Untangling the effects of multiple human stressors and their impacts on fish assemblages in European running waters

The impacts of alien species on river bioassessment

The extent of alien taxa impacts on river ecosystem health is unclear, but their frequency continues to rise. We investigated 1) the prevalence of including alien taxa in common bioindicators used in river bioassessment, 2) the effect of alien taxa on the richness and abundance of natives, and 3) whether including alien taxa in bioassessment tools increased their sensitivity to river degradation. In the 17 countries analyzed fish represented the greatest number of alien species (1726), followed by macrophytes (925), macroinvertebrates (556), and diatoms (7). Yet, alien species are only distinguished from natives in some fish and macrophyte indices. In addition, the analyses of 8 databases with fish, macroinvertebrate, or macrophyte data showed that abundance of alien taxa was associated with different stressors and pressures resulting in river degradation, and had a significant effect on native community composition. When alien species were accounted for, there was a strong negative correlation between the values of a fish index with alien richness and abundance while when alien taxa was not or only partially considered the results varied. Thus, we recommend: 1) Include specific metrics for alien species in biological quality indices. 2) Increase the investigation of alien taxa of small organisms (e.g. diatoms, small benthic invertebrates). 3) Eliminate sites with confirmed biological invasions for use as reference sites. 4) Remove alien from calculations of total richness and diversity. 5) Identify to the species level in biomonitoring programs. 6) Avoid legislation and management that protect alien species. 7) Encourage behaviors that prevent alien invasions of aquatic biota.

Freshwater fish in mid and northern German rivers - Long-term trends and associated species traits

To understand biodiversity change and support conservation decision-making, estimates of species' long-term population trends at regional and national scales are essential. However, such estimates are missing for many freshwater taxa, despite the diverse range of threats that they face. For this study, we mobilised monitoring data on riverine freshwater fish abundance collected across different regions of Germany. We applied generalized mixed effect models to estimate the population trends for 55 native species and 11 non-native species between 2004 and 2020. In addition, we used boosted regression trees to identify trait-based predictors of species trends and assessed their predictive ability. We found evidence of increasing abundance trends for 14 species and decreasing trends for 15 species; while the remaining species were mostly stable (26 species). Native species were more typically decreasing than increasing (14 vs 10 species); while non-native species were more often increasing (4 vs 1 species). Important traits associated with trends were maximum length, spawning temperature, and water quality tolerance, with small species, those spawning at high temperatures, and those preferring unpolluted waters, being most likely to have positive trends. Despite these associations, overall trait-based models showed limited power to predict trends in terms of direction as well as magnitude. Our results highlight the ongoing change in riverine fish communities and the importance of on-going species-level monitoring. The trait-based associations also indicate climate change and invasive species as important drivers of change in European freshwater rivers.

Multiple climate change stressors reduce the emergence success of gravel-spawning fish species and alter temporal emergence patterns

Climate change, with its profound effects on stream sediment, hydrological, and temperature dynamics, will exacerbate impacts on habitat conditions for many species, particularly those with vulnerable early life stages relying on the hyporheic zone, such as gravel-spawning fishes. Due to the complex and interactive nature of multiple stressor effects, we employed large-scale outdoor mesocosms to systemically test how the reproductive success of three gravel-spawning fish species brown trout (Salmo trutta), nase, (Chrondrostoma nasus) and Danube salmon (Hucho hucho) was affected by individual and combined effects of warming (+3-4 °C), fine sediment (increase in <0.85 mm by 22 %) and low-flow (eightfold discharge-reduction). Fine sediment had the most detrimental effect on emergence rate and fry length in all three species, reducing the emergence rate to zero in brown trout, 9 % in nase, and 4 % in Danube salmon. The emergence mortality caused by fine sediment surpassed that of hatching distinctly, suggesting that negative effects due to hypoxia were considerably exacerbated by entombment. Warming had only minor effects as a single stressor, but low flow reduced emergence rates of the spring spawning species nase and Danube salmon by 8 and 50 %, respectively. In combined treatments including fine sediment, however, the emergence success of all three species responded strongly negatively, even in the cyprinid species nase, which showed little interactive effects between stressors regarding hatching success. Warming and fine sediment also led to the earlier emergence of fry, implying a risk of asynchrony with available food resources. This study dramatically shows that climate change can have deleterious impacts on the reproductive success of gravel-spawning fish species, irrespective of taxonomic or ecological traits.

Development of a probabilistic risk model for pharmaceuticals in the environment under population and wastewater treatment scenarios

Preparing for future environmental pressures requires projections of how relevant risks will change over time. Current regulatory models of environmental risk assessment (ERA) of pollutants such as pharmaceuticals could be improved by considering the influence of global change factors (e.g., population growth) and by presenting uncertainty more transparently. In this article, we present the development of a prototype object-oriented Bayesian network (BN) for the prediction of environmental risk for six high-priority pharmaceuticals across 36 scenarios: current and three future population scenarios, combined with infrastructure scenarios, in three Norwegian counties. We compare the risk, characterized by probability distributions of risk quotients (RQs), across scenarios and pharmaceuticals. Our results suggest that RQs would be greatest in rural counties, due to the lower development of current wastewater treatment facilities, but that these areas consequently have the most potential for risk mitigation. This pattern intensifies under higher population growth scenarios. With this prototype, we developed a hierarchical probabilistic model and demonstrated its potential in forecasting the environmental risk of chemical stressors under plausible demographic and management scenarios, contributing to the further development of BNs for ERA. Integr Environ Assess Manag 2024;20:1715-1735. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Linking wastewater treatment plant effluents to water quality and hydrology: Effects of multiple stressors on fish communities

Wastewater treatment plants (WWTPs) are essential for maintaining a good water quality of surface waters. However, WWTPs are also associated with water quality deterioration and hydro-morphological alteration. Riverine communities respond to these stressors with changes in their community structure, abundance and diversity. In this study, we used a dataset of 94 monitoring sites across North Rhine-Westphalia, Germany to investigate the influence of WWTPs on the water quality and hydro-morphological quality in river sections downstream of WWTP effluents. More specifically, we analyzed the effects of the percentage of WWTP effluents (in relation to median base flow) on four stressor groups (physico-chemistry, micropollutants, hydrological and morphological alteration) using Linear Mixed Models (LMM). Furthermore, we assessed the impact of a selection of twelve ecologically relevant stressor variables reflecting water quality deterioration and hydro-morphological alteration on reference fish communities using Canonical Correspondence Analysis (CCA). The percentage of WWTP effluents was correlated with water quality, especially with toxic units of a wide range of pharmaceuticals including diclofenac, venlafaxine and sulfamethoxazole (R² up to 0.54) as well as specific pesticides (e.g., terbutryn: R² = 0.33). The correlation of percent WWTP effluents with hydro-morphological alteration was weaker and most pronounced for the frequency of high flow (R² = 0.24) and flow variability (R² = 0.19). About 40 % of the variance in the fish community structure were explained by 12 stressor variables in the CCA models. Water quality and hydrological, but not morphological stressors showed strong albeit highly variable effects on individual fish species. The results indicate that water quality degradation and hydrological alteration are important factors determining the ecological status of fish communities. In this context, WWTP effluents can impose relevant point sources of pollution that affect water quality but also cause alterations of the hydrological regime. Further management measures addressing both stressor groups are needed to improve the ecological status.

