Lakes at Risk of Chloride Contamination

Chloride accumulation in inland rivers of China and its toxic impact on cotton

The escalation of major ion concentrations in freshwater and soil poses diverse effects on ecosystems and the environment. Excessive ions can exhibit toxicity to aquatic organisms and terrestrial plants. Currently, research on ion toxicity primarily focuses on cation toxicity. Notably, there is a noticeable research gap in understanding the impact of chloride ion (Cl-) on plant growth and development, as well as on the defense mechanisms against Cl- toxicity. In the present study, sampling was conducted on major rivers in China to measure Cl- concentrations. The results revealed that certain rivers exhibited excessive levels of Cl-, emphasizing the critical need to address Cl- toxicity issues. Subsequently, when salt-tolerant cotton seedlings were subjected to various chloride treatments, it was observed that excessive Cl- severely hindered plant growth and development. A combined analysis of transcriptomic and metabolomic data shed light on significantly enriched pathways related to galactose metabolism, arginine and proline metabolism, carotenoid metabolism, and alpha-linolenic acid metabolism under chloride stress. In summary, this research provides a scientific foundation and references for environmental management and water resource protection and offers novel insights for mitigating the adverse impacts of Cl-, thereby contributing to the preservation of ecosystem health.

Impacts of sequential salinity and heat stress are recovery time-specific in freshwater crustacean, Daphnia pulicaria

Stressors can interact to affect animal fitness, but we have limited knowledge about how temporal variation in stressors may impact their combined effect. This limits our ability to predict the outcomes of pollutants and future dynamic environmental changes. Elevated salinity in freshwater ecosystems has been observed worldwide. Meanwhile, heatwaves have become more frequent and intensified as an outcome of climate change. These two stressors can jointly affect organisms; however, their interaction has rarely been explored in the context of freshwater ecosystems. We conducted lab experiments using Daphnia pulicaria, a key species in lakes, to investigate how elevated salinity and heatwave conditions collectively affect freshwater organisms. We also monitored the impacts of various recovery times between the two stressors. Daphnia physiological conditions (metabolic rate, Na+-K+-ATPase (NKA) activity, and lipid peroxidation level) and life history traits (survival, fecundity, and growth) in response to salt stress as well as mortality in heat treatment were examined. We found that Daphnia responded to elevated salinity by upregulating NKA activity and increasing metabolic rate, causing a high lipid peroxidation level. Survival, fecundity, and growth were all negatively affected by this stressor. These impacts on physiological conditions and life history traits persisted for a few days after the end of the exposure. Heat treatments caused mortality in Daphnia, which increased with rising temperature. Results also showed that individuals that experienced salt exposure were more susceptible to subsequent heat stress, but this effect decreased with increasing recovery time between stressors. Findings from this work suggest that the legacy effects from a previous stressor can reduce individual resistance to a subsequent stressor, adding great difficulties to the prediction of outcomes of multiple stressors. Our work also demonstrates that cross-tolerance/susceptibility and the associated mechanisms remain unclear, necessitating further investigation.

Urban lake water quality responses to elevated road salt concentrations

Road salt runoff from de-icing applications is increasingly impacting water quality around the globe. Excess salt (especially chloride) concentrations can negatively impact the biological, chemical, and physical properties of freshwater ecosystems. Though road salt pollution is a prevalent issue affecting many northern temperate lakes, there are few studies on how freshwater salinization interacts with other ecological stressors such as eutrophication. We investigated how chloride from road deicers influences water quality in an urban lake. We sampled a tributary and lake receiving large amounts of road salt runoff from a nearby highway in Grand Rapids, Michigan over a 20-month period. Chloride concentrations in the deepest part of the lake consistently exceeded the US EPA chloride chronic toxicity threshold of 230 mg/L, at times reaching up to 331 mg/L. These high chloride concentrations appear to be responsible for preventing part of the lake from complete mixing, and causing hypoxia in the deepest regions of the lake. Total phosphorus concentrations near the surface averaged 35 μg/L but exceeded 7500 μg/L in the deepest part of the lake, which occupies 3-5 % of total lake volume. Phosphorus release rates from the sediments were low and unlikely to be a current source of the high phosphorus concentrations. Rather, both phosphorus and chloride likely have been accumulating in the hypolimnion over a relatively long period of time. Lake management actions will require control of both internal and external phosphorus and chloride sources in the future. We recommend that phosphorus be addressed first to avoid the extremely high phosphorus concentrations from reaching the photic zone and stimulating algal blooms, which would occur if salt was removed first and the halocline broke down. Our findings and recommendations are applicable to other lakes facing similar issues.

