Salinisation of rivers: an urgent ecological issue

It's all about the fluxes: Temperature influences ion transport and toxicity in aquatic insects

Many freshwater ecosystems are becoming saltier and/or warmer, but our understanding of how these factors interact and affect the physiology and life history outcomes of most aquatic species remain unknown. We hypothesize that temperature modulates ion transport rates. Since ion transport is energetically expensive, increases in salinity and/or temperature may influence ion flux rates and ultimately, organismal performance. Radiotracer (²²Na⁺, ³⁵SO₄^-2, and ⁴⁵Ca²⁺) experiments with lab-reared mayflies (N. triangulifer) and other field-collected insects showed that increasing temperature generally increased ion transport rates. For example, increasing temperature from 15 °C to 25 °C, increased ²²Na⁺ uptake rates by two-fold (p < 0.0001) and ³⁵SO₄^-2 uptake rates by four-fold (p < 0.0001) in the caddisfly, Hydropsyche sparna. Smaller changes in ²²Na⁺ and ³⁵SO₄^-2 uptake rates were observed in the mayflies, Isonychia sayi and Maccaffertium sp., suggesting species-specific differences in the thermal sensitivity of ion transport. Finally, we demonstrated that the toxicity of SO₄ was influenced by temperature profoundly in a 96-h bioassay. Under the saltiest conditions (1500 mg L^-1 SO₄), mayfly survival was 78 % at 15 °C, but only 44 % at 25 °C (p < 0.0036). Conceivably, the energetic cost of osmoregulation in warmer, saltier environments may cause significant major ion toxicity in certain freshwater insects.

Investigating Industrial Effluent Impact on Municipal Wastewater Treatment Plant in Vaal, South Africa

Industrial effluents with high concentrations of toxic heavy metals are of great concern because of their persistence and non-degradability. However, poor operation and maintenance of wastewater treatment infrastructure is a great concern in South Africa. In this study, physico-chemical parameters and heavy metals (HMs) concentration of wastewater from five different industries, Leeuwkuil wastewater treatment plant (WWTP) inflow and effluent, and Vaal River water samples were monitored between January and September 2017, to investigate the correlation between heavy metal pollution and the location of industries and ascertain the effectiveness of the municipal WWTP. Physico-chemical variables such as pH, biological oxygen demand (BOD), dissolved oxygen (DO), chemical oxygen demand (COD), total dissolved solids (TDS) and electrical conductivity (EC) exhibited both temporal and spatial variations with the values significantly higher in the industrial samples. Inductively coupled plasma optical emission spectrometry (ICP-OES) results also showed that aluminium (Al), copper (Cu), lead (Pb) and zinc (Zn) were significantly higher in industrial effluents (p < 0.05), with only Zn and Al exhibiting significant seasonal variability. Statistical correlation analysis revealed a poor correlation between physicochemical parameters and the HMs compositional quality of wastewater. However, toxic HMs (Zn, Cu and Pb) concentrations in treated wastewater from WWTP were above the permissible limits. Although the WWTP was effective in maintaining most of the wastewater parameters within South African Green drop Standards, the higher Cu, Zn, Pb and COD in its final effluent is a concern in terms of Vaal river health and biological diversity. Therefore, we recommend continuous monitoring and maintenance of the WWTPs infrastructure in the study area.

Flow-Mediated Vulnerability of Source Waters to Elevated TDS in an Appalachian River Basin

Widespread salinization—and, in a broader sense, an increase in all total dissolved solids (TDS)—is threatening freshwater ecosystems and the services they provide (e.g., drinking water provision). We used a mixed modeling approach to characterize long-term (2010–2018) spatio-temporal variability in TDS within the Monongahela River basin and used this information to assess the extent and drivers of vulnerability. The West Fork River was predicted to exceed 500 mg/L a total of 133 days. Occurrence and duration (maximum = 28 days) of—and thus vulnerability to—exceedances within the West Fork River were driven by low flows. Projected decreases in mean daily discharge by ≤10 cfs resulted in an additional 34 days exceeding 500 mg/L. Consistently low TDS within the Tygart Valley and Cheat Rivers reduced vulnerability of the receiving Monongahela River to elevated TDS which was neither observed (maximum = 419 mg/L) nor predicted (341 mg/L) to exceed the secondary drinking water standard of 500 mg/L. Potential changes in future land use and/or severity of low-flow conditions could increase vulnerability of the Monongahela River to elevated TDS. Management should include efforts to increase assimilative capacity by identifying and decreasing sources of TDS. Upstream reservoirs could be managed toward low-flow thresholds; however, further study is needed to ensure all authorized reservoir purposes could be maintained.

Advanced biofilm analysis in streams receiving organic deicer runoff

Prolific heterotrophic biofilm growth is a common occurrence in airport receiving streams containing deicers and anti-icers, which are composed of low-molecular weight organic compounds. This study investigated biofilm spatiotemporal patterns and responses to concurrent and antecedent (i.e., preceding biofilm sampling) environmental conditions at stream sites upstream and downstream from Milwaukee Mitchell International Airport in Milwaukee, Wisconsin, during two deicing seasons (2009-2010; 2010-2011). Biofilm abundance and community composition were investigated along spatial and temporal gradients using field surveys and microarray analyses, respectively. Given the recognized role of Sphaerotilus in organically enriched environments, additional analyses were pursued to specifically characterize its abundance: a consensus sthA sequence was determined via comparison of whole metagenome sequences with a previously identified sthA sequence, the primers developed for this gene were used to characterize relative Sphaerotilus abundance using quantitative real-time PCR, and a Sphaerotilus strain was isolated to validate the determined sthA sequence. Results indicated that biofilm abundance was stimulated by elevated antecedent chemical oxygen demand concentrations, a surrogate for deicer concentrations, with minimal biofilm volumes observed when antecedent chemical oxygen demand concentrations remained below 48 mg/L. Biofilms were composed of diverse communities (including sheathed bacterium Thiothrix) whose composition appeared to shift in relation to antecedent temperature and chemical oxygen demand. The relative abundance of sthA correlated most strongly with heterotrophic biofilm volume (positive) and dissolved oxygen (negative), indicating that Sphaerotilus was likely a consistent biofilm member and thrived under low oxygen conditions. Additional investigations identified the isolate as a new strain of Sphaerotilus montanus (strain KMKE) able to use deicer components as carbon sources and found that stream dissolved oxygen concentrations related inversely to biofilm volume as well as to antecedent temperature and chemical oxygen demand. The airport setting provides insight into potential consequences of widescale adoption of organic deicers for roadway deicing.

High-Frequency Data Reveal Deicing Salts Drive Elevated Specific Conductance and Chloride along with Pervasive and Frequent Exceedances of the U.S. Environmental Protection Agency Aquatic Life Criteria for Chloride in Urban Streams

Increasing specific conductance (SC) and chloride concentrations [Cl] negatively affect many stream ecosystems. We characterized spatial variability in SC, [Cl], and exceedances of Environmental Protection Agency [Cl] criteria using nearly 30 million high-frequency observations (2-15 min intervals) for SC and modeled [Cl] from 93 sites across three regions in the eastern United States: Southeast, Mid-Atlantic, and New England. SC and [Cl] increase substantially from south to north and within regions with impervious surface cover (ISC). In the Southeast, [Cl] weakly correlates with ISC, no [Cl] exceedances occur, and [Cl] concentrations are constant with time. In the Mid-Atlantic and New England, [Cl] and [Cl] exceedances strongly correlate with ISC. [Cl] criteria are frequently exceeded at sites with greater than 9-10% ISC and median [Cl] higher than 30-80 mg/L. Tens to hundreds of [Cl] exceedances observed annually at most of these sites help explain previous research where stream ecosystems showed changes at (primarily nonwinter) [Cl] as low as 30-40 mg/L. Mid-Atlantic chronic [Cl] exceedances occur primarily in December-March. In New England, exceedances are common in nonwinter months. [Cl] is increasing at nearly all Mid-Atlantic and New England sites with the largest increases at sites with higher [Cl].

