate an unprecedented breadth and depth of data: from individual changes in fitness and population shifts in size structure to food web complexity. We will show how the order of multiple stressor events (i.e. previous exposure) affects community resistance and resilience to change. These unique data sets will allow us to ask numerous novel questions in pure and applied ecology, and to characterise the little known multiple impacts of multiple stressors on freshwater food webs. Such a comprehensive coverage of responses has never been attempted before and this study will address this glaring gap in our knowledge of stressor impacts.

Climate change and human activities are expected to change the quantity of water entering rivers and streams, with potentially dramatic impacts on animals and plants resident in these ecosystems. In many regions, climate change is expected to reduce rainfall and bring about drought conditions, and water abstraction and river diversions may also reduce flows in rivers and streams. To date, relatively little work has been done to determine the effect of hydrologic droughts on aquatic biodiversity, and less is known about impacts on important processes, such as decomposition and nutrient cycling, that affect water quality and productivity of aquatic life. Our study will use novel experiments to understand the ecological effects of hydrologic droughts in streams, with a view to predicting future change. We will use a series of artificial stream channels to directly manipulate flows, thereby simulating drought episodes, and measure the responses of flora and fauna, and a series of processes that reflect the ecological health of the ecosystem. We will establish a series of experimental drought treatments which differ in the extent of flow reduction, from unaltered reference conditions to extreme low flows that cause habitat loss. We will also examine how the physical nature of the stream bed affects the extent to which animals and plants can withstand periods of drought, and how quickly these communities recover from these events. With a project student, we will investigate how water abstraction, a leading anthropogenic cause of stream drought, affects biodiversity and functioning across a suite of lowland streams in south west England. Together, the results will give valuable insights into the ways in which the environment responds to change brought about through human activities and the likely effects of climate change.

Lake ecosystems face multiple stresses including nutrient enrichment, climate change and invasion of nonindigenous species. This latter stress is widely recognised as having a major impact on biodiversity and the functioning of ecosystems worldwide and its effects are increasing because human activity has enhanced rates of dispersal and climate change is opening new niches at high latitudes. Windermere, comprising two basins, is England's largest lake and one of the best studied in the world with detailed records extending back for up to 70 years. A marked deterioration in water quality has been observed in the last 10 to 15 years despite continued removal of a key nutrient, phosphorus, at the wastewater treatment works. For example, summer algal blooms have increased and concentrations of oxygen at depth have decreased. The numbers of the rare and protected fish, the Arctic charr, have also declined dramatically in recent years. These changes have coincided with the population expansion of a lower-latitude, nonindigenous species, the roach. In this project we will test the hypotheses that the roach expansion is a result of the documented warmer waters in Windermere and that the population increase has triggered a 'trophic cascade' leading to greater predation on the zooplankton, which in turn has reduced the algae from control by their grazer. We will also test whether the decline in Arctic charr numbers results from competition with roach, since both feed on the zooplankton. We are in a unique position to assess the long term ramifications of multiple stressors because of the wealth ecological and environmental data and preserved samples collected from Windermere for most of the last century. The project will involve targeted, detailed analysis of the long-term data, analysis of the historical archived fish and zooplankton samples, identification of food sources of the different fish populations and food-web structure using stable isotope analysis and studies of fish gut contents, hydroacoustic analysis to estimate fish density and location and modelling to estimate roach ecological niche, zooplankton grazing and algal growth. The project is relevant to current general ecological issues such as the importance of top-down-control, the effects of multiple stressors and possible species extinction caused by species invasion. The results will also be highly relevant to the management of lakes since if our hypotheses are correct, nutrient removal will need to be even more stringent in the face of climate change and disruption of food-chains caused by invasion of nonindigenous species.