A genome catalogue of lake bacterial diversity and its drivers at continental scale

Lakes are heterogeneous ecosystems inhabited by a rich microbiome whose genomic diversity is poorly defined. We present a continental-scale study of metagenomes representing 6.5 million km2 of the most lake-rich landscape on Earth. Analysis of 308 Canadian lakes resulted in a metagenome-assembled genome (MAG) catalogue of 1,008 mostly novel bacterial genomospecies. Lake trophic state was a leading driver of taxonomic and functional diversity among MAG assemblages, reflecting the responses of communities profiled by 16S rRNA amplicons and gene-centric metagenomics. Coupling the MAG catalogue with watershed geomatics revealed terrestrial influences of soils and land use on assemblages. Agriculture and human population density were drivers of turnover, indicating detectable anthropogenic imprints on lake bacteria at the continental scale. The sensitivity of bacterial assemblages to human impact reinforces lakes as sentinels of environmental change. Overall, the LakePulse MAG catalogue greatly expands the freshwater genomic landscape, advancing an integrative view of diversity across Earth's microbiomes.

Toxicity and chemical composition of commercial road palliatives versus oil and gas produced waters

Across the United States, road palliatives are applied to roads for maintenance operations that improve road safety. In the winter, solid rock salts and brine solutions are used to reduce the accumulation of snow and ice, while in the summer, dust suppressants are used to minimize fugitive dust emissions. Many of these products are chloride-based salts that have been linked to freshwater salinization, toxicity to aquatic organisms, and damage to infrastructure. To minimize these impacts, organic products have been gaining attention, though their widespread adoption has been limited due to their higher cost. In some states, using produced water from conventionally drilled oil and gas wells (OGPWs) on roads is permitted as a cost-effective alternative to commercial products, despite its typically elevated concentrations of heavy metals, radioactivity, and organic micropollutants. In this study, 17 road palliatives used for winter and summer road maintenance were collected and their chemical composition and potential human toxicity were characterized. Results from this study demonstrated that liquid brine solutions had elevated levels of trace metals (Zn, Cu, Sr, Li) that could pose risks to human and environmental health. The radium activity of liquid calcium chloride products was comparable to the activity of OGPWs and could be a significant source of radium to the environment. The organic fractions of evaluated OGPWs and chloride-based products posed little risk to human health. However, organic-based dust suppressants regulated toxicity pathways related to xenobiotic metabolism, lipid metabolism, endocrine disruption, and oxidative stress, indicating their use could lead to environmental harm and health risks to operators handing these products and residents living near treated roads.

Novel insights into the multistep chlorination of silver nanoparticles in aquatic environments

Due to the increasing applications, silver nanoparticles (AgNPs) are inevitably released into the environments and are subjected to various transformations. Chloride ion (Cl^-) is a common and abundant anion with a wide range of concentration in aquatic environments and exhibits a strong affinity for silver. The results indicate that AgNPs experienced multistep chlorination, which was dependent on the concentration of Cl^- in a non-linear manner. The dissolution of AgNPs was accelerated at Cl/Ag ratio of 1 and the intensive etching effect of Cl^- contributed to the significant morphology changes of AgNPs. The dissolved Ag⁺ quickly precipitated with Cl^- to form an amorphous and passivating AgCl(s) layer on the surface of AgNPs, thus the dissolution rate of AgNPs decreased at higher Cl/Ag ratios (100 and 1000). As the Cl/Ag ratio further increased to 10,000, the overall transformation rate increased remarkably due to the complexation of Cl^- with AgCl(s) to form soluble AgCl_x^(x-1)- species, which was verified by the reaction of AgCl nanoparticles with Cl^-. Besides, several environmental factors (electrolytes, surfactants and natural organic matter) affected AgNPs dissolution and the following chlorination. These results will expand the understanding of the environmental fate and potential risks of AgNPs in natural chloride-rich waters.

