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Salting of roadways contaminates local waterways via snowmelt and precipitation runoff, eliciting various toxicological impacts on aquatic ecosystems. Recently, "eco‐friendly" deicing alternatives have been introduced in hopes of mitigating environmental impacts of deicing agents, while maintaining human safety. These "eco‐friendly" alternatives may pose their own set of environmental concerns that require further study. While the potential toxicity of road salts has been evaluated for various aquatic species, the environmental factors that may influence this toxicity are less understood; and for emerging deicing alternatives, there is a lack of literature documenting these potential implications. For aquatic organisms, the highest exposure to road salts may coincide with reduced food availability, namely during the winter months. The present study evaluates the effect of a conditioning diet on the sensitivity of adult Hyalella azteca to an "eco‐friendly"‐labeled beet deicer (Snow Joe MELT Beet‐IT). Various conditioning diets were examined, including TetraMinTM, TetraMin and diatom (Thalassiosira weissflogii) combinations, and TetraMin and conditioned Acer sacharum leaves. For each diet type, 48‐ and 96‐h water‐only toxicity bioassays were conducted with adult H. azteca. These results were compared to organisms which experienced a 96‐h starvation period prior to exposure and culture organisms. Diet types representing excess quality and quantity of food significantly decreased the toxicity of beet deicer to the organisms. However, starvation likely increases the toxicity of road salts to H. azteca. Therefore, the quantity and quality of food available to H. azteca may influence their sensitivity to deicing agents. Environ Toxicol Chem 2024;43:2608–2615. © 2024 SETAC [ABSTRACT FROM AUTHOR]

Worldwide freshwater salinization may significantly alter ecosystem functioning and affect biodiversity. We studied the effect of salinity on hatching success of resting eggs of different ages and life history traits of post-diapause females of cladoceran Moina macrocopa. The hatching success of resting eggs was almost unaffected by the salinity we tested (up to 5.9 g L−1). We observed a distinct difference in the hatching success of resting eggs of different ages, which was lower by ca. 50% for 4-year-old resting eggs compared with 1-year-old resting eggs. We found a decrease in the values of the life-history traits of the females hatched from 4-year-old resting eggs compared with the females hatched from 1-year-old resting eggs exposed to salinity within the range of 0.08–3.5 g L−1: a shorter lifespan, a lower juvenile somatic growth rate, a delay in the day of the first reproduction, and a smaller number of parthenogenetic clutches produced. Analysis showed that at a salinity of 3.5 g L−1, females hatched from resting eggs are more vulnerable to the effect of salinity than directly developing females. The post-diapause females exposed to salinity showed a shorter lifespan and reduced reproduction parameters (produced fewer clutches and neonates) compared with the directly developing females. Thus, the hatching of resting eggs can be considered as a critical period of the development of a species with the diapause in its life cycle due to the vulnerability of post-diapause females to increased salinity, which can be crucially important for the ecological success of the population. [ABSTRACT FROM AUTHOR]

Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed chemical cocktails, in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.

Freshwater salinization increasingly threatens river ecosystems in arid regions. In situ studies on effects of salinity on freshwater communities are still scarce, especially in largely understudied areas of Africa. To compare macroinvertebrate communities in differing salinity levels, we conducted a confluence-based study in the Draa River basin in Morocco by focusing on two tributaries and their joint downstream sections, in the immediate vicinity of three confluences. Our study revealed that α-diversity differed only minimal. Although only around five taxa comprised over 90% of specimens per section, the more saline sections exhibited proportionally more salt-tolerant generalist species. There was lower β-diversity between the downstream section and each tributary compared to between tributaries, indicating a mixed community after the confluence. The trait profile of the saline El Mellah displayed more resistance and resilience traits to disturbances than the less saline Iriri. Furthermore, low water flow reduced the abundance of sensitive taxa. Overall, we observed minimal differences in macroinvertebrate community composition, due to low γ-diversity in the basin. However, the confluence-based study design remains valuable for investigating effects of specific stressors on ecosystems by excluding large-scale geographic patterns, as compared sites are close and therefore share the same climate, geology, and altitude. [ABSTRACT FROM AUTHOR]

Human activity is increasing salt concentrations in freshwaters worldwide, but effects of freshwater salinity gradients on biogeochemical cycling are less understood than in saline, brackish, or marine environments. Using controlled microcosm experiments, we characterized (1) short-term (one to five days) biogeochemical responses and (2) water column metabolism along a freshwater salinity gradient of multiple salt types. After one day, microcosms were oxic (4.48–7.40 mg O2 L−1) but became hypoxic (1.20–3.31 mg L−1) by day five. After one day in oxic conditions, microbial respiration in magnesium-, sodium-, and sea salt-based salinity treatments showed a subsidy-stress response, with respiration increasing by over 100% as salinity increased from 30 to 350–800 µS cm−1. Conversely, respiration consistently increased along a calcium-based salinity gradient, peaking at 1500 µS cm−1. By day five, an inverse subsidy-stress response was observed with elevated respiration at upper or lower ends of the gradient except for the magnesium treatment, which had the lowest respiration at the highest salinity. Calcium- and magnesium-based salinity treatments also caused considerable changes in phosphorus concentrations and C:P and N:P. In a separate experiment, microbial respiration and water column primary production also displayed subsidy-stress responses, but imbalances in effect sizes caused consistently declining net community production with increasing salinity. Collectively, our results establish that short-term exposure to different salt ion concentrations can enhance freshwater biogeochemical cycling at relatively low concentrations and alter resource stoichiometry. Furthermore, the nature of effects of freshwater salinization may also change with oxygen availability. [ABSTRACT FROM AUTHOR]

