Abstract Environmental tracers, including both elemental concentrations and isotope ratios, are widely used to reconstruct the movement patterns of animals throughout landscapes. The methodology involves creating a map that describes the distribution of the environmental tracer across the landscape, an isoscape and then matching the values of the same tracer in the tissue of the animal (teeth, fish otolith, feathers) to determine provenance at one or more life stages. Classification models are commonly used to assign an individual to different areas of the isoscape. However, many of the current classification models are data intensive and may not account for (i) spatial autocorrelation (i.e. where an animal has moved is a function of where it was previously) inherent to data sets that use environmental tracers, (ii) species' movement ability which can influence region assignment or (iii) the propagation of errors from misallocation of locations early in the otolith time series. Here, we introduce a Bayesian classification model to estimate large‐scale movement patterns over the lifetime of freshwater fish that has relatively low data requirements, integrates spatial autocorrelation, offers an avenue to include movement capabilities and quantifies the uncertainty associated with the classification of fish movement throughout its life. We use a simulation study to test the accuracy of this model and then demonstrate functionality using a small otolith microchemistry data set (four species of fish collected at two sites) and a 87Sr/86Sr isoscape from the Mitchell River (Queensland, Australia) that accounts for spatial and temporal variation in water 87Sr/86Sr using water and mussel shell samples. The probabilistic framework of the Monte Carlo simulation allows uncertainty to be incorporated at each life stage, reducing the cumulative impact of misclassification and providing a more reliable reconstruction of lifetime movement patterns.

Riparian restoration can improve water quality, reduce sources of risk and pathogen load in raw drinking water, sequester carbon, and increase biodiversity. However, there is uncertainty around the return on investment (ROI) in riparian restoration for decision-makers, as studies that estimate the spatial and temporal aspects of flood related and non-flood related monetary benefits to specific human populations are rare. We established the baseline cost and risk to the drinking water of Brisbane (population ∼ 1 million) due to variation in turbidity during floods with a range of magnitudes. Subsequently, the merits of 95 km of targeted riparian restoration were explored from the perspective of: 1. A ROI for the water supply utility that included avoided treatment costs and revenue from sale of carbon sequestration credits with biodiversity co-benefits; 2. A risk analysis for the water supply utility using reduced variability of treatment costs and disaster risk following riparian restoration; 3. A ROI for the region that included avoided population disease burden; and 4. A ROI for society that included avoided social cost of carbon. Substantial time was required before saplings delivered maximum water quality benefits. However, carbon sequestration and biodiversity enhancement benefits were delivered while trees were growing, and their inclusion allowed a modest positive ROI to the utility based on avoided operating costs alone. Including benefits from avoided population disease burden and avoided social cost of carbon increased the ROI further. Results demonstrated that if riparian restoration was appropriately targeted, it could address complex water challenges and contribute to global sustainability goals.

High-quality, yet less abundant, food sources sustain river food webs. However, the impacts of dams on these sources and their nutritional contributions to aquatic animals are not well understood. A substantial knowledge gap exists in understanding the quantitative contribution of basal food sources, such as periphyton, to aquatic animal diets and their role in supporting aquatic animals. This study fills this gap by using long-chain polyunsaturated fatty acids as biomarkers to quantify the dietary contributions of periphyton, submerged leaves, and macrophytes to macroinvertebrates and fish in dammed rivers. Our results showed that periphyton contributed more to the lipid diets of macroinvertebrates and fish than submerged leaves and macrophytes, regardless of dams, and was a high-quality food source supporting aquatic food webs. The dietary contribution of periphyton to macroinvertebrates decreased at the downstream sites of small dams compared to the upstream sites, while the contribution of leaves and macrophytes increased due to dam-induced changes in periphyton nutritional quality. The nutritional quality of periphyton correlated positively with its contribution to invertebrate grazers and filterers, while its contribution to fish varied depending on the feeding strategies of fish species. Our findings underscore periphyton as a high-quality dietary source in dammed river ecosystems and suggest that improving its nutritional quality enhances its contribution to the lipid diets of macroinvertebrates and fish.

