Salty fertile lakes: how salinization and eutrophication alter the structure of freshwater communities

Weak salinization alleviates the harmful impact of cyanobacteria on water fleas

Climate change and anthropogenic activities are driving dramatic changes in aquatic ecosystems. In this context, cyanobacterial blooms and freshwater salinization have recently received much attention, however, the comprehensive effects of these stressors on aquatic organisms are complex and have yet to be accurately clarified. This study tested the harmful effects of cyanobacteria and increasing salinity on zooplankton by characterizing the life-history traits and gut microbiota composition in the large-bodied Daphnia pulex and small-bodied Simocephalus vetulus. In 0 g L-1 salinity, the intrinsic growth rates of both species decreased when fed a diet containing cyanobacteria. Without cyanobacteria, the intrinsic growth rate of D. pulex was highest at 1 g L-1 salinity and lowest at 2 g L-1, whereas that of S. vetulus declined as salinity levels rose. Salinity alleviated the negative effects of cyanobacteria on water fleas. D. pulex performed worse than S. vetulus under high salinity (2 g L-1), cyanobacteria or their combined effects. Salinity changed the dominant gut microbiota in the water fleas when fed cyanobacteria. Bacteroides sp. and Cetobacterium sp. in D. pulex and Cetobacterium sp. in S. vetulus were most abundant when the water fleas were reared at low salinity (1 g L-1). The consistent changes in the dominant bacterial groups and the life-history traits indicate that the gut microbiota might adjust their tolerance to cyanobacteria.

Multiple stressors affect function rather than taxonomic structure of freshwater microbial communities

Microbial community responses to environmental stressors are often characterised by assessing changes in taxonomic structure, but such changes, or lack thereof, may not reflect functional changes that are critical to ecosystem processes. We investigated the individual and combined effects of nutrient enrichment ( + 10 mg/L N, + 1 mg/L P) and salinisation ( + 15 g/L NaCl)-key stressors in freshwater systems-on the taxonomic structure and metabolic function of benthic microbial communities using 1000 L open freshwater ponds established >10 years ago in the field. Combined stressors drove strong decreases in maximum and mean total carbon metabolic rates and shifted carbon metabolic profiles compared to either stressor individually and compared to ambient conditions. These metabolic functional changes did not recover through time and occurred without significant alterations in bacterial community taxonomic structure. These results imply that critical functions, including organic carbon release, are likely to be impaired under multiple stressors, even when taxonomic structure remains stable.

Road salt chloride exposure in urban streambeds and links to groundwater - surface water interactions and salt sources

Groundwater transport of chloride (Cl) containing road salt deicers is an important contributor to salinization of fresh surface waters in temperate climates. While mass loading of salt to streams via groundwater has received greater recognition lately, only a few studies have demonstrated the unique risk posed by the direct discharge of salt-laden groundwater to aquatic life residing in the benthic zone (e.g., macroinvertebrates, mussels, plant roots, fish eggs). These studies revealed high Cl concentrations in stream porewater but provided limited information on its spatio-temporal variability and factors influencing it. To address this gap, this study conducted a detailed 2-year field investigation on 100-m scale reaches of two streams adjacent to salt-receiving roads in London, Ontario, applying year-round streambed porewater sampling, streambed surface electrical conductivity mapping, and electrical imaging (geophysics) techniques. Areas of probable groundwater upwelling and surface water downwelling were identified using streambed temperature mapping and hydraulic head gradients. Benthic porewater Cl concentrations were commonly above water quality guidelines, but also varied substantially over small spatial scales, ranging from <100 to >5000 mg/L over ≤10 m distances. Generally, porewater Cl concentrations were relatively stable seasonally compared to the stream water, leading to benthic areas with year-round exposure to elevated Cl and limiting prospects for benthic zone refugia. Key factors affecting this spatial and temporal variability included proximity to roads, impacts from point sources (snow storage locations), and surface water downwelling (hyporheic flows). These observations have direct implications for the health of urban stream benthic communities and for designing management and remediation strategies.