Five state factors control progressive stages of freshwater salinization syndrome

Factors driving freshwater salinization syndrome (FSS) influence the severity of impacts and chances for recovery. We hypothesize that spread of FSS across ecosystems is a function of interactions among five state factors: human activities, geology, flowpaths, climate, and time. (1) Human activities drive pulsed or chronic inputs of salt ions and mobilization of chemical contaminants. (2) Geology drives rates of erosion, weathering, ion exchange, and acidification-alkalinization. (3) Flowpaths drive salinization and contaminant mobilization along hydrologic cycles. (4) Climate drives rising water temperatures, salt stress, and evaporative concentration of ions and saltwater intrusion. (5) Time influences consequences, thresholds, and potentials for ecosystem recovery. We hypothesize that state factors advance FSS in distinct stages, which eventually contribute to failures in systems-level functions (supporting drinking water, crops, biodiversity, infrastructure, etc.). We present future research directions for protecting freshwaters at risk based on five state factors and stages from diagnosis to prognosis to cure.

The effects of salinity and N:P on N-rich toxins by both an N-fixing and non-N-fixing cyanobacteria

Freshwater ecosystems are experiencing increased salinization. Adaptive management of harmful algal blooms (HABs) contribute to eutrophication/salinization interactions through the hydrologic transport of blooms to coastal environments. We examined how nutrients and salinity interact to affect growth, elemental composition, and cyanotoxin production/release in two common HAB genera. Microcystis aeruginosa (non-nitrogen (N)-fixer and microcystin-LR producer; MC-LR) and Aphanizomenon flos-aquae (N-fixer and cylindrospermopsin producer; CYN) were grown in N:phosphorus (N:P) 4 and 50 (by atom) for 21 and 33 days, respectively, then dosed with a salinity gradient (0 - 10.5 g L-1). Both total MC-LR and CYN were correlated with particulate N. We found Microcystis MC-LR production and release was affected by salinity only in the N:P 50 treatment. However, Aphanizomenon CYN production and release was affected by salinity regardless of N availability. Our results highlight how cyanotoxin production and release across the freshwater - marine continuum are controlled by eco-physiological differences between N-acquisition traits.

Long-term impacts of impervious surface cover change and roadway deicing agent application on chloride concentrations in exurban and suburban watersheds

Roadway deicing agents, including rock salt and brine containing NaCl, have had a profound impact on the water quality and aquatic health of rivers and streams in urbanized areas with temperate climates. Yet, few studies evaluate impacts to watersheds characterized by relatively low impervious surface cover (ISC; < 15 %). Here, we use long-term (1997-2019), monthly streamwater quality data combined with daily streamflow for six exurban and suburban watersheds in southeastern Pennsylvania to examine the relations among chloride (Cl^-) concentrations and ISC. Both flow-normalized Cl^- concentrations and ISC increased over time in each of the six watersheds, consistent with changes in watershed management (e.g., ISC, road salt application, etc.). The watersheds that experienced the greatest changes in percent ISC (e.g., agriculture replaced by residential and commercial development) experienced the greatest changes in flow-normalized Cl^- concentrations. We also utilized a comprehensive mass-balance model (2011-2018) that indicated Cl^- inputs exceeded the outputs for the study watersheds. Road salt applied to state roads, non-state roads, and other impervious surfaces accounted for the majority of Cl^- inputs to the six watersheds. Furthermore, increasing Cl^- concentrations during baseflow conditions confirm impacts to shallow groundwater. Although flow-normalized Cl^- concentrations are below the U.S. Environmental Protection Agency's chronic threshold value for impacts to aquatic organisms, year-round exceedances may result before the end of this century based on current trends. Though reduced Cl^- loading to streams may be achieved by limiting the expansion of impervious surfaces in exurban and suburban watersheds, changes in baseflow concentrations are likely to be gradual because of the accumulated Cl^- in groundwater.