The ecological roles of large predators are well recognized, but quantifying their functional impacts remains an active area of research. In this study, we examined the metabolic requirements and nutrient outputs of the estuarine crocodile population (Crocodylus porosus) in northern Australia over a 50-year period, during which the population increased from a few thousand to over 100 000 individuals. Bioenergetic modelling showed that during this period, the crocodile population's annual prey consumption increased from <20 kg km-2 in 1979 to approximately 180 kg km-2 in 2019. Further, the prey consumption increase was accompanied by a significant dietary shift from predominantly aquatic prey (approx. 65% in 1979) to a terrestrial-based diet (approx. 70% in 2019). A substantial portion of these terrestrial-derived nutrients was excreted into the water, significantly increasing the input rates of nitrogen (186-fold) and phosphorus (56-fold). The study shows that, despite being ectothermic, the high biomass of crocodiles within the environment generated nutrient inputs comparable to terrestrial endothermic predator populations. While crocodiles are apex predators, they are not considered to influence ecosystems in the same manner that large-bodied endothermic predators do. However, in the oligotrophic freshwater systems of northern Australia, the large volume of crocodile biomass is likely to impact the ecosystem through top-down and bottom-up processes.

Safe and just Earth system boundaries (ESBs) for surface water and groundwater (blue water) have been defined for sustainable water management in the Anthropocene. Here we assessed whether minimum human needs could be met with surface water from within individual river basins alone and, where this is not possible, quantified how much groundwater would be required. Approximately 2.6 billion people live in river basins where groundwater is needed because they are already outside the surface water ESB or have insufficient surface water to meet human needs and the ESB. Approximately 1.4 billion people live in river basins where demand-side transformations would be required as they either exceed the surface water ESB or face a decline in groundwater recharge and cannot meet minimum needs within the ESB. A further 1.5 billion people live in river basins outside the ESB, with insufficient surface water to meet minimum needs, requiring both supply- and demand-side transformations. These results highlight the challenges and opportunities of meeting even basic human access needs to water and protecting aquatic ecosystems.

Hydrological connectivity in river systems facilitates the movement of animals across riverine landscapes and influences fish habitat but is threatened by climate change and water resource developments. We studied fish assemblages across a large wet–dry tropical river system in northern Australia that is under consideration for new agricultural and water resource developments, which are expected to alter hydrological connectivity. We explored relationships between the environment and several biotic variables; fish taxonomic composition, species turnover, and species trait presence, quantifying how they were related to hydrological connectivity. Environmental dissimilarity of sites was influenced by hydrological connectivity variables, including flow, elevation, and river distance. Environment characteristics and hydrological connectivity together were important predictors of fish taxonomic composition. Fish species turnover was highest in headwater sites, and species presence absence was related to feeding and reproductive traits. Our results suggest that habitat specialists and species with reproductive traits that depend on hydrological connectivity, such as diadromous species, are most vulnerable to declines in relative abundance following a reduction in connectivity, which would lead to range contractions within catchments. Maintaining habitats that support taxonomically and functionally unique fish assemblages, such as wetlands and headwater streams, is important for maintaining biodiversity.

While eutrophication has led to serious habitat degradation and biotic shifts in freshwater ecosystems, most current studies have focused on changes in community assemblages, with few considering the effect of eutrophication on food webs. We conducted a field study in subtropical headwater streams with a gradient of nutrient levels to examine the effect of increasing water nutrients on food webs by using the long-chain polyunsaturated fatty acid eicosapentaenoic acid (EPA) as a measure of the nutritional quality of food. Basal food resources (macrophytes, submerged leaf litter, and periphyton), and aquatic consumers (macroinvertebrates and fish) were collected, and their fatty acid (FA) profiles were analyzed. Our results showed that periphyton was the dominant source of EPA for macroinvertebrates and fish, and a high-quality resource for consumers. As water nutrient concentrations increased, nutritional quality of periphyton significantly decreased and, in turn, the correlation between FA profiles of periphyton and macroinvertebrates declined. However, periphyton FA profiles did not account for the variability of fish FA, which may be induced by the increasing proportions of omnivorous fish in eutrophic streams that derive EPA from other sources. Further, the reduced periphyton EPA was associated with decreased trophic links and simplified stream food webs. Our study highlights the importance of high-quality food resources for aquatic food webs as water nutrients increase in stream ecosystems and provides a nutritional perspective to understand the mechanisms how eutrophication affects aquatic ecosystems.