Geographical origin determines responses to salinity of Mediterranean caddisflies

Many freshwater ecosystems worldwide, and particularly Mediterranean ones, show increasing levels of salinity. These changes in water conditions could affect abundance and distribution of inhabiting species as well as the provision of ecosystem services. In this study we conduct laboratory experiments using the macroinvertebrate Smicridea annulicornis as a model organism. Our factorial experiments were designed to evaluate the effects of geographical origin of organisms and salinity levels on survival and behavioral responses of caddisflies. The experimental organisms were captured from rivers belonging to three hydrological basins along a 450 Km latitudinal gradient in the Mediterranean region of Chile. Animals were exposed to three conductivity levels, from 180 to 1400 μS/cm, close to the historical averages of the source rivers. We measured the behavioral responses to experimental stimuli and the survival time. Our results showed that geographical origin shaped the behavioral and survival responses to salinity. In particular, survival and activity decreased more strongly with increasing salinity in organisms coming from more dilute waters. This suggests local adaptation to be determinant for salinity responses in this benthic invertebrate species. In the current scenario of fast temporal and spatial changes in water levels and salt concentration, the conservation of geographic intra-specific variation of aquatic species is crucial for lowering the risk of salinity-driven biodiversity loss.

Sulphate contamination in groundwater and its remediation: an overview

Most abundant form of sulphur in the geosphere has been sulphate. Sulphate, with sulphur in the plus six oxidation state is very stable. Sources of sulphate in groundwater include mineral dissolution, atmospheric deposition and other anthropogenic sources (mining, fertilizer, etc.). Gypsum is an important contributor to the high levels of sulphate in many aquifer of the world. Sulphate is not as much as toxic, but it can cause catharsis, dehydration and diarrhoea, and when ingested in higher amount through dietary absorption, the levels of methaemoglobin and sulphaemoglobin are changed in human and animal body. The role of sulphate in aqueous phase and sedimentary phase has been discussed. There is only limited work on sulphate pollution remediation in groundwater at national and international level; therefore, in the light of rising attention in sulphate as a contaminant, different sources of sulphate, its distribution and available different remediation techniques for groundwater system reported so far have been discussed in the present paper. Abiologic processes' thermochemical sulphate reduction (TSR) also plays significant role in reduction of sulphate.

Potash mining effluents induce moderate effects on histopathological and physiological endpoints of adult zebrafish (Danio rerio)

Stress in fish can be caused by a variety of factors and has the potential to evoke stress responses leading to a reduction of physical condition and of health. The river Werra (Germany) presents a severe case of secondary salinisation caused by potash mining activities. The model organism Danio rerio was exposed to different ion-concentrations depicting current (HT) and future (LT) threshold values of the Werra, as well as to solutions with single-exceeding ions (Mg²⁺ + K⁺ (KMg), Mg²⁺ (Mg) and K⁺ (K)). After a six-week exposure period, cortisol levels, growth and weight were measured, gills and gonads were histologically analysed and mRNA expression of follicle stimulating hormone (FSH), luteinising hormone (LH), growth hormone (GH) and prolactin (PRL) were determined. Cortisol was still elevated in fish in the HT and K group, indicating moderate stress. However, gills revealed structural changes in zebrafish in all exposure groups, size of oocytes differed in the LT and K group, male FSH mRNA levels were elevated in the HT and LT group whereas PRL mRNA levels were lower in HT and LT for both, male and female fish. These results suggest that ion-stress induces moderate effects on a variety of biological parameters that mainly serve to adapt to elevated ion concentrations. For these reasons current and even future thresholds should be reconsidered, including thresholds for total as well as single ion concentrations. Future research looking at the effects on local fish species is needed, along with regular and long-term monitoring of environmental conditions, species abundance and diversity.

Acute and chronic toxicity of magnesium to the early life stages of two tropical freshwater mussel species

Magnesium (Mg) is a common contaminant in mine water discharges. Although Mg is an essential element in biological processes, increased concentrations from anthropogenic sources can stress aquatic ecosystems. Additionally, studies evaluating the effects of Mg on north Australian freshwater species have indicated that in very soft waters there is a high risk to some species. Freshwater mussels are an ecologically and culturally important taxon in many freshwater environments, but knowledge of their sensitivity to Mg is limited. In the present study, the acute and chronic sensitivity of two freshwater mussel species, Velesunio angasi and an undescribed Velesunio species, to Mg was assessed (using MgSO₄) on their early life stages, larval glochidia and post-parasitic juveniles. Acute 24-h exposures with glochidia generated a mean median lethal (LC50) toxicity estimate of 284mg/L for the five tests with V. angasi, and a mean LC50 of 300mg/L for the three tests with Velesunio sp. Mean chronic 14-d toxicity estimates resulting in 50% (EC50) and 10% (EC10) growth rate reductions for juveniles were 241 and 88mg/L respectively for the three tests with V. angasi juveniles, and 232 and 87mg/L respectively for the three tests with Velesunio sp. juveniles. The results represent the first acute and chronic Mg toxicity data for tropical freshwater mussels, and indicated that V. angasi and Velesunio sp. exhibited similar sensitivity and were moderately sensitive to Mg when compared to other tropical species. These results are a valuable contribution to the small existing dataset for Mg toxicity to tropical freshwater species, which can be used to inform water management in areas where Mg is a contaminant of concern, and ensure the protection of these taxa.

Response of the mollusc communities to environmental factors along an anthropogenic salinity gradient

Anthropogenic salinisation of freshwater ecosystems is frequent across the world. The scale of this phenomenon remains unrecognised, and therefore, monitoring and management of such ecosystems is very important. We conducted a study on the mollusc communities in inland anthropogenic ponds covering a large gradient of salinity located in an area of underground coal mining activity. A total of 14 gastropod and 6 bivalve species were noted. No molluscs were found in waters with total dissolved solids (TDS) higher than 17.1 g L^-1. The share of alien species in the communities was very high in waters with elevated salinity and significantly lower in the freshwaters. Canonical correspondence analysis (CCA) showed that TDS, pH, alkalinity, nitrate nitrogen, ammonium nitrogen, iron, the content of organic matter in sediments, the type of substrate and the content of sand and gravel in sediments were the variables that were significantly associated with the distribution of molluscs. The regression analysis revealed that total mollusc density was positively related to alkalinity and negatively related to nitrate nitrogen. The taxa richness was negatively related to TDS, which is consistent with previous studies which indicated that a high salinity level is a significant threat to freshwater malacofauna, causing a loss of biodiversity and contributing to the colonisation and establishment of alien species in aquatic ecosystems.

Agricultural impacts on streams near Nitrate Vulnerable Zones: A case study in the Ebro basin, Northern Spain

Agricultural intensification during the last century has caused river degradation across Europe. From the wide range of stressors derived from agricultural activities that impact rivers, diffuse agricultural pollution has received most of the attention from managers and scientists. The aim of this study was to determine the main stressors exerted by intensive agriculture on streams around Nitrate Vulnerable Zones (NVZs), which are areas of land that drain into waters polluted by nitrates according to the European Nitrate Directive (91/676/EEC). The study area was located in the NW of La Rioja (Northern Spain), which has some of the highest nitrate concentrations within the Ebro basin. The relationships between 40 environmental variables and the taxonomic and functional characteristics of the macroinvertebrate assemblages (which are useful indicators of water quality) were analyzed in 11 stream reaches differentially affected by upstream agricultural activity. The streams affected by a greater percentage of agricultural land cover in the surrounding catchment had significantly higher nitrate concentrations than the remaining sites. However, hydromorphological alteration (i.e. channel simplification, riparian forest and habitat degradation), which is closely linked to agricultural practices, was the main factor affecting macroinvertebrate assemblages. We suggest that “good agricultural practices” should be implemented in streams affected by NVZs to reverse stream degradation, in concordance with the European Water Framework Directive (WFD).

