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

Coping with stress: Salt type, concentration, and exposure history limit life history tradeoffs in response to road salt salinization

Substantial increases in the salinity of freshwater ecosystems has occurred around the globe from causes such as climate change, industrial operations, and the application of road deicing salts. We know very little about how plastic responses in life history traits or rapid evolution of new traits among freshwater organisms could promote stability in ecological communities affected by salinization. We performed a cohort life history analysis from birth to death with 180 individuals of a ubiquitous freshwater zooplankter to understand how life history traits are affected by exposure to two common salt types causing salinization-sodium chloride (NaCl) and calcium chloride (CaCl2)-across two environmentally relevant concentrations. We also tested if a multi-generational exposure history to high salinity altered life-history responses. We tracked and measured lifespan, time to maturation, brood size, brood interval, and body size. We found smaller brood sizes but slightly longer lifespans occurred at a low concentration of NaCl (230 mg Cl-/L). The longer lifespans led to more, albeit smaller broods, which generated a similar lifetime reproductive output compared to the no-salt control populations. At higher concentrations of NaCl and CaCl2, we found lifetime reproductive output was reduced by 23 % to 83 % relative to control populations because no tradeoff among life history traits occurred. In CaCl2, we observed shorter life spans, longer time intervals between smaller broods, and smaller body sizes leading to reduced lifetime reproductive output. We also found that a multi-generational exposure to the salt types did not convey any advantages for lifetime reproductive output. In some cases, the exposure history worsened the life history trait responses suggesting maladaptation. Our findings suggest that life history tradeoffs for freshwater species can occur in response to salinization, but these tradeoffs will largely depend on salt type and concentration, which will have implications for biodiversity and ecological stability.

Salt Belt Index (SBI): A biotic index for streams within the North American "salt belt," with proposed baseline chloride thresholds

Road salt (commonly NaCl, CaCl2, and MgCl2) is widely used in the northern United States as a deicing agent for roadways and other byways. Millions of tons of road salt are used annually in the United States, resulting in drastic increases in freshwater salinity. This study aims to determine the chloride optima and tolerance ranges of macroinvertebrates using publicly accessible stream monitoring data from the US EPA. We assigned taxa region-specific tolerance values, which we then used to calculate the Salt Belt Index (SBI). In addition to the SBI, we determined new, region-specific, chronic Cl- thresholds, determined using threshold indicator taxa analysis (TITAN). Using generalized linear models, we found the SBI was highly accurate at estimating chloride concentration (mg/L Cl-) across the salt belt states. Macroinvertebrate community richness exhibited a significant negative relationship with increasing chloride concentrations. Newly proposed chloride thresholds, based on the richness-chloride relationship, were far lower than current thresholds. The SBI was able to differentiate between Low-, Medium-, and High-Impact sites, grouped based on proposed chloride thresholds. Based on our findings, it is clear current salinity thresholds are too high, and management practices should factor in regional variability, taxon-specific physiology, and historical instream chemistry when implementing salinity thresholds.

Salt pollution reduces turbidity, dissolved organic matter, and cyanobacteria in experimental vernal pool communities

Anthropogenic activities such as the over-application of road deicers are causing an increase in the concentration of salts in historically fresh waters. Experimental and field investigations demonstrate that freshwater salinization disrupts ecosystem functions and services, causing the death of freshwater organisms and changes to nutrient conditions. Wetland habitats are one system negatively affected by salt pollution, including ephemeral wetlands (vernal pools) that fill with salt-polluted water after snowmelt. In urbanized areas, the degradation of these ecosystems could result in irreversible ecological damage including reduced water quality and a reduction in biodiversity. To investigate the effects of freshwater salinization on vernal pool communities, we exposed soils from vernal pools to water containing no salt (control), or four concentrations of three salts standardized by chloride concentration (50 mg Cl- L-1, 100 mg Cl- L-1, 200 mg Cl- L-1, and 400 mg Cl- L-1; magnesium chloride, calcium chloride, and sodium chloride). The results of this experiment suggest that emerging zooplankton communities in vernal pools are sensitive to low concentrations of salt pollution, and that alternative salts such as magnesium chloride and calcium chloride are more toxic than sodium chloride. We did not find positive or negative changes in the abundance of eukaryotic phytoplankton but did find negative effects of salt on cyanobacteria abundance, possibly due to corresponding reductions in turbidity which might be needed as a fixation site for cyanobacteria to form heterocysts. Finally, we found that salt pollution likely caused flocculation of Dissolved Organic Matter (DOM), resulting in reduced concentrations of DOM which could alter the buffering capacity of freshwater systems, light attenuation, and the populations of planktonic heterotrophs.