Large predators have disproportionate effects on their underlying food webs. Thus, appropriately assigning trophic positions has important conservation implications both for the predators themselves and for their prey. Large-bodied predators are often referred to as apex predators, implying that they are many trophic levels above primary producers. However, theoretical considerations predict both higher and lower trophic position with increasing body size. Nitrogen stable isotope values (delta N-15) are increasingly replacing stomach contents or behavioral observations to assess trophic position and it is often assumed that ontogenetic dietary shifts result in higher trophic positions. Intraspecific studies based on delta N-15 values found a positive relationship between size and inferred trophic position. Here, we use datasets of predatory vertebrate ectotherms (crocodilians, turtles, lizards and fishes) to show that, although there are positive intraspecific relationships between size and delta N-15 values, relationships between stomach-content-based trophic level (TPdiet) and size are undetectable or negative. As there is usually no single value for N-15 trophic discrimination factor (TDF) applicable to a predator species or its prey, estimates of trophic position based on delta N-15 in ectotherm vertebrates with large size ranges, may be inaccurate and biased. We urge a reconsideration of the sole use of delta N-15 values to assess trophic position and encourage the combined use of isotopes and stomach contents to assess diet and trophic level.

Globally, there is an increasing trade-off between efforts to attain human water security and minimise ecosystem threats. Poorly planned efforts to meet current human water demands may jeopardise the ecological integrity of the river basins and may put future water security at risk. This article assesses the trade-offs between the future human and ecosystem water security threats in the South East Queensland Region, Australia. The article evaluates future water security threats based on a set of water and ecosystem services-related indicators and an ensemble of all climate change scenarios and models on water flow. It predicts the hotspots with higher human water security and ecosystem threats in 2040. The article finds that 52.1 per cent of the region may be affected in 2040 with either higher human water security risk or ecosystem services risk and, in some cases, with both. The challenge of managing the trade-offs is complex in regions with higher economic activities, which cover 16.3 per cent of the region. The research recommends that mitigation measures that address human water provision and nature conservation will need to be balanced and put at the same level of importance in these regions that are more affected.

Organisms at the base of stream food webs are typically poor in long-chain polyunsaturated fatty acids (LC-PUFA), especially in docosahexaenoic acid (DHA), whereas consumers at higher trophic levels are often rich in LC-PUFA. For example, fish tissues, especially the brain, are DHA-rich. This obvious mismatch between consumer LC-PUFA and their basal dietary supply may result from selective retention and/or endogenous conversion of dietary precursors to LC-PUFA. To determine which is more likely, we investigated compound-specific carbon stable isotopes in PUFA (delta C-13(PUFA)) of potential basal resources (stream epilithon, leaf litter) and consumers (invertebrates, European bullhead, and two salmonid species and their brain, eye, liver, and muscle tissues). We predicted that consumer-PUFA, depleted in C-13 values relative to their dietary sources, would indicate internal de novo PUFA synthesis. Alternatively, higher consumer-delta C-13(PUFA) values would imply selective retention of aquatic (epilithon and conditioned leaves) rather than terrestrial resources or internal production, irrespective of trophic levels. Invertebrate grazers and predators resembled delta C-13 values of essential fatty acids (delta C-13(EFA)) of benthic algae, while shredder-delta C-13(EFA) values reflected those of conditioned rather than fresh leaves. Lower eye-delta C-13(EFA) values of salmonids than in livers indicated high retention of dietary PUFA sources (invertebrates and epilithon). Stable isotope values of eicosapentaenoic acid suggest that all consumers retained algal EPA, while insectivorous fish produced DHA in their liver. There is no further evidence from carbon stable isotopes for local PUFA conversion within neural tissues. Our study demonstrates that delta C-13(PUFA) can be used to track sources of these highly functional molecules in aquatic consumers and highlights the importance of algal derived-PUFA for these consumers in oligotrophic headwater streams.

