Salinized rivers: degraded systems or new habitats for salt-tolerant faunas?

Steady-State Land Cover but Non-Steady-State Major Ion Chemistry in Urban Streams

Sources of many major ions in urban streams remain ambiguous, particularly for ions unrelated to deicing salt use, and temporal patterns in concentrations are unstudied. We used 16 years of water chemistry data based on weekly samples from the Baltimore, MD, USA, metropolitan area and the Weighted Regressions on Time, Discharge, and Season approach to investigate connections between major ions, land cover, and time. All watersheds were underlain by silicate bedrock, contained no regulated point sources, and had stable land cover. Major ion concentrations were higher with greater urban land cover. Notably, concentrations of most ions increased with time in (sub)urban streams and had higher annual variability than in watersheds without impervious surface cover. Nonpoint source contributions from deicing salt and concrete were the predominant influences on major ion concentrations and produced stream chemistry that was distinctly different from forested streams. The novel finding that concentrations of most major ions were not only elevated but increasing in urban streams even with no substantial changes in land cover during the study period has important implications for ecosystem health and water quality, particularly given recent work demonstrating the high correlation between elevated ion concentrations and changes in freshwater biotic communities.

A field-based model of the relationship between extirpation of salt-intolerant benthic invertebrates and background conductivity

Field-collected measures of dissolved salts and occurrences of aquatic invertebrates have been used to develop protective levels. However, sufficiently large field data sets of exposures and biota are often not available. Therefore, a model was developed to predict the exposure extirpating 5% of benthic invertebrate genera using only measures of specific conductivity (SC) as the independent variable. The model is based on 3 assumptions: (1) a genus will rarely occur where the background exceeds its upper physiological limit; (2) the lowest possible tolerance limit of a genus in a region is defined by the natural background; and (3) as a result, there will be a regular association between natural background SC and the SC at which salt-intolerant genera are present. Three steps were used to develop the model. First, background SC was characterized as the 25th centile of sampled sites for each of 24 areas in the United States with streams dominated by bicarbonate and sulfate ions. Second, the extirpation concentration (XC₉₅), an estimate of the upper tolerance limit with respect to SC, was calculated for genera in 24 data sets. Next, the lower 5th centile of each set of XC₉₅ values (XCD₀₅) was identified for the most salt-intolerant members in each data set. Finally, the relationship between the 24 background SC and the 24 XCD₀₅ values was empirically modeled to develop a background-to-criterion model. The least squares regression of XCD₀₅ values on log background SC (log Y = 0.658logX + 1.071) yields a strong linear relationship (r = 0.93). The regression model makes it possible to use SC background to predict the SC likely to extirpate the most salt-intolerant genera in an area. The results also suggest that species distribute along natural background gradients of SC and that this relationship can be used to develop criteria for ionic concentration.

A field-based model of the relationship between extirpation of salt-intolerant benthic invertebrates and background conductivity

Field-collected measures of dissolved salts and occurrences of aquatic invertebrates have been used to develop protective levels. However, sufficiently large field data sets of exposures and biota are often not available. Therefore, a model was developed to predict the exposure extirpating 5% of benthic invertebrate genera using only measures of specific conductivity (SC) as the independent variable. The model is based on 3 assumptions: (1) a genus will rarely occur where the background exceeds its upper physiological limit; (2) the lowest possible tolerance limit of a genus in a region is defined by the natural background; and (3) as a result, there will be a regular association between natural background SC and the SC at which salt-intolerant genera are present. Three steps were used to develop the model. First, background SC was characterized as the 25th centile of sampled sites for each of 24 areas in the United States with streams dominated by bicarbonate and sulfate ions. Second, the extirpation concentration (XC₉₅), an estimate of the upper tolerance limit with respect to SC, was calculated for genera in 24 data sets. Next, the lower 5th centile of each set of XC₉₅ values (XCD₀₅) was identified for the most salt-intolerant members in each data set. Finally, the relationship between the 24 background SC and the 24 XCD₀₅ values was empirically modeled to develop a background-to-criterion model. The least squares regression of XCD₀₅ values on log background SC (log Y = 0.658logX + 1.071) yields a strong linear relationship (r = 0.93). The regression model makes it possible to use SC background to predict the SC likely to extirpate the most salt-intolerant genera in an area. The results also suggest that species distribute along natural background gradients of SC and that this relationship can be used to develop criteria for ionic concentration.