Acetic acid: a cost-effective agent for mitigation of seawater-induced salt toxicity in mung bean

The current study sought the effective mitigation measure of seawater-induced damage to mung bean plants by exploring the potential roles of acetic acid (AA). Principal component analysis (PCA) revealed that foliar application of AA under control conditions improved mung bean growth, which was interlinked to enhanced levels of photosynthetic rate and pigments, improved water status and increased uptake of K+, in comparison with water-sprayed control. Mung bean plants exposed to salinity exhibited reduced growth and biomass production, which was emphatically correlated with increased accumulations of Na+, reactive oxygen species and malondialdehyde, and impaired photosynthesis, as evidenced by PCA and heatmap clustering. AA supplementation ameliorated the toxic effects of seawater, and improved the growth performance of salinity-exposed mung bean. AA potentiated several physio-biochemical mechanisms that were connected to increased uptake of Ca2+ and Mg2+, reduced accumulation of toxic Na+, improved water use efficiency, enhanced accumulations of proline, total free amino acids and soluble sugars, increased catalase activity, and heightened levels of phenolics and flavonoids. Collectively, our results provided new insights into AA-mediated protective mechanisms against salinity in mung bean, thereby proposing AA as a potential and cost-effective chemical for the management of salt-induced toxicity in mung bean, and perhaps in other cash crops.

Chloride inputs to the North Saskatchewan River watershed: the role of road salts as a potential driver of salinization downstream of North America's northern most major city (Edmonton, Canada)

Multiple anthropogenic activities are driving the salinization of freshwater environments threatening water resources worldwide. Accordingly, this research will first examine the spatial and temporal variability of major ions (i.e. Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄^2-, CO₃^2-, and HCO₃^-) upstream and downstream of the northernmost major city in North America (Edmonton, Canada). Second, this research will estimate the relative contributions of the major sources of chloride (Cl), the main constituent of road deicers, to the sub-basin around Edmonton. Monthly water quality data was for three sites on the North Saskatchewan River (NSR): Rocky Mountain House (RMH - downstream of the Rocky Mountain headwaters), Devon Bridge (upstream of Edmonton) and Pakan Bridge (downstream of Edmonton). Change ratios investigate the downstream alterations of major ions at Pakan and Devon, relative to RMH. Seasonal Kendall tests examine temporal trends (1987-2017). A mass-balance approach then quantifies the major sources of Cl in the sub-basin of the NSR between Devon and Pakan. Progressing from the Rocky Mountain headwaters (at RMH) to downstream of Edmonton (at Pakan), Cl^- increased by >5.5 times, Na⁺ by 4.8 times and K by 2.7 times. No significant temporal trends for Cl^-, Na⁺ and K⁺ were evident at Devon (upstream of Edmonton), whereas all three significantly increased at Pakan (downstream of Edmonton). Deicers (54%), agriculture (20%), along with waste water treatment plant and industrial effluent (15%) were the largest Cl sources in the NSR Devon-Pakan sub-basin. In total, 77 Gg yr^-1 of Cl (or 6 t km² yr^-1) is added to the Devon-Pakan sub-basin, of which, 43 Gg yr^-1 is retained. Understanding and managing the major drivers of freshwater salinization will be of increasing importance in the 21^st century owing to the potential salinization of freshwater resources in the context of a changing climate.

Potash mining effluents and ion imbalances cause transient stress in adult common roach, Rutilus rutilus

A present ecological issue causing secondary salinization in different countries is the discharge of effluents by the potash mining industry. In Germany, the River Werra is used as a sink for potash mining discharges containing high concentrations of ions, predominantly Cl^-, K⁺, Na⁺, and Mg²⁺ resulting in a strong decline of the biodiversity and abundance of local species. However, hardly anything is known about the acute and chronic physiological effects of high concentrations and imbalances of ions being prevalent in potash mining effluents in fish. Therefore, the stress response and selected immune and growth parameters were investigated in standardized laboratory experiments. A native freshwater fish species, Rutilus rutilus, was exposed to concentrations of the high currently allowed (HT) and lowered future thresholds (LT) and three different ion solutions (containing high Mg²⁺ (Mg), high K⁺ (K) and high Mg²⁺+K⁺ (Mg+K) concentrations) for four different exposure times (24 h, 7 d, 21 d, 8 wk). Tank water (additionally after 9 and 12 h) and plasma cortisol, glucose and protein, hematocrit and hemoglobin were determined after each exposure time. Furthermore, plasma lysozyme and head kidney leucocyte respiratory burst activity (only after 21 d) were evaluated as well as growth parameters. A transient stress response was induced in almost all groups. Tank water cortisol was elevated after 9 h in HT, LT and Mg+K and in HT after 12 h, whereas glucose concentrations increased after 24 h in all exposure groups except K. HT led to enhanced hematocrit and hemoglobin content after 24 h. Plasma protein, immune system and growth were not affected in any group. None of the ion solutions induced acute toxicity but most triggered typical acute stress reactions. Rather the sum of high ion concentrations than single ions challenged the fish. Even though the effects observed in adult roach were only transient and indicate acclimatization under laboratory conditions, adverse effects observed in the river are evident and further research on physiological endpoints including reproductive parameters and impacts on younger life stages seem to be needed to scientifically base protective thresholds.

Qualifying the effects of single and multiple stressors on the food web structure of Dutch drainage ditches using a literature review and conceptual models

In September 2017, a workshop was held at Wageningen University and Research to determine the current state of knowledge of multiple stressor effects on aquatic ecosystems and to assess how to improve prediction of these effects. We developed a theoretical framework that integrates species-level responses to stressors to predict how these effects propagate through higher levels of biological organisation. Here, we present the application of the framework for drainage ditch ecosystems in the Netherlands. We used food webs to assess single and multiple stressor effects of common stressors on ditch communities. We reviewed the literature for the effects of targeted stressors (nutrients, pesticides, dredging and mowing, salinization, and siltation) on each functional group present in the food web and qualitatively assessed the relative sensitivity of groups. Using this information, we created a stressor-response matrix of positive and negative direct effects of each stressor-functional group combination. Fungicides, salinization, and sedimentation were identified as particularly detrimental to most groups, although destructive management practices, such as dredging with almost complete community removal, would take precedence depending on frequency. Using the stressor-response matrix we built, first, a series of conceptual null models of single stressor effects on food web structure and, second, a series of additive null models to illustrate potential paired-stressor effects. We compared these additive null models with published studies of the same pairs of combined single stressors to explore more complex interactions. Our approach serves as a first-step to considering multiple stressor scenarios in systems that are understudied or data-poor and as a baseline from which more complex models that include indirect effects and quantitative data may be developed. We make specific suggestions for appropriate management strategies that could be taken to support the biodiversity of these systems for individual stressors and their combined impacts.

Freshwater tributaries provide refuge and recolonization opportunities for mussels following salinity reversal

Reversing the effects of secondary salinization, and its impacts on aquatic biodiversity, is a growing global challenge, and particularly prevalent in Mediterranean-climate regions. Remnant freshwater tributaries in salinized landscapes provide significant biodiversity values, including discrete areas of refuge, dilution of salinized reaches, and potential source populations for recolonisation. The importance of these areas for aquatic fauna is widely accepted but rarely evaluated in the field. This study explored how spatial distribution of southwestern Australia's only freshwater mussel species, Westralunio carteri, has responded to the ongoing salinity trend in the Kent River catchment. Our results showed that salinity in the river has begun to reverse following improved catchment management, and also detected the first evidence of an associated recovery of the freshwater mussel population. Mussels in the mainstem were limited to sites around and downstream of a permanently flowing freshwater tributary, suggesting that dilution from this source provides a refuge in the lower reach. At two of those sites, all individuals were <15 years of age, indicative of recolonisation coinciding with salinity reversal around the turn of the century. Interestingly, mussels clearly persisted in other parts of the lower reach throughout the peak salinity period, when salinities regularly exceeded laboratory derived toxicity thresholds for the species. Mussels were not found in the majority of the mainstem or in highly acidic parts of the freshwater tributaries. The presence of old shells at those sites shows that the species was once widespread, and that the current distribution probably reflects a contraction due to historical salinization as well as acidification. Overall, our results show that the W. carteri population in the catchment has taken a first step towards recovery, and highlights the importance of freshwater tributaries in providing both refuge from disturbance and a source of new recruits.