A systematic review of approaches to assess fish health responses to anthropogenic threats in freshwater ecosystems

Anthropogenic threats such as water infrastructure, land-use changes, overexploitation of fishes and other biological resources, invasive species and climate change present formidable challenges to freshwater biodiversity. Historically, management of fish and fishery species has largely been based on studies of population- and community-level dynamics; however, the emerging field of conservation physiology promotes the assessment of individual fish health as a key management tool. Fish health is highly sensitive to environmental disturbances and is also a fundamental driver of fitness, with implications for population dynamics such as recruitment and resilience. However, the mechanistic links between particular anthropogenic disturbances and changes in fish health, or impact pathways, are diverse and complex. The diversity of ways in which fish health can be measured also presents a challenge for researchers deciding on methods to employ in studies seeking to understand the impact of these threats. In this review, we aim to provide an understanding of the pathway through which anthropogenic threats in freshwater ecosystems impact fish health and the ways in which fish health components impacted by anthropogenic threats can be assessed. We employ a quantitative systematic approach to a corpus of papers related to fish health in freshwater and utilize a framework that summarizes the impact pathway of anthropogenic threats through environmental alterations and impact mechanisms that cause a response in fish health. We found that land-use changes were the most prolific anthropogenic threat, with a range of different health metrics being suitable for assessing the impact of this threat. Almost all anthropogenic threats impacted fish health through two or more impact pathways. A robust understanding of the impact pathways of anthropogenic threats and the fish health metrics that are sensitive to these threats is crucial for fisheries managers seeking to undertake targeted management of freshwater ecosystems.

Multiple stressors alter greenhouse gas concentrations in streams through local and distal processes

Streams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in-situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2-equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams.

Multiple lines of evidence point to pesticides as stressors affecting invertebrate communities in small streams in five United States regions

Multistressor studies were performed in five regions of the United States to assess the role of pesticides as stressors affecting invertebrate communities in wadable streams. Pesticides and other chemical and physical stressors were measured in 75 to 99 streams per region for 4 weeks, after which invertebrate communities were surveyed (435 total sites). Pesticides were sampled weekly in filtered water, and once in bed sediment. The role of pesticides as a stressor to invertebrate communities was assessed by evaluating multiple lines of evidence: toxicity predictions based on measured pesticide concentrations, multivariate models and other statistical analyses, and previously published mesocosm experiments. Toxicity predictions using benchmarks and species sensitivity distributions and statistical correlations suggested that pesticides were present at high enough concentrations to adversely affect invertebrate communities at the regional scale. Two undirected techniques-boosted regression tree models and distance-based linear models-identified which pesticides were predictors of (respectively) invertebrate metrics and community composition. To put insecticides in context with known, influential covariates of invertebrate response, generalized additive models were used to identify which individual pesticide(s) were important predictors of invertebrate community condition in each region, after accounting for natural covariates. Four insecticides were identified as stressors to invertebrate communities at the regional scale: bifenthrin, chlordane, fipronil and its degradates, and imidacloprid. Fipronil was particularly important in the Southeast region, and imidacloprid, bifenthrin, and chlordane were important in multiple regions. For imidacloprid, bifenthrin, and fipronil, toxicity predictions were supported by mesocosm experiments that demonstrated adverse effects on naïve aquatic communities when dosed under controlled conditions. These multiple lines of evidence do not prove causality-which is challenging in the field under multistressor conditions-but they make a strong case for the role of insecticides as stressors adversely affecting invertebrate communities in streams within the five sampled regions.

Cross-continental evaluation of landscape-scale drivers and their impacts to fluvial fishes: Understanding frequency and severity to improve fish conservation in Europe and the United States

Fluvial fishes are threatened globally from intensive human landscape stressors degrading aquatic ecosystems. However, impacts vary regionally, as stressors and natural environmental factors differ between ecoregions and continents. To date, a comparison of fish responses to landscape stressors over continents is lacking, limiting understanding of consistency of impacts and hampering efficiencies in conserving fishes over large regions. This study addresses these shortcomings through a novel, integrative assessment of fluvial fishes throughout Europe and the conterminous United States. Using large-scale datasets, including information on fish assemblages from more than 30,000 locations on both continents, we identified threshold responses of fishes summarized by functional traits to landscape stressors including agriculture, pasture, urban area, road crossings, and human population density. After summarizing stressors by catchment unit (local and network) and constraining analyses by stream size (creeks vs. rivers), we analyzed stressor frequency (number of significant thresholds) and stressor severity (value of identified thresholds) within ecoregions across Europe and the United States. We document hundreds of responses of fish metrics to multi-scale stressors in ecoregions across two continents, providing rich findings to aid in understanding and comparing threats to fishes across the study regions. Collectively, we found that lithophilic species and, as expected, intolerant species are most sensitive to stressors in both continents, while migratory and rheophilic species are similarly strongly affected in the United States. Also, urban land use and human population density were most frequently associated with declines in fish assemblages, underscoring the pervasiveness of these stressors in both continents. This study offers an unprecedented comparison of landscape stressor effects on fluvial fishes in a consistent and comparable manner, supporting conservation of freshwater habitats in both continents and worldwide.

Identifying the impact of toxicity on stream macroinvertebrate communities in a multi-stressor context based on national ecological and ecotoxicological monitoring databases

In situ bioassays are used to measure the harmful effects induced by mixtures of toxic chemicals in watercourses. In France, national-scale biomonitoring data are available including invertebrate surveys and in-field chemical toxicity measures with caged gammarids to assess environmental toxicity of mixtures of chemicals. The main objective of our study is to present a proof-of-concept approach identifying possible links between in-field chemical toxicity, stressors and the ecological status. We used two active biomonitoring databases comprising lethal toxicity (222 in situ measures of gammarid mortality) and sublethal toxicity (101 in situ measures of feeding inhibition). We measured the ecological status of each active biomonitoring site using the I₂M₂ metric (macroinvertebrate-based multimetric index), accounted for known stressors of nutrients and organic matter, hydromorphology and chemical toxicity. We observed a negative relationship between stressors (hydromorphology, nutrients and organic matter, and chemical toxicity) and the good ecological status. This relationship was aggravated in watercourses where toxicity indicators were degraded. We validated this hypothesis for instance with nutrients and organic matter like nitrates or hydromorphological conditions like percentage of vegetation on banks. Future international assesments concerning the role of in-field toxic pollution on the ecological status in a multi-stressor context are now possible via the current methodology.

Identifying Key Stressors Driving Biological Impairment in Freshwater Streams in the Chesapeake Bay Watershed, USA

Biological communities in freshwater streams are often impaired by multiple stressors (e.g., flow or water quality) originating from anthropogenic activities such as urbanization, agriculture, or energy extraction. Restoration efforts in the Chesapeake Bay watershed, USA seek to improve biological conditions in 10% of freshwater tributaries and to protect the biological integrity of existing healthy watersheds. To achieve these goals, resource managers need to better understand which stressors are most likely driving biological impairment. Our study addressed this knowledge gap through two approaches: 1) reviewing and synthesizing published multi-stressor studies, and 2) examining 303(d) listed impairments linked to biological impairment as identified by jurisdiction regulatory agencies (the states within the watershed and the District of Columbia). Results identified geomorphology (i.e., physical habitat), salinity, and toxic contaminants as important for explaining variability in benthic community metrics in the literature review. Geomorphology (i.e., physical habitat and sediment), salinity, and nutrients were the most reported stressors in the jurisdictional impairment analysis. Salinity is likely a major stressor in urban and mining settings, whereas geomorphology was commonly reported in agricultural settings. Toxic contaminants, such as pesticides, were rarely measured; more research is needed to quantify the extent of their effects in the region. Flow alteration was also highlighted as an important urban stressor in the literature review but was rarely measured in the literature or reported by jurisdictions as a cause of impairment. These results can be used to prioritize stressor monitoring by managers, and to improve stressor identification methods for identifying causes of biological impairment.

Perceived multiple stressor effects depend on sample size and stressor gradient length

Multiple stressors are continuously deteriorating surface waters worldwide, posing many challenges for their conservation and restoration. Combined effect types of multiple stressors range from single-stressor dominance to complex interactions. Identifying prevalent combined effect types is critical for environmental management, as it helps to prioritise key stressors for mitigation. However, it remains unclear whether observed single and combined stressor effects reflect true ecological processes unbiased by sample size and length of stressor gradients. Therefore, we examined the role of sample size and stressor gradient lengths in 158 paired-stressor response cases with over 120,000 samples from rivers, lakes, transitional and marine ecosystems around the world. For each case, we split the overall stressor gradient into two partial gradients (lower and upper) and investigated associated changes in single and combined stressor effects. Sample size influenced the identified combined effect types, and stressor interactions were less likely for cases with fewer samples. After splitting gradients, 40 % of cases showed a change in combined effect type, 30 % no change, and 31 % showed a loss in stressor effects. These findings suggest that identified combined effect types may often be statistical artefacts rather than representing ecological processes. In 58 % of cases, we observed changes in stressor effect directions after the gradient split, suggesting unimodal stressor effects. In general, such non-linear responses were more pronounced for organisms at higher trophic levels. We conclude that observed multiple stressor effects are not solely determined by ecological processes, but also strongly depend on sampling design. Observed effects are likely to change when sample size and/or gradient length are modified. Our study highlights the need for improved monitoring programmes with sufficient sample size and stressor gradient coverage. Our findings emphasize the importance of adaptive management, as stress reduction measures or further ecosystem degradation may change multiple stressor-effect relationships, which will then require associated changes in management strategies.

