Laboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés

Assessing trait responses to environmental gradients requires the simultaneous analysis of the information contained in three tables: L (species distribution across samples), R (environmental characteristics of samples), and Q (species traits). Among the available methods, the so-called fourth-corner and RLQ methods are two appealing alternatives that provide a direct way to test and estimate trait-nvironment relationships. Both methods are based on the analysis of the fourth-corner matrix, which crosses traits and environmental variables weighted by species abundances. However, they differ greatly in their outputs: RLQ is a multivariate technique that provides ordination scores to summarize the joint structure among the three tables, whereas the fourth-corner method mainly tests for individual trait-environment relationships (i.e., one trait and one environmental variable at a time). Here, we illustrate how the complementarity between these two methods can be exploited to promote new ecological knowledge and to improve the study of trait-environment relationships. After a short description of each method, we apply them to real ecological data to present their different outputs and provide hints about the gain resulting from their combined use.

Current understanding of nano- and microplastic movement, propagation and potential effects on biota in freshwater environments is developing rapidly. Still, there are significant disconnects in the integration of knowledge derived from laboratory and field studies. This review synthesises the current understanding of nano- and microplastic impacts on freshwater biota from field studies and combines it with the more mechanistic insights derived from laboratory studies. Several discrepancies between the field and laboratory studies, impacting progress in process understanding, were identified including that the most prevalent plastic morphologies found in the field (fibres) are not those used in most of the laboratory studies (particles). Solutions to overcome these disparities are proposed to aid comparability of future studies. For example, environmental sampling and separation of biota into its constituents is encouraged when conducting field studies to map microplastic uptake preferences. In laboratory studies, recommendations include performing toxicity studies to systematically test possible factors affecting toxicity of nano- and microplastics, including morphology, chemical makeup (e.g., additives) and effects of plastic size. Consideration should be given to environmentally relevant exposure factors in laboratory studies, such as realistic exposure medium and effects of plastic ageing. Furthermore, based on this comprehensive review recommendations of principal toxicity endpoints for each of the main trophic levels (microbes, primary producers, primary consumers and secondary consumers) that should be reported to make toxicity studies more comparable in the future are given.

Dispersal is a central process in ecology and evolution. It strongly influences the dynamics of spatially structured populations and affects evolutionary processes by shaping patterns of gene flow. For these reasons, dispersal has received considerable attention from ecologists, evolutionary biologists, and conservationists. Although it has been studied extensively in taxa such as birds and mammals, much less is known about dispersal in vertebrates with complex life cycles such as pond-breeding amphibians. Over the past two decades, researchers have taken an ever-increasing interest in amphibian dispersal and initiated both basic and applied studies, using a broad range of experimental and observational approaches. This body of research reveals complex dispersal patterns, causations, and syndromes, with dramatic consequences for the demography and genetics of amphibian populations. In this review, our goals are to: redefine and clarify the concept of amphibian dispersal; review current knowledge about the effects of individual (i.e., condition-dependent dispersal) and environmental (i.e., context-dependent dispersal) factors during the three stages of dispersal (i.e., emigration, transience, and immigration); identify the demographic and genetic consequences of dispersal in spatially structured amphibian populations; and propose new research avenues to extend our understanding of amphibian dispersal.

Reliable chemical identification of specific polymers in environmental samples represents a major challenge in plastic research, especially with the wide range of commercial polymers available, along with variable additive mixtures. Thermogravimetric analysis-Fourier transform infrared-gas chromatography-mass spectrometry (TGA-FTIR-GC-MS) offers a unique characterization platform that provides both physical and chemical properties of the analyzed polymers. This study presents a library of 11 polymers generated using virgin plastics and post-consumer products. TGA inflection points and mass of remaining residues following pyrolysis, in some cases, proved to be indicative of the polymer type. FTIR analysis of the evolved gas was able to differentiate between all but polypropylene (PP) and polyethylene (PE). Finally, GC-MS was able to differentiate between the unique chemical fingerprints of all but one polymer in the library. This library was then used to characterize real environmental samples of mesoplastics collected from beaches in the U.K. and South Africa. Unambiguous identification of the polymer types was achieved, with PE being the most frequently detected polymer and with South African samples indicating variations that potentially resulted from aging and weathering.