Combined effect of freshwater salinization and harmful algae on the benthic invertebrate Chironomus pallidivittatus

Global climate change as well as human activities have been reported to increase the frequency and severity of both salinization and harmful algal blooms (HABs) in many freshwater systems, but their co-effect on benthic invertebrates has rarely been studied. This study simultaneously examined the joint toxicity of salinity and different cyanobacterial diets on the behavior, development, select biomarkers, and partial life cycle of Chironomus pallidivittatus (Diptera). High concentrations of salts (e.g., 1 g/L Ca2+ and Mg2+) and toxic Microcystis had synergistic toxicity, inhibiting development, burrowing ability and causing high mortality of C. pallidivittatus, especially for the Mg2+ treatment, which caused around 90% death. Low Ca2+ concentration (e.g., 0.01 g/L) promoted larval burrowing ability and inhibited toxin accumulation, which increased the tolerance of Chironomus to toxic Microcystis. However, low Mg2+ concentration (e.g., 0.01 g/L) was shown to inhibit the behavior, development and increase algal toxicity to Chironomus. Toxic Microcystis resulted in microcystin (MC) accumulation, inhibited the burrowing ability of larvae, and increased the proportion of male adults (>50%). The combined toxicity level from low to high was verified by the weight of evidence and the grey TOPSIS model, which integrated five lines of evidence to increase the risk assessment accuracy and efficiency. This is the first study that provided insights into ecological risk arising from the joint effect of salinity and harmful algae on benthic organisms. We suggest that freshwater salinization and HABs should be considered together when assessing ecological threats that arise from external stress.

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.

Sensitivity of Daphnia spinulata Birabén, 1917 to glyphosate at different salinity levels

Daphnia spinulata Birabén, 1917 is an endemic cladoceran species, frequent in the zooplankton communities of the shallow lakes of the Pampean region of Argentina. These lakes have varying salinity levels and, being located in agricultural areas, are frequently subject to pesticide pollution. This study aimed to determine the effects of the herbicide glyphosate (Panzer Gold®) in combination with different salinity levels on the biological parameters of D. spinulata and its recovery ability after a short exposure. Three types of assays were performed: an acute toxicity test, a chronic assessment to determine survival, growth and reproduction, and recovery assays under optimal salinity conditions (1 g L-1). The LC50-48 h of glyphosate was 7.5 mg L-1 (CL 3.15 to 11.72). Longevity and the number of offspring and clutches were significantly reduced due to the combined exposure of glyphosate and increased salinity. The timing of the first offspring did not recover after glyphosate exposure. Our results reveal that D. spinulata is sensitive to the herbicide Panzer Gold® at concentrations well below those indicated in the safety data sheet of this commercial formulation, which causes stronger negative effects in conditions of higher salinity. Further research is needed to shed light on the sensitivity of this cladoceran to glyphosate and its variability under other interactive stress factors.

Lake water chemistry and local adaptation shape NaCl toxicity in Daphnia ambigua

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.

The relative importance of salinization in lowland stream zooplankton: Implications of the ecosystem nutrient status

Salinization of aquatic systems is predicted to increase due to climate and land use changes. Nevertheless, community responses may be different according to the ecosystem characteristics and contextual scenarios. Small flowing waters are particularly vulnerable to salinization, which may impact on the biodiversity and ecosystem processes, but this remains unclear. We conducted a study in 42 lowland streams characterized by overall high nutrient levels along a salinity gradient between 2 and 160 g L-1 to analyze changes in zooplankton structural and functional metrics, and the grazing effects of zooplankton on phytoplankton affecting the energy transfer. Generalized additive models revealed that the analyzed metrics were relatively influenced by salinity, with factors related to trophic conditions playing an important role as well. Total abundance and biomass decreased along the salinity gradient while increasing at intermediate soluble reactive phosphorous concentrations (SRP) in the former and with a linear increase in the SRP in the latter case. Taxonomic richness decreased with salinity and dissolved inorganic nitrogen, with species replacement toward saline-tolerant ones according to the compositional and optimums analyses. In opposite, functional richness did not display any specific trend within the environmental gradients. This explains why zooplankton compositional changes were not reflected into shifts in the grazing pattern on phytoplankton, which was in turn driven by SRP and dissolved oxygen concentrations. Further research is a critical requirement in these poorly studied ecosystems for planning mitigation actions to the co-occurrence of eutrophication and salinization in a fast changing world.