Protist Diversity and Metabolic Strategy in Freshwater Lakes Are Shaped by Trophic State and Watershed Land Use on a Continental Scale

Protists play key roles in aquatic food webs as primary producers, predators, nutrient recyclers, and symbionts. However, a comprehensive view of protist diversity in freshwaters has been challenged by the immense environmental heterogeneity among lakes worldwide. We assessed protist diversity in the surface waters of 366 freshwater lakes across a north temperate to subarctic range covering nearly 8.4 million km² of Canada. Sampled lakes represented broad gradients in size, trophic state, and watershed land use. Hypereutrophic lakes contained the least diverse and most distinct protist communities relative to nutrient-poor lakes. Greater taxonomic variation among eutrophic lakes was mainly a product of heterotroph and mixotroph diversity, whereas phototroph assemblages were more similar under high-nutrient conditions. Overall, local physicochemical factors, particularly ion and nutrient concentrations, elicited the strongest responses in community structure, far outweighing the effects of geographic gradients. Despite their contrasting distribution patterns, obligate phototroph and heterotroph turnover was predicted by an overlapping set of environmental factors, while the metabolic plasticity of mixotrophs may have made them less predictable. Notably, protist diversity was associated with variation in watershed soil pH and agricultural crop coverage, pointing to human impact on the land-water interface that has not been previously identified in studies on smaller scales. Our study exposes the importance of both within-lake and external watershed characteristics in explaining protist diversity and biogeography, critical information for further developing an understanding of how freshwater lakes and their watersheds are impacted by anthropogenic stressors.

IMPORTANCE Freshwater lakes are experiencing rapid changes under accelerated anthropogenic stress and a warming climate. Microorganisms underpin aquatic food webs, yet little is known about how freshwater microbial communities are responding to human impact. Here, we assessed the diversity of protists and their myriad ecological roles in lakes varying in size across watersheds experiencing a range of land use pressures by leveraging data from a continental-scale survey of Canadian lakes. We found evidence of human impact on protist assemblages through an association with lake trophic state and extending to agricultural activity and soil characteristics in the surrounding watershed. Furthermore, trophic state appeared to explain the distributions of phototrophic and heterotrophic protists in contrasting ways. Our findings highlight the vulnerability of lake ecosystems to increased land use and the importance of assessing terrestrial interfaces to elucidate freshwater ecosystem dynamics.

Road Salt versus Urban Snow Effects on Lake Microbial Communities

Freshwater salinization is an ongoing concern for north temperate lakes; however, little is known about its impacts on microbial communities, particularly for bacteria. We tested the hypotheses that road de-icing salt induces changes in the microbial community structure of lake plankton, and that changes due to chloride would differ from those due to urban snowmelt because of additional chemicals in the snowmelt. In a laboratory incubator experiment, an overwintering plankton community in lake water was exposed for two weeks to either NaCl or municipal road snow with the same level of chloride. Microbial community structure as determined by 16S (prokaryotes) and 18S (eukaryotes) rRNA transcript analysis showed changes in response to the chloride-only enrichment, with some rare taxa becoming more prominent. Consistent with our hypothesis, the salt and the snow treatments induced different community changes. These results indicate that ecotoxicology assays based on a single salt addition may not reflect the in situ effects of salt-contaminated urban snow, and that the combined chemical effects of urban snowmelt require direct testing.

Current water quality guidelines across North America and Europe do not protect lakes from salinization

The salinity of freshwater ecosystems is increasing worldwide. Given that most freshwater organisms have no recent evolutionary history with high salinity, we expect them to have a low tolerance to elevated salinity caused by road deicing salts, agricultural practices, mining operations, and climate change. Leveraging the results from a network of experiments conducted across North America and Europe, we showed that salt pollution triggers a massive loss of important zooplankton taxa, which led to increased phytoplankton biomass at many study sites. We conclude that current water quality guidelines established by governments in North America and Europe do not adequately protect lake food webs, indicating an immediate need to establish guidelines where they do not exist and to reassess existing guidelines.