Mitochondria are known to play a central role in life history processes, being the main source of reactive oxygen species (ROS), which promote oxidative constraint. Surprisingly, although the main role of the mitochondria is to produce ATP, the plasticity of mitochondrial ATP generation has received little attention in life history studies. Yet, mitochondrial energy transduction represents the physiological link between environmental resources and energy allocated to animal performance. Studying both facets of mitochondrial functioning (ATP and ROS production) would allow better understanding of the proximate mechanisms underlying life history. We have experimentally modulated the mitochondrial capacity to generate ROS and ATP during larval development of Rana temporaria tadpoles, via chronic exposure (34 days) to a mitochondrial uncoupler (2,4-dinitrophenol, dNP). The aim was to better understand the impact of mitochondrial uncoupling on both responses in terms of oxidative balance, energy input (oxygen and feeding consumption) and energy output (growth and development of the tadpole). Exposure to 2,4-dNP reduced mitochondrial ROS generation, total antioxidant defences and oxidative damage in treated tadpoles compared with controls. Despite the beneficial effect of dNP on oxidative status, development and growth rates of treated tadpoles were lower than those in the control group. Treatment of tadpoles with 2,4-dNP promoted a mild mitochondrial uncoupling and enhanced metabolic rate. These tadpoles did not increase their food consumption, and thus failed to compensate for the energy loss elicited by the decrease in the efficiency of ATP production. These data suggest that the cost of ATP production, rather than the oxidative balance, is the parameter that constrains growth/development of tadpoles, highlighting the central role of energy transduction in larval performance.

The presence of microplastics in cosmetics and personal care products (C&PCPs) has been increasingly in the public eye since the early 2010s. Despite increasing research into the potential environmental and health effects of microplastics, most research to date on microplastics in C&PCPs has investigated "rinse-off" products, while the potential impacts of "leave-on" C&PCPs have been largely neglected, despite these products being purchased in greater volumes and often having two or more microplastic ingredients in their formulations(CosmeticsEurope, 2018b). This review aims to synthesize the current knowledge of microplastic in C&PCPs, assessing the potential environmental and human health impacts of C&PCPs and discussing the regulatory implications. The lack of studies on leave-on C&PCPs is significant, suggesting a severe knowledge gap regarding microplastic presence in, and emissions from, C&PCPs. There is a noticeable lack of studies on the (eco)toxicological consequences of microplastic exposure from C&PCPs. As a result, significant aspects of microplastic contamination may be overlooked in the microplastic legislations emerging globally (including from the European Commission), which intend to restrict microplastic use in C&PCPs but focus on rinse-off C&PCPs only. This review highlights the potential consequences of microplastics in leave-on C&PCPs for regulatory decision-making, particularly as alternatives to microplastics are considered during the phase-out periods and spotlights the need for sufficient monitoring and research on these alternatives, to avoid unforeseen consequences.

BACKGROUND: Within-generational plasticity (WGP) and transgenerational plasticity (TGP) are mechanisms allowing rapid adaptive responses to fluctuating environments without genetic change. These forms of plasticity have often been viewed as independent processes. Recent evidence suggests that WGP is altered by the environmental conditions experienced by previous generations (i.e., TGP). In the context of inducible defenses, one of the most studied cases of plasticity, the WGP x TGP interaction has been poorly investigated. RESULTS: We provide evidence that TGP can alter the reaction norms of inducible defenses in a freshwater snail. The WGP x TGP interaction patterns are trait-specific and lead to decreased slope of reaction norms (behaviour and shell thickness). Offspring from induced parents showed a higher predator avoidance behaviour and a thicker shell than snails from non-induced parents in no predator-cue environment while they reached similar defenses in predator-cue environment. The WGP x TGP interaction further lead to a switch from a plastic towards a constitutive expression of defenses for shell dimensions (flat reaction norm). CONCLUSIONS: WGP-alteration by TGP may shape the adaptive responses to environmental change and then has a substantial importance to understand the evolution of plasticity.

This study investigates ancient alkenone producers among the late Oligocene–early Miocene coccolithophores recorded at Deep Sea Drilling Project (DSDP) Site 516. Contrary to common assumptions, Reticulofenestra was not the most important alkenone producer throughout the studied time interval. The comparison between coccolith species‐specific absolute abundances and alkenone contents in the same sedimentary samples shows that Cyclicargolithus abundances explain 40% of the total variance of alkenone concentration and that the species Cyclicargolithus floridanus was a major alkenone producer, although other related taxa may have also contributed to the alkenone production at DSDP Site 516. The distribution of the different alkenone isomers (MeC 37:2 , EtC 38:2 , and MeC 38:2 ) remained unchanged across distinct changes in species composition, suggesting similar diunsaturated alkenone compositions within the Noelaerhabdaceae family during the late Oligocene–early Miocene. However, the overall larger cell size of Cyclicargolithus may have implications for the alkenone‐based reconstruction of past partial pressure of CO 2. Our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods (>1.85 Ma) predating the first occurrence of contemporary alkenone producers (i.e., Emiliania huxleyi and Gephyrocapsa oceanica ).