Private water distribution as a potential everyday risk: The case of Goba, Dar es Salaam

A large number of peri-urban settlements in developing countries, including Goba in Tanzania, fall short of government supplied water. The inability of the Government to budget and prioritise its budget poses a serious problem to meet the water demand, a few residents in peri-urban settlements use other sources of water, including groundwater. However, the quality and suitability safety of such groundwater are questionable. This research of Goba settlement was undertaken to explore the reality of what happens and how problems can be resolved. The research methodology deployed in-depth interviews, physical observations, photographing and mapping and analysing and testing various water samples in a laboratory. From the study several conclusions could be drawn: most of the current laws relating to groundwater and sanitation were violated, the distribution of water to neighbours compounded problems. The major diseases typhoid, diarrhoea and dysentery were common. People themselves can take the necessary precaution by the choice of locating waste water so that it does not contaminate, boiling drinking water, etc. The devolution of the power from the Central Government and Ministries to District and down to the wards would go a long way to bring public/private partnership to be meaningful. The evidence from Goba points to the prevailing situation and what could be done to bring improvements.

Anuran tadpoles inhabiting a fluoride-rich stream: diets and morphological indicators

We in situ assessed the influence of natural fluoride concentrations in lotic freshwater ecosystems on diet and morphology of Boana cordobae tadpoles. Two streams were sampled in Argentina: Los Vallecitos stream (LF-LV) and Los Cerros Negros stream (HF-CN) with low and high fluoride levels, respectively. We captured tadpoles of B. cordobae in each stream using nets. Body weight (BW), total length (TL) and body condition (BC) of tadpoles was registered. Food items were identified to genus level and assigned to functional traits. Tadpoles showed significant differences in TL between streams, with smaller individuals in HF-CN, while did not show differences in BW and BC. The diet of tadpoles consisted mostly of microalgae. In tadpoles from HF-CN stream the proportion of cyanobacteria was lower than tadpoles from LF-LV. In relation to functional traits, small algae, high profile and colonial algae were more abundant in HF-CN. Algae attached by pads showed a higher proportion in HF-CN diets and stalked algae were more abundant in LF-LV. The differences in TL and diet of tadpoles can be attributed to differences in algal community composition, with genera that are affected by high concentrations of natural fluoride; for example cyanobacteria. The low algal richness registered in HF-CN stream does not affect the physiological state of the tadpoles, possibly because of a higher algal density in HF-CN diets. However, in further studies it would be important to evaluate the population status of B. cordobae from the HF-CN, because a small body length of tadpoles could have consequences at the population level.

Mayflies (Ephemeroptera) and Their Contributions to Ecosystem Services

This work is intended as a general and concise overview of Ephemeroptera biology, diversity, and services provided to humans and other parts of our global array of freshwater and terrestrial ecosystems. The Ephemeroptera, or mayflies, are a small but diverse order of amphinotic insects associated with liquid freshwater worldwide. They are nearly cosmopolitan, except for Antarctica and some very remote islands. The existence of the subimago stage is unique among extant insects. Though the winged stages do not have functional mouthparts or digestive systems, the larval, or nymphal, stages have a variety of feeding approaches-including, but not limited to, collector-gatherers, filterers, scrapers, and active predators-with each supported by a diversity of morphological and behavioral adaptations. Mayflies provide direct and indirect services to humans and other parts of both freshwater and terrestrial ecosystems. In terms of cultural services, they have provided inspiration to musicians, poets, and other writers, as well as being the namesakes of various water- and aircraft. They are commemorated by festivals worldwide. Mayflies are especially important to fishing. Mayflies contribute to the provisioning services of ecosystems in that they are utilized as food by human cultures worldwide (having one of the highest protein contents of any edible insect), as laboratory organisms, and as a potential source of antitumor molecules. They provide regulatory services through their cleaning of freshwater. They provide many essential supporting services for ecosystems such as bioturbation, bioirrigation, decomposition, nutrition for many kinds of non-human animals, nutrient cycling and spiraling in freshwaters, nutrient cycling between aquatic and terrestrial systems, habitat for other organisms, and serving as indicators of ecosystem health. About 20% of mayfly species worldwide might have a threatened conservation status due to influences from pollution, invasive alien species, habitat loss and degradation, and climate change. Even mitigation of negative influences has benefits and tradeoffs, as, in several cases, sustainable energy production negatively impacts mayflies.

Macroinvertebrate Responses to Conductivity in Different Bioregions of Victoria, Australia

The use of field data to derive guideline water quality trigger values is likely to be more environmentally relevant than laboratory estimates. In the present study, macroinvertebrate responses to conductivity (specific conductance at 25 °C) within 5 bioregions in Victoria, Australia, were derived from 19 yr of macroinvertebrate field data. Varying response to electrical conductivity (EC) occurred among taxa. Ninety-five percent extirpation concentrations (XC95) for EC were calculated for each genus and species and ranged from 25 to 23 600 µS/cm. Hazardous concentration 5th percentiles (HC05) were calculated for each bioregion from species sensitivity distributions developed using genus and species XC95 values. Genus HC05 values varied substantially between bioregions: bioregion 1 (29 µS/cm), 2 (78 µS/cm), 3 (143 µS/cm), 4 (1068 µS/cm), and 5 (2226 µS/cm). No substantial differences in HC05 values were shown between genus- and species-level calculations in bioregions 1 to 3 and 5; however, a decrease of approximately 300 µS/cm was shown for bioregion 4. The substantial differences in HC05 values between bioregions supports the need for region-specific determination of effects of EC. We explore the use of HC05 values as water quality guidelines across a bioregion gradient and provide a comprehensive analysis of macroinvertebrate responses to changes in EC, with important implications for waterway management. Environ Toxicol Chem 2019;38:1334-1342. © 2019 SETAC.

Potash mining effluents and ion imbalances cause transient osmoregulatory stress, affect gill integrity and elevate chronically plasma sulfate levels in adult common roach, Rutilus rutilus

Secondary salinization is a growing global ecological issue. One cause is the discharge of effluents by the potash mining industry into surface waters such as the River Werra in Germany. Increases of major ions require various physiological responses of freshwater organisms to maintain the hydromineral balance of body fluids. However, only little is known about the acute and chronic effects of high concentrations and imbalances of ions on osmoregulation in freshwater teleosts. The present study aimed to elucidate the effects of potash mining effluents and different cation ratios on the osmoregulatory capacity and gill histopathology of a native fish species. Individuals of Rutilus rutilus were exposed to the currently allowed (HT) and intended future (LT) thresholds as well as to high concentrations of Mg²⁺ (Mg), K⁺ (K), and Mg²⁺ and K⁺ (Mg + K) for a period of 24 h, 7 d, 21 d and 8 wk. Plasma osmolarity, [Na⁺], [Mg²⁺], [K⁺], [Ca²⁺], [Cl^-] and [SO₄^2-] and branchial Na⁺/K⁺-ATPase activity were determined. Moreover, histological gill alterations after 21 d and muscle water content after 8 wk were examined. HT transiently (24 h) elevated plasma osmolarity, plasma [Na⁺] and [Ca²⁺], whereas [SO₄^2-] was chronically increased even after 8 wk. Exposure to LT, Mg and Mg + K led to increased [SO₄^2-] levels for at least 21 d. It seems that [SO₄^2-] is mainly disturbed by multiple ions at high concentrations and long-term effects are unknown. Hydromineral homeostasis was maintained as indicated by unchanged Na⁺/K⁺-ATPase activity and muscle water content. However, mild structural alterations of the gills were observed in all exposure groups suggesting adaptational responses but with the potential to affect gas exchange capacity. Hence, the current thresholds for potash mining effluents affect osmomineral regulation in roach and further investigations should address potential impacts on reproduction in native fish species and physiological effects of SO₄^2-.

The influence of potential stressors on oviposition site selection and subsequent growth, survival and emergence of the non-biting midge (Chironomus tepperi)

Theory predicts that animals should prefer habitats where their fitness is maximized but some mistakenly select habitats where their fitness is compromised, that is, ecological traps. Understanding why this happens requires knowledge of the habitat selection cues animals use, the habitats they prefer and why, and the fitness costs of habitat selection decisions. We conducted experiments with a freshwater insect, the non-biting midge Chironomus tepperi to ask: (a) whether females respond to potential oviposition cues, (b) to explore whether oviposition is adaptive in relation to metal pollution and conductivity, and (c) whether individuals raised in poor quality sites are more likely to breed in similarly poor locations. We found the following: (a) females responded to some cues, especially conductivity and conspecifics, (b) females preferred sites with higher concentrations of bioavailable metals but suffered no consequences to egg/larval survival, (c) females showed some avoidance of high conductivities, but they still laid eggs resulting in reduced egg hatching, larval survival, and adult emergence, and (d) preferences were independent of natal environment. Our results show that C. tepperi is susceptible to ecological traps, depending on life stage and the relative differences in conductivities among potential oviposition sites. Our results highlight that (a) the fitness outcomes of habitat selection need to be assessed across the life cycle and (b) the relative differences in preference/suitability of habitats need to be considered in ecological trap research. This information can help determine why habitat preferences and their fitness consequences differ among species, which is critical for determining which species are susceptible to ecological traps.