Impervious surface cover increases peak flows and degrades stream health, contributing to a variety of hydrologic, water quality, and ecological symptoms, collectively known as the urban stream syndrome. Strategies to combat the urban stream syndrome often employ engineering approaches to enhance stream-floodplain reconnection, dissipate erosive forces from urban runoff, and enhance contaminant retention, but it is not always clear how effective such practices are or how to monitor for their effectiveness. In this study, we explore applications of longitudinal stream synoptic (LSS) monitoring (an approach where multiple samples are collected along stream flowpaths across both space and time) to narrow this knowledge gap. Specifically, we investigate (1) whether LSS monitoring can be used to detect changes in water chemistry along longitudinal flowpaths in response to stream-floodplain reconnection and (2) what is the scale over which restoration efforts improve stream quality. We present results for four different classes of water quality constituents (carbon, nutrients, salt ions, and metals) across five watersheds with varying degrees of stream-floodplain reconnection. Our work suggests that LSS monitoring can be used to evaluate stream restoration strategies when implemented at meter to kilometer scales. As streams flow through restoration features, concentrations of nutrients, salts, and metals significantly decline (p < 0.05) or remain unchanged. This same pattern is not evident in unrestored streams, where salt ion concentrations (e.g., Na+, Ca2+, K+) significantly increase with increasing impervious cover. When used in concert with statistical approaches like principal component analysis, we find that LSS monitoring reveals changes in entire chemical mixtures (e.g., salts, metals, and nutrients), not just individual water quality constituents. These chemical mixtures are locally responsive to restoration projects, but can be obscured at the watershed scale and overwhelmed during storm events. [ABSTRACT FROM AUTHOR]

The increasing application of road deicing agents (e.g., NaCl) has caused widespread salinization of freshwater environments. Chronic exposure to toxic NaCl levels can impact freshwater biota at genome to ecosystem scales, yet the degree of harm caused by road salt pollution is likely to vary among habitats and populations. The background ion chemistry of freshwater environments may strongly impact NaCl toxicity, with greater harm occurring in ion‐poor, soft water conditions. In addition, populations exposed to salinization may evolve increased NaCl tolerance. Notably, if organisms are adapted to the water chemistry of their natal environment, toxicity responses may also vary among populations in a given test medium. We examined the potential for this evolutionary and environmental context to interact in shaping NaCl toxicity with a pair of laboratory reciprocal transplant toxicity experiments, using natural populations of the water flea Daphnia ambigua collected from three lakes that vary in ion availability and composition. We observed a strong effect of the lake water environment on NaCl toxicity in both trials. NaCl caused a much greater decline in reproduction and r in lake water from a low‐ion/calcium‐poor environment (20 μS/cm specific conductance; 1.7 mg/L Ca2+) compared with water from both a Ca2+‐rich lake (55 μS/cm; 7.2 mg/L Ca2+) and an ion‐rich coastal lake (420 μS/cm; 3.4 mg/L Ca2+). Daphnia from this coastal lake were most robust to the effects of NaCl on reproduction and r. A significant interaction between the population and lake water environment shaped survival in both trials, suggesting that local adaptation to the test waters used may have contributed to toxicity responses. Our findings that the lake water environment, adaptation to that environment, and adaptation to a contaminant of interest may shape toxicity demonstrate the importance of considering environmental and biological complexity in mitigating pollution impacts. [ABSTRACT FROM AUTHOR]

Freshwater salinization, due to road salt and other increased anthropogenic activities, has become a significant threat to freshwater organisms. However, whether freshwater salinization affects the response of aquatic organisms to their predators, especially prey that have been acclimated to salinity environments for a long time, remains unclear. In the present study, we investigated the changes in anti-predator defense of Daphnia magna with and without salinity acclimation at five different salinities (0, 0.6, 0.8, 0.10, and 0.12 M). Results showed that freshwater salinization weakened the induced defense response of D. magna, regardless of whether it had undergone long-term salinity acclimation. Specifically, induced defense traits such as smaller body size, higher relative spine length, more relative reproductive output, and smaller body size neonates disappeared at ≥ 0.08 M salinities. In addition, there were no significant differences in most traits of induced defense strength between D. magna with and without salinity acclimation at the same salinity. Importantly, the integrated induced defense response index decreased with increasing salinity. Our study showed that salinity-tolerant organisms do not recover their induced defense at high salinities, underlining the importance of incorporating interspecific interactions when estimating the effects of freshwater salinization on organisms. [ABSTRACT FROM AUTHOR]

Human activities are not only increasing salinization of rivers, they might also be altering the temporal dynamics of salinity. Here, we assess the effect of human activities on the temporal dynamics of electrical conductivity (EC) in 91 Spanish rivers using daily measures of EC from 2007 to 2011. We expected rivers weakly affected by human activities to have low and constant ECs, whereas rivers strongly affected by human activities should have high and variable ECs throughout the year. We collected information on land use, climate, and geology that could explain the spatiotemporal variation in EC. We identified four groups of rivers with differences in EC trends that covered a gradient of anthropogenic pressure. According to Random Forest analysis, temporal EC patterns were mainly driven by agriculture, but de-icing roads, mining, and wastewater discharges were also important to some extent. Linear regressions showed a moderate relationship between EC variability and precipitation, and a weak relationship to geology. Overall, our results show strong evidence that human activities disrupt the temporal dynamics of EC. This could have strong effects on aquatic biodiversity (e.g., aquatic organisms might not adapt to frequent and unpredictable salinity peaks) and should be incorporated into monitoring and management plans. [ABSTRACT FROM AUTHOR]