Ratio of abundances of ciliates behavioral groups as an indicator of the treated wastewater impact on rivers

A method for assessing the degree of impact of wastewater treatment plant discharge on receiving rivers was proposed, based on the structural indicators of the population of ciliated protozoa. It was shown that the ratio of attached, crawling and free-swimming forms in bottom sediments changes under the influence of discharge. In the points subject to organic pollution, the share of attached filter-feeding bacteriovorous ciliates increases in the assemblage of ciliated protozoa. The proposed Attached Form Index (AFI) takes this ratio into account. The use of AFI makes it possible to assess the restructuring of the assemblage of ciliated protozoa under the influence of point sources of pollution, to establish a zone of negative influence of runoff, to assess the degree of restoration of the aquatic ecosystem, as the influence of the pollution source weakened.

Protected area characteristics that help waterbirds respond to climate warming

Protected area networks help species respond to climate warming. However, the contribution of a site's environmental and conservation-relevant characteristics to these responses is not well understood. We investigated how composition of nonbreeding waterbird communities (97 species) in the European Union Natura 2000 (N2K) network (3018 sites) changed in response to increases in temperature over 25 years in 26 European countries. We measured community reshuffling based on abundance time series collected under the International Waterbird Census relative to N2K sites' conservation targets, funding, designation period, and management plan status. Waterbird community composition in sites explicitly designated to protect them and with management plans changed more quickly in response to climate warming than in other N2K sites. Temporal community changes were not affected by the designation period despite greater exposure to temperature increase inside late-designated N2K sites. Sites funded under the LIFE program had lower climate-driven community changes than sites that did not received LIFE funding. Our findings imply that efficient conservation policy that helps waterbird communities respond to climate warming is associated with sites specifically managed for waterbirds.

River habitat assessment and restoration in high dam flood discharge systems with total dissolved gas supersaturation

The success of river habitat restoration relies on accurate assessment proxies. However, determining how to quantitatively assess the impact of multiple stressors during flood discharge from high dams in riverine ecosystems and where and how to implement more reliable recovery interventions remain challenges. Here, we developed a bottom-up mechanistic framework for assessing the effects of total dissolved gas supersaturation (TDGS) and hydrodynamics on fish habitat quality and applied it to the downstream river reach of the Xiangjiaba Dam in Southwest China. The results showed that the available habitat area of river sturgeon was the smallest, while Chinese sucker had the largest available habitat area among the three target species under all discharge scenarios. Although the TDGS levels were evenly mixed laterally, the habitat suitability index indicated that the suitable habitats were primarily within both sides of the river reach under all scenarios, which is contrary to findings based on the traditional TDGS risk assessment model. The traditional TDGS risk assessment model overestimates the impact of dams on habitats. This divergence reflected the sensitivity of the habitat assessment to fish habitat preferences, fish tolerance to TDGS and the biological response of fish under TDGS. Additionally, the priority areas for restoration can be identified by habitat suitability index with lower values. We simulated twenty-four schemes and found that interventions such as stone groups, ecological spur dike, water-retaining weir and river dredging can enhance habitat suitability for fish species under multiple stressors, providing novel insights into where and how to mitigate the impact of TDGS. Our findings offer a transferable framework for the quantitative evaluation of fish habitat and implementation of restoration management during dam flood discharge periods, thus providing a new perspective for biodiversity conservation and habitat restoration in dam-regulated rivers with TDGS around the world.

Physical habitat in conterminous US streams and rivers, Part 1: Geoclimatic controls and anthropogenic alteration

Anthropogenic alteration of physical habitat structure in streams and rivers is increasingly recognized as a major cause of impairment worldwide. As part of their assessment of the status and trends in the condition of rivers and streams in the U.S., the U.S. Environmental Protection Agency's (USEPA) National Aquatic Resource Surveys (NARS) quantify and monitor channel size and slope, substrate size and stability, instream habitat complexity and cover, riparian vegetation cover and structure, anthropogenic disturbance activities, and channel-riparian interaction. Like biological assemblages and water chemistry, physical habitat is strongly controlled by natural geoclimatic factors that can obscure or amplify the influence of human activities. We developed a systematic approach to estimate the deviation of observed river and stream physical habitat from that expected in least-disturbed reference conditions. We applied this approach to calculate indices of anthropogenic alteration of three aspects of physical habitat condition in the conterminous U.S. (CONUS): streambed sediment size and stability, riparian vegetation cover, and instream habitat complexity. The precision and responsiveness of these indices led the USEPA to use them to evaluate physical habitat condition in CONUS rivers and streams. The scores of these indices systematically decreased with greater anthropogenic disturbance at river and stream sites in the CONUS and within ecoregions, which we interpret as a response of these physical habitat indices to anthropogenic influences. Although anthropogenic activities negatively influenced all three physical habitat indices in the least-disturbed sites within most ecoregions, natural geoclimatic and geomorphic factors were the dominant influences. For sites over the full range of anthropogenic disturbance, analyses of observed/expected sediment characteristics showed augmented flood flows and basin and riparian agriculture to be the leading predictors of streambed instability and excess fine sediments. Similarly, basin and riparian agriculture and non-agricultural riparian land uses were the leading predictors of reduced riparian vegetation cover complexity in the CONUS and within ecoregions. In turn, these reductions in riparian vegetation cover and complexity, combined with reduced summer low flows, were the leading predictors of instream habitat simplification. We conclude that quantitative measures of physical habitat structure are useful and important indicators of the impacts of human activities on stream and river condition.

Nutrient limitation in Atlantic salmon rivers and streams: Causes, consequences, and management strategies

Freshwater catchments can experience nutrient deficits that result in reduced primary and secondary productivity. The most commonly limiting nutrients are nitrogen and phosphorus, either separately or together. This review considers the impact of increasing nutrient limitation in temperate basin stream and river systems, focusing on upland areas that currently or previously supported wild Atlantic salmon (Salmo salar) populations.Anthropogenic changes to land use and increases in river barriers have altered upland nutrient dynamics, with particular impacts on salmon and other migratory fish species which may be net importers of nutrients to upland streams. Declining salmon populations may further reduce nutrient sources, reducing ecosystem and fisheries productivity below desired levels.Experimental manipulations of nutrient levels have examined the impacts of this cultural oligotrophication. There is evidence that growth and biomass of juvenile salmon can be increased via appropriate additions of nutrients, offering potential as a conservation tool. However, further research is required to understand the long-term effects of these additions on salmon populations and stream ecosystems, and to assess the vulnerability of downstream habitats to eutrophication as a result.Although purposeful nutrient addition with the aim of enhancing and conserving salmonid populations may be justified in some cases, it should be undertaken in an adaptive management framework. In addition, nutrient addition should be linked to nutrient retention and processing, and integrated into large-scale habitat restoration and recovery efforts.Both the scientific and the management community should recognize that the ecological costs and benefits associated with adding nutrients to salmon streams may change in a non-stationary world.