Toxicity of various road-deicing salts to Asian clams (Corbicula fluminea)

Humans are altering environments by destroying habitats, introducing species, and releasing pollution. One emergent pollutant is the salinization of freshwater habitats from road-deicing salts. Government agencies have set thresholds to protect freshwater ecosystems, yet these values are exceeded in many systems. The present study investigated the tolerance of Asian clams (Corbicula fluminea), a common invasive bivalve, to the common road salt (sodium chloride [NaCl]) and 2 alternatives (magnesium chloride [MgCl₂ ] and calcium chloride [CaCl₂ ]). Experiments conducted at 4 and 8 d revealed that Asian clams are very salt tolerant. The median lethal concentration after 4 d of exposure (LC50_4-d ) estimate was 2162 mg Cl^- /L for MgCl₂ , 3554 mg Cl^- /L for CaCl₂ , and more than 22 581 mg Cl^- /L for NaCl, which were all significantly different from each other (p ≤ 0.05). The LC50_8-d values were significantly different (p ≤ 0.05) from each other and from the LC50_4-d values, and were estimated to be 1769 mg Cl^- /L for MgCl₂ , 2235 Cl^- /L for CaCl₂ , and 10 069 mg Cl^- /L for NaCl. Mortality was determined using 2 methods: either no response after exposure or no response after being in freshwater following exposure. For the majority of the LC50s, these methods were not significantly different (p > 0.05). The high salt tolerance of Asian clams is a concern because of their transportation in ballast water between aquatic ecosystems. Furthermore, salt-tolerant organisms may outcompete sensitive organisms in salinized ecosystems, which may alter ecosystem services. Environ Toxicol Chem 2018;37:1839-1845. © 2018 SETAC.

Increasing salinity drastically reduces hatching success of crustaceans from depression wetlands of the semi-arid Eastern Cape Karoo region, South Africa

Salinity is an important factor affecting freshwater aquatic species distribution and diversity. The semi-arid Eastern Cape Karoo region of South Africa has been earmarked for shale gas development through hydraulic fracturing. The process uses large amounts of water and produces briny wastewater. When not managed properly, these wastewaters may lead to salinisation of surface freshwater bodies in the region. Therefore, the effect of salinity on the hatching success of crustacean resting eggs was examined using sediments from four depression wetlands found in the region. The sediments were exposed for 28 days to salinity levels of 0.5 g L⁻¹, 2.5 g L⁻¹, 5 g L⁻¹ and 10 g L⁻¹. Control aquaria in which no salt was added were also set up. There was a significant decrease in the emerged taxa richness and abundances at salinities of 2.5 g L⁻¹ and above. Anostraca, Notostraca and Spinicaudata hatchlings were abundant at salinities of 0.5 g L⁻¹ and below, while Copepoda, Daphniidae (Cladocera) and Ostracoda were observed in the highest salinity, but their densities were still lower with increased salinities. Given the importance of large branchiopods in the trophic balance of depression wetlands, their loss may alter the ecological balance and function of these ecosystems.

The effects of hydraulic works and wetlands function in the Salado-River basin (Buenos Aires, Argentina)

Man-made activities exert great influences on fluvial ecosystems, with lowland rivers being substantially modified through agricultural land use and populations. The recent construction of drainage canals in the upper stretch of the Salado-River basin caused the mobilization of huge amounts of salts formerly stored in the groundwater. The main aim of this work was to analyze the effect of the discharges of those canals into the Salado-River water, under different hydrologic conditions, and the role of the wetlands and shallow lakes placed along the canals' system. Physicochemical variables were measured and water samples were taken during times of high water, mean flows, drought, and extreme drought. The environmental variables and the plankton development were related to the hydrologic regime and reached minimum values during floods because of low temperatures and dilution. Local effects on the water's ionic composition became pronounced during droughts because of groundwater input. Nutrient concentrations were mainly associated with point wastewater discharges. Conductivity, ion concentrations, total plankton biomass, and species richness increased in the Salado-River downstream site, after the canals' discharges. The artificial-drainage system definitely promotes the incorporation of salts into the Salado-River basin. In this scenario, a careful hydraulic management is needed to take into account this issue of secondary salinization that threatens the economic exploitation of the region. The wetlands present in this study acted as service environments not only helping to reduce salt, nutrient, and suspended-solid concentrations downstream but also contributing a plethora of species and plankton biomass into the Salado-River main course.