Salinity tolerance of a rare and endangered unionid mussel, Popenaias popeii (Texas Hornshell) and its implications for conservation and water management

Unionid mussels are considered sensitive to salinity and there is growing concern in arid and semi-arid regions that declining flows coupled with anthropogenic impacts are amplifying natural salinity levels. In this study, we tested the effects of varying salinity concentrations (3.0, 4.0, 5.0, 6.0, 7.0 and 10.0 ppt NaCl) on survival of adult Popenaias popeii, (Texas Hornshell). This species occurs in the Rio Grande basin of Texas and northern Mexico, an arid to semi-arid stream plagued by salinization, and was recently listed as Endangered under the U.S. Endangered Species Act. We performed 2, 4, and 10-day toxicity tests on individuals from two disjunct populations: Laredo, TX, and the Lower Canyons of the Rio Grande near Big Bend National Park. We found no significant differences in LC50 estimates between populations at 96-hrs or 10-days but significant differences in TUD50s at 5 ppt between populations, which indicates that tolerance does not vary but sensitivity may between these populations. Overlaying LC50 estimates at 10-days for both populations on plots of salinity (ppt) measured over time, we show parts of the Rio Grande periodically approach or exceed 4.0 ppt, indicating these reaches are becoming unsuitable for P. popeii and populations within them at risk.

Spatio-temporal impact of salinated mine water on Lake Jormasjärvi, Finland

The salinization of freshwater environments is a global concern, and one of the largest sources of salinated water is the mining industry. An increasing number of modern mines are working with low grade sulfide ores, resulting in increased volumes of potentially harmful saline drainage. We used water monitoring data, together with data on sedimentary fossil remains (cladoceran, diatom and chironomid), to analyze the spatio-temporal (5 sampling locations and 3 sediment depths) impact of salinated mine water originating from the Talvivaara/Terrafame open cast mine on multiple components of the aquatic ecosystem of Lake Jormasjärvi, Finland. Lake Jormasjärvi is the fourth and largest lake in a chain of lakes along the path of the mine water. Despite the location and large water volume, the mine water has changed the chemistry of Lake Jormasjärvi, reflected in increased electrical conductivity values since 2010. The ecological impact is significant around the inflow region of the lake, as all biological indicator groups show a rapid and directional shift towards new species composition. There is a clear trend in improved water quality as one moves further from the point of inflow, and as one looks back in time. Our results show that salinated mine water may induce rapid and large scale changes, even far downstream along a chain of several sinking basins. This is of special importance in cases where large amounts of waste water are processed in the vicinity of protected habitats.

Combined effects of NaCl and fluoxetine on the freshwater planarian, Schmidtea mediterranea (Platyhelminthes: Dugesiidae)

Increasing salinity levels in freshwaters due to natural and anthropogenic sources pose risk to exposed aquatic organisms. However, there is a paucity of information on how salinity may influence the effects of other chemical stressors especially psychiatric pharmaceuticals. Freshwater planarians which have been suggested as bioindicator species in aquatic habitats were used in this study to evaluate toxic effects of sodium chloride (NaCl) used here as a surrogate for increasing salinity, and its influence on the effects of the antidepressant, fluoxetine. Effects of NaCl on Schmidtea mediterranea were evaluated using survival, regeneration, locomotion, feeding, and reproduction as endpoints. Subsequently, combined effects of NaCl and fluoxetine on planarians' locomotion and reproduction were also evaluated. Result showed that exposure to increased NaCl concentrations is toxic to planarians with 48 and 96 h LC₅₀ of 9.15 and 7.55 g NaCl L^-1 respectively and exposure to sub-lethal concentrations led to reductions in feeding (LOEC of 0.75 g NaCl L^-1 or 1906 μS cm^-1 at 20 °C) and reproduction (LOEC 3.0 g NaCl L^-1 or 5530 μS cm^-1 at 20 °C), delayed head regeneration (LOEC of 1.5 g NaCl L^-1 or 3210 μS cm^-1 at 20 °C), and also slight decreases in locomotor activity. Moreover, some developmental malformations were observed in regenerating planarians, as well as delayed or inhibition of wound healing and degeneration after fissioning and during head regeneration. A significant interaction between fluoxetine and NaCl was observed for locomotor activity and unlike planarians exposed to fluoxetine alone, fissioned planarians and their pieces from the combined exposure treatments were also unable to regenerate missing portions. Results show that S. mediterranea can be highly sensitive to low NaCl concentrations and that this stressor can alter the effects of fluoxetine. The implication of these effects for planarian populations in the natural habitat is discussed as well as the need for more research on the effects of neuroactive pharmaceuticals under relevant exposure scenarios.

Threshold Responses in the Taxonomic and Functional Structure of Fish Assemblages to Land Use and Water Quality: A Case Study from the Taizi River

Biological functional traits help to understand specific stressors that are ignored intaxonomic data analysis. A combination of biological functional traits and taxonomic data ishelpful in determining specific stressors which are of significance for fish conservation and riverbasin management. In the current study, the Taizi River was used as a case study to understand therelationships between the taxonomic and functional structure of fish and land use and waterquality, in addition to determining the thresholds of these stressors. The results showed thattaxonomic structure was significantly affected by the proportion of urban land and specificconductivity levels, while functional metrics were influenced by the proportions of farmland andforest. Threshold indicator taxa analysis found that Phoxinus lagowskii, Barbatula barbatula nuda,Odontobutis obscura, and Cobitis granoei had negative threshold responses along the gradients ofurban developments and specific conductivity. There was a significant change in fish taxonomiccomposition when the proportion of urban land exceeded a threshold of 2.6–3.1%, or specificconductivity exceeded a threshold of 369.5–484.5 μS/cm. Three functional features—habitatpreference, tolerance to disturbances, and spawning traits—showed threshold responses to theproportion of farmland and forest. The abundance of sensitive species should be monitored as partof watershed management, as sensitive species exhibit an earlier and stronger response to stressorsthan other functional metrics. Sensitive species had a positive threshold response to the proportionof forest at 80.1%. These species exhibited a negative threshold response to the proportion offarmland at 13.3%. The results of the current study suggest that the taxonomic and functionalstructure of fish assemblages are affected by land use and water quality. These parameters shouldbe integrated into routine monitoring for fish conservation and river basin management in the TaiziRiver. In addition, corresponding measures for improving river habitat and water quality shouldbe implemented according to the thresholds of these parameters.

Influence of salinity on spermatogenesis in adult Nile tilapia (Oreochromis niloticus) testis

Continental waters salinisation is a global threat that has grown because of climate change and human activities, but little is known about how and what biological tracts are affected. The aim of this study was to investigate the influence of different water salinities on the expression of HSP70, PCNA and caspase-3 during spermatogenesis of Nile tilapia. Adult males were submitted to four salinity treatments: (S₀) fresh water, (S₇) 7 g L^-1, (S₁₄) 14 g L^-1, and (S₂₁) 21 g L^-1 for 1, 4, and 9 days. All specimens were in spermatogenic activity and the highest values of the gonadosomatic index (GSI) occurred in the S₀ and S₇. In the morphometric analysis, spermatocytes were the most frequent germ cell detected in all treatments (>50%) and spermatids achieved about 20% of the testicular proportion, with few variations among treatments. Spermatozoa were significantly reduced only in S14 compared to S7. Leydig cells were significantly increased in S₁₄ when compared to S7 but plasma concentrations of 11-KT showed no significant difference among treatments. ELISA assay showed higher testicular expression of HSP70 at 1 day in all groups, followed by a significant decrease at days 4 and 9 in S₁₄ and S₂₁. The expression of PCNA was significantly lower while the activity of caspase-3 was higher in S₁₄ and S₂₁ when compared to S₀ and S₇. These results indicate that higher salinities in S₁₄ and S₂₁ interfere with the relationship between testicular HSP70, PCNA, and caspase-3, but with few effects over spermatogenesis dynamics of Nile tilapia.