Stakeholders Perceptions of Non-indigenous Freshwater Fish Species: A Case Study from a Mediterranean Biodiversity Hotspot

Even though the ecological impacts of invasive alien species (IAS) have been well studied, social aspects of IAS in freshwater ecosystems are still poorly explored. This study aimed to assess the perceptions associated with non-indigenous freshwater fish species (NIFS) among environmental-related professional and non-professional stakeholders in a Mediterranean country that displays high levels of fish species endemism, by using a questionnaire survey approach. Overall, 203 individuals participated, from which, the majority (n = 144) were related to environmental sciences. Most of the respondents perceived NIFS to be a threat of national importance, possibly attributed to the emotional effect of the term non-indigenous (conceived as intruder). However, NIFS were indicated as the least important threat affecting aquatic ecosystems when compared with other pressures. This contrasting perception could be explained by the fact that broad-scale impacts (i.e., climate change) exert stronger reactions compared to NIFS. Interestingly, non-professionals were more successful in identifying NIFS and native species compared to professionals, however both indicated low overall identification success. The majority of the respondents, both professionals and non-professionals, considered that: (a) the official authorities do not implement actions for NIFS management, (b) government spending should be increased to manage NIFS even if it should be reduced for other needs, and (c) early detection is the best way to manage NIFS. The establishment of a nationwide network consisting of all related stakeholders on NIFS issues, aiming to public awareness and preventive management actions to limit the spread and impacts of NIFS should be set as a priority.

A comprehensive petrochemical vulnerability index for marine fishes in the Gulf of Mexico

Oil and gas extraction activities occur across the globe, yet species-specific toxicological information on the biological and ecological impacts of exposure to petrochemicals is lacking for the vast majority of marine species. To help prioritize species for recovery, mitigation, and conservation in light of significant toxicological data gaps, a trait-based petrochemical vulnerability index was developed and applied to the more than 1700 marine fishes present across the entire Gulf of Mexico, including all known bony fishes, sharks, rays and chimaeras. Using life history and other traits related to likelihood of exposure, physiological sensitivity to exposure, and population resiliency, final calculated petrochemical vulnerability scores can be used to provide information on the relative sensitivity, or resilience, of marine fish populations across the Gulf of Mexico to oil and gas activities. Based on current knowledge of traits, marine fishes with the highest vulnerability scores primarily occur in areas of high petrochemical activity, are found at or near the surface, and have low reproductive turnover rates and/or highly specialized diet and habitat requirements. Relative population vulnerability scores for marine fishes can be improved with additional toxicokinetic studies, including those that account for the synergistic or additive effect of multiple stressors, as well as increased research on ecological and life history traits, especially for deep living species.

Biomarker responses in perch (Perca fluviatilis) under multiple stress: Parasite co-infection and multicomponent metal mixture exposure

Parasitic infections may cause damage to the host immune system (i.e. fish), thereby endangering its health and weakening its responses to other types of stressors. Therefore, exposure to different kinds of natural or anthropogenic stressors can lead to unexpected toxicity outcomes in aquatic organisms. This study examined the haematological, genotoxic and cytotoxic effects of the co-infection with the protozoan parasite (Trichodina sp.) and the pathogenic oomycete (Saprolegnia parasitica) in Perca fluviatilis alone and in combination with chemical stress (environmentally-relevant aqueous concentrations of metal mixtures). Haematological analyses such as red cell and white cell indices revealed that chemical and biological stressors, used singly and in combination, exerted adverse effects on fish health. Changes in haematological indices induced by exposure to each of the above-mentioned stressors separately and by combined exposure to all of them suggested the multiple stress-induced inflammation process in the exposed fish. The cytogenetic damage inflicted by the S. parasitica and Trichodina sp. co-infection and multiple stress was revealed in fish erythrocytes. This information is expected to contribute to the elucidation of how multiple stressors impact on responses of haematic indices, geno- and cytotoxicity endpoints in P. fluviatilis. Assessment of the risk associated with multiple stressors is expected to prove valuable for the effective aquatic environment management (Løkke et al., 2013 and references therein).

Multiple in-stream stressors degrade biological assemblages in five U.S. regions

Biological assemblages in streams are affected by a wide variety of physical and chemical stressors associated with land-use development, yet the importance of combinations of different types of stressors is not well known. From 2013 to 2017, the U.S. Geological Survey completed multi-stressor/multi-assemblage stream ecological assessments in five regions of the United States (434 streams total). Diatom, invertebrate, and fish communities were enumerated, and five types of potential stressors were quantified: habitat disturbance, excess nutrients, high flows, basic water quality, and contaminants in water and sediment. Boosted regression tree (BRT) models for each biological assemblage and region generally included variables from all five stressor types and multiple stressors types in each model was the norm. Classification and regression tree (CART) models then were used to determine thresholds for each BRT model variable above which there appeared to be adverse effects (multi-metric index (MMI) models only). In every region and assemblage there was a significant inverse relation between the MMI and the number of stressors exerting potentially adverse effects. The number of elevated instream stressors often varied substantially for a given level of land-use development and the number of elevated stressors was a better predictor of biological condition than was development. Using the adverse effects-levels that were developed based on the BRT model results, 68% of the streams had two or more stressors with potentially adverse effects and 35% had four or more. Our results indicate that relatively small increases in the number of stressors of different types can have a large effect on a stream ecosystem.

Incorporating the life stages of fish into habitat assessment frameworks: A case study in the Baihetan Reservoir

Although it is widely accepted that the construction of dams may alter fish habitats, few studies have followed the life cycles of fish and combined the environmental conditions with the ecological behaviors and habit preferences of fish during reproductive processes to assess its effects of dam construction. In this study, we call for more sophisticated and holistic assessment framework, including effectiveness of technologies intended to mitigate environmental impacts in different life stages. An assessment framework that considers the swimming ability, perception ability of water flow and environmental preference of different fish species during migration, spawning and hatching was proposed. We used the Baihetan Reservoir as an example environment to assess the impoundment effect on the habitat of a tributary upstream of the reservoir. We observed shifts in the habitats of target fish in different life stages which is dominated by reservoir operation of the Baihetan Dam. Combined with the response of fish activities to impoundment, the selection of suitable positions for artificial breeding and release projects and the outlet of the fish transportation system were recommended measures to improve the migration possibilities. Our reassessment results also demonstrated the theoretical possibility and feasibility of joint improvements in spawning and hatching periods using instream structures. Our framework provides a complete set of "assessment-solution" processes for developers and managers to address the aquatic ecological degradation caused by resource development, and its use is strongly recommended for assessments or assessments of damming effects in other regions and on other fish species.

Major threats to European freshwater fish species

In Europe, freshwater fish are the richest group amongst European vertebrates and the second most threatened animal group, surpassed only by freshwater molluscs. The identification of threats is a major benefit for conservation efforts, as it allows actions to be bespoke to specific threats imperilling fish communities in sensitive areas. In this work, we analyse all threats identified under the International Union for Conservation of Nature (IUCN) Red List of Threatened Species for all European native freshwater dependent fish and lamprey species and relate them with the species distribution, conservation status and migratory phenology. Results show that the current level of imperilment of European freshwater fish fauna is high, especially in the Iberian Peninsula fish communities where low richness is combined with a proportion of threatened species surpassing 50% in several catchments The most relevant threats affecting European freshwater fish are: "Dams & Water Management/Use", "Droughts", "Invasive Non-Native/Alien Species/Diseases", "Agricultural & Forestry Effluents" and "Fishing & Harvesting Aquatic Resources". The present work contributes to the ultimate goal of species conservation by highlighting the main threats affecting freshwater fish species in Europe and by demonstrating how specific regions need particular attention. Increasing longitudinal connectivity stands out as a measure with the potential to increase species' resilience to the several threats affecting them, and it should be coupled with additional efforts to reduce water pollution, control alien species and effectively manage fishing.

Time marches on, but do the causal pathways driving instream habitat and biology remain consistent?