By altering or eliminating delicate ecological relationships, non-indigenous species are considered a major threat to biodiversity, as well as a driver of environmental change. Global climate change affects ecosystems and ecological communities, leading to changes in the phenology, geographic ranges, or population abundance of several species. Thus, predicting the impacts of global climate change on the current and future distribution of invasive species is an important subject in macroecological studies. The African clawed frog (Xenopus laevis), native to South Africa, possesses a strong invasion potential and populations have become established in numerous countries across four continents. The global invasion potential of X. laevis was assessed using correlative species distribution models (SDMs). SDMs were computed based on a comprehensive set of occurrence records covering South Africa, North America, South America and Europe and a set of nine environmental predictors. Models were built using both a maximum entropy model and an ensemble approach integrating eight algorithms. The future occurrence probabilities for X. laevis were subsequently computed using bioclimatic variables for 2070 following four different IPCC scenarios. Despite minor differences between the statistical approaches, both SDMs predict the future potential distribution of X. laevis, on a global scale, to decrease across all climate change scenarios. On a continental scale, both SDMs predict decreasing potential distributions in the species' native range in South Africa, as well as in the invaded areas in North and South America, and in Australia where the species has not been introduced. In contrast, both SDMs predict the potential range size to expand in Europe. Our results suggest that all probability classes will be equally affected by climate change. New regional conditions may promote new invasions or the spread of established invasive populations, especially in France and Great Britain.

Summary Rivers and floodplains are among the most threatened ecosystems. Hydroelectric power plants and embankments have reduced the hydrological connectivity between rivers and their floodplain channels, leading to loss of freshwater habitats and biological communities. To prevent and reverse such loss, numerous restoration programmes have aimed at rejuvenating the lateral hydrological connectivity between rivers and floodplain channels. Despite considerable global attention, we know remarkably little about the ecological benefits of floodplain restoration programmes. We analysed the functional diversity of different macroinvertebrate groups (natives and aliens) along a gradient of lateral hydrological connectivity on the Rhône river in France. We used 36 sampling sites to describe the functional diversity (Rao's quadratic entropy) before and after the enhancement of the lateral hydrological connectivity by restoration. The effects of restoration on functional diversity were tested for each macroinvertebrate group and at multiple spatial levels (alpha and beta). Before restoration, alpha functional diversity of the entire macroinvertebrate community peaked in sites with a high lateral connectivity. The contribution of the native groups to functional diversity was higher than that of the alien group. The latter was not constrained by high values of lateral hydrological connectivity and reached a maximum in highly connected sites. After restoration, within‐site functional diversity (alpha FD ) declined linearly following the enhancement of lateral hydrological connectivity. The restoration operations increased the contribution of the aliens to functional diversity and reduced the contribution of a group of native taxa. In addition, among‐sites functional diversity (beta FD ) was successfully enlarged by restoration. Synthesis and applications . The lateral hydrological connectivity (LHC) represents a key parameter for explaining the functional diversity (FD) of macroinvertebrates in a floodplain ecosystem. Our results demonstrate that restoration‐induced changes to functional diversity can be predicted. Controversially, restoration‐induced enhancement of lateral hydrological connectivity increased the functional diversity of the alien macroinvertebrates. However, these species contributed only to a small part of the total macroinvertebrate functional diversity. We recommend that restoration programmes diversify the levels of lateral hydrological connectivity among the channels to ensure an optimal functional diversity at the floodplain scale.