Road salt (NaCl) is commonly used as a deicer during winter to improve road safety, resulting in freshwater salinization. Such elevated chloride concentrations may have a strong effect on zooplankton, which are key elements in freshwater food webs. The aim of this study was to investigate the effect of chloride ion concentration and inter-pond environmental variability on zooplankton density in four urban ponds over 3 years differing in winter conditions. Analysis of variance showed significant differences in chloride ion concentration and zooplankton density regarding study year and ponds. Redundancy analysis of column water data showed that both the environmental variables (including chloride ion concentration) and the pond location significantly contributed to the model explaining the variability in zooplankton densities. However, the environment variable (referring to the environmental variables) had a smaller contribution (11%) than the POND variable (related to the spatial configuration of the ponds), which contributed 62%, indicating that the dynamics of zooplankton density depended primarily on ecosystem characteristics. Even so, although chloride content was not the main determinant of zooplankton density, their presence in aquatic ecosystems and potential interactions with other types of pollutants could have negative biological and ecological effects. [ABSTRACT FROM AUTHOR]

Potash fertilizer production is one of the most important economic activities. Historically, potash mining has had a significant impact on the environment, often with catastrophic consequences. The purpose of this paper is to summarize the results of studies on the environmental impact of potash mining using the example of the Verkhnekamskoe potash deposit. The deposit is located in the central part of the Solikamsk depression in the Pre-Ural foredeep (Perm Krai, Russia). All the main features and problems of underground mining of water-soluble ores and potassium fertilizer production are considered using the example of one of the world’s largest potash deposits. This paper looks into the specifics of the material composition of waste, its disposal, underground mining issues associated with the solubility of salts, and the risks of groundwater inflow into the mine workings, which causes flooding of mines. The results of all surveys show that potash mining affects the atmosphere, surface water, groundwater, soil, and vegetation. The most effective measure to reduce the adverse environmental impact of potash mining at the Verkhnekamskoe Deposit is hydraulic backfilling of mine chambers, which protects the underground mines from flooding, minimizes ground subsidence, and reduces the area of potash waste.

Potash fertilizer production is one of the most important economic activities. Historically, potash mining has had a significant impact on the environment, often with catastrophic consequences. The purpose of this paper is to summarize the results of studies on the environmental impact of potash mining using the example of the Verkhnekamskoe potash deposit. The deposit is located in the central part of the Solikamsk depression in the Pre-Ural foredeep (Perm Krai, Russia). All the main features and problems of underground mining of water-soluble ores and potassium fertilizer production are considered using the example of one of the world's largest potash deposits. This paper looks into the specifics of the material composition of waste, its disposal, underground mining issues associated with the solubility of salts, and the risks of groundwater inflow into the mine workings, which causes flooding of mines. The results of all surveys show that potash mining affects the atmosphere, surface water, groundwater, soil, and vegetation. The most effective measure to reduce the adverse environmental impact of potash mining at the Verkhnekamskoe Deposit is hydraulic backfilling of mine chambers, which protects the underground mines from flooding, minimizes ground subsidence, and reduces the area of potash waste. [ABSTRACT FROM AUTHOR]

Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed “chemical cocktails”, in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.

Study region: The Amu Darya River (ADR) basin in Central Asia. Study focus: To understand the spatiotemporal patterns and underlying driving mechanisms of river salinization in arid environments, this study gathered 50 years (1970–2019) of water chemistry data from 12 locations along the ADR. The variations in discharge and salinity were assessed by a linear regression model and violin plot. The salinity-discharge relationships were evaluated by a general hyperbolic model and Spearman’s rank correlation coefficient. Random forest models were also constructed to identify the predominant drivers of river water salinization. Finally, a conceptual model of river water salinization was constructed. New hydrological insights for the region: The water salinity (S) in the upper stream of the ADR was 541–635 mg/L. Salinity showed an increasing trend along the river course, reaching 751–1560 mg/L downstream. In the downstream, the river salinity before the 1990 s (751–1128 mg/L) was slightly lower than that after the 1990 s (983–1560 mg/L). Generally, water salinity was notably correlated with river discharge (Q) in upstream, exhibiting a relationship of S= 17,497Q−0.62, p

Frequent and severe occurrences of harmful algal blooms increasingly threaten human health by the release of microcystins (MCs). Urgent attention is directed toward managing MCs, as evidenced by rising HAB-related do not drink/do not boil advisories due to unsafe MC levels in drinking water. UV/chlorine treatment, in which UV light is applied simultaneously with chlorine, showed early promise for effectively degrading MC-LR to values below the World Health Organization's guideline limits. Still, much is unknown regarding potential disinfection byproduct formation and associated toxicity, which can occur from the reaction of chlorine and other reactive species with MCs and algal and natural organic matter. To ensure UV/chlorine guarding drinking water for human consumption, the degradation and detoxification of four of the most problematic MC variants, namely, MC-LR, -RR, -YR, and -LA, which differ in amino acid substituents, were evaluated using UV/chlorine and compared to results from chlorination. Overall, UV/chlorine effectively enhanced MC degradation kinetics and generated less halogenated disinfection byproducts in the target analysis of 11 types of DBPs&#95;C 1-3 from 7 classes, total organic chlorine, and nontarget analysis revealing 35 higher molecular weight DBPs&#95;C 46-52 , which maintained the MC structures. Reactivity and cytotoxicity changes varied based on the individual amino acid moieties within the cyclic heptapeptide structure common to all MCs. Analogous trends in MC reactivity were observed in degradation kinetics and mixed MC competition reactions, aligning with individual amino acid structure-reactivity. Cytotoxicity results indicated no significant unintended toxic consequences from MC&#95;DBPs. Our results suggest that UV/chlorine treatment offers an efficient strategy for treating MCs in drinking water.