Stream ecosystems are complex networks of interacting terrestrial and aquatic drivers. To untangle these ecological networks, efforts evaluating the direct and indirect effects of landscape, climate, and instream predictors on biological condition through time are needed. We used structural equation modeling and leveraged a stream survey program to identify and compare important predictors driving condition of benthic macroinvertebrate and fish assemblages. We used data resampled 14 years apart at 252 locations across Maryland, USA. Sample locations covered a wide range of conditions that varied spatiotemporally. Overall, the relationship directions were consistent between sample periods, but their relative strength varied temporally. For benthic macroinvertebrates, we found that the total effect of natural landscape (e.g., elevation, longitude, latitude, geology) and land use (i.e., forest, development, agriculture) predictors was 1.4 and 1.5 times greater in the late 2010s compared to the 2000s. Moreover, the total effect of water quality (e.g., total nitrogen and conductivity) and habitat (e.g., embeddedness, riffle quality) was 1.2 and 4.8 times lower in the 2010s, respectively. For fish assemblage condition, the total effect of land use-land cover predictors was 2.3 times greater in the 2010s compared to the 2000s, while the total effect of local habitat was 1.4 times lower in the 2010s, respectively. As expected, we found biological assemblages in catchments with more agriculture and urban development were generally comprised of tolerant, generalist species, while assemblages in catchments with greater forest cover had more-specialized, less-tolerant species (e.g., Ephemeroptera, Plecoptera, and Trichoptera taxa, clingers, benthic and lithophilic spawning fishes). Changes in the relative importance of landscape and land-use predictors suggest other correlated, yet unmeasured, proximal factors became more important over time. By untangling these ecological networks, stakeholders can gain a better understanding of the spatiotemporal relationships driving biological condition to implement management practices aimed at improving stream condition.

Making waves. Bridging theory and practice towards multiple stressor management in freshwater ecosystems

Despite advances in conceptual understanding, single-stressor abatement approaches remain common in the management of fresh waters, even though they can produce unexpected ecological responses when multiple stressors interact. Here we identify limitations restricting the development of multiple-stressor management strategies and address these, bridging theory and practice, within a novel empirical framework. Those critical limitations include that (i) monitoring schemes fall short of accounting for theory on relationships between multiple-stressor interactions and ecological responses, (ii) current empirical modelling approaches neglect the prevalence and intensity of multiple-stressor interactions, and (iii) mechanisms of stressor interactions are often poorly understood. We offer practical recommendations for the use of empirical models and experiments to predict the effects of freshwater degradation in response to changes in multiple stressors, demonstrating this approach in a case study. Drawing on our framework, we offer practical recommendations to support the development of effective management strategies in three general multiple-stressor scenarios.

Skin damage caused by scale loss modifies the intestine of chronically stressed gilthead sea bream (Sparus aurata, L.)

The present study was designed to test if the damage caused by scale loss provokes a change in other innate immune barriers such as the intestine and how chronic stress affects this response. Sea bream (Sparus aurata) were kept in tanks at low density (16 kg m^-3, LD) or exposed to a chronic high density (45 kg m^-3, HD) stress for 4 weeks. Scales were then removed (approximately 50%) from the left flank in the LD and HD fish. Intestine samples (n = 8/group) were examined before and at 12 h, 3 days and 7 days after scale removal. Changes in the morphology of the intestine revealed that chronic stress and scale loss was associated with intestinal inflammation. Specifically, enterocyte height and the width of the lamina propria, submucosa and muscle layer were significantly increased (p < 0.05) 3 days after skin damage in fish under chronic stress (HD) compared to other treatments (LDWgut3d or HDgut0h). This was associated with a significant up-regulation (p < 0.05) in the intestine of gene transcripts for cell proliferation (pcna) and anti-inflammatory cytokine tgfβ1 and down-regulation of gene transcripts for the pro-inflammatory cytokines tnf-α and il1β (p < 0.05) in HD and LD fish 3 days after scale removal compared to the undamaged control (LDgut0h). Furthermore, a significant up-regulation of kit, a marker of mast cells, in the intestine of HDWgut3d and LDWgut3d fish suggests they may mediate the crosstalk between immune barriers. Skin damage induced an increase in cortisol levels in the anterior intestine in HDWgut12 h fish and significant (p < 0.05) down-regulation of mr expression, irrespective of stress. These results suggest glucocorticoid levels and signalling in the intestine of fish are modified by superficial cutaneous wounds and it likely modulates intestine inflammation.

Big data challenges in overcoming China's water and air pollution: relevant data and indicators

Big data are potentially useful for environmental management planning and actions that can be directed toward pollution control. China is using big data approaches to help reduce its current levels of pollution. However, also needed are better environmental indicators, measurement technologies, data management and reporting, and adaptive management and enforcement. Based on continental-extent monitoring and assessment programs in Europe and the USA, we recommend three major programmatic changes for China. (1) Establish long-term systemic environmental and human health objectives and indicators. (2) Adopt national standard methods for survey designs, sampling and analytical protocols, statistical analyses, and collaborative sampling programs. (3) Provide a transparent process for reporting and correcting data errors.

Variability in Population Traits of a Sentinel Iberian Fish in a Highly Modified Mediterranean-Type River

Human pressures on water resources have been suggested as a driver of biological traits that induce changes in native fish populations. This study highlighted the interplay between environmental stress factors, mostly related to flow regulation, and the longitudinal river gradient in biological traits such as the growth, size structure and somatic condition of a sentinel fish, Luciobarbus sclateri. We found an increase in size-related metrics and somatic condition at population levels associated with downstream reaches, although fragmentation and habitat alteration, flow regime alteration and the abundance of non-native fish were also significantly involved in their variability. Age-related parameters and growth were only explained by flow regime alterations and the abundance of non-native fish species. The high plasticity observed in L. sclateri population traits suggests that this is a key factor in the species adaptability to resist in a strongly altered Mediterranean river basin. However, the interplay of multiple stressors plays an important role in fish population dynamics and could induce complex responses that may be essential for long-term monitoring in sentinel species.

Intraspecific variability of responses to combined metal contamination and immune challenge among wild fish populations

Wild organisms are increasingly exposed to multiple anthropogenic and natural stressors that can interact in complex ways and lead to unexpected effects. In aquatic ecosystems, contamination by trace metals has deleterious effects on fish health and commonly co-occurs with pathogens, which affect similar physiological and behavioral traits. However, the combined effects of metal contamination and parasitism are still poorly known. In addition, the sensitivity to multiple stressors could be highly variable among different fish populations depending on their evolutionary history, but this intraspecific variability is rarely taken into account in existing ecotoxicological studies. Here, we investigated i) the interactive effects of metal contamination (i.e., realistic mixture of Cd, Cu and Zn) and immune challenge mimicking a parasite attack on fish health across biological levels. In addition, we compared ii) the physiological and behavioral responses among five populations of gudgeon fish (Gobio occitaniae) having evolved along a gradient of metal contamination. Results show that single stressors exposure resulted in an increase of immune defenses and oxidative stress at the expense of body mass (contamination) or fish swimming activity (immune challenge). Multiple stressors had fewer interactive effects than expected, especially on physiological traits, but mainly resulted in antagonistic effects on fish swimming activity. Indeed, the immune challenge modified or inhibited the effects of contamination on fish behavior in most populations, suggesting that multiple stressors could reduce behavioral plasticity. Interestingly, the effects of stressors were highly variable among populations, with lower deleterious effects of metal contamination in populations from highly contaminated environments, although the underlying evolutionary mechanisms remain to be investigated. This study highlights the importance of considering multiple stressors effects and intraspecific variability of sensitivity to refine our ability to predict the effects of environmental contaminants on aquatic wildlife.

Plasticity in Reproductive Traits, Condition and Energy Allocation of the Non-Native Pyrenean Gudgeon Gobio lozanoi in a Highly Regulated Mediterranean River Basin

The invasion success of non-native fish, such as Pyrenean gudgeon Gobio lozanoi in several Iberian rivers, is often explained by the expression of its life history traits. This study provides the first insights into the reproductive traits, fish condition, and energy allocation (protein and lipid contents of tissues) of this species, along a longitudinal gradient in one of the most regulated river basins in the Iberian Peninsula, the Segura river. Larger sizes of first maturity, higher fecundity and larger oocytes were found in fluvial sectors with the most natural flow regimes, characterised by a low base flow with high flow peaks in spring and autumn. A delay in the reproductive period, lower fish condition and no differences in sex-ratio were observed in fluvial sectors with a high increase in base flow and notable inversion in the seasonal pattern of flow regime. Lipid contents in the liver and gonads were stable during the reproductive cycle and decreases in muscle were noted, whereas ovarian and liver proteins increased. In relation to energy allocation for G. lozanoi, an intermediate energy strategy was observed between income and capital breeding. Our results support the hypothesis that the high plasticity of G. lozanoi population traits plays a significant role in its success in a highly regulated Mediterranean river basin. Understanding the mechanisms by which flow regulation shapes fish populations in Mediterranean type-rivers could inform management actions.