Weather fluctuations have been demonstrated to affect demographic traits in many species. In long-lived organisms, their impact on adult survival might be buffered by the evolution of traits that reduce variation in interannual adult survival. For example, skipping breeding is an effective behavioral mechanism that may limit yearly variation in adult survival when harsh weather conditions occur; however, this in turn would likely lead to strong variation in recruitment. Yet, only a few studies to date have examined the impact of weather variation on survival, recruitment and breeding probability simultaneously in different populations of the same species. To fill this gap, we studied the impact of spring temperatures and spring rainfall on survival, on reproductive skipping behavior and on recruitment in five populations of a long-lived amphibian, the yellow-bellied toad (Bombina variegata). Based on capture-recapture data, our findings demonstrate that survival depends on interactions between age, population and weather variation. Varying weather conditions in the spring result in strong variation in the survival of immature toads, whereas they have little effect on adult toads. Breeding probability depends on both the individual's previous reproductive status and on the weather conditions during the current breeding season, leading to high interannual variation in recruitment. Crucially, we found that the impact of weather variation on demographic traits is largely context dependent and may thus differ sharply between populations. Our results suggest that studies predicting the impact of climate change on population dynamics should be taken with caution when the relationship between climate and demographic traits is established using only one population or few populations. We therefore highly recommend further research that includes surveys replicated in a substantial number of populations to account for context-dependent variation in demographic processes.

The use of functional information in the form of species traits plays an important role in explaining biodiversity patterns and responses to environmental changes. Although relationships between species composition, their traits, and the environment have been extensively studied on a case-by-case basis, results are variable, and it remains unclear how generalizable these relationships are across ecosystems, taxa and spatial scales. To address this gap, we collated 80 datasets from trait-based studies into a global database for metaCommunity Ecology: Species, Traits, Environment and Space; "CESTES". Each dataset includes four matrices: species community abundances or presences/absences across multiple sites, species trait information, environmental variables and spatial coordinates of the sampling sites. The CESTES database is a live database: it will be maintained and expanded in the future as new datasets become available. By its harmonized structure, and the diversity of ecosystem types, taxonomic groups, and spatial scales it covers, the CESTES database provides an important opportunity for synthetic trait-based research in community ecology.

In the vadose zone, preferential flow and transport are much more common than uniform water flow and solute transport. In recent decades, several models have been developed for preferential water flow and physical nonequilibrium solute transport. Among these models, the dual-permeability approach is an interesting tool for the conceptualization and modeling of preferential flow. However, this approach has been mainly studied from a numerical point of view. In this study, we developed a new analytical model for water infiltration into dual-permeability soils. The model is based on the analytical model originally proposed for single-permeability soils. The proposed model relies on the assumption that the water exchange rate at the interface between the matrix and fast-flow regions does not change cumulative infiltration at the soil surface, so that the total cumulative infiltration can be set equal to the sum of independent cumulative infiltrations into each region. This assumption was investigated using numerically generated data. The proposed analytical model was then used to evaluate the effects of fast-flow region hydraulic properties and hydraulic conditions on total cumulative infiltration for the case of single- and multi-tension water infiltration experiments. Finally, both single- and dual-permeability models were evaluated with respect to their ability to fit experimental data and associated problems of non-uniqueness in optimized parameters. The proposed model could serve as a new tool for modeling and characterizing preferential flow in the vadose zone.

Because it modulates the fitness returns of possible options of energy expenditure at each ontogenetic stage, environmental stochasticity is usually considered a selective force in driving or constraining possible life histories. Divergent regimes of environmental fluctuation experienced by populations are expected to generate differences in the resource allocation schedule between survival and reproductive effort and outputs. To our knowledge, no study has previously examined how different regimes of stochastic variation in environmental conditions could result in changes in both the temporal variation and mean of demographic parameters, which could then lead to intraspecific variation along the slow-fast continuum of life history tactics. To investigate these issues, we used capture-recapture data collected on five populations of a long-lived amphibian (Bombina variegata) experiencing two distinct levels of stochastic environmental variation: (1) constant availability of breeding sites in space and time (predictable environment), and (2) variable spatio-temporal availability of breeding sites (unpredictable environment). We found that female breeding propensity varied more from year to year in unpredictable than in predictable environments. Although females in unpredictable environments produced on average more viable offspring per year, offspring production was more variable between years. Survival at each ontogenetic stage was slightly lower and varied significantly more from year to year in unpredictable environments. Taken together, these results confirm that increased environmental stochasticity can modify the resource allocation schedule between survival and reproductive effort and outputs and may lead to intraspecific variation along the slow-fast continuum of life history tactics.