Chloride concentrations in freshwater are rising, with toxic effects on aquatic life. In temperate regions with cold winters, road salt used for deicing paved surfaces is a primary cause. There is evidence that water hardness can modify salt toxicity, but data are insufficient to inform policy. Because calcium is a primary ion influencing water hardness and there is widespread calcium decline in lakes, we examined the effects of varying calcium concentrations on acute salt toxicity in three Daphnia species to gain a greater understanding of the water hardness-salt toxicity relationship. We conducted 48-hr acute sodium chloride (NaCl) toxicity tests, using chloride concentrations as our metric, on neonates less than 24 hrs old in six calcium treatments: 1.5 to 128 mg/L (hardness ∼7 to 323 mgCaCO3/L). We determined the effective concentration of chloride that was lethal to 10%, 25%, and 50% of the sample populations from each iso-female line in each calcium treatment. Acute NaCl toxicity decreased as calcium concentrations increased. The relationship between NaCl toxicity and calcium concentration differed among Daphnia, such that Daphnia catawba and Daphnia pulex were more sensitive to NaCl in lower calcium treatments and less sensitive in higher calcium treatments compared to Daphnia pulicaria. Our results provide evidence that water quality guidelines are not protective enough for aquatic life in very soft water (≤3 mg Ca2+/L, 11.3 mg CaCO3/L) because most ECxx values we found for Daphnia were significantly lower than Canada's national guidelines for short-term chloride exposure. There are already many lakes with calcium concentrations below 3 mg/L, and global widespread calcium decline may put more aquatic ecosystems at risk of experiencing NaCl toxicity., (© The Author(s) 2025. Published by Oxford University Press on behalf of the Society of Environmental Toxicology and Chemistry.)

The increasing salinization of freshwater streams from anthropogenic land uses and activities is a growing global environmental problem. Increases in individual ions (such as sodium or chloride) and combined measures such as total dissolved solids (TDS) threaten drinking water supplies, agricultural and economic interests, and the ecological health of freshwater streams. Because the toxicity of high ionic strength waters depends on the specific ion composition, few water quality standards exist to protect freshwater streams from salinization. In the present study, we used a novel approach to develop site‐specific and ecologically relevant TDS thresholds for the protection of aquatic life. The first step of the approach was to characterize the ion composition of the waterbody or region of interest and prepare artificial samples to match that composition. Using a combination of standardized toxicity test species and more ecologically relevant field‐collected species, toxicity tests were then conducted on these artificial samples prepared at a range of TDS concentrations. The advantage of this approach is that water quality criteria can be developed for easy‐to‐measure generalized parameters such as TDS while ensuring that the criteria are protective of instream aquatic life and account for the complex interactions of the various ions contributing to salinization. We tested this approach in Sand Branch, Loudoun County, Virginia, USA, where salinization from hard rock mining and urban runoff has impaired aquatic life. Acute and chronic TDS thresholds of 938 and 463 mg/L, respectively, were developed in this stream and used for total maximum daily load development in the watershed. The approach provides a potential model for establishing protective thresholds for other waterbodies impacted by salinization. Environ Toxicol Chem 2022;41:2782–2796. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

Freshwater salinization is a common yet underappreciated environmental problem in rivers, yet how this process impacts the transport of dissolved oxygen (DO) in the hyporheic zone remains unknown. Using flume experiments and numerical simulations, we have demonstrated that the morphological dynamics of the DO transition zone are controlled by the dimensionless number Da·Ra−2, a measure of the rate of aerobic respiration to free convection. At low Da·Ra−2, the oxic front breaks up into fingers which subsequently grow, and leads to accelerating of DO transport and shrinking of the anaerobic zone in the underlying sediment. As Da·Ra−2 increases, the growth rate of the instability decreases, and the DO plume fingers are suppressed and delayed compared with the saltwater‐freshwater interface, the mixing area between saltwater and freshwater. These results indicate that the freshwater salinization syndrome can have significant impacts on the functioning of aquifer ecosystems unless regulated and managed effectively. Plain Language Summary: Increases in salinity, alkalinity, and major ions occur in a wide range of freshwater ecosystems around the world, and the freshwater salinization syndrome is becoming a serious water quality problem and a major chemical signature of the Anthropocene. The goal of this study is to understand how dissolved oxygen (DO), which is beneficial to the survival of benthic invertebrates, is transported in the sediment in response to freshwater salinity fluctuation. Flume experiments and numerical simulations are performed to investigate the development of DO plume in the riverbed. The experimental outcomes, obtained through high‐resolution imagining techniques, show that the convection induced by saltwater density gradients in the hyporheic zone could influence DO plume development. Our numerical model is capable of describing the transport behaviors of DO before and after salinization in the surface water. A dimensionless number Da·Ra−2 is proposed to predict DO transport dynamics in the river with freshwater salinization syndrome. The implications and potential threats of freshwater salinization for the river ecosystem, including the biogeochemical processes and contaminant attenuation in the hyporheic zone, are also discussed. Key Points: Experiments and simulations are performed to investigate the morphology of dissolved oxygen (DO) transition zone in a salinity‐impacted bedformAsynchronous transport of salt and DO is controlled by a dimensionless number Da·Ra−2Whether density‐driven flow can be a DO pump depends on the intensity of aerobic respiration [ABSTRACT FROM AUTHOR]

Salinization poses an increasing problem worldwide, threatening freshwater organisms and raising questions about their ability to adapt. We explored the mechanisms enabling a planktonic crustacean to tolerate elevated salinity. By gradually raising water salinity in clonal cultures from 185 Daphnia magna populations, we showed that salt tolerance strongly correlates with native habitat salinity, indicating local adaptation. A genome-wide association study (GWAS) further revealed a major effect of the Alpha,alpha-trehalose-phosphate synthase (TPS) gene, suggesting that trehalose production facilitates salinity tolerance. Salinity-tolerant animals showed a positive correlation between water salinity and trehalose concentrations, while intolerant animals failed to produce trehalose. Animals with a non-functional TPS gene, generated through CRISPR-Cas9, supported the trehalose role in salinity stress. Our study highlights how a keystone freshwater animal adapts to salinity stress using an evolutionary mechanism known in bacteria, plants, and arthropods. [Display omitted] • Salinity tolerance is a locally adapted trait in Daphnia magna • The trehalose TPS gene shows a strong association with salinity tolerance • Trehalose content increases with salinity, allowing survival of tolerant genotypes • Disabling the TPS gene reduces salinity tolerance by preventing trehalose synthesis Santos et al. show that tolerance to elevated salinity in D. magna is mediated by trehalose. Genomic analysis and CRISPR-Cas9 technique provide evidence for the crucial role of the TPS gene in trehalose production and local adaptation. These findings reveal how this species uses a mechanism known in certain bacteria, plants, and other invertebrates. [ABSTRACT FROM AUTHOR]