What happens when salinization meets eutrophication? A test using stream microcosms

Nutrient and salt pollution often co-occur in rivers and streams due to human activities (e.g., agriculture, urbanization). Thus, understanding the interactive effects of nutrients and salinity on freshwater ecosystems is critical for environmental management. We experimentally assessed the interactive effects of nutrient and salt pollution on stream microcosms using biofilm and macroinvertebrates as model systems. Six treatments were performed in triplicate: control (C: N-NH4+ = 0.05; P- PO43- = 0.037; Cl- = 33.5 mg L-1), intermediate nutrient (IN: N-NH4+ = 0.4; P- PO43- = 0.271; Cl- = 33. 5 mg L-1), high nutrient (HN: N-NH4+ = 0.84; P- PO43- = 0.80; Cl- = 33.5 mg L-1), salt (S: N-NH4+ = 0.05; P- PO43- = 0.037; Cl- = 3000 mg L-1), salt with intermediate nutrient (SIN: N-NH4+ = 0.4; P- PO43- = 0.27; Cl- = 3000 mg L-1) and salt with high nutrient (SHN: N-NH4+ = 0.84; P- PO43- = 0.80; Cl- = 3000 mg L-1). After 14 days of exposure, biofilm chlorophyll-a increased across all treatments, with cyanobacteria replacing diatoms and green algae. Treatments with no added nutrients (C and S) had more P uptake capacity than the rest. The indicator species analysis showed 8 significant taxa, with Orthocladius (Orthocladius) gr. Wetterensis and Virganytarsus significantly associated with the salinity treatment. Overall, salt pollution led to a very strong decline in macroinvertebrate richness and diversity. However, salt toxicity seemed to be ameliorated by nutrient addition. Finally, both structural equation models and biotic-abiotic interaction networks showed that complex biological interactions could be modulating the response of the biological communities to our treatments. Thus, our study calls for species-level assessments of salt and nutrient effects on river ecosystems and advocates for better management of co-occurring pollutants.

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.

Lake water chemistry and population of origin interact to shape fecundity and growth in Daphnia ambigua

Freshwater environments vary widely in ion availability, owing to both natural and anthropogenic drivers. Field and laboratory work point to the importance of overall salinity, as well as cation depletion, in shaping the physiology, behavior, and ecology of freshwater taxa. Yet, we currently have a poor understanding of the degree to which populations may vary in response to ion availability. Using Daphnia collected from three lakes that differ greatly in salinity and calcium availability, we conducted a laboratory reciprocal transplant experiment to assess how animals representing these populations vary in fecundity, body size, and survival when reared in lake water from each environment. The lake water environment and population of origin strongly interacted to shape Daphnia growth and reproduction. Surprisingly, we found only modest evidence that lake water with abundant calcium (5.5 vs. 1.2-2.3 mg/L) increased Daphnia growth or reproduction. By contrast, water from a relatively ion-rich lake (400 μS/cm specific conductance) strongly boosted Daphnia fecundity over lower-ion lake water (20-50 μS/cm), especially for the population originating from the high-ion environment. Our results suggest that ion-poor conditions common in regions around the world may exert stress on freshwater organisms, even for populations inhabiting these environments. Meanwhile, moderate salt enrichment may not prove harmful but could even benefit freshwater taxa in these ion-poor regions. The context dependence of how and when lake water chemistry affects Daphnia and other freshwater taxa deserves greater attention, in both ion-depleted and ion-rich conditions. Daphnia are key members of lake food webs and serve as an important model for ecology, evolution, and toxicology research. Consideration of how lake water chemistry may influence how Daphnia populations respond to abiotic and biotic stress may improve the ability to evaluate and predict ecological and evolutionary dynamics in lakes of varying chemical composition.