Exposure to unpredictable environmental stressors could influence animal health and fitness by inducing oxidative stress, potentially through downstream effects of glucocorticoid stress hormones (e.g. corticosterone) on mitochondrial function. Yet, it remains unclear whether species that have evolved in stochastic and challenging environments may present adaptations to alleviate the effects of stress exposure on oxidative stress. We tested this hypothesis in wild king penguins by investigating mitochondrial and oxidative stress responses to acute restraint-stress, and their relationships with baseline (potentially mirroring exposure to chronic stress) and stress-induced increase in corticosterone levels. Acute restraint-stress did not significantly influence mitochondrial function. However, acute restraint-stress led to a significant increase in endogenous antioxidant defences, while oxidative damage levels were mostly not affected or even decreased. High baseline corticosterone levels were associated with an up-regulation of the glutathione antioxidant system and a decrease in mitochondrial efficiency. Both processes might contribute to prevent oxidative damage, potentially explaining the negative relationship observed between baseline corticosterone and plasma oxidative damage to proteins. While stress exposure can represent an oxidative challenge for animals, protective mechanisms like up-regulating antioxidant defences and decreasing mitochondrial efficiency seem to occur in king penguins, allowing them to cope with their stochastic and challenging environment.

The classic understanding of organisms focuses on genes as the main source of species evolution and diversification. The recent concept of genetic accommodation questions this gene centric view by emphasizing the importance of phenotypic plasticity on evolutionary trajectories. Recent discoveries on epigenetics and symbiotic microbiota demonstrated their deep impact on plant survival, adaptation and evolution thus suggesting a novel comprehension of the plant phenotype. In addition, interplays between these two phenomena controlling plant plasticity can be suggested. Because epigenetic and plant-associated (micro-) organisms are both key sources of phenotypic variation allowing environmental adjustments, we argue that they must be considered in terms of evolution. This 'non-conventional' set of mediators of phenotypic variation can be seen as a toolbox for plant adaptation to environment over short, medium and long time-scales.

Abstract An overview of the current state of knowledge on the pollution of agricultural soils with microplastic and nanoplastic (MnP) particles is provided and the main MnP sources are discussed. MnP transport mechanisms from soil to groundwater, as well as the potential impact of MnPs on soil structure are considered, and the relevance of co-contaminants such as agrochemicals is further highlighted. We elaborate on why MnPs in soil and groundwater are understudied and how analytical capabilities are critical for furthering this crucial research area. We point out that plastic fragmentation in soils can generate secondary MnPs, and that these smaller particles potentially migrate into aquifers. The transport of MnP in soils and groundwater and their migration and fate are still poorly understood. Higher MnP concentrations in agricultural soils can influence the sorption behavior of agrochemicals onto soil grains while attachment/detachment of MnPs onto soil grains and MnP-agrochemical interactions can potentially lead to enhanced transport of both MnP particles and agrochemicals towards underlying groundwater systems.

Plant Growth-Promoting Bacteria (PGPB) of the genus Azospirillum are known to enhance root growth and yield in many plant species including cereals. To probe the underlying mechanisms, correlations between modifications of yield and 6-leaf plantlet characteristics were estimated on maize in four fields with contrasting soil properties over two consecutive years using the commercial isolate A. lipoferum CRT1. In both years, plantlet metabolome, photosynthetic potential and organ morphology were found to display field- and inoculation-specific signatures. Metabolomic analyses revealed that A. lipoferum CRT1 mostly affected sugar metabolism with no suggested impact on N and P assimilation. Mineral nitrogen feeding increased yield but did not affect yield enhancement by the bacterial partner. However, greater improvements of leaf photosynthetic potential correlated with yield diminutions and larger plantlets in all of their proportions correlated with yield enhancements. Bacterial inoculation restored proper seed-to-adult plant ratio when it accidentally dropped below 80%. Only in these cases did it raise yield. All in all, securing mature plant density is hypothesized as being the primary driver of A. lipoferum CRT1-mediated yield enhancement in maize fields.