Freshwater salinization is an escalating global environmental issue that threatens freshwater biodiversity, including fish populations. This study aims to uncover the molecular basis of salinity physiological responses in a non-native minnow species (Phoxinus septimaniae x P. dragarum) exposed to saline effluents from potash mines in the Llobregat River, Barcelona, Spain. Employing high-throughput mRNA sequencing and differential gene expression analyses, brain, gills, and liver tissues collected from fish at two stations (upstream and downstream of saline effluent discharge) were examined. Salinization markedly influenced global gene expression profiles, with the brain exhibiting the most differentially expressed genes, emphasizing its unique sensitivity to salinity fluctuations. Pathway analyses revealed the expected enrichment of ion transport and osmoregulation pathways across all tissues. Furthermore, tissue-specific pathways associated with stress, reproduction, growth, immune responses, methylation, and neurological development were identified in the context of salinization. Rigorous validation of RNA-seq data through quantitative PCR (qPCR) underscored the robustness and consistency of our findings across platforms. This investigation unveils intricate molecular mechanisms steering salinity physiological response in non-native minnows confronting diverse environmental stressors. This comprehensive analysis sheds light on the underlying genetic and physiological mechanisms governing fish physiological response in salinity-stressed environments, offering essential knowledge for the conservation and management of freshwater ecosystems facing salinization. [Display omitted] • Minnows adapt to salt stress, unveiling crucial molecular responses for survival. • Transcriptomics reveal salinity stress in gills, liver, and brain pathways. • The brain plays a pivotal role in stressor adaptation. • Transcriptomic analysis suggests potential strategies in conservation. [ABSTRACT FROM AUTHOR]

Anthropogenic sources of total phosphorus (TP) and chloride (Cl−) to lakes and rivers have been issues of concern for many decades in the Great Lakes Basin with northern Boreal Shield headwater tributaries less well studied. In the Sturgeon River – Lake Nipissing – French River basin, a headwater basin of Georgian Bay, Lake Huron, water quality monitoring of major inflows to Lake Nipissing, the third largest inland lake located entirely within Ontario, is only available from the mid-1960s to the 1990s. During the period of 2015–2018, we conducted monthly water quality surveys of major and minor inflows for two water years and have generated the first chloride (Cl−) and total phosphorus (TP) elemental budgets for the lake. Review of available long-term concentration data indicate decreasing TP concentrations by decade in major inflows, but select inflows continue to exhibit concentrations above provincial objectives, including inflows from agricultural areas that are no longer part of provincial monitoring programs. Some inflows also show high average Cl− concentrations with potential influences (e.g., road salt, agricultural activities) to stream water quality throughout the year. Water and elemental budgets indicate that while specific runoff (l/s/km2) is quite similar across contributing catchments, yields of Cl− and TP (kg/ha/yr) are disproportionately higher in catchments with urban and agricultural activities. While uncertainties in the water balance and elemental yields remain, this first effort to quantify annual elemental budgets of Lake Nipissing highlights the need to develop community-based, spatially distributed water quality surveying for long-term ecosystem monitoring and future planning. [ABSTRACT FROM AUTHOR]

Freshwater salinization is a global ecological concern because of the alarming biodiversity declines associated with increases in major ion concentrations. Loss of mayfly diversity appears to be a common ecological response to anthropogenic salinization worldwide. Remarkably few regulatory standards exist to protect aquatic life from major ions, and antiquated approaches for setting such standards rely on traditional laboratory toxicity tests, which do not address sensitivities of mayflies at different larval stages. The lab-reared mayfly Neocloeon triangulifer (McDunnough, 1931) has emerged as one of the very few useful aquatic insect models for studying the effects of environmental stressors, including salinity, in the laboratory. Here, we asked if different larval life stages are differentially sensitivity to ion concentrations by conducting traditional 96-h toxicity tests with NaCl, CaCl2, and Ca/MgSO4. We used a general linear model to determine if survivorship differed among larval stages as well as ion type and concentration. We also calculated median lethal concentrations (LC50) for each larval stage. Larval sensitivity to NaCl decreased slightly with age (2–6, 9–13, and 17–21 d, with LC50 values of 401, 441, and 570 mg/L, respectively, when expressed as Na concentrations). Similarly, larval sensitivity to Ca/MgSO4 differed slightly among age groups (LC50 = 748, 1503, and 1439 mg/L, respectively, when expressed as SO4 concentrations). Reliable confidence intervals on LC50 values for CaCl2 could not be calculated because of high survivorship. However, our general linear model revealed that age played a moderate role in survival (p = 0.0065) across all salts of interest. To assess the potential changes in ion flux between larval stages, we used radiotracers (22Na, 35SO4, or 45Ca) in 18- and 25-d-old larvae and found no strong differences in ion uptake rates. We also qualitatively examined morphological differences between larval life stages, including the appearance of gills and number of ionocytes. Our results indicate that younger N. triangulifer larvae may be more sensitive to major ions than mature larvae. These results should be considered when experimentally using larger, late-stage N. triangulifer larvae to study the physiological effects and acute toxicity of salinity. [ABSTRACT FROM AUTHOR]