The Biological Assessment and Rehabilitation of the World's Rivers: An Overview

The biological assessment of rivers i.e., their assessment through use of aquatic assemblages, integrates the effects of multiple-stressors on these systems over time and is essential to evaluate ecosystem condition and establish recovery measures. It has been undertaken in many countries since the 1990s, but not globally. And where national or multi-national monitoring networks have gathered large amounts of data, the poor water body classifications have not necessarily resulted in the rehabilitation of rivers. Thus, here we aimed to identify major gaps in the biological assessment and rehabilitation of rivers worldwide by focusing on the best examples in Asia, Europe, Oceania, and North, Central, and South America. Our study showed that it is not possible so far to draw a world map of the ecological quality of rivers. Biological assessment of rivers and streams is only implemented officially nation-wide and regularly in the European Union, Japan, Republic of Korea, South Africa, and the USA. In Australia, Canada, China, New Zealand, and Singapore it has been implemented officially at the state/province level (in some cases using common protocols) or in major catchments or even only once at the national level to define reference conditions (Australia). In other cases, biological monitoring is driven by a specific problem, impact assessments, water licenses, or the need to rehabilitate a river or a river section (as in Brazil, South Korea, China, Canada, Japan, Australia). In some countries monitoring programs have only been explored by research teams mostly at the catchment or local level (e.g., Brazil, Mexico, Chile, China, India, Malaysia, Thailand, Vietnam) or implemented by citizen science groups (e.g., Southern Africa, Gambia, East Africa, Australia, Brazil, Canada). The existing large-extent assessments show a striking loss of biodiversity in the last 2-3 decades in Japanese and New Zealand rivers (e.g., 42% and 70% of fish species threatened or endangered, respectively). A poor condition (below Good condition) exists in 25% of South Korean rivers, half of the European water bodies, and 44% of USA rivers, while in Australia 30% of the reaches sampled were significantly impaired in 2006. Regarding river rehabilitation, the greatest implementation has occurred in North America, Australia, Northern Europe, Japan, Singapore, and the Republic of Korea. Most rehabilitation measures have been related to improving water quality and river connectivity for fish or the improvement of riparian vegetation. The limited extent of most rehabilitation measures (i.e., not considering the entire catchment) often constrains the improvement of biological condition. Yet, many rehabilitation projects also lack pre-and/or post-monitoring of ecological condition, which prevents assessing the success and shortcomings of the recovery measures. Economic constraints are the most cited limitation for implementing monitoring programs and rehabilitation actions, followed by technical limitations, limited knowledge of the fauna and flora and their life-history traits (especially in Africa, South America and Mexico), and poor awareness by decision-makers. On the other hand, citizen involvement is recognized as key to the success and sustainability of rehabilitation projects. Thus, establishing rehabilitation needs, defining clear goals, tracking progress towards achieving them, and involving local populations and stakeholders are key recommendations for rehabilitation projects (Table 1). Large-extent and long-term monitoring programs are also essential to provide a realistic overview of the condition of rivers worldwide. Soon, the use of DNA biological samples and eDNA to investigate aquatic diversity could contribute to reducing costs and thus increase monitoring efforts and a more complete assessment of biodiversity. Finally, we propose developing transcontinental teams to elaborate and improve technical guidelines for implementing biological monitoring programs and river rehabilitation and establishing common financial and technical frameworks for managing international catchments. We also recommend providing such expert teams through the United Nations Environment Program to aid the extension of biomonitoring, bioassessment, and river rehabilitation knowledge globally.

Framing biophysical and societal implications of multiple stressor effects on river networks

Urbanization, agriculture, and the manipulation of the hydrological cycle are the main drivers of multiple stressors affecting river ecosystems across the world. Physical, chemical, and biological stressors follow characteristic patterns of occurrence, intensity, and frequency, linked to human pressure and socio-economic settings. The societal perception of stressor effects changes when moving from broad geographic regions to narrower basin or waterbody scales, as political and ecologically based perspectives change across scales. Current approaches relating the stressor effects on river networks and human societies fail to incorporate complexities associated to their co-occurrence, such as: i) the evidence that drivers can be associated to different stressors; ii) their intensity and frequency may differ across spatial and temporal scales; iii) their differential effects on biophysical receptors may be related to their order of occurrence; iv) current and legacy stressors may produce unexpected outcomes; v) the potentially different response of different biological variables to stressor combinations; vi) the conflicting effects of multiple stressors on ecosystem services; and, vii) management of stressor effects should consider multiple occurrence scales. We discuss how to incorporate these aspects to present frameworks considering biophysical and societal consequences of multiple stressors, to better understand and manage the effects being caused on river networks.

Direct and indirect effects of multiple environmental stressors on fish health in human-altered rivers

Freshwater fish face multiple challenges in human-altered rivers such as trace metal contamination, temperature increase and parasitism. These multiple stressors could have unexpected interactive effects on fish health due to shared physiological pathways, but few studies investigated this question in wild fish populations. In this study, we compared 16 populations of gudgeon (Gobio occitaniae) distributed along perturbation gradients in human-altered rivers in the South of France. We tested the effects of single and combined stressors (i.e., metal contamination, temperature, parasitism) on key traits linked to fish health across different biological levels using a Structural Equation Modelling approach. Parasitism and temperature alone had limited deleterious effects on fish health. In contrast, fish living in metal-contaminated sites had higher metal bioaccumulation and higher levels of cellular damage in the liver through the induction of an inflammatory response. In addition, temperature and contamination had interactive negative effects on growth. These results suggest that trace metal contamination has deleterious effects on fish health at environmentally realistic concentrations and that temperature can modulate the effects of trace metals on fish growth. With this study, we hope to encourage integrative approaches in realistic field conditions to better predict the effects of natural and anthropogenic stressors on aquatic organisms.

Unravelling the effects of multiple stressors on diatom and macroinvertebrate communities in European river basins using structural and functional approaches

Rivers suffer from more severe decreases in species diversity compared to other aquatic and terrestrial ecosystems due to a variety of pressures related to human activities. Species provide different roles in the functioning of the ecosystem, and their loss may reduce the capacity of the ecosystems to respond to multiple stressors. The effects on diversity will differ based on the type, combination and severity of stressors, as well as on the characteristics of the community composition and tolerance. Multiple trait-based approaches (MTBAs) can help to unravel the effects of multiple stressors on communities, providing a mechanistic interpretation, and, thus, complementing traditional biodiversity assessments using community structure. We studied the relationships between diversity indexes and trait composition of macroinvertebrate and diatom communities, as well as environmental variables that described the hydrological and geomorphological alterations and toxic pollution (pesticides and pharmaceuticals) of three different European river basins: the Adige, the Sava, and the Evrotas. These river basins can be considered representative cases of different situations in European freshwater systems. Hydrological variables were the main drivers determining the community structure and function in the rivers, for both diatoms and macroinvertebrates. For diatom communities, pharmaceutical active compound (PhAC) toxic units were also identified as a very important driver of diversity changes, explaining up to 57% of the variance in taxonomic richness. For macroinvertebrates, river geomorphology was an important driver of structural changes, particularly affecting Plecoptera richness. In addition, PhAC and pesticide toxic units were also identified as stressors for macroinvertebrate communities. MTBA provided a detailed picture of the effects of the stressors on the communities and confirmed the importance of hydrological variables in shaping the functional attributes of the communities.