Leaf litter microbial decomposition in salinized streams under intermittency

Human-induced salinization of freshwaters constitutes a growing global problem, whose consequences on streams functioning are largely unknown. Climate change projections predict enhanced evaporation, as well as an increase in extreme events and in variability of precipitation. This will result in more frequent, extended and severe drought periods that may aggravate water salinization of streams and rivers. In this study we conducted a microcosm experiment to assess the combined effects of three drought regimes - abrupt (AD), slow (SD) and very slow transition to dryness (VSD) - and three levels of salinization (0, 4, 6 g L^-1 NaCl) on microbial-mediated oak leaf decomposition over ten weeks. Salinization did not affect mass loss and associated microbial respiration of colonized oak leaves but significantly reduced the biomass and eliminated the sporulating capacity of fungi. Desiccation negatively affected leaf decomposition regardless of regime. Even though microbial respiration did not react to the different treatments, lower fungal biomass, diversity, and conidial production were observed under AD; for fungal biomass these effects were amplified at higher salt concentrations (particularly at 6 g L^-1). Our results indicate that effects of leaf litter desiccation depend on the rate of transition between wet and dry conditions and on the level of salt in the water. The two factors jointly affect decomposer survival and activity and, by extension, the dynamics of detrital food webs in streams.

Does short-term salinization of freshwater alter the behaviour of the Iberian barbel (Luciobarbus bocagei, Steindachner 1864)?

Stream salinization is a great environmental hazard being aggravated by anthropogenic disturbances. Harmful conditions, as increasing salinity in freshwater systems, may negatively affect river fish fauna and possibly influence fish behaviour, such as boldness and/or cerebral lateralization. Salinity has been proven to affect behavioural expression, despite the tolerance of some species. It is thus relevant to study these behaviours, as the salinity exposure effects could represent greater environmental consequences. The impact of salinity stress was evaluated by exposing Iberian barbels, Luciobarbus bocagei (Steindachner, 1864) (Cypriniformes, Cyprinidae), to three levels of salinity (0.9, 9 and 19 mS/cm, using NaCl) and by conducting boldness and lateralization experiments, regarding population trends. Results show that, with increased salinity, fish diverged to the extremes of the shy-bold gradient, the population was slightly lateralized to the left, and seemed to become more lateralized with increasing salinity. However, there were no statistical differences between the treatments. Fish living in a Mediterranean climate are especially resilient to various stressors, which may confer them additional tolerance, and in this case, acute punctual exposure to increased salinity may not be detrimental for behaviour maintenance. We encourage the expansion of the research to different freshwater fish species that would help to recognise salinity thresholds and use them to implement effective conservation measures and appropriate ecological restoration actions for these sensible systems.

Spatial distribution of prokaryotic communities in hypersaline soils

Increasing salinization in wetland systems is a major threat to ecosystem services carried out by microbial communities. Thus, it is paramount to understand how salinity drives both microbial community structures and their diversity. Here we evaluated the structure and diversity of the prokaryotic communities from a range of highly saline soils (EC_1:5 from 5.96 to 61.02 dS/m) from the Odiel Saltmarshes and determined their association with salinity and other soil physicochemical features by analyzing 16S rRNA gene amplicon data through minimum entropy decomposition (MED). We found that these soils harbored unique communities mainly composed of halophilic and halotolerant taxa from the phyla Euryarchaeota, Proteobacteria, Balneolaeota, Bacteroidetes and Rhodothermaeota. In the studied soils, several site-specific properties were correlated with community structure and individual abundances of particular sequence variants. Salinity had a secondary role in shaping prokaryotic communities in these highly saline samples since the dominant organisms residing in them were already well-adapted to a wide range of salinities. We also compared ESV-based results with OTU-clustering derived ones, showing that, in this dataset, no major differences in ecological outcomes were obtained by the employment of one or the other method.

Drought impacts on river salinity in the southern US: Implications for water scarcity

Hydrological droughts have a diverse range of effects on water resources. Whilst the impacts of drought on water quantity are well studied, the impacts on water quality have received far less attention. Similarly, quantifications of water scarcity have typically lacked water quality dimensions, whilst sectoral water uses are associated with both water quantity and quality requirements. Here we aim to combine these two elements, focussing on impacts of droughts on river salinity levels and including a salinity dimension in quantifications of water scarcity during drought and extreme drought conditions. The impact of historical droughts on river salinity (electrical conductivity (EC) was studied at 66 monitoring stations located across the Southern USA for 2000-2017. Salinity was found to increase strongly (median increase of 21%) and statistically significantly (p ≤ 0.05) during drought conditions for 59/66 stations compared to non-drought conditions. In a next step, a salinity dimension was added to water scarcity quantifications for 15 river basins in Texas. Water scarcity was quantified using data of sector water uses, water availability, river salinity levels and salinity thresholds for sector water uses. Results showed that the dominant factor driving water scarcity highly differed per basin. Increases in water scarcity were further compounded by drought-induced decreases in water availability, increases in sectoral water demands and increases in river water salinity. This study demonstrates that droughts are associated with important increases in river salinity, in addition to reduced water availability, and that both of these aspects should be considered when quantifying water scarcity. Alleviating water scarcity should therefore not only focus on increasing water availability and reducing water demands (quantity aspects), but also on improving water quality.

Life Cycle Impact and Benefit Trade-Offs of a Produced Water and Abandoned Mine Drainage Cotreatment Process

A cotreatment process for produced water and abandoned mine drainage (AMD) has been established and demonstrated at the pilot-scale. The present study evaluates the potential of the proposed process to aid in management of two high volume wastewater resources in Pennsylvania. A systems-level approach is established to evaluate the primary trade-offs, including cotreatment process environmental impacts, transportation impacts, and environmental benefits realized from precluding direct AMD release to the environment. Life cycle impact assessment was used to quantify the environmental and human health impacts as well as to identify "hot spots" of the cotreatment process. Electricity use was found to be the dominant contributor to all impact categories. Extending the system boundary to include transportation of the two wastewaters to a to-be-determined cotreatment site revealed the important impact of transportation. An optimization approach was employed (using the region of Southwest Pennsylvania) to evaluate minimization of transportation distance considering the location and number of treatment sites. Finally, a quantitative analysis of environmental benefits realized by precluding direct AMD release to the environment was performed. The results suggest that the magnitude of benefit realized in treating a highly polluted AMD is greater than the magnitude of impacts from the cotreatment process.

Are fungal strains from salinized streams adapted to salt-rich conditions?

Anthropogenic salinization of freshwater is a global problem with largely unknown consequences for stream functions. We compared the effects of salt addition (6 g l^-1 NaCl) in microcosms on leaf mass loss and microbial parameters in single- and multispecies assemblages of fungal strains (Heliscus lugdunensis, HELU; Tetracladium marchalianum, TEMA; Flagellospora curta, FLCU) isolated from a reference (R) or salinized (S) stream. Fungal growth and interactions were also assessed. Salinization inhibited leaf decomposition and fungal biomass, but no differences were observed between species, strains or species combinations. Sporulation rates in monocultures were not affected by added salt, but differed among species (FLCU > HELU > TEMA), with S strains releasing more conidia. Fungal assemblages did not differ significantly in total conidia production (either between strains or medium salt concentration). HELU was the dominant species, which also had highest growth and most pronounced antagonistic behaviour. Fungal species, irrespective of origin, largely maintained their function in salinized streams. Strains from salt-contaminated streams did not trade-off conidial production for vegetative growth at high salt levels. The expected reduction of fungal diversity and potential changes in nutritional litter quality owing to salinization may impact leaf incorporation into secondary production in streams.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Temperature affects acute mayfly responses to elevated salinity: implications for toxicity of road de-icing salts