Road salt runoff is a leading cause of secondary freshwater salinization in north temperate climates. Increased chloride concentrations in freshwater can be toxic and lead to changes in organismal behavior, lethality, biotic homogenization, and altered food webs. High chloride concentrations have been reported for winter months in urban centers, as road density is highest in cities. However, summer chloride conditions are not typically studied as road salt is not actively applied outside of winter months, yet summer is when many taxa reproduce and are most sensitive to chloride. In our study, we test the spatial variability of summer chloride conditions across four watersheds in Toronto, Canada. We find 89% of 214 sampled sites exceeded the federal chronic exposure guidelines for chloride, and 13% exceeded the federal acute guidelines. Through a model linking concentration to cumulative proportion of impacted species, we estimate 34% of sites show in excess of one-quarter of all species may be impacted by their site-specific chloride concentrations, with up to two-thirds of species impacted at some sites. Our results suggest that even presumed low seasons for chloride show concentrations sufficient to cause significant negative impacts to aquatic communities.

Many ecosystems are losing biodiversity, raising concern for the services they provide. However, the extent of loss is uncertain, especially for diverse insects, because of incomplete sampling. Modeling techniques for estimating upper bounds on diversity are needed to assess benthic freshwater insect abundances, taxa richness, and diversity because some species are declining at alarming rates while others are increasing. In central Appalachian streams of the U.S.A., aquatic insect communities have lost diversity following salinization caused by mining activities. However, the number of taxa observed are dependent upon sampling effort. Incomplete sampling could misrepresent biodiversity and functional losses. Our goal was to use taxon sampling curves to estimate sampling effort required to maximize the probability of accurate benthic macroinvertebrate characterization in dominant riffle habitats of headwater streams. We collected 5 to 10 quantitative benthic macroinvertebrate samples in each of six, first-order streams in the central Appalachian region. For our single-habitat, mesoscale approach, we predicted: (1) macroinvertebrate taxa richness would be the most robust indicator of salinization response compared to diversity, evenness, and density, (2) less sampling effort would be needed to capture taxonomic richness in salinized streams compared to reference streams, and (3) response diversity would also be lower in salinized streams because select trait states would be represented by fewer taxa. Results suggested nominally lower taxa richness, evenness, and diversity in some salinized streams but not in all despite greater or more variable within-stream densities. Nonetheless, sampling effort required to characterize macroinvertebrate communities did not differ between reference and salinized streams, though uneven within-stream taxa distributions corresponded with greater sampling effort requirements for complete characterization. Benthic macroinvertebrate community characteristics were different where higher densities and richness of small-bodied and fast-developing taxa were more common in salinized streams. Response diversity depended on trait states. For example, only about five shredder taxa were represented in salinized streams compared to eight taxa in reference streams. Despite some indication of lower response diversity across some functional feeding groups, more than five samples were needed for robust comparisons. Taxon and trait-based sampling curves suggest that greater overall sampling effort is needed and equal samples per stream was critical for complete diversity assessments regardless of the level of mining-induced salinization. Model-based sampling curves can serve as a tool to assess upper bounds on diversity metrics and sample-effort rigor.

Stream salinization is a global issue, yet few models can provide reliable salinity estimates for unmonitored locations at the time scales required for ecological exposure assessments. Machine learning approaches are presented that use spatially limited high-frequency monitoring and spatially distributed discrete samples to estimate the daily stream-specific conductance across a watershed. We compare the predictive performance of space- and time-unaware Random Forest models and space- and time-aware Recurrent Graph Convolution Neural Network models (KGE: 0.67 and 0.64, respectively) and use explainable artificial intelligence methods to interpret model predictions and understand salinization drivers. These models are applied to the Delaware River Basin, a developed watershed with diverse land uses that experiences anthropogenic salinization from winter deicer applications. These models capture seasonality for the winter first flush of deicers, and the streams with elevated predictions correspond well with indicators of deicer application. This result suggests that these models can be used to identify potential salinity-impaired streams for winter best management practices. Daily salinity predictions are driven primarily by land cover (urbanization) trends that may represent anthropogenic salinization processes and weather at time scales up to three months. Such modeling approaches are likely transferable to other watersheds and can be applied to further understand salinization risks and drivers.

Much scientific research dedicated to understanding the effects of freshwater salinization caused by road de-icing salts has utilized static exposures, with many tests conducted at winter or spring temperatures. While relevant for lentic ecosystems, pulsed patterns of salinity occur in lotic environments, particularly in summer months where precipitation can decrease elevated salinity levels caused by retention of residual salts. The current study aimed to evaluate the effects of pulsed patterns of salinity on the emergence, sex ratio, and fecundity of Chironomus dilutus over two generations of laboratory exposure. Three road de-icing salt treatments, including a control, modeled after environmental monitoring data of two local streams, were used to determine the ecological effects of periodic declines in salinity on C. dilutus at summer temperatures. No significant effects were observed on emergence success or sex ratios within or across generations, but fecundity of C. dilutus in the high salt treatment was reduced regardless of generation (P < 2e−16), possibly due to increased osmoregulatory stress caused by increased salinities. The intermediate and decreasing salinities may account for the lack of negative effects on emergence success and sex ratios by protecting sensitive life stages. More research is needed on long-term effects of reduced fecundity on population viability. The current study suggests more research using a similar experimental design is needed to fully evaluate the influence of road de-icing salts in lotic environments, as static laboratory exposures may not accurately reflect environmental changes in salinity. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are under increasing threat of salinization due to human activity. Given the contributions of microbial communities to stream ecosystems, it is critical to understand how these communities are affected by the increasing presence of salt in the environment. We used an artificial stream system to investigate how salt concentrations representing the 95th- and 99th-percentile of concentrations observed in NE Ohio streams affect bacterial community succession and what implications this has on community-level functional capabilities. We hypothesized that the successional trajectory of community functionality (in the form of extracellular enzyme activity) and structure (via denitrification gene abundances and community 16S rRNA gene profiles) would be altered in response to increasing salt concentrations. We observed considerable structural changes in bacterial composition among treatments that corresponded with niche expansion by more salt-tolerant taxa. Increases in denitrification gene abundances and modifications to extracellular enzyme activity were also observed. These data suggest that continued salt pollution can dramatically affect community structure and has the potential to modify the functional contributions of the bacterial community to the ecosystem. [ABSTRACT FROM AUTHOR]