Stream salinization and fungal-mediated leaf decomposition: A microcosm study

Salinization is of major global concern due to its effect on stream biota, and ecosystem functions and services. In small streams, litter decomposition is a key ecosystem-level process driven by decomposers, mainly fungi (aquatic hyphomycetes), which link litter and invertebrates. Here we assessed the effects of an environmentally relevant range of salt additions (0, 2, 4, 8, 16gL^-1 NaCl) on (1) fungal growth and species-specific reproductive output and (2) fungal mediated-decomposition of Quercus robur leaves. Growth rates of eight out of nine species of aquatic hyphomycetes were negatively affected by salinity at concentrations ≥4gL^-1. EC50s were species-specific and ≥7.80gL^-1. Distinct thresholds were observed for reproduction: only five species sporulated at 2gL^-1, and a single one (Flagellospora curta) sporulated at 4 and 8gL^-1 NaCl. Based on these results, we evaluated if tolerant fungal assemblages, with increasingly fewer species (9, 5, 1), were able to maintain similar functional functions and processes at the different salt levels. No significant differences were found in oak mass loss or sporulation rates at 0 or 2gL^-1 NaCl; a clear inhibition of both parameters was observed at the highest concentrations (i.e., 4 and 8gL^-1 NaCl). Different dominance patterns in multi-species fungal assemblages may determine bottom-up impacts on the stream food webs through effects on detritivore feeding preferences. Specific growth rate, characterized by RNA concentration, was higher in the single species, at the highest salt-concentration, and lower in the 9-species assemblage. Respiration was almost 2-times higher in mixed assemblages without added salt. Under salt-contamination, trade-offs between growth and sporulation seem to guarantee high levels of fungal growth and decomposition, particularly in multi-species assemblages. In the presence of salt contamination, aquatic hyphomycetes, even at reduced diversity, remain important drivers of leaf decomposition and ensure organic matter recycling.

Evolution to environmental contamination ablates the circadian clock of an aquatic sentinel species

Environmental contamination is a common cause of rapid evolution. Recent work has shown that Daphnia pulex, an important freshwater species, can rapidly evolve increased tolerance to a common contaminant, sodium chloride (NaCl) road salt. While such rapid evolution can benefit organisms, allowing them to adapt to new environmental conditions, it can also be associated with unforeseen tradeoffs. Given that exposure to environmental contaminants can cause circadian disruption, we investigated whether the circadian clock was affected by evolving a tolerance to high levels of road salt. By tracking the oscillations of a putative clock gene, period, we demonstrated that D. pulex express per mRNA with approximately 20-hr oscillations under control conditions. This putative circadian rhythm was ablated in response to high levels of salinity; populations adapted to high NaCl concentrations exhibited an ablation of period oscillation. Moreover, we showed that while gene expression is increased in several other genes, including clock, actin, and Na⁺/K⁺-ATPase, upon the adaptation to high levels of salinity, per expression is unique among the genes we tracked in that it is the only gene repressed in response to salt adaptation. These results suggest that rapid evolution of salt tolerance occurs with the tradeoff of suppressed circadian function. The resultant circadian disruption may have profound consequences to individuals, populations, and aquatic food webs by affecting species interactions. In addition, our research suggests that circadian clocks may also be disrupted by the adaptation to other environmental contaminants.

Species Diversity Hinders Adaptation to Toxicants

Environmental toxicants such as pesticides exert strong selection pressure on many species. While the resulting development of pesticide resistance in agricultural pest species is well-known, reports on the extent of adaptation in exposed nontarget species are contradictory. These contradictory reports highlight a continuing challenge in understanding the relevant ecological mechanisms that facilitate or hinder adaptation to toxicants in the field. Here we show that species diversity hinders the adaptation to toxicants. In agricultural streams with low diversity, we observed an up to 8-fold increase in insecticide tolerance in a total of 17 macroinvertebrate species that was not observed in more diverse communities under similar contamination. High species diversity occurred independently from adjacent nonpolluted refuge areas. Therefore, the low level of adaptation in diverse streams cannot be explained by an increased recolonization of sensitive individuals from refuge areas. Instead, high intraspecific competition may facilitate the selection for increased tolerance in low-diverse communities. In diverse communities, by contrast, species interactions may reduce intraspecific competition and, thus, the potential for developing toxicant resistance. We suggest that this mechanism may be the general case in adaptation to environmental stressors. Additionally, we conclude that the current framework for risk assessment of pesticides is not protective against selection for tolerant organisms and the associated risk of genetic erosion.