Effect of Different Water Salinities on the Larvae of the Blue Bream Ballerus ballerus (Linnaeus, 1758) during Rearing

The influence of water salinities of 3, 5, and 7 ppt on the growth and survival of Ballerus ballerus (L.) larvae was studied. The control group was fish reared in freshwater (0 ppt). The larvae showed high tolerance to water salinities of 5–7 ppt. The mean final weight of the larvae ranged from 48.6 to 64.1 mg, with corresponding mean total lengths from 18.9 to 22.6 mm, depending on the water salinity level. The best larval length increments were recorded in water with salinity of 3 ppt. They were only slightly lower in 0 ppt water, and there were no statistically significant differences between the breeding rates calculated for larvae reared in 3 ppt water. Depending on the salinity level of the water, the final survival rate of the blue bream larvae ranged from 83.5 to 98.6%. The blue bream larvae reared in water with salinity levels of 5 and 7 ppt were statistically smaller than the others, but the results obtained were equally satisfactory.

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.

Contaminants disrupt aquatic food webs via decreased consumer efficiency

Changes in consumer-resource dynamics due to environmental stressors can alter energy flows or key interactions within food webs, with potential for cascading effects at population, community, and ecosystem levels. We conducted a meta-analysis to quantify the direction and magnitude of changes in consumption rates following exposure of consumer-resource pairs within freshwater-brackish and marine systems to anthropogenic CO₂, heavy metals, microplastics, oil, pesticides, or pharmaceuticals. Across all contaminants, exposure generally decreased consumption rates, likely due to reduced consumer mobility or search efficiency. These negative effects on consumers appeared to outweigh co-occurring reductions in prey vigilance or antipredator behaviors following contaminant exposure. Consumption was particularly dampened in freshwater-brackish systems, for consumers with sedentary prey, and for lower-trophic-level consumers. This synthesis indicates that energy flow up the food web, toward larger - often ecologically and economically prized - taxa may be dampened as aquatic contaminant loads increase.

Salty freshwater macrophytes: the effects of salinization in freshwaters upon non-halophyte aquatic plants

Salinization is a threat that affects aquatic ecosystems worldwide. As primary producers, freshwater macrophytes are of paramount importance in these ecosystems, however, information regarding the potential impacts of salinization upon these organisms is still scarce. In this review we provide a comprehensive and updated discussion of how freshwater macrophytes deal with salinity increase in freshwaters. We reviewed the salinity tolerance of widespread non-halophyte macrophytes through an overview of salinity tolerance mechanisms, their tolerance classification, and salinity effects at different levels of organization: from individuals to ecosystems. Thus, we demonstrated that widespread macrophytes that inhabit freshwaters display efficient salinity tolerance to salinity levels between 5 and 10 g L^-1, and only a few species display tolerance to salinities higher than 10 g L^-1. Widespread macrophytes demonstrated salinity tolerance of approximately 5 g L^-1. Widespread macrophytes demonstrated salinity tolerance of approximately 5 g L^-1. Emergent, floating and submerged species showed no significant difference in salinity tolerance. Salinity stress symptoms in freshwater macrophytes are somewhat similar to those of terrestrial plants and can show up even at slight salinity increases. Salinities higher than 1 g L^-1 can negatively affect both physiology and diversity of non-halophyte macrophytes and cause long-term - and not well understood - changes in freshwater ecosystems. Salinization of freshwater ecosystems, among others threats, in combination with climate change, raise concerns about the future ecological status of freshwater ecosystems and the services they can provide.

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.

A Novel Approach to Developing Thresholds for Total Dissolved Solids Using Standardized and Experimental Toxicity Test Methods

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.

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.