Salty chemical cocktails as water quality signatures: Longitudinal trends and breakpoints along different U.S. streams

Along urban streams and rivers, various processes, including road salt application, sewage leaks, and weathering of the built environment, contribute to novel chemical cocktails made up of metals, salts, nutrients, and organic matter. In order to track the impacts of urbanization and management strategies on water quality, we conducted longitudinal stream synoptic (LSS) monitoring in nine watersheds in five major metropolitan areas of the U.S. During each LSS monitoring survey, 10-53 sites were sampled along the flowpath of streams as they flowed along rural to urban gradients. Results demonstrated that major ions derived from salts (Ca2+, Mg2+, Na+, and K+) and correlated elements (e.g. Sr2+, N, Cu) formed 'salty chemical cocktails' that increased along rural to urban flowpaths. Salty chemical cocktails explained 46.1% of the overall variability in geochemistry among streams and showed distinct typologies, trends, and transitions along flowpaths through metropolitan regions. Multiple linear regression predicted 62.9% of the variance in the salty chemical cocktails using the six following significant drivers (p < 0.05): percent urban land, wastewater treatment plant discharge, mean annual precipitation, percent silicic residual material, percent volcanic material, and percent carbonate residual material. Mean annual precipitation and percent urban area were the most important in the regression, explaining 29.6% and 13.0% of the variance. Different pollution sources (wastewater, road salt, urban runoff) in streams were tracked downstream based on salty chemical cocktails. Streams flowing through stream-floodplain restoration projects and conservation areas with extensive riparian forest buffers did not show longitudinal increases in salty chemical cocktails, suggesting that there could be attenuation via conservation and restoration. Salinization represents a common urban water quality signature and longitudinal patterns of distinct chemical cocktails and ionic mixtures have the potential to track the sources, fate, and transport of different point and nonpoint pollution sources along streams across different regions.

Salinity is an important factor in carbon emissions from an inland lake in arid region

Saline lakes, serving as the ultimate destination for most hydrological systems, accumulate substantial amounts of nutrients and organic matter from basins, and act as vast carbon reservoirs. These lakes exhibit exceptionally active biogeochemical cycling processes of carbon dioxide (CO2) and methane (CH4), and constitute integral components of the global carbon cycle. However, understanding of greenhouse gas emissions from saline lakes remains unclear mostly due to scarce data. In this study, we obtained CO2 and CH4 diffusive fluxes and biogeochemical parameters during ice-free period of 2021 at Bosten Lake, which is a representative inland saline lake located in China's arid region. Results revealed that Bosten Lake was a significant source of atmospheric gas carbon emissions, with average diffusion emissions of 12.645 ± 3.475 mmol m-2 d-1 for CO2 and 0.279 ± 0.069 mmol m-2 d-1 for CH4. Temporally, field measurements found a positive correlation between conductivity (Spc, a proxy of salinity) and CO2 emissions (R2 = 0.50, p < 0.01). Furthermore, the CH4 diffusive fluxes increased with the trophic state index (TSI, R2 = 0.31, p < 0.01). Spatially, exogenous inputs led to the spatial heterogeneity of carbon emissions. Our results highlighted that temporal variations in salinity constitute a crucial factor influencing CO2 emissions, and the saline lake has greater global warming potential compared to freshwater. The study provides an in-depth analysis of greenhouse gas emissions and driving factors in saline lakes of arid regions, and supports a further understanding of the carbon cycle in different types of lakes.

