Effects of anthropogenic salinization on biological traits and community composition of stream macroinvertebrates

Coping with salinity extremes: Gill transcriptome profiling in the black-chinned tilapia (Sarotherodon melanotheron)

Steeper and sometimes extreme salinity gradients increasingly affect aquatic organisms because of climate change. Hypersalinity habitats demand powerful physiological adaptive strategies. Few teleost species have the capacity to spend their whole life cycle in salinities way over seawater levels. Focusing on the multifunctional gill, we unraveled the tilapia S. melanotheron key strategies to cope with different environmental conditions, ranging from freshwater up to hypersaline habitats. De novo transcriptome assembly based on RNAseq allowed for the analysis of 40,967 annotated transcripts among samples collected in three wild populations at 0, 40 and 80 ‰. A trend analysis of the expression patterns revealed responses across the salinity gradient with different gene pathways involved. Genes linked to ion transport, pH regulation and cell surface receptor signaling were mainly upregulated in the high salinity habitat. We identified tight junction proteins that were critical in high salinity habitats and that were different from the well-known tightening junctional proteins identified and expressed in fresh water. Expression profiles also suggest a change in the vascular tone that could be linked to an osmorespiratory compromise not only in fresh water, but also in high salinity environments. A striking downregulation of genes linked to the immune system and to the heat shock response was observed suggesting an energetic trade-off between immunity and acclimation/adaptation in the hypersaline habitat. The high expression of transcripts coding for immune and heat shock response in the freshwater habitat suggests the establishment of powerful mechanisms to protect gills from environmental threats and to maintain protein integrity. Non-directional expression trends were also detected with an upregulation of genes only in the hypersaline habitat (80 ‰) or only in the marine habitat (40 ‰). Unravel physiological strategies in S. melanotheron populations will help to better understand the molecular basis of fish euryhalinity in salinity-contrasted environments.

Assessing the response of an urban stream ecosystem to salinization under different flow regimes

Urban streams are exposed to a variety of anthropogenic stressors. Freshwater salinization is a key stressor in these ecosystems that is predicted to be further exacerbated by climate change, which causes simultaneous changes in flow parameters, potentially resulting in non-additive effects on aquatic ecosystems. However, the effects of salinization and flow velocity on urban streams are still poorly understood as multiple-stressor experiments are often conducted at pristine rather than urban sites. Therefore, we conducted a mesocosm experiment at the Boye River, a recently restored stream located in a highly urbanized area in Western Germany, and applied recurrent pulses of salinity along a gradient (NaCl, 9 h daily of +0 to +2.5 mS/cm) in combination with normal and reduced current velocities (20 cm/s vs