Interacting stressors and the potential for adaptation in a changing world: responses of populations and individuals

To accurately predict the impact of environmental change, it is necessary to assay effects of key interacting stressors on vulnerable organisms, and the potential resiliency of their populations. Yet, for the most part, these critical data are missing. We examined the effects of two common abiotic stressors predicted to interact with climate change, salinity and temperature, on the embryonic survival and development of a model freshwater vertebrate, the rough-skinned newt (Taricha granulosa) from different populations. We found that salinity and temperature significantly interacted to affect newt embryonic survival and development, with the negative effects of salinity most pronounced at temperature extremes. We also found significant variation among, and especially within, populations, with different females varying in the performance of their eggs at different salinity-temperature combinations, possibly providing the raw material for future natural selection. Our results highlight the complex nature of predicting responses to climate change in space and time, and provide critical data towards that aim.

Effects of potash mining on river ecosystems: An experimental study

In spite of being a widespread activity causing the salinization of rivers worldwide, the impact of potash mining on river ecosystems is poorly understood. Here we used a mesocosm approach to test the effects of a salt effluent coming from a potash mine on algal and aquatic invertebrate communities at different concentrations and release modes (i.e. press versus pulse releases). Algal biomass was higher in salt treatments than in control (i.e. river water), with an increase in salt-tolerant diatom species. Salt addition had an effect on invertebrate community composition that was mainly related with changes in the abundance of certain taxa. Short (i.e. 48 h long) salt pulses had no significant effect on the algal and invertebrate communities. The biotic indices showed a weak response to treatment, with only the treatment with the highest salt concentration causing a consistent (i.e. according to all indices) reduction in the ecological quality of the streams and only by the end of the study. Overall, the treatment's effects were time-dependent, being more clear by the end of the study. Our results suggest that potash mining has the potential to significantly alter biological communities of surrounding rivers and streams, and that specific biotic indices to detect salt pollution should be developed.

Salinization triggers a trophic cascade in experimental freshwater communities with varying food-chain length

The application of road deicing salts in northern regions worldwide is changing the chemical environment of freshwater ecosystems. Chloride levels in many lakes, streams, and wetlands exceed the chronic and acute thresholds established by the United States and Canada for the protection of freshwater biota. Few studies have identified the impacts of deicing salts in stream and wetland communities and none have examined impacts in lake communities. We tested how relevant concentrations of road salt (15, 100, 250, 500, and 1000 mg Cl^- /L) interacted with experimental communities containing two or three trophic levels (i.e., no fish vs. predatory fish). We hypothesized that road salt and fish would have a negative synergistic effect on zooplankton, which would then induce a trophic cascade. We tested this hypothesis in outdoor mesocosms containing filamentous algae, periphyton, phytoplankton, zooplankton, several macroinvertebrate species, and fish. We found that the presence of fish and high salt had a negative synergistic effect on the zooplankton community, which in turn caused an increase in phytoplankton. Contributing to the magnitude of this trophic cascade was a direct positive effect of high salinity on phytoplankton abundance. Cascading effects were limited with respect to impacts on the benthic food web. Periphyton and snail grazers were unaffected by the salt-induced trophic cascade, but the biomass of filamentous algae decreased as a result of competition with phytoplankton for light or nutrients. We also found direct negative effects of high salinity on the biomass of filamentous algae and amphipods (Hyalella azteca) and the mortality of banded mystery snails (Viviparus georgianus) and fingernail clams (Sphaerium simile). Clam mortality was dependent on the presence of fish, suggesting a non-consumptive interactive effect with salt. Our results indicate that globally increasing concentrations of road salt can alter community structure via both direct and indirect effects.