Toxicant Responses and Culturing Characteristics of Long-Term Laboratory-Reared and Field Populations of Ceriodaphnia dubia

Ceriodaphnia dubia is a standardized test organism for regulatory toxicity testing of surface waters and commercial chemicals because of its simplicity to culture and responsiveness to toxicants. For testing convenience, C. dubia is often cultured for extended periods in the laboratory with little knowledge of the impact on subsequent generations. Extended laboratory rearing could impact how they respond to stressors and decrease the accuracy of test results. The present study investigated if C. dubia cultured for an extended period were representative of three recently collected field populations by comparing their culturing characteristics and sensitivities to toxicants. For culturing characteristics, the field cultures were more challenging because they had shorter body lengths, fewer neonates, and higher mortality rates than the laboratory culture. Comparative chronic toxicity tests with sodium chloride and the neonicotinoid insecticide thiamethoxam indicated that the laboratory and field organisms did not differ much in their toxicological responses but did differ in the variability of responses (percentage of coefficient of variation). The differences between the laboratory and field cultures found in the present study highlight the challenges of addressing discrepancies between laboratory and field applications in existing standardized methodologies. Environ Toxicol Chem 2024;43:159-169. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Environment and Host Genetics Influence the Biogeography of Plant Microbiome Structure

To understand how microbiota influence plant populations in nature, it is important to examine the biogeographic distribution of plant-associated microbiomes and the underlying mechanisms. However, we currently lack a fundamental understanding of the biogeography of plant microbiomes across populations and the environmental and host genetic factors that shape their distribution. Leveraging the broad distribution and extensive genetic variation in duckweeds (the Lemna species complex), we identified key factors that governed plant microbiome diversity and compositional variation geographically. In line with the microbial biogeography of free-living microbiomes, we observed higher bacterial richness in temperate regions relative to lower latitudes in duckweed microbiomes (with 10% higher in temperate populations). Our analyses revealed that higher temperature and sodium concentration in aquatic environments showed a negative impact on duckweed bacterial richness, whereas temperature, precipitation, pH, and concentrations of phosphorus and calcium, along with duckweed genetic variation, influenced the biogeographic variation of duckweed bacterial community composition. Analyses of plant microbiome assembly processes further revealed that niche-based selection played an important role (26%) in driving the biogeographic variation of duckweed bacterial communities, alongside the contributions of dispersal limitation (33%) and drift (39%). These findings add significantly to our understanding of host-associated microbial biogeography and provide important insights for predicting plant microbiome vulnerability and resilience under changing climates and intensifying anthropogenic activities.

Can long-term salinity acclimation eliminate the inhibitory effect of salinization on anti-predation defense of Daphnia?

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.

The Anthropogenic Salt Cycle

Increasing salt production and use is shifting the natural balances of salt ions across Earth systems, causing interrelated effects across biophysical systems collectively known as freshwater salinization syndrome. In this Review, we conceptualize the natural salt cycle and synthesize increasing global trends of salt production and riverine salt concentrations and fluxes. The natural salt cycle is primarily driven by relatively slow geologic and hydrologic processes that bring different salts to the surface of the Earth. Anthropogenic activities have accelerated the processes, timescales and magnitudes of salt fluxes and altered their directionality, creating an anthropogenic salt cycle. Global salt production has increased rapidly over the past century for different salts, with approximately 300 Mt of NaCl produced per year. A salt budget for the USA suggests that salt fluxes in rivers can be within similar orders of magnitude as anthropogenic salt fluxes, and there can be substantial accumulation of salt in watersheds. Excess salt propagates along the anthropogenic salt cycle, causing freshwater salinization syndrome to extend beyond freshwater supplies and affect food and energy production, air quality, human health and infrastructure. There is a need to identify environmental limits and thresholds for salt ions and reduce salinization before planetary boundaries are exceeded, causing serious or irreversible damage across Earth systems.