Salinity in freshwater ecosystems has increased significantly at numerous locations throughout the world, and this increase often reflects the use or production of salts from road de-icing, mining/oil and gas drilling activities, or agricultural production. When related to de-icing salts, highest salinity often occurs in winter when water temperature is often low relative to mean annual temperature at a site. Our study examined acute (96 h) responses to elevated salinity (NaCl) concentrations at five to seven temperature treatments (5-25°C) for four mayfly species (Baetidae: Neocloeon triangulifer, Procloeon fragile; Heptageniidae: Maccaffertium modestum; Leptophlebiidae: Leptophlebia cupida) that are widely distributed across eastern North America. Based on acute LC50s at 20°C, P. fragile was most sensitive (LC50 = 767 mg l^-1, 1447 µS cm^-1), followed by N. triangulifer (2755 mg l^-1, 5104 µS cm^-1), M. modestum (2760 mg l^-1, 5118 µS cm^-1) and L. cupida (4588 mg l^-1, 8485 µS cm^-1). Acute LC50s decreased as temperature increased for all four species (n = 5-7, R ² = 0.65-0.88, p = 0.052-0.002). Thus, acute salt toxicity is strongly temperature dependent for the mayfly species we tested, which suggests that brief periods of elevated salinity during cold seasons or in colder locations may be ecologically less toxic than predicted by standard 20 or 25°C laboratory bioassays.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Predicting combined effects of land use and climate change on river and stream salinity

Agricultural, industrial and urban development have all contributed to increased salinity in streams and rivers, but the likely effects of future development and climate change are unknown. I developed two empirical models to estimate how these combined effects might affect salinity by the end of this century (measured as electrical conductivity, EC). The first model predicts natural background from static (e.g. geology and soils) and dynamic (i.e. climate and vegetation) environmental factors and explained 78% of the variation in EC. I then compared the estimated background EC with current measurements at 2001 sites chosen probabilistically from all conterminous USA streams. EC was more than 50% greater at 34% of these sites. The second model predicts deviation of EC from background as a function of human land use and environmental factors and explained 60% of the variation in alteration from background. I then predicted the effects of climate and land use change on EC at the end of the century by replacing dynamic variables with published projections of future conditions based on the A2 emissions scenario. By the end of the century, the median EC is predicted to increase from 0.319 mS cm^-1 to 0.524 mS cm^-1 with over 50% of streams having greater than 50% increases in EC and 35% more than doubling their EC. Most of the change is related to increases in human land use, with climate change accounting for only 12% of the increase. In extreme cases, increased salinity may make water unsuitable for human use, but widespread moderate increases are likely a greater threat to stream ecosystems owing to the elimination of low EC habitats.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Salinization effects on coastal ecosystems: a terrestrial model ecosystem approach

In coastal areas, intrusion/irrigation with seawater can threaten biodiversity along with crop yields, and the leaching of salts from areas affected by these processes can increase the salinity of water bodies nearby. The aims of this study were to evaluate the effects of salinization on coastal soil ecosystems due to saline intrusion/irrigation. Terrestrial model ecosystems were used to simulate two soil salinization scenarios: (i) seawater intrusion and irrigation with distilled water and (ii) seawater intrusion and irrigation with saline water. Three sampling periods were established: T0-after acclimation period; T1-salinization effects; and T2-populations' recovery. In each sampling period, the abundance of nematodes, enchytraeids, springtails, mites and earthworms, and plant biomass were measured. Immediate negative effects on enchytraeid abundance were detected, especially at the higher level of saltwater via intrusion+irrigation. Eight weeks after the cessation of saline irrigation, the abundance of enchytraeids fully recovered, and some delayed effects were observed in earthworm abundance and plant biomass, especially at the higher soil conductivity level. The observed low capacity of soil to retain salts suggests that, particularly at high soil conductivities, nearby freshwater bodies can also be endangered. Under saline conditions similar to the ones assayed, survival of some soil communities can be threatened, leading to the loss of biodiversity.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Salt in freshwaters: causes, effects and prospects - introduction to the theme issue

Humans are globally increasing the salt concentration of freshwaters (i.e. freshwater salinization), leading to significant effects at the population, community and ecosystem level. The present theme issue focuses on priority research questions and delivers results that contribute to shaping the future research agenda on freshwater salinization as well as fostering our capacity to manage salinization. The issue is structured along five topics: (i) the estimation of future salinity and evaluation of the relative contribution of the different drivers; (ii) the physiological responses of organisms to alterations in ion concentrations with a specific focus on the osmophysiology of freshwater insects and the responses of different organisims to seawater intrusion; (iii) the impact of salinization on ecosystem functioning, also considering the connections between riparian and stream ecosystems; (iv) the role of context in moderating the response to salinization. The contributions scrutinise the role of additional stressors, biotic interactions, the identify of the ions and their ratios, as well as of the biogeographic and evolutionary context; and (v) the public discourse on salinization and recommendations for management and regulation. In this paper we introduce the general background of salinization, outline research gaps and report key findings from the contributions to this theme issue.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Novel 'chemical cocktails' in inland waters are a consequence of the freshwater salinization syndrome

Widespread changes in water temperatures, salinity, alkalinity and pH have been documented in inland waters in North America, which influence ion exchange, weathering rates, chemical solubility and contaminant toxicity. Increasing major ion concentrations from pollution, human-accelerated weathering and saltwater intrusion contribute to multiple ecological stressors such as changing ionic strength and pH and mobilization of chemical mixtures resulting in the freshwater salinization syndrome (FSS). Here, we explore novel combinations of elements, which are transported together as chemical mixtures containing salts, nutrients and metals as a consequence of FSS. First, we show that base cation concentrations have increased in regions primarily in North America and Europe over 100 years. Second, we show interactions between specific conductance, pH, nitrate and metals using data from greater than 20 streams located in different regions of the USA. Finally, salinization experiments and routine monitoring demonstrate mobilization of chemical mixtures of cations, metals and nutrients in 10 streams draining the Washington, DC-Baltimore, MD metropolitan regions. Freshwater salinization mobilizes diverse chemical mixtures influencing drinking water quality, infrastructure corrosion, freshwater CO2 concentrations and biodiversity. Most regulations currently target individual contaminants, but FSS requires managing mobilization of multiple chemical mixtures and interacting ecological stressors as consequences of freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Are sulfate effects in the mayfly Neocloeon triangulifer driven by the cost of ion regulation?

Elevated major ion concentrations in streams are commonly observed as a consequence of resource extraction, de-icing and other anthropogenic activities. Ecologists report biodiversity losses associated with increasing salinity, with mayflies typically being highly responsive to increases of different major ions. In this study, we evaluated the performance of the mayfly Neocloeon triangulifer reared for its entire larval phase in a gradient of sulfate concentrations. Two natural waters were amended with SO₄ as a blend of CaSO₄ and MgSO₄ and exposures ranged from 5 to 1500 mg l^-1 SO_4. Survival (per cent successful emergence to the subimago stage) was significantly reduced at the highest SO₄ concentration in both waters, while development was significantly delayed at 667 mg l^-1 SO₄ Final sub-adult body weights were consistent across treatments, except at the highest treatment concentration. Despite evidence for sulfate uptake rates increasing with exposure concentrations and not being saturated at even extremely high SO₄ concentrations, total body sulfur changed little in subimagos. Together, these results suggest that elevated SO₄ imposes an energetic demand associated with maintaining homeostasis that is manifested primarily as reduced growth rates and associated developmental delays. We identified two genes related to sulfate transport in N. trianguliferThis article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Why are mayflies (Ephemeroptera) lost following small increases in salinity? Three conceptual osmophysiological hypotheses

The salinity of many freshwaters is increasing globally as a result of human activities. Associated with this increase in salinity are losses of Ephemeroptera (mayfly) abundance and richness. The salinity concentrations at which Ephemeroptera decline in nature are lower than their internal salinity or haemolymph osmolality. Many species also suffer substantial mortality in single species laboratory toxicity tests at salinities lower than their internal salinity. These findings are problematic as conventional osmoregulation theory suggests that freshwater animals should not experience stress where external osmolality is greater than haemolymph osmolality. Here I explore three hypotheses to explain salt sensitivity in Ephemeroptera. These conceptual hypotheses are based on the observations that as the external sodium ion (Na⁺) concentration increases so does the Na⁺ turnover rate (both uptake and elimination rates increase). Sulphate ([Formula: see text]) uptake in mayflies also increases with increasing external [Formula: see text] although, unlike Na⁺, its rate of increase decreases with increasing external [Formula: see text] The first hypothesis is premised on ion turnover being energetically costly. The first hypothesis proposes that individuals must devote a greater proportion of their energy to ion homeostasis at the expense of other uses including growth and development. Lethal levels of salinity presumably result from individuals not being able to devote enough energy to maintain ion homeostasis without critical loss of other vital functions. The second hypothesis is premised on the uptake of Na⁺ exchanged for (an outgoing) H⁺, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Na⁺ toxicity or poisoning with increased Na turnover as salinity increases. None of the proposed hypotheses is without potential problems, yet all are testable, and research effort should be focused at attempting to falsify them.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Lost in translation: the German literature on freshwater salinization