Policymakers are keen to know where and when to combine green infrastructure investments with grey infrastructure to mitigate the growing water security threats. The paper uses a simulated dynamic optimization model to evaluate grey (traditional water infrastructure) and green (reforestation and nature conservation) investment allocation over time and space to mitigate water security threats. The paper illustrates the model with a case study in the Velhas river basin, Brazil, and assesses the economic impact of the integration between green and grey infrastructure in the river basin. The study finds that the spatial and temporal combination of green and grey investments can save 24% of the total cost compared to when water security threats are mitigated only with grey investments. The paper pinpoints where the investment in grey infrastructure could be deferred with appropriate green investment in place.

Dams have disrupted natural river systems worldwide and although population and community level effects on aquatic biota have been well documented, food web responses remain poorly understood and difficult to characterize. The application of stable isotope analysis (SIA) provides a means to assess the effect of dams on food webs. Here we review the effect of dams on aquatic food webs using SIA, aiming to detect knowledge gaps in the field of dam impacts on aquatic food webs and propose a conceptual framework to help formulate hypotheses about dam impacts on food webs guided by food web theory. Dams can affect aquatic food webs via two pathways: a bottom-up pathway with altered basal food sources and their transfer to consumers through changes in flow, nutrients, temperature and sediment, and a top-down pathway with consumer species composition altered mainly through habitat fragmentation and related physiochemical changes. Taking these mechanisms into consideration, the impact of dams on food web attributes derived from SIA was evaluated. These studies generally apply mixing models to determine how dams alter the dominant carbon sources supporting food webs, use δ15N to examine how dams alter food-chain length, or use Layman metrics of isotope variability to assess niche changes for invertebrate and fish assemblages. Most studies compare the patterns of SIA metrics spatially (e.g. upstream vs reservoir vs downstream of dams; regulated vs unregulated rivers) and temporally (before vs after dam construction), without explicit hypotheses and/or links to theoretical concepts of food webs. We propose several steps to make SIA studies of dam impacts more rigorous and enhance their potential for producing novel insights. Future studies should quantify the shape and strength of the effect of dams on SIA-measured food web response, be conducted at larger temporal and spatial scales (particularly along the river longitudinal continuum and the lateral connected ecosystems (e.g., floodplains)), and consider effects of dams on food web resilience and tipping points.

The stability and resilience of the Earth system and human well-being are inseparably linked1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future.

Traditionally, trophic ecology research on aquatic ecosystems has focused more on the quantity of dietary energy flow within food webs rather than food quality and its effects on organisms at various trophic levels. Recent studies emphasize that food quality is central to consumer growth and reproduction, and the importance of food quality for aquatic ecosystems has become increasingly well recognized. It is timely to synthesise these findings and identify potential future research directions. We conducted a systematic review of omega-3 polyunsaturated fatty acids (ω3-PUFAs) as a crucial component of high-quality food sources in freshwater ecosystems to evaluate their impact on a variety of consumers, and explore the effects of global change on these high-quality food sources and their transfer to higher trophic consumers within and across ecosystems. In freshwater ecosystems, algae rich in ω3 long-chain PUFAs, such as diatoms, dinoflagellates and cryptophytes, represent important high-quality food sources for consumers, whereas cyanobacteria, green algae, terrestrial vascular plants and macrophytes low in ω3 long-chain PUFAs are low-quality food sources. High-quality ω3-PUFA-containing food sources usually lead to increased growth and reproduction of aquatic consumers, e.g. benthic invertebrates, zooplankton and fish, and also provide ω3 long-chain PUFAs to riparian terrestrial consumers via emergent aquatic insects. Consumers feeding on high-quality ω3-PUFA-containing foods in turn represent high-quality food for their own predators. However, the ω3-PUFA content of food sources is sensitive to global environmental changes. Warming, eutrophication, increased light intensity (e.g. from loss of riparian shading), and pollutants potentially inhibit the synthesis of algal ω3-PUFAs while at the same time promoting the growth of lower-quality foods, such as cyanobacteria and green algae. These factors combined could lead to a significant reduction in the availability of ω3-PUFAs for consumers and constrain their overall fitness. Although the effect of individual environmental factors on high-quality ω3-PUFA-containing food sources has been investigated, multiple environmental factors (e.g. climate change, human activities, pollution) will act in combination and any synergistic effects on aquatic food webs remain unclear. Identifying the sources and fate of ω3-PUFAs within and across ecosystems could represent an important approach to understand the impact of multiple environmental factors on trophic relationships and the implications for populations of freshwater and riparian consumers. Maintaining the availability of high-quality ω3-PUFA-containing food sources may also be key to mitigating freshwater biodiversity loss due to global change.