Longitudinal stream synoptic monitoring tracks chemicals along watershed continuums: a typology of trends

There are challenges in monitoring and managing water quality due to spatial and temporal heterogeneity in contaminant sources, transport, and transformations. We demonstrate the importance of longitudinal stream synoptic (LSS) monitoring, which can track combinations of water quality parameters along flowpaths across space and time. Specifically, we analyze longitudinal patterns of chemical mixtures of carbon, nutrients, greenhouse gasses, salts, and metals concentrations along 10 flowpaths draining 1,765 km2 of the Chesapeake Bay region. These 10 longitudinal stream flowpaths are drained by watersheds experiencing either urban degradation, forest and wetland conservation, or stream and floodplain restoration. Along the 10 longitudinal stream flowpaths, we monitored over 300 total sampling sites along a combined stream length of 337 km. Synoptic monitoring along longitudinal flowpaths revealed: (1) increasing, decreasing, piecewise, or no trends and transitions in water quality with increasing distance downstream, which provide insights into water quality processes along flowpaths; (2) longitudinal trends and transitions in water quality along flowpaths can be quantified and compared using simple linear and non-linear statistical relationships with distance downstream and/or land use/land cover attributes, (3) attenuation and transformation of chemical cocktails along flowpaths depend on: spatial scales, pollution sources, and transitions in land use and management, hydrology, and restoration. We compared our LSS patterns with others from the global literature to synthesize a typology of longitudinal water quality trends and transitions in streams and rivers based on hydrological, biological, and geochemical processes. Applications of LSS monitoring along flowpaths from our results and the literature reveal: (1) if there are shifts in pollution sources, trends, and transitions along flowpaths, (2) which pollution sources can spread further downstream to sensitive receiving waters such as drinking water supplies and coastal zones, and (3) if transitions in land use, conservation, management, or restoration can attenuate downstream transport of pollution sources. Our typology of longitudinal water quality responses along flowpaths combines many observations across suites of chemicals that can follow predictable patterns based on watershed characteristics. Our typology of longitudinal water quality responses also provides a foundation for future studies, watershed assessments, evaluating watershed management and stream restoration, and comparing watershed responses to non-point and point pollution sources along streams and rivers. LSS monitoring, which integrates both spatial and temporal dimensions and considers multiple contaminants together (a chemical cocktail approach), can be a comprehensive strategy for tracking sources, fate, and transport of pollutants along stream flowpaths and making comparisons of water quality patterns across different watersheds and regions.

Lethal and Sub-lethal Implications of Sodium Chloride Exposure for Adult Unionid Mussel Species: Eurynia dilatata and Lasmigona costata

The elevated use of salt as a de-icing agent on roads in Canada is causing an increase in the chloride concentration of freshwater ecosystems. Freshwater Unionid mussels are a group of organisms that are sensitive to increases in chloride levels. Unionids have greater diversity in North America than anywhere else on Earth, but they are also one of the most imperiled groups of organisms. This underscores the importance of understanding the effect that increasing salt exposure has on these threatened species. There are more data on the acute toxicity of chloride to Unionids than on chronic toxicity. This study investigated the effect of chronic sodium chloride exposure on the survival and filtering activity of two Unionid species (Eurynia dilatata, and Lasmigona costata) and assessed the effect on the metabolome in L. costata hemolymph. The concentration causing mortality after 28 days of exposure was similar for E. dilatata (1893 mg Cl^-/L) and L. costata (1903 mg Cl^-/L). Significant changes in the metabolome of the L. costata hemolymph were observed for mussels exposed to non-lethal concentrations. For example, several phosphatidylethanolamines, several hydroxyeicosatetraenoic acids, pyropheophorbide-a, and alpha-linolenic acid were significantly upregulated in the hemolymph of mussels exposed to 1000 mg Cl^-/L for 28 days. While no mortality occurred in the treatment, elevated metabolites in the hemolymph are an indicator of stress.