Environmental factors affecting the nursery function for fish in the main estuaries of the Gulf of Cadiz (south-west Iberian Peninsula)

Hydrological, geomorphological, physicochemical and biological factors influence the nursery function of estuaries. Our study compared the environmental conditions and the assemblages of early life stages of fish in the main four estuaries of the Gulf of Cadiz (Cadiz Bay, Guadalquivir, Odiel-Tinto and Guadiana). Samples were taken within each estuary and on their adjacent coast, during the dry-warm seasons of 2016, 2017 and 2018. Results showed that rivers with smaller basins had a very low freshwater input and their estuaries, Odiel-Tinto and Cadiz Bay, were essentially sea extensions into the land, containing similar physicochemical conditions to nearshore zones, as well as similar assemblages and densities of early life stages of fish. Open water masses of these estuaries do not have important nursery functions. In contrast, inner zones of estuaries with bigger basins and higher freshwater discharges, Guadalquivir and Guadiana, have different environmental characteristics and a long transition zone with a well-defined salinity gradient. Their assemblages and densities of early life stages of fish were different between them and with other estuaries. The Guadalquivir estuary held the highest abundance of larval and early juvenile fish, as well as macrozooplankton biomass. The most abundant fish species in all zones of every estuary was the anchovy Engraulis encrasicolus; the Guadalquivir inner zone had the highest density. High concentration of suspended organic matter, provided by freshwater input and correlated with total suspended solid, suspended inorganic matter and turbidity, was the physicochemical characteristic more typical of the Guadalquivir. This characteristic, in addition to the salinity gradient, could explain the highest densities of macrozooplankton found in this estuary, and consequently, of early fish stages. Recurrent jellyfish blooms were observed in Cadiz Bay and the inner zone of Guadiana, affecting their nursery functions. Odiel-Tinto showed altered physicochemical and biological characteristics, which may need further specific research.

Functional response of fish assemblage to multiple stressors in a highly regulated Mediterranean river system

Mediterranean rivers are characterised by strong environmental constrains and species-poor, highly endemic fish fauna. In Europe, these systems are exposed to multiple stressors due to extensive human activities. Studies on the effects of some stressors on riverine fish are available but complex responses of fish assemblages to interplay of flow alteration with physical habitat changes and invasive species have not been evaluated up to date. This study analysed the response of functional diversity of fish assemblages to multiple stressors in the Segura River system in the southern Spain. Fish assemblages were sampled in 16 sites in two consecutive periods (2009-2010 and 2013-2015). Subsequently, we assessed the responses of functional specialisation, originality and entropy (based on nine functional traits and abundances) as well as species richness and abundance to interplay of flow regime alteration and ecological status, fragmentation as well as non-native species abundance across spatial and temporal scales. The governing role of flow regime in structuring fish assemblage was superimposed on physical habitat changes, water quality deterioration and fragmentation as well as the presence of non-native fish species. We found an increase of species richness and abundance but decrease of functional specialisation and originality in river reaches with high level of base flow and more stable hydrological conditions. Opposite pattern was observed in reaches with severe reduction of base flow and marked inversion in the seasonal pattern of high and low flows. We postulate that the use of tools that consider the functional identity of the species as method to assess the effects of environmental alterations on fish biodiversity could improve conservation measures for Mediterranean fish fauna. Furthermore, design flows that mimic natural flow regime patterns characteristic for Mediterranean rivers are a promising tool to provide environmental conditions that would favour native fish within the assemblage and benefit their conservation.

Multiple-Line Identification of Socio-Ecological Stressors Affecting Aquatic Ecosystems in Semi-Arid Countries: Implications for Sustainable Management of Fisheries in Sub-Saharan Africa

Water resources are among the fundamental resources that are the most threatened worldwide by various pressures. This study applied the Driver–Pressure–State–Impact–Response (DPSIR) framework as an innovative tool to better understand the dynamic interlinkages between the different sources of multiple stressors on aquatic ecosystems in Burkina Faso. The triangulation of evidences from interviews, literature reviews, and strategic simulations shows that several human impacts as well as climate change and its effects (such as the decrease of the water level, and the increase of the surface water temperature) are detrimental to fish productivity, abundance, and average size. Furthermore, the ongoing demographic and nutritional transition is driving cumulative pressures on water and fish resources. In this context, the development of aquaculture could offer alternative livelihoods and help fish stocks in natural ecosystems to recover, thereby reducing fishermen’s vulnerability and easing overfishing pressures. Further, the empowerment of the actors and their participation to reinforce fisheries regulation are required to escape the current “regeneration trap” and to achieve a sustainable management of aquatic ecosystems in Burkina Faso.

The Relation of Lotic Fish and Benthic Macroinvertebrate Condition Indices to Environmental Factors Across the Conterminous USA

National and regional ecological assessments are essential for making rational decisions concerning water body conservation and management at those spatial extents. We analyzed data from 4597 samples collected from 3420 different sites across the conterminous USA during the U.S. Environmental Protection Agency's 2008-2009 and 2013-2014 National Rivers and Streams Assessment. We evaluated the relationship between both fish and macroinvertebrate multimetric index (MMI) condition scores and 38 environmental factors to assess the relative importance of natural versus anthropogenic predictors, contrast site-scale versus watershed-scale predictors, and examine ecoregional and assemblage differences. We found that most of the environmental factors we examined were related to either fish and/or macroinvertebrate MMI scores in some fashion and that the factors involved, and strength of the relationship, varied by ecoregion and between assemblages. Factors more associated with natural conditions were usually less important in explaining MMI scores than factors more directly associated with anthropogenic disturbances. Local site-scale factors explained more variation than watershed-scale factors. Random forest and multiple regression models performed similarly, and the fish MMI-environment relationships were stronger than macroinvertebrate MMI-environment relationships. Among ecoregions, the strongest environmental relationships were observed in the Northern Appalachians and the weakest in the Southern Plains. The fish and macroinvertebrate MMIs were only weakly correlated with each other, and they generally responded more strongly to different groups of variables. These results support the use of multiple assemblages and the sampling of multiple environmental indicators in ecological assessments across large spatial extents.

Challenges to saving China's freshwater biodiversity: Fishery exploitation and landscape pressures

China has over 1320 freshwater fish species, 877 of which are endemic. In recent decades, over-exploitation and landscape pressures have threatened them and led to a severe aquatic biodiversity crisis. In response, large-scale fishing bans have been promulgated to protect freshwater biodiversity in major Chinese rivers since the early 1980s. Here, we present the historical background and current challenges to the fishing bans. Implementing large-scale fishing bans may help improve China's current freshwater biological resources and biodiversity to some extent. But implementing fishing bans alone is not sufficient to solve the crisis because of shortcomings of the current bans and expanding human pressures in most river basins. Thus, we recommend regulating other anthropogenic pressures, expanding duration and extent of current fishing regulations, establishing a comprehensive monitoring program, and initiating basin-scale ecological rehabilitation. These programs are also needed in other developing countries facing similar biodiversity crises and human pressures.

How Do Eutrophication and Temperature Interact to Shape the Community Structures of Phytoplankton and Fish in Lakes?

Freshwater ecosystems are among the systems most threatened and impacted by anthropogenic activities, but there is still a lack of knowledge on how this multi-pressure environment impacts aquatic communities in situ. In Europe, nutrient enrichment and temperature increase due to global change were identified as the two main pressures on lakes. Therefore, we investigated how the interaction of these two pressures impacts the community structure of the two extreme components of lake food webs: phytoplankton and fish. We modelled the relationship between community components (abundance, composition, size) and environmental conditions, including these two pressures. Different patterns of response were highlighted. Four metrics responded to only one pressure and one metric to the additive effect of the two pressures. Two fish metrics (average body-size and biomass ratio between perch and roach) were impacted by the interaction of temperature and eutrophication, revealing that the effect of one pressure was dependent on the magnitude of the second pressure. From a management point of view, it appears necessary to consider the type and strength of the interactions between pressures when assessing the sensitivity of communities, otherwise their vulnerability (especially to global change) could be poorly estimated.