The effects of road salt on freshwater ecosystems and solutions for mitigating chloride pollution - A review

Road salt (mainly NaCl) is commonly used during the winter to ensure road and pavement safety; however, the long-term application of NaCl has negative consequences on soil and the water environment. The aims of the present review were to evaluate the impact of road salt on catchment processes which accelerate the eutrophication of waters, and to identify a possible approach for reducing the impact of winter salt treatments of roads and sidewalks, on water body quality. The objectives were implemented in accordance with the ecohydrological approach, which recommends using hierarchical steps to solve problems. The first step was the monitoring of threats, in which the causes of high chloride (Cl) concentrations in groundwater and surface water were identified. The results indicate that long-term winter application of road salt increases the annual mean concentrations of Cl in rivers and lakes, due to Cl entering groundwater. The second step was a cause-effect analysis of the impact of NaCl on the abiotic processes in soil and water, and on the biotic response to chloride exposure. Chlorides appear to decrease the biodiversity of aquatic animals and plants but favour the growth of phytoplankton, especially cyanobacteria. Moreover, Cl reduces the self-purification processes of water by decreasing nutrient accumulation in macrophytes, decreasing the denitrification rate and reducing organic matter decomposition. The third step was to evaluate possible solutions for reducing the negative impact of NaCl on the environment, and to improve the effectiveness of alternative de-icing agents. An analysis of available literature indicates that a system-based approach integrating engineering knowledge with an understanding of biological and hydrological processes is necessary to indicate solutions for reducing environmental risks from road salt use.

Competition alters species' plastic and genetic response to environmental change

Species react to environmental change via plastic and evolutionary responses. While both of them determine species' survival, most studies quantify these responses individually. As species occur in communities, competing species may further influence their respective response to environmental change. Yet, how environmental change and competing species combined shape plastic and genetic responses to environmental change remains unclear. Quantifying how competition alters plastic and genetic responses of species to environmental change requires a trait-based, community and evolutionary ecological approach. We exposed unicellular aquatic organisms to long-term selection of increasing salinity-representing a common and relevant environmental change. We assessed plastic and genetic contributions to phenotypic change in biomass, cell shape, and dispersal ability along increasing levels of salinity in the presence and absence of competition. Trait changes in response to salinity were mainly due to mean trait evolution, and differed whether species evolved in the presence or absence of competition. Our results show that species' evolutionary and plastic responses to environmental change depended both on competition and the magnitude of environmental change, ultimately determining species persistence. Our results suggest that understanding plastic and genetic responses to environmental change within a community will improve predictions of species' persistence to environmental change.

The combined effects of macrophytes and three road salts on aquatic communities in outdoor mesocosms

Because of environmental and societal concerns, new strategies are being developed to mitigate the effects of road salt. These include new deicers that are alternatives to or mixtures with the most common road salt, sodium chloride (NaCl), improved techniques and equipment, and biotic mitigation methods. Using outdoor mesocosms, we investigated the impacts of NaCl and two common alternatives, magnesium chloride (MgCl₂) and calcium chloride (CaCl₂) on freshwater communities. We also investigated the mitigation ability of a common macrophyte, Elodea. We hypothesized that road salt exposure reduces filamentous algae, zooplankton, and macrocrustaceans, but results in increases in phytoplankton and gastropods. We also hypothesized that MgCl₂ is the most toxic salt to communities, followed by CaCl₂, and then NaCl. Lastly, we hypothesized that macrophytes mitigate some of the effects of road salt, specifically the effects on primary producers. We found that all three salts reduced filamentous algal biomass and amphipod abundance, but only MgCl₂ reduced Elodea biomass. MgCl₂ had the largest and longest lasting effects on zooplankton, specifically cladocerans and copepods, which resulted in a significant increase in phytoplankton and rotifers. CaCl₂ increased ostracods and decreased snail abundance, but NaCl increased snail abundance. Lastly, while we did not find many interactions between road salt and macrophyte treatments, macrophytes did counteract many of the salt effects on producers, leading to decreased phytoplankton, increased filamentous algae, and altered abiotic responses. Thus, at similar chloride concentrations, NaCl alternatives, specifically MgCl₂, are not safer for aquatic ecosystems and more research is needed to find safer road management strategies to protect freshwater ecosystems.

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.