Applied winter biology: threats, conservation and management of biological resources during winter in cold climate regions

Winter at high latitudes is characterized by low temperatures, dampened light levels and short photoperiods which shape ecological and evolutionary outcomes from cells to populations to ecosystems. Advances in our understanding of winter biological processes (spanning physiology, behaviour and ecology) highlight that biodiversity threats (e.g. climate change driven shifts in reproductive windows) may interact with winter conditions, leading to greater ecological impacts. As such, conservation and management strategies that consider winter processes and their consequences on biological mechanisms may lead to greater resilience of high altitude and latitude ecosystems. Here, we use well-established threat and action taxonomies produced by the International Union of Conservation of Nature-Conservation Measures Partnership (IUCN-CMP) to synthesize current threats to biota that emerge during, or as the result of, winter processes then discuss targeted management approaches for winter-based conservation. We demonstrate the importance of considering winter when identifying threats to biodiversity and deciding on appropriate management strategies across species and ecosystems. We confirm our expectation that threats are prevalent during the winter and are especially important considering the physiologically challenging conditions that winter presents. Moreover, our findings emphasize that climate change and winter-related constraints on organisms will intersect with other stressors to potentially magnify threats and further complicate management. Though conservation and management practices are less commonly considered during the winter season, we identified several potential or already realized applications relevant to winter that could be beneficial. Many of the examples are quite recent, suggesting a potential turning point for applied winter biology. This growing body of literature is promising but we submit that more research is needed to identify and address threats to wintering biota for targeted and proactive conservation. We suggest that management decisions consider the importance of winter and incorporate winter specific strategies for holistic and mechanistic conservation and resource management.

Salinization and heavy metal cadmium impair growth but have contrasting effects on defensive colony formation of Scenedesmus obliquus

Driven by anthropogenic activities, freshwater salinization has become an emerging global environmental issue. Recent studies indicate that salinization increases the mobility of heavy metals in soil and causes higher flux into surface waterbodies. The present study assessed the combined effects of salinization (0, 3, 6 PSU) and the heavy metal Cd²⁺ (0, 0.2, 0.4 mg L^-1) on the anti-grazing colony formation and population growth of Scenedesmus obliquus, a common freshwater alga. The results showed that the increase in salinity promoted colony formation of S. obliquus with or without the presence of grazing cues and, in contrast, Cd²⁺ contamination depressed the defensive colony formation of S. obliquus to Daphnia filtrate. The increase in both salinity and Cd²⁺ concentration depressed the population growth of S. obliquus, including impaired photosynthesis and a decreased population growth rate. Salinization moderated the negative effects of Cd²⁺ on defensive colony formation of S. obliquus, suggesting increased absorption of Cd²⁺ ions by a thicker outer layer of the algal cell wall under saltier conditions. As a result, larger defensive colonies of S. obliquus under freshwater salinization may cause higher bioaccumulation of heavy metals by algal cells and heavier influence on zooplankton. This study provides evidence that freshwater salinization could interfere with plankton interactions by affecting algal defense and growth, which may lead to bottom-up cascading effects on freshwater food webs.