Algal biofilms play an essential role in ecosystem functioning, but their physiological and biochemical changes during salt stress remain poorly understood. This study aimed at investigating how biofilm communities react to different salinity in a microcosm experiment and to offer insights into the potential effects of future freshwater salinization. Biofilm samples collected from Iriri River (Drâa basin, Southern Morocco) were exposed to water gradually enriched with sodium chloride to obtain three final concentrations (1, 10, and 30 g/L) over 14 days, and the changes were compared with control. Several biofilm responses were analyzed throughout the experiment, including biofilm biomass, changes in community structure and composition via eDNA metabarcoding, physiological feedback, biochemical parameters, antioxidant activities, non-enzymatic salt stress markers, osmoprotection indicators, and nitrogen assimilation. High salt concentrations significantly altered biofilm communities' composition, structure, and function, leading to an overall decrease in alpha diversity and a decrease in diatom and cyanobacterial biomass, while green algae remained unchanged. Chlorophyll a content increased in biofilm exposed to low salt concentration (1 g/L) and decreased at high salt concentrations while carotenoid significantly increased even at high salinity (10 g/L). Biofilm also showed an increase in carbohydrates and a decrease in protein content under high salinities (10 and 30 g/L). In addition, salt stress-induced ROS generation in the biofilm, with an increase in H 2 O 2 content, caused oxidative damage and lipid peroxidation with an increase in MDA levels. To prevent this damage, elevated proline content was recorded, and additionally, the biofilm exhibited antioxidant defense mechanisms by rising activity of ROS-scavenger enzymes such as SOD, CAT, and POD. Our experimental results highlighted the salinity effect on algal biofilm as well as physiological and biochemical responses. This study could help predict the consequences of freshwater salinization on river biofilms and its potential impact at the ecosystem level. • Salinity induced shifts in algal biofilm composition and reduced alpha diversity. • Benthic cyanobacteria and diatoms were more sensitive to salt stress. • Lower salinity (1 g/L) increase biofilm biomass and pigments contents. • Biofilm carbohydrates increase and proteins decrease under high salinities. • Salt-stress induced ROS generation in biofilm with increasing in H 2 O 2 content. [ABSTRACT FROM AUTHOR]

Premise: Outcomes of species interactions, especially mutualisms, are notoriously dependent on environmental context, and environments are changing rapidly. Studies have investigated how mutualisms respond to or ameliorate anthropogenic environmental changes, but most have focused on nutrient pollution or climate change and tested stressors one at a time. Relatively little is known about how mutualisms may be altered by or buffer the effects of multiple chemical contaminants, which differ fundamentally from nutrient or climate stressors and are especially widespread in aquatic habitats. Methods: We investigated the impacts of two contaminants on interactions between the duckweed Lemna minor and its microbiome. Sodium chloride (salt) and benzotriazole (a corrosion inhibitor) often co‐occur in runoff to water bodies where duckweeds reside. We tested three L. minor genotypes with and without the culturable portion of their microbiome across field‐realistic gradients of salt (3 levels) and benzotriazole (4 levels) in a fully factorial experiment (24 treatments, tested on each genotype) and measured plant and microbial growth. Results: Stressors had conditional effects. Salt decreased both plant and microbial growth and decreased plant survival more as benzotriazole concentrations increased. In contrast, benzotriazole did not affect microbial abundance and even benefited plants when salt and microbes were absent, perhaps due to biotransformation into growth‐promoting compounds. Microbes did not ameliorate duckweed stressors; microbial inoculation increased plant growth, but not at high salt concentrations. Conclusions: Our results suggest that multiple stressors matter when predicting responses of mutualisms to global change and that beneficial microbes may not always buffer hosts against stress. [ABSTRACT FROM AUTHOR]

A series of urban stream and snowmelt samples collected 2017-2018 provide a localized understanding of seasonal chloride concentrations in urban streams and establishes baseline knowledge of conditions in Cedar Rapids and Coralville, Iowa. Results indicate both variability in chloride concentrations based on location as well as conditions under which water was sampled. [ABSTRACT FROM AUTHOR]