Response of the microbial community to salt stress and its stratified effect in constructed wetlands

Nitrogen removal in constructed wetlands (CWs) may be inhibited by salinity. The clarification of the response of microbial community to salt stress is a premise for developing strategies to improve nitrogen removal efficiency in CWs under saline conditions. Results showed that the ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total nitrogen (TN) removal percentages significantly (p < 0.05) decreased in CWs with increasing salinity. The structure and abundance of the microbial community varied with different salinity levels and sampling depths in CWs. Compared with a non-saline condition, the abundances of some bacteria with a denitrification function (e.g., Arthrobacter) significantly (p < 0.05) decreased in CWs under saline conditions (i.e., EC of 15 and 30 mS/cm). Aerobic bacteria (e.g., Sphingomonas) exhibited more abundance in soil and upper gravel samples in CWs than those in bottom gravel samples, while the abundance of some denitrifying bacteria (e.g., Thauera and Azoarcus) was significantly (p < 0.05) higher in bottom gravel samples compared with soil and upper gravel samples, respectively. This study provides both microbiological evidence for explaining the impact of salt stress on nitrogen removal in CWs and scientific reference for developing enhanced strategies to improve the nitrogen removal capacity of CWs.

Invertebrate turnover along gradients of anthropogenic salinisation in rivers of two German regions

Rising salinity in freshwater ecosystems can affect community composition. Previous studies mainly focused on changes in freshwater communities along gradients of absolute levels of electrical conductivity (EC). However, both geogenic and anthropogenic drivers contribute to the EC level and taxa may regionally be adapted to geogenic EC levels. Therefore, we examined the turnover in freshwater invertebrates along gradients of anthropogenic EC change in two regions of Germany. The anthropogenic change of EC was estimated as the difference between the measured EC and the modeled background EC driven by geochemical and climate variables. Turnover in freshwater invertebrates (β-diversity) was estimated using the Jaccard index (JI). We found that invertebrate turnover between EC gradient categories is generally greater than 47%, with a maximum of approximately 70% in sites with a more than 0.4 mS cm^-1 change compared to the baseline (i.e. no difference between predicted and measured EC). The invertebrates Amphinemura sp., Anomalopterygella chauviniana and Leuctra sp. were reliable indicators of low EC change, whereas Potamopyrgus antipodarum indicated sites with the highest EC change. Variability within categories of EC change was slightly lower than within categories of absolute EC. Elevated nutrient concentrations that are often linked to land use may have contributed to the observed change of the invertebrate richness and can exacerbate effects of EC on communities in water. Overall, our study suggests that the change in EC, quantified as the difference between measured EC and modeled background EC, can be used to examine the response of invertebrate communities to increasing anthropogenic salinity concentrations in rivers. However, due to the strong correlation between EC change and observed EC in our study regions, the response to these two variables was very similar. Further studies in areas where EC change and observed EC are less correlated are required. In addition, such studies should consider the change in specific ions.

How Do Indirect Effects of Contaminants Inform Ecotoxicology? A Review

Indirect effects in ecotoxicology are defined as chemical- or pollutant-induced alterations in the density or behavior of sensitive species that have cascading effects on tolerant species in natural systems. As a result, species interaction networks (e.g., interactions associated with predation or competition) may be altered in such a way as to bring about large changes in populations and/or communities that may further cascade to disrupt ecosystem function and services. Field studies and experimental outcomes as well as models indicate that indirect effects are most likely to occur in communities in which the strength of interactions and the sensitivity to contaminants differ markedly among species, and that indirect effects will vary over space and time as species composition, trophic structure, and environmental factors vary. However, knowledge of indirect effects is essential to improve understanding of the potential for chemical harm in natural systems. For example, indirect effects may confound laboratory-based ecological risk assessment by enhancing, masking, or spuriously indicating the direct effect of chemical contaminants. Progress to better anticipate and interpret the significance of indirect effects will be made as monitoring programs and long-term ecological research are conducted that facilitate critical experimental field and mesocosm investigations, and as chemical transport and fate models, individual-based direct effects models, and ecosystem/food web models continue to be improved and become better integrated.