Impacts of water hardness and road deicing salt on zooplankton survival and reproduction

Rising salinity from road deicing salts threatens the survival and reproduction of freshwater organisms. We conducted two experiments to address how Daphnia pulex survival and reproduction were affected by road salt concentration (control, 120, 640 and 1200 mg Cl^-/L) crossed with three concentrations of water hardness (20, 97, 185 mg CaCO₃ /L). D. pulex survival was poor in our hard water treatment in both experiments (185 mg CaCO₃ /L), potentially indicating a low tolerance to hard water for the strain used in our experiments. With the remaining two hardness treatments (20 and 97 mg CaCO₃ /L), we found no evidence of an interactive effect between salt concentration and water hardness on D. pulex survival. In our population-level experiment, D. pulex survival was reduced by > 60% at 120 mg Cl^-/L compared to the control. In the individual experiment, survival was similar between the control and 120 mg Cl^-/L, but ≤ 40% of individuals survived in 640 and 1200 mg Cl^-/L. For the surviving individuals across all treatments, the number of offspring produced per individual declined with increasing Cl^- concentration and in hard water. Our results indicate that current Cl^- thresholds may not protect some zooplankton and reduced food availability per capita may enhance the negative impacts of road salt.

Resurrection genomics provides molecular and phenotypic evidence of rapid adaptation to salinization in a keystone aquatic species

Ecologists and evolutionary biologists are increasingly cognizant of rapid adaptation in wild populations. Rapid adaptation to anthropogenic environmental change is critical for maintaining biodiversity and ecosystems services into the future. Anthropogenic salinization of freshwater ecosystems is quickly emerging as a primary threat, which is well documented in the northern temperate ecoregion. Specifically, many northern temperate lakes have undergone extensive salinization because of urbanization and the associated increase in impervious surfaces causing runoff, and the extensive use of road deicing salts (e.g., NaCl). It remains unclear whether increasing salinization will lead to extirpation of species from these systems. Using a "resurrection genomics" approach, we investigated whether the keystone aquatic herbivore, Daphnia pulicaria, has evolved increased salinity tolerance in a severely salinized lake located in Minnesota, USA. Whole-genome resequencing of 54 Daphnia clones from the lake and hatched from resting eggs that represent a 25-y temporal contrast demonstrates that many regions of the genome containing genes related to osmoregulation are under selection in the study population. Tolerance assays of clones revealed that the most recent clones are more tolerant to salinity than older clones; this pattern is concomitant with the temporal pattern of stabilizing salinity in this lake. Together, our results demonstrate that keystone species such as Daphnia can rapidly adapt to increasing freshwater salinization. Further, our results indicate that rapid adaptation to salinity may allow lake Daphnia populations to persist in the face of anthropogenic salinization maintaining the food webs and ecosystem services they support despite global environmental change.

Destabilized microbial networks with distinct performances of abundant and rare biospheres in maintaining networks under increasing salinity stress

Global changes such as seawater intrusion and freshwater resource salinization increase environmental stress imposed on the aquatic microbiome. A strong predictive understanding of the responses of the aquatic microbiome to environmental stress will help in coping with the "gray rhino" events in the environment, thereby contributing to an ecologically sustainable future. Considering that microbial ecological networks are tied to the stability of ecosystem functioning and that abundant and rare biospheres with different biogeographic patterns are important drivers of ecosystem functioning, the roles of abundant and rare biospheres in maintaining ecological networks need to be clarified. Here we showed that, with the increasing salinity stress induced by the freshwater-to-seawater transition, the microbial diversity reduced significantly and the taxonomic structure experienced a strong succession. The complexity and stability of microbial ecological networks were diminished by the increasing stress. The composition of the microorganisms supporting the networks underwent sharp turnovers during the freshwater-to-seawater transition, with the abundant biosphere behaving more robustly than the rare biosphere. Notably, the abundant biosphere played a much more important role than the rare biosphere in stabilizing ecological networks under low-stress environments, but the difference between their relative importance narrowed significantly with the increasing stress, suggesting that the environmental stress weakened the "Matthew effect" in the microbial world. With in-depth insights into the aquatic microbial ecology under stress, our findings highlight the importance of adjusting conservation strategies for the abundant and rare biospheres to maintain ecosystem functions and services in response to rising environmental stress.

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.

Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions

Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L^-1 was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.

Can Common Pool Resource Theory Catalyze Stakeholder-Driven Solutions to the Freshwater Salinization Syndrome?