Human activities have globally increased and altered the ion concentration of freshwater ecosystems. The proliferation of potash mines in Germany (especially intense in the early 1900s) constitutes a good example of it. The effluents and runoff coming from potash mines led to extreme salt concentrations (e.g. 72 g l^-1 of total salt content, approx. 149 mS cm^-1) in surrounding rivers and streams, causing ecosystem degradation (e.g. massive algal blooms and fish kills). This promoted scientific research that was mostly published in German, thereby being neglected by the wide scientific community. Here, the findings of the German literature on freshwater salinization are discussed in the light of current knowledge. German studies revealed that at similar ion concentrations potassium (K⁺) can be the most toxic ion to freshwater organisms, whereas calcium (Ca²⁺) could have a toxicity ameliorating effect. Also, they showed that salinization could lead to biodiversity loss, major shifts in the composition of aquatic communities (e.g. dominance of salt-tolerant algae, proliferation of invasive species) and alter organic matter processing. The biological degradation caused by freshwater salinization related to potash mining has important management implications, e.g. it could prevent many European rivers and streams from reaching the good ecological status demanded by the Water Framework Directive. Within this context, German publications show several examples of salinity thresholds and biological indices that could be useful to monitor and regulate salinization (i.e. developing legally enforced salinity and ion-specific standards). They also provide potential management techniques (i.e. brine collection and disposal) and some estimates of the economic costs of freshwater salinization. Overall, the German literature on freshwater salinization provides internationally relevant information that has rarely been cited by the English literature. We suggest that the global editorial and scientific community should take action to make important findings published in non-English literature more widely available.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Insect communities in saline waters consist of realized but not fundamental niche specialists

Considering how organisms adapt to stress is essential if we are to anticipate biological responses to global change in ecosystems. Communities in stressful environments can potentially be assembled by specialists (i.e. species that only occur in a limited range of environmental conditions) and/or generalist species with wider environmental tolerances. We review the existing literature on the salinity tolerance of aquatic insects previously identified as saline specialists because they were exclusively found in saline habitats, and explore if these saline realized niche specialists are also specialists in their fundamental niches or on the contrary are fundamental niche generalist species confined to the highest salinities they can tolerate. The results suggest that species inhabiting saline waters are generalists in their fundamental niches, with a predominant pattern of high survival in freshwater-low salinity conditions, where their fitness tends to be similar or even higher than in saline waters. Additionally, their performance in freshwater tends to be similar to related strictly freshwater species, so no apparent trade-off of generalization is shown. These results are discussed in the framework of the ecological and evolutionary processes driving community assembly across the osmotic stress gradient, and their potential implications for predicting impacts from saline dilution and freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Do all roads lead to Rome? Exploring community trajectories in response to anthropogenic salinization and dilution of rivers

Abiotic stress shapes how communities assemble and support ecological functions. However, it remains unclear whether artificially increasing or decreasing stress levels would lead to communities assembling predictably along a single axis of variation or along multiple context-dependent trajectories of change. In response to stress intensity alterations, we hypothesize that a single trajectory of change occurs when trait-based assembly prevails, while multiple trajectories of change arise when dispersal-related processes modify colonization and trait-filtering dynamics. Here, we tested these hypotheses using aquatic macroinvertebrates from rivers exposed to gradients of natural salinity and artificially diluted or salinized ion contents. Our results showed that trait-filtering was important in driving community assembly in natural and diluted rivers, while dispersal-related processes seemed to play a relevant role in response to salinization. Salinized rivers showed novel communities with different trait composition, while natural and diluted communities exhibited similar taxonomic and trait compositional patterns along the conductivity gradient. Our findings suggest that the artificial modification of chemical stressors can result in different biological communities, depending on the direction of the change (salinization or dilution), with trait-filtering, and organism dispersal and colonization dynamics having differential roles in community assembly. The approach presented here provides both empirical and conceptual insights that can help in anticipating the ecological effects of global change, especially for those stressors with both natural and anthropogenic origins.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Drivers of spatio-temporal patterns of salinity in Spanish rivers: a nationwide assessment

The salinization of freshwaters is a global water quality problem that leads to the biological degradation of aquatic ecosystems. However, little is known about the spatial extent of freshwater salinization and the relative contribution of each human activity (e.g. agriculture, urbanization, mining or shale-gas extraction). Here, we investigated environmental factors that explain spatio-temporal patterns of water salinity and examined the causes, the extent and the degree of salinization of Spanish rivers. Results showed a strong variation in water salinity among river typologies and between river reaches in good and poor ecological status according to the Water Framework Directive. The variation in water salinity was largely explained by a combination of natural (i.e. climate and geology) and anthropogenic (i.e. land use) factors. By contrast, land use factors as urbanization and agriculture were the main drivers of salinization, which affected more than one quarter of the rivers and streams in Spain, especially those in the most arid regions (central and southern regions) and in the main courses of the largest rivers such as the Ebro, Douro and Tajo rivers. The information provided here can be relevant to set priority regions and actions to ameliorate freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Effects of salinity changes on aquatic organisms in a multiple stressor context

Under global change, the ion concentration of aquatic ecosystems is changing worldwide. Many freshwater ecosystems are being salinized by anthropogenic salt inputs, whereas many naturally saline ones are being diluted by agricultural drainages. This occurs concomitantly with changes in other stressors, which can result in additive, antagonistic or synergistic effects on organisms. We reviewed experimental studies that manipulated salinity and other abiotic stressors, on inland and transitional aquatic habitats, to (i) synthesize their main effects on organisms' performance, (ii) quantify the frequency of joint effect types across studies and (iii) determine the overall individual and joint effects and their variation among salinity-stressor pairs and organism groups using meta-analyses. Additive effects were slightly more frequent (54%) than non-additive ones (46%) across all the studies (n = 105 responses). However, antagonistic effects were dominant for the stressor pair salinity and toxicants (44%, n = 43), transitional habitats (48%, n = 31) and vertebrates (71%, n = 21). Meta-analyses showed detrimental additive joint effects of salinity and other stressors on organism performance and a greater individual impact of salinity than the other stressors. These results were consistent across stressor pairs and organism types. These findings suggest that strategies to mitigate multiple stressor impacts on aquatic ecosystems should prioritize restoring natural salinity concentrations.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Biological interactions mediate context and species-specific sensitivities to salinity

Toxicants have both sub-lethal and lethal effects on aquatic biota, influencing organism fitness and community composition. However, toxicant effects within ecosystems may be altered by interactions with abiotic and biotic ecosystem components, including biological interactions. Collectively, this generates the potential for toxicant sensitivity to be highly context dependent, with significantly different outcomes in ecosystems than laboratory toxicity tests predict. We experimentally manipulated stream macroinvertebrate communities in 32 mesocosms to examine how communities from a low-salinity site were influenced by interactions with those from a high-salinity site along a gradient of salinity. Relative to those from the low-salinity site, organisms from the high-salinity site were expected to have greater tolerance and fitness at higher salinities. This created the potential for both salinity and tolerant-sensitive organism interactions to influence communities. We found that community composition was influenced by both direct toxicity and tolerant-sensitive organism interactions. Taxon and context-dependent responses included: (i) direct toxicity effects, irrespective of biotic interactions; (ii) effects that were owing to the addition of tolerant taxa, irrespective of salinity; (iii) toxicity dependent on sensitive-tolerant taxa interactions; and (iv) toxic effects that were increased by interactions. Our results reinforce that ecological processes require consideration when examining toxicant effects within ecosystems.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.