Human-induced salinization caused by the use of road deicing salts, agricultural practices, mining operations, and climate change is a major threat to the biodiversity and functioning of freshwater ecosystems. Yet, it is unclear if freshwater ecosystems are protected from salinization by current water quality guidelines. Leveraging an experimental network of land-based and in-lake mesocosms across North America and Europe, we tested how salinization—indicated as elevated chloride (Cl⁻) concentration—will affect lake food webs and if two of the lowest Cl⁻ thresholds found globally are sufficient to protect these food webs. Our results indicated that salinization will cause substantial zooplankton mortality at the lowest Cl⁻ thresholds established in Canada (120 mg Cl⁻/L) and the United States (230 mg Cl⁻/L) and throughout Europe where Cl⁻ thresholds are generally higher. For instance, at 73% of our study sites, Cl⁻ concentrations that caused a ≥50% reduction in cladoceran abundance were at or below Cl⁻ thresholds in Canada, in the United States, and throughout Europe. Similar trends occurred for copepod and rotifer zooplankton. The loss of zooplankton triggered a cascading effect causing an increase in phytoplankton biomass at 47% of study sites. Such changes in lake food webs could alter nutrient cycling and water clarity and trigger declines in fish production. Current Cl⁻ thresholds across North America and Europe clearly do not adequately protect lake food webs. Water quality guidelines should be developed where they do not exist, and there is an urgent need to reassess existing guidelines to protect lake ecosystems from human-induced salinization.

Impacts of salinization on aquatic communities: Abrupt vs. gradual exposures

Increasing chloride concentrations from road salt applications are an emerging threat to freshwater diversity in cold weather regions. Few studies have focused on how road salt affects freshwater biota and even fewer have focused on how the rate of exposure alters organism responses. We hypothesized that road salt concentrations delivered gradually would result in slower population declines and more rapid rebounds due to evolved tolerance. To test this hypothesis, we examined the responses of freshwater lake organisms to four environmentally relevant salt concentrations (100, 230, 860, and 1600 mg Cl^-/L) that differed in application rate (abrupt vs. gradual). We used outdoor aquatic mesocosms containing zooplankton, filamentous algae, phytoplankton, periphyton, and macroinvertebrates. We found negative effects of road salt on zooplankton and macroinvertebrate abundance, but positive effects on phytoplankton and periphyton, likely resulting from reduced grazing. Only rarely did we detect a difference between abrupt vs gradual salt applications and the directions of those differences were not consistent. This affirms the need for additional research on how road salt pollution entering ecosystems at different frequencies and magnitudes will alter freshwater communities.

Salinization of Alpine rivers during winter months

Human-induced (i.e., secondary) salinization affects aquatic biodiversity and ecosystem functioning worldwide. While agriculture or resource extraction are the main drivers of secondary salinization in arid and semi-arid regions of the world, the application of deicing road salt in winter can be an important source of salts entering freshwaters in cold regions. Alpine rivers are probably affected by salinization, especially in highly populated mountain regions, although this remains to be explored. In this study, we analyzed multi-year conductance time series from four rivers in the European Alps and demonstrated that the application of deicing road salt is linked to peaking rivers' salinity levels during late winter/early spring. Especially in small catchments with more urban surfaces close to the rivers, conductance increased during constant low-flow periods in late winter and was less correlated with discharge than in summer. Thus, our results suggest that small rivers highly connected to urban infrastructures are prone to considerable salinity peaks during late winter/early spring. Given the low natural level of salinities in Alpine rivers, the aquatic biodiversity might be significantly affected by the recorded changes in conductance, with potential consequences on ecosystem functioning. Thereby, we urge the research community to assess the impact of secondary salinization in Alpine rivers and call for an implementation of management practices to prevent the degradation of these pristine and valuable ecosystems.