Combined effects of temperature increase and immune challenge in two wild gudgeon populations

In the context of global changes, aquatic ecosystems are increasingly exposed to multiple stressors that can have unexpected interactive effects on aquatic organisms. Among these stressors, the occurrence of heat waves and pathogens is changing rapidly in freshwater rivers, but their combined effects on fish health are still understudied. In this study, we experimentally tested the crossed effects of increased temperature (mimicking a heat wave) and a standardized immune challenge (mimicking a parasite attack) on wild gudgeon (Gobio occitaniae) physiology and behaviour across biological levels from molecules to the whole individual. We also investigated the potential variation of sensitivity among populations by comparing two wild populations from contrasted thermal regimes. Combined stressors (i.e. temperature increase and immune challenge) had contrasted effects on fish physiology and behaviour compared to single stressors, but only at the individual level. In particular, the immune challenge inhibited the effect of the temperature on fish behaviour (activity, exploration and foraging) but amplified the negative effect of temperature on fish survival. No interactions were found at other biological levels. This study thus shows that it is essential to consider biotic stressors such as pathogens to better anticipate the effects of global changes on aquatic organisms. In addition, there was a high variability of response between the two gudgeon populations, suggesting that future studies should take into account population variability to better predict the responses of aquatic wildlife to current and future stressors.

Validating anthropogenic threat maps as a tool for assessing river ecological integrity in Andean-Amazon basins

Anthropogenic threat maps are commonly used as a surrogate for the ecological integrity of rivers in freshwater conservation, but a clearer understanding of their relationships is required to develop proper management plans at large scales. Here, we developed and validated empirical models that link the ecological integrity of rivers to threat maps in a large, heterogeneous and biodiverse Andean–Amazon watershed. Through fieldwork, we recorded data on aquatic invertebrate community composition, habitat quality, and physical-chemical parameters to calculate the ecological integrity of 140 streams/rivers across the basin. Simultaneously, we generated maps that describe the location, extent, and magnitude of impact of nine anthropogenic threats to freshwater systems in the basin. Through seven-fold cross-validation procedure, we found that regression models based on anthropogenic threats alone have limited power for predicting the ecological integrity of rivers. However, the prediction accuracy improved when environmental predictors (slope and elevation) were included, and more so when the predictions were carried out at a coarser scale, such as microbasins. Moreover, anthropogenic threats that amplify the incidence of other pressures (roads, human settlements and oil activities) are the most relevant predictors of ecological integrity. We concluded that threat maps can offer an overall picture of the ecological integrity pattern of the basin, becoming a useful tool for broad-scale conservation planning for freshwater ecosystems. While it is always advisable to have finer scale in situ measurements of ecological integrity, our study shows that threat maps provide fast and cost-effective results, which so often are needed for pressing management and conservation actions.

Effects of multiple stressors on river biofilms depend on the time scale

Global change exposes ecosystems to a myriad of stressors differing in their spatial (i.e. surface of stressed area) and temporal (i.e. exposure time) components. Among freshwater ecosystems, rivers and streams are subject to physical, chemical and biological stressors, which interact with each other and might produce diverging effects depending on exposure time. We conducted a manipulative experiment using 24 artificial streams to examine the individual and combined effects of warming (1.6 °C increase in water temperature), hydrological stress (simulated low-flow situation) and chemical stress caused by pesticide exposure (15.1-156.7 ng L-1) on river biofilms. We examined whether co-occurring stressors could lead to non-additive effects, and if these differed at two different exposure times. Specifically, structural and functional biofilm responses were assessed after 48 hours (short-term effects) and after 30 days (long-term effects) of exposure. Hydrological stress caused strong negative impacts on river biofilms, whereas effects of warming and pesticide exposure were less intense, although increasing on the long term. Most stressor combinations (71%) resulted in non-significant interactions, suggesting overall additive effects, but some non-additive interactions also occurred. Among non-additive interactions, 59% were classified as antagonisms after short-term exposure to the different stressor combinations, rising to 86% at long term. Our results indicate that a 30-day exposure period to multiple stressors increases the frequency of antagonistic interactions compared to a 48-hour exposure to the same conditions. Overall, the impacts of multiple-stressor occurrences appear to be hardly predictable from individual effects, highlighting the need to consider temporal components such as duration when predicting the effects of multiple stressors.

Effects of large-river restoration measures on ecological fish guilds and focal species of conservation in a large European river (Danube, Austria)

River engineering measures of the last two centuries have severely impacted the fish coenoses of all larger rivers in Europe by creating river barriers, replacing natural shores with riprap-reinforced banks, and installing artificial structures that changed the hydrology of inshore habitats. As a result, fluvial specialists have declined and were replaced by generalists. Consequently, there is an urgent need to develop functioning means and measures to restore stream habitats and re-establish near-natural conditions. In this study, we analyze the effectiveness of four different restoration measures to increase the abundance and ratio of the rheophilic fish guild in general and of two fluvial focal species of conservation, the barbel and nase, in particular. The sampling took place in a free-flowing part of the Austrian Danube, during the monitoring program of a large hydro-engineering project with both economic and ecological targets. We applied a BACI design and used data from three extensive sampling years. The ecologically orientated measures included groynefield adaptations, a bank re-naturalization, and a side arm re-connection. All measures increased the abundance of nase and barbel and also provided suitable conditions for young-of-the-year fishes. The bank re-naturalization, significantly increased both the abundance and the ratio of rheophilic fishes. The side arm re-connection significantly improved the ratio of rheophilic fishes. Both groyne adaptation measures showed positive, but non-significant effects. Consequently, replacing riprap sections with near-natural gravel bars and re-connecting former side arms can be recommended as appropriate measures to support the rheophilic fish guild, whereas groyne adaptations appear to be of limited value. The evaluated restoration measures significantly improved conditions for local populations of target species and coenoses and can be used as building blocks in upscaled, river-wide restoration efforts. A sustainable restoration of the whole river ecosystem, however, requires a holistic approach that also addresses floodplains and catchment-wide issues.

Stress responses in fish: From molecular to evolutionary processes

In the context of global changes, fish are increasingly exposed to multiple stressors that have cascading effects from molecules to the whole individual, thereby affecting wild fish populations through selective processes. In this review, we synthetize recent advances in molecular biology and evolutionary biology to outline some potentially important effects of stressors on fish across biological levels. Given the burgeoning literature, we highlight four promising avenues of research. First, (1) the exposure to multiple stressors can lead to unexpected synergistic or antagonistic effects, which should be better taken into account to improve our predictions of the effects of actual and future human activities on aquatic organisms. Second, (2) we argue that such interactive effects might be due to switches in energy metabolism leading to threshold effects. Under multiple stress exposure, fish could switch from a "compensation" strategy, i.e. a reallocation of energy to defenses and repair to a "conservation" strategy, i.e. blocking of stress responses leading to strong deleterious effects and high mortality. Third, (3) this could have cascading effects on fish survival and population persistence but multiscale studies are still rare. We propose emerging tools merging different levels of biological organization to better predict population resilience under multiple stressors. Fourth (4), there are strong variations in sensitivity among populations, which might arise from transgenerational effects of stressors through plastic, genetic, and epigenetic mechanisms. This can lead to local adaptation or maladaptation, with strong impacts on the evolutionary trajectories of wild fish populations. With this review, we hope to encourage future research to bridge the gap between molecular ecology, ecotoxicology and evolutionary biology to better understand the evolution of responses of fishes to current and future multiple stressors in the context of global changes.

Food deprived largemouth bass (Micropterus salmoides) are inactive and stressed, but do not show changes in lure inspections

Coping style traits, including physiology and behavior, can be used to predict if fish are vulnerable to capture by hook-and-line angling. Typically, fish with proactive coping styles are selectively captured, but effects of environmental influences, such as food availability, on the completion of each step leading to a successful angling capture (i.e., activity rates, encountering a lure, lure inspection, lure-striking, and ingestion) have not been quantified. Therefore, the objective of this study was to quantify the effects among activity behavior, stress (cortisol) responsiveness, and food availability on lure inspection behaviors of largemouth bass. No relationships were found between activity, stress responsiveness, and food availability to determine lure inspections. However, food deprivation decreased activity rates and increased baseline cortisol concentrations of largemouth bass. Additionally, after feeding treatments, fish with low baseline cortisol concentrations were more likely to inspect lures in both the fed and food deprived treatments. Results further discuss the implications of study findings to help fisheries managers predict the evolutionary impacts of angling.