Freshwater salinity is rising across many regions of the United States as well as globally, a phenomenon called the freshwater salinization syndrome (FSS). The FSS mobilizes organic carbon, nutrients, heavy metals, and other contaminants sequestered in soils and freshwater sediments, alters the structures and functions of soils, streams, and riparian ecosystems, threatens drinking water supplies, and undermines progress toward many of the United Nations Sustainable Development Goals. There is an urgent need to leverage the current understanding of salinization's causes and consequences─in partnership with engineers, social scientists, policymakers, and other stakeholders─into locally tailored approaches for balancing our nation's salt budget. In this feature, we propose that the FSS can be understood as a common pool resource problem and explore Nobel Laureate Elinor Ostrom's social-ecological systems framework as an approach for identifying the conditions under which local actors may work collectively to manage the FSS in the absence of top-down regulatory controls. We adopt as a case study rising sodium concentrations in the Occoquan Reservoir, a critical water supply for up to one million residents in Northern Virginia (USA), to illustrate emerging impacts, underlying causes, possible solutions, and critical research needs.

Hydrogeochemical processes controlling the salinity of surface water and groundwater in an inland saline-alkali wetland in western Jilin, China

Understanding the hydrochemical evolutionary mechanisms of surface water and groundwater in saline-alkali wetlands in arid and semi-arid regions is necessary for assessing how wetland water resource utilization and restoration processes may affect the natural interface between wetland salinity and water. The Momoge National Nature Reserve (MNNR) is an inland wetland in northeastern China that is mainly fed by irrigation water and floods from the Nenjiang River. The purpose of the present study is to describe the spatial distribution characteristics of surface water and groundwater hydrochemistry and salinity in the MNNR and analyze the main processes controlling these parameters. The composition of stable isotopes (δ2H and δ18O) and water chemistry, including the levels of Na, K, Ca, Mg, HCO3, SO4, and Cl, of 156 water samples were analyzed. The results show that the lake water in the MNNR is at a risk of salinization owing to a high degree of evaporation. The analysis of the ion ratio and mineral saturation index showed that the ions in water are primarily derived from aquifer leaching, and the precipitation of Ca2+ and Mg2+ resulted in lower Ca2+ and Mg2+ levels in lake water than in groundwater. Hydrogen and oxygen stable isotope and deuterium excess analyses show that evaporation is the dominant factor controlling the hydrochemistry and salinity of lake water in the MNNR. Long-term effective monitoring of lake water and groundwater must be developed to provide an early warning for the salinization of lake water and a scientific basis for the protection and restoration of wetland ecosystem functions within the MNNR.

The joint effects of salt and 6PPD contamination on a freshwater herbivore

Using sodium chloride (NaCl) for de-icing roads is known to have severe consequences on freshwater organisms when washed into water bodies. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, also known as 6PPD, is an antiozonant mainly found in automobile tire rubber to prevent ozone mediated cracking or wear-out. Especially the ozonated derivate, 6PPD-quinone, which is washed into streams after storm events, has been found to be toxic for coho salmon. Studies on other freshwater organisms could not confirm those findings, pointing towards distinct species-specific differences. Storm events result in greater run-offs from all water-soluble contaminants into freshwater bodies, potentially enhancing the concentrations of both chloride and 6PPD during winter. Here we show that these two contaminants have synergistic negative effects on the population growth of the rotifer Brachionus calyciflorus, a common freshwater herbivore. Hence, while only high concentrations of 6PPD and even higher concentrations of 6PPD-quinone, beyond environmentally relevant concentrations, had lethal effects on rotifers, the addition of NaCl enhanced the sensitivity of the rotifers towards the application of 6PPD so that their negative effects were more pronounced at lower concentrations. Similarly, 6PPD increased the lethal effect of NaCl. Our results support the species-specific toxicity of 6PPD and demonstrate a synergistic effect of the antiozonant on the toxicity of other environmentally relevant stressors, such as road salt contamination.