Rapid evolution of tolerance to road salt in zooplankton

Organisms around the globe are experiencing novel environments created by human activities. One such disturbance of growing concern is the salinization of freshwater habitats from the application of road deicing salts, which creates salinity levels not experienced within the recent evolutionary history of most freshwater organisms. Moreover, salinization can induce trophic cascades and alter the structure of freshwater communities, but knowledge is still scarce about the ability of freshwater organisms to adapt to elevated salinity. We examined if a common zooplankton of freshwater lakes (Daphnia pulex) could evolve a tolerance to the most commonly used road deicing salt (sodium chloride, NaCl). Using a mesocosm experiment, we exposed freshwater communities containing Daphnia to five levels of NaCl (15, 100, 200, 500, and 1000 mg Cl^- L^-1). After 2.5 months, we collected Daphnia from each mesocosm and raised them in the lab for three generations under low salt conditions (15 mg Cl^- L^-1). We then conducted a time-to-death experiment with varying concentrations of NaCl (30, 1300, 1500, 1700, 1900 mg Cl^- L^-1) to test for evolved tolerance. All Daphnia populations exhibited high survival when subsequently exposed to the lowest salt concentration (30 mg Cl^- L^-1). At the intermediate concentration (1300 mg Cl^- L^-1), however, populations previously exposed to elevated concentrations (i.e.100-1000 mg Cl^- L^-1) had higher survival than populations previously exposed to natural background levels (15 mg Cl^- L^-1). All populations survived poorly when subsequently exposed to the highest concentrations (1500, 1700, and 1900 mg Cl^- L^-1). Our results show that the evolution of tolerance to moderate levels of salt can occur within 2.5 months, or 5-10 generations, in Daphnia. Given the importance of Daphnia in freshwater food webs, such evolved tolerance might allow Daphnia to buffer food webs from the impacts of freshwater salinization.

Chloride and sulphate toxicity to Hydropsyche exocellata (Trichoptera, Hydropsychidae): Exploring intraspecific variation and sub-lethal endpoints

The rivers and streams of the world are becoming saltier due to human activities. In spite of the potential damage that salt pollution can cause on freshwater ecosystems, this is an issue that is currently poorly managed. Here we explored intraspecific differences in the sensitivity of freshwater fauna to two major ions (Cl(-) and SO4(2-)) using the net-spinning caddisfly Hydropsyche exocellata Dufour 1841 (Trichoptera, Hydropsychidae) as a model organism. We exposed H. exocellata to saline solutions (reaching a conductivity of 2.5mScm(-1)) with Cl(-):SO4(2-) ratios similar to those occurring in effluents coming from the meat, mining and paper industries, which release dissolved salts to rivers and streams in Spain. We used two different populations, coming from low and high conductivity streams. To assess toxicity, we measured sub-lethal endpoints: locomotion, symmetry of the food-capturing nets and oxidative stress biomarkers. According to biomarkers and net building, the population historically exposed to lower conductivities (B10) showed higher levels of stress than the population historically exposed to higher conductivities (L102). However, the differences between populations were not strong. For example, net symmetry was lower in the B10 than in the L102 only 48h after treatment was applied, and biomarkers showed a variety of responses, with no discernable pattern. Also, treatment effects were rather weak, i.e. only some endpoints, and in most cases only in the B10 population, showed a significant response to treatment. The lack of consistent differences between populations and treatments could be related to the high salt tolerance of H. exocellata, since both populations were collected from streams with relatively high conductivities. The sub-lethal effects tested in this study can offer an interesting and promising tool to monitor freshwater salinization by combining physiological and behavioural bioindicators.

Rapid changes in water hardness and alkalinity: Calcite formation is lethal to Daphnia magna

There is growing concern that freshwater ecosystems may be negatively affected by ever-increasing anthropogenic inputs of extremely hard, highly alkaline effluent containing large quantities of Ca(2+), Mg(2+), CO3(2-), and HCO3(-) ions. In this study, the toxicity of rapid and extreme shifts in water hardness (38-600mg/L as CaCO3) and alkalinity (30-420mg/L as CaCO3) to Daphnia magna was tested, both independently and in combination. Within these ranges, where no precipitation event occurred, shifts in water hardness and/or alkalinity were not toxic to D. magna. In contrast, 98-100% of D. magna died within 96h after exposure to 600mg/L as CaCO3 water hardness and 420mg/L as CaCO3 alkalinity (LT50 of 60h with a 95% CI of 54.2-66.0h). In this treatment, a CaCO3 (calcite) precipitate formed in the water column which was ingested by and thoroughly coated the D. magna. Calcite collected from a mining impacted stream contained embedded organisms, suggesting field streams may also experience similar conditions and possibly increased mortality as observed in the lab tests. Although further investigation is required to determine the exact fate of aquatic organisms exposed to rapid calcite precipitation in the field, we caution that negative effects may occur more quickly or at lower concentrations of water hardness and alkalinity in which we observed effects in D. magna, because some species, such as aquatic insects, are more sensitive than cladocerans to changes in ionic strength. Our results provide evidence that both calcite precipitation and the major ion balance of waters should be managed in industrially affected ecosystems and we support the development of a hardness+alkalinity guideline for the protection of aquatic life.