1. Variation in river flow is a strong behavioural determinant for the movement of many freshwater fish species and often is linked to key aspects of their life cycle. The alteration of natural flow regimes to meet human water demands can result in changes to this variability, and cause declines in water-dependent biota. Environmental flows are used as a remediation tool in some regulated rivers with the intention of restoring aspects of the natural flow regime to benefit riverine species, although empirical data are required to inform the efficacy of these interventions. 2. Using acoustic telemetry, we quantified the movement responses of two large-bodied native fish species (freshwater catfish, Tandanus tandanus and Murray cod, Maccullochella peelii) to variations in river flow over 4 years in two intermittent regulated rivers in the northern Murray-Darling Basin, Australia. Both rivers received periodic environmental flow releases and also there were several large natural flow events during the study period. 3. Both species displayed a range of intra-specific movement behaviours. Analysis of individuals' movements revealed five distinct functional groups, which were represented in both species. We found that periods of environmental flow delivery played an important role in the movement behaviour of both species. Murray cod were more likely to move during periods of environmental flow releases, whereas freshwater catfish were less likely to move on an environmental release following higher antecedent flows. 4. No large-scale philopatric movements were observed, yet Murray cod were more likely to move during the breeding period, indicative of nest site selection. We also found that the likelihood of movement in both species was higher in the smaller of the two rivers for a given magnitude of flow. 5. Our results suggest that environmental flows may benefit certain fish species by facilitating, rather than cueing breeding behaviours, allowing individuals improved access to, and provision of, higher-quality nesting habitats. 6. These findings will aid water managers in creating economical and targeted environmental flow releases, timing larger flow pulses for the species with flow-cued reproductive strategies, and providing a continuity of smaller baseflows for species whose reproduction is not dependent on flow cues but may be enhanced by greater connectivity.

The Amazon River basins present distinct natural and anthropogenic characteristics that influence the structure of stream habitats and their associated biota. The influence of these characteristics can be evaluated through different spatial scales. We aimed to assess the influence (with and without the effect of spatial-geographical factors) of local, macroscale, and land-use variables in the structure of stream fish assemblages of Amazonian catchments with different deforestation levels. A partial redundancy analysis and a reduced metrics model were used to assess these influences. With geographic-spatial effects, we verified that the macroscale and local variables explained the variation in fish composition, and, without the effects, land use also explained the variation in this composition. In the forested catchments, the biota was associated with streams with natural characteristics (e.g., leaf banks). In the deforested catchments, it was associated with land use, sandy catchments with higher soil density (higher capacity of degradation), and less complex streams (fewer leaf banks, more sand). The associated fish have life features linked to these characteristics (e.g., Gymnorhamphichthys rondoni associated with sand). This configuration seems to be a result of both the impact of land use in the catchment (i.e., increased erosion, increased sedimentation) and the naturally sandy constitution of the catchment as well, reflecting the sandy substrate.