The world faces an invisible crisis of water quality. Its impacts are wider, deeper, and more uncertain than previously thought and require urgent attention—The World Bank—(Damania et al., 2019). Healthy rivers provide vital services for humans and other life on Earth. Water pollution can seem like a 20th century problem: solved and sorted. In reality, gains in water quality have been hard won and far from universal, with many pollutants persisting or even increasing. Without widespread awareness and action, growing anthropogenic pressures could threaten anew the integrity of our water resources. Over the last half century, water quality has improved—most notably in upper income countries (UICs)—with declining pollution attributed to better monitoring, treatment and regulation such as the EU Water Framework Directive, US Clean Water Act, Chinese Water Pollution Control Law, and Ghana National Water Policy. Globally, there are fewer deaths now from waterborne pathogens, it is rare for rivers to catch fire from industrial waste, and advisories against swimming and fishing have been lifted in many regions (Landrigan et al., 2018). As we congratulate ourselves on abatement of ‘classical’ pollutants (human waste, nutrients, sediment), it is tempting to assume that adequate standards of water quality have been achieved. Unfortunately, poor water quality remains a pervasive problem. Water pollution still causes 2 M deaths each year and yields an additional critical burden of chronic diseases (Landrigan et al., 2018). Across Europe, 34% of the 130 000 water bodies surveyed in 2020 failed to meet “good” chemical status. Notably, 100% of rivers in England, Germany, Belgium and Sweden failed standards, and less than one-third of rivers met comparable ratings used in the USA (Kristensen et al., 2018). Moreover, deteriorating water quality is evident for Asian, African and South American rivers (UNEP, 2016). These reports demonstrate that we have not solved our water quality woes. The current state of river water quality reflects an intertwined history of human development and governance. We propose river pollution can be conceptualized in three historical ‘Phases’, characterized by distinct contaminant types and mitigation methods: Phase 1. Chronic organic pollution and pathogens associated with limited treatment of faecal waste, exacerbated by a rapidly increasing population density. Phase 2. Point source and diffuse pollution associated with the intensification of primary (agriculture, mining, forestry) and secondary (textiles, manufacturing, petroleum refining) industry. Phase 3. Emerging contaminants associated with industrial (per- and poly-fluoroalkyl substances, nanomaterials), medical and veterinary (pharmaceuticals) advances. In UICs, these Phases occurred sequentially over several decades or even centuries, tracking industrialisation and technological advances (Figure 1; Arden & Jawitz, 2019). This enabled development of infrastructure such as wastewater treatment facilities and capacity to monitor and regulate contaminants in Phases 1 and 2 (Figure 2). Today, many lower- and middle-income countries (LMICs) are facing pressures from compressed and overlapping water pollution phases. Sanitation challenges from rapid urbanization coincide with industrial development fuelled by outsourcing of manufacturing and agriculture from UICs to LMICs. With even the highest income countries struggling to reduce Phase 2 and 3 pollutants, it is unsurprising that many countries lack the resources to address the multiplicative pressures of all three Phases simultaneously. With water technologies at an all-time high, what accounts for this lack of progress and even degradation of water quality worldwide? While water quantity challenges have attracted attention due to their visually dramatic manifestation (floods, drought), water quality issues are often inconspicuous or invisible. Several converging factors are making the three Phases of water pollution increasingly visible and impossible to ignore: Thousands of pollutants now exist at detectable concentrations in the environment (Figure 1a). Agricultural applications (fertilizers, pesticides, pharmaceuticals) are increasing worldwide, and freshwater environments are affected by salinization due to irrigation and sea level rise. Meat consumption continues to increase, with its associated nutrient and pharmacological burdens. Surges in pollution are generated by unforeseen global crises, such as plastic pollution linked to personal protective equipment against COVID-19 (Prata et al., 2020). The diversity and concentration of pollutants can result in non-additive interactions—mixtures that affect mobility, toxicity and bioavailability of the various ingredients (Niu et al., 2020). Land use and climate change are short-circuiting the water cycle (Levia et al., 2020). Extreme storms and altered surface and subsurface drainage accelerate pollution transport and reduce ecosystem removal processes. Moreover, human disturbance can result in long-term release of legacy contaminants into soils, aquifers, and rivers (Van et al., 2018). At the same time, abrupt increases in global trade have supercharged transnational transport of livestock, crops, manufactured goods, and waste. This has resulted in imbalances in nutrients, metals, plastics, and other contaminants (Figure 1b). Because resource extraction and waste disposal are concentrated in LMICs, they bear the water quality burden of global markets and consumption in UICs. As LMICs typically have less capacity to treat waste (Figure 1c), this results in more water pollution per tonne and much higher human exposure. Although the widespread detection of long-banned pollutants such as Polychlorinated biphenyls could signal worsening pollution, often it reflects improved measurement capabilities. Sensitive methods now detect a wide array of pollutants at low concentrations. As egregious water pollutants have declined, we have tightened acceptable exposure standards with new knowledge of lethal and detrimental impacts on the environment and society (Ward et al., 2018). In some cases, our failures are a consequence of better-informed and increasingly stringent standards rather than absolute decreases in water quality. The last two centuries of water problems and solutions (Figure 2) demonstrate that we must be proactive in managing river pollution rather than create new pollution legacies for future generations. In the face of intermeshed phases of pollutants, we need compressed and overlapping solutions that: To address complex water quality challenges, rivers and their hinterlands must be managed as connected systems. This requires improved understanding of linkages between human activities on the landscape and water quality across space–time scales. Knowledge of water science is needed to balance public expectations and inform policy. Short-term interventions may take decades to result in improvements, while mismanagement may trigger new issues far into the future (Van et al., 2018). Actions to address poor water quality should begin with identifying the specific places, times and conditions that degrade disproportionately water quality and work to redress stoichiometric imbalances introduced by globalized manufacturing and trade (Figure 1b; Peters et al., 2008). To date, most chemicals are regulated individually. This can initiate a legislative wild-goose chase whereby slight changes to chemical composition circumvent regulation. A new EU model is emerging whereby chemicals are regulated based on their combined impact, such as regulating ‘total estrogenicity’ as opposed to individual compounds. For this approach to be effective for more emerging contaminants, we need improved knowledge of potential acute and chronic toxicity of multi-contaminant cocktails. This will account for cumulative risks from exposure to many stressors, such as the emerging ‘exposome concept’ (Landrigan et al., 2018). Environmental regulation has been informed by manual sampling and in situ monitoring. Insights from satellite imagery and unoccupied aerial vehicles are expanding monitoring in inaccessible areas, enabling detection of sources and consequences of pollution (Huang et al., 2018). CubeSat missions are complementing longstanding satellite missions with higher spatial resolution and higher frequency data (Cooley et al., 2017), providing new pathways for regional to global monitoring and modelling. Yet, there is a parallel need to extend long-term monitoring to track progress and ground-truth newer methods. However, these records' integrity is threatened by declining funding for monitoring networks, inconsistent approaches to data collection and lack of open data sharing (Lovett et al., 2007). A combination of conventional and cutting-edge monitoring methods is needed. Working directly with impacted communities to monitor water quality augments observational capabilities and empowers local people (Nardi et al., 2021). Stakeholder engagement and citizen science initiatives can lead to improved decision-making and behavioural change through community cohesion around relevant issues, particularly when information is transparent and accessible to stakeholders. Equally important is building trust and data literacy for decision makers and stakeholders, to ensure findings are understood and the best available science is used in decision-making. If knowledge and management actions are aligned with well-designed, effectively implemented and enforceable regulations, we can illuminate invisible water challenges, producing healthier river environments for the benefit of ecosystems and society. Through support of international cooperation and by considering water pollution challenges in their global context, LMICs may benefit from the experiences in UICs as well as from technological and legal advances (Figure 2), resulting in faster progress on solutions with less environmental degradation to generate a leapfrog effect for human wellbeing. DMH holds the UNESCO Chair in Water Science at the University of Birmingham and SK and DMH are leading the UNESCO UniTwin network on ‘Ecohydrological Interfaces’, which supported this work. The work of SK, DMH and IL is supported by the Institute of Global Innovation at the University of Birmingham under the Water Challenges theme. The work of CK is supported by the Foundational Program (Grant No. 12432010) from the USDA National Institute of Food and Agriculture.

While microplastic inputs into rivers are assumed to be correlated with anthropogenic activities and to accumulate towards the sea, the impacts of water management on downstream microplastic transport are largely unexplored. A comparative study of microplastic abundance in Boulder Creek (BC), and its less urbanized tributary South Boulder Creek (SBC), (Colorado USA), characterized the downstream evolution of microplastics in surface water and sediments, evaluating the effects of urbanization and flow diversions on the up-to-downstream profiles of microplastic concentrations and loads. Water and sediment samples were collected from 21 locations along both rivers and microplastic properties determined by fluorescence microscopy and Raman spectroscopy. The degree of catchment urbanization affected microplastic patterns, as evidenced by greater water and sediment concentrations and loads in BC than the less densely populated SBC, which is consistent with the differences in the degree of urbanization between both catchments. Microplastic removal through flow diversions was quantified, showing that water diversions removed over 500 microplastic particles per second from the river, and caused stepwise reductions of downstream loads at diversion points. This redistribution of microplastics back into the catchment should be considered in large scale models quantifying plastic fate and transport to the oceans.