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+, SO 4 2−, CO 3 2−, and HCO 3 −) 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 21st century owing to the potential salinization of freshwater resources in the context of a changing climate. Unlabelled Image • Deicers often drive freshwater salinization in northern regions of the world. • Less is known in areas where temperatures may be too cold for NaCl to be effective. • Cl is increasing downstream of North America's northernmost major city (Edmonton). • Deicers (54%) and agriculture (20%) are main sources of chloride. • Important to identify and manage all potential drivers of freshwater salinization [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract An influx of chloride ions from road de-icing solutions can result in toxicological effects to organisms in terrestrial and aquatic environments. As such, "eco-friendly" de-icing alternatives are sought to mitigate environmental impacts of de-icing impervious surfaces, while maintaining human safety. While many alternative de-icers are economically impractical for municipal use, the residential commercial market is flooded with de-icing formulations claiming to be "eco-friendly". Given the little regulation and guidance that surrounds eco-labeling, the meaning of "eco-friendly" remains unclear in the context of biological systems. The objective of the current study was to determine the toxicity of three "eco-friendly" de-icing formulations to Chironomus dilutus using 10 d toxicity tests. The toxicity of these three formulations was compared to a traditional formulation composed entirely of chloride salts. Two of the "eco-friendly" de-icers demonstrated LC 50 s of 6.61 and 6.32 g/L, which were similar in toxicity to the traditional sodium chloride formulation with a LC 50 6.29 g/L. The comparable toxicities of these formulations is likely due to the presence of chloride salts in each of the "eco-friendly" de-icers. The third "eco-friendly" formulation, a urea-based de-icer, demonstrated toxicity an order of magnitude higher than that of the traditional formulation with an LC 50 of 0.63 g/L. While C. dilutus may not have been the intended endpoint in consideration when marketing these products as "eco-friendly", consideration of how eco-labeling is utilized and the role of environmental scientists in determining the meaning of such claims must be considered to ensure continued and future protection of the environment. Graphical abstract Image 1 Highlights • Chloride salt de-icers pose risk to aquatic and terrestrial environments 12. • Eco-friendly de-icers reduce or replace chloride salts with alternative ingredients 13. • Eco-friendly de-icers were not protective of Chironomus dilutus 14. • Ambiguity in eco-friendly claims undermines such products in commercial markets 15. • A clearer definition of "eco-friendly" is required for green marketing to progress. Development of "eco-friendly" alternatives to chloride salt de-icers are purported to better protect aquatic and terrestrial environments while maintaining human safety, yet ambiguity in eco-friendly claims leads to difficulty in evaluating product safety for eco-conscious consumers. [ABSTRACT FROM AUTHOR]

Over the last century, anthropogenic activities have increased global carbon emissions and freshwater use, intensifying critical environmental problems such as global warming and water scarcity. This study unveils the metabolic and physiological adaptation of Chlorella vulgaris under extreme NaCl and CO 2 concentrations, suggesting a possible halotolerance mechanism for this freshwater species. Under hypertonic conditions (up to 3.5% NaCl - 600 mM), C. vulgaris controllably induces Na+ uptake and sharply increases intracellular carbohydrate content. This combination, along with an unexpected cell volume plasticity, effectively regulates osmotic equilibrium, ensuring cell viability and functionality. After the initial salinity shock, osmoadaptation mechanisms tend to actively eliminate Na+ influx and restore cell turgidity. Interestingly, photosynthetic activity and fluorescence induction measurements, revealed that exposure to a 30% CO 2 atmosphere could partially alleviate photosystem II excitation pressure and increase photosynthetic efficiency. Therefore, enrichment of readily available energy reserves significantly reduces salinity stress while simultaneously, changes in lipid composition result in high-added value biomass. This euryhaline bioenergetic strategy observed in C. vulgaris could pave the way for numerous sustainable biotechnological applications while dealing with major environmental issues. • Freshwater algae Chlorella vulgaris can tolerate salinities up to 3.5% NaCl. • Salinity shock induces Na+ uptake and carbohydrates increase, ensuring viability. • Osmotic equilibrium is reached by a controllable reduction of cell volume. • Osmoadaptation tends to restore cellular Na+ levels along with cell size. • Extreme CO 2 concentrations mitigate salt stress by relieving PSII excitation pressure. [ABSTRACT FROM AUTHOR]

Increasing chloride concentrations in freshwater streams throughout many areas of North America have raised concern over freshwater salinization. Road salt has been implicated; however, chloride source identification is lacking in the Toronto region. We assessed trends in chloride concentrations in streams and groundwater between 2000 and 2021 within the Toronto region and identified factors contributing to trends. Chloride concentrations increased in 36 of 47 streams and in 5 of 13 groundwater wells. There were no significant relationships between changes in stream chloride concentrations and changes in winter climatic conditions over the study period; however, changes in stream chloride concentrations were positively related to changes in road density. Chloride:bromide ratios indicated that road salt and/or septic effluent was the dominant source of chloride in streams. Inputs varied throughout the year with road salt and/or septic effluent having a higher proportion of inputs during the salting season compared to the non-salting season where a higher proportion of inputs were from basin brines and/or animal waste, landfill leachate, and pristine aquifer. Commercial, industrial, institutional, and medium density residential land uses were also positively correlated with stream chloride. Parking lot cover increased in several catchments suggesting that these too may contribute to trends. These results highlight the importance of continued and enhanced investment in long-term monitoring of freshwater ecosystems. It also highlights the need for urgent action to better control and monitor road salt usage by public and private applicators since urban growth continues to drive increasing chloride trends. [ABSTRACT FROM AUTHOR]

Excessive use of road salts to maintain safe winter travel conditions leads to increasing chloride (Cl) concentrations in streams, damaging the structure and function of freshwater ecosystems. Long-term increasing stream Cl trends are generally attributed to increases in urban land cover, however recent research shows that even relatively rural streams can retain Cl and exceed water quality guidelines in summer after road salting has stopped. Untangling the relative influences of long-term changes in streamflow and urban growth on Cl trends is critical for making informed decisions about road salt management. The portion of Cl trends not explained by changes in streamflow or urban growth could be due to changes in road salt application rates and/or legacy Cl in groundwater that is slowly making its way to streams. This study assessed seasonal, long-term stream Cl trends across the Province of Ontario, Canada, where urbanization accelerated and road salt management plans started to develop since early 2000s. We compared stream Cl trends over salting and non-salting seasons with urban growth estimates from two independent time periods, 1965–1995 and 2002–2018. For a subset of sites with sufficient flow data in the periods analyzed, we parsed the seasonal trends into flow and management trend components. We found that most of the variance in the management trend component in the winter salting season could be explained by urbanization, while about half of it could be explained in the summer non-salting season. We further analyzed Cl estimates in low-flow conditions to explore the extent of subsurface contributions to Cl trends, and concluded with a summary of challenges and recommendations for future studies on road salt legacy in streams.

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'. [ABSTRACT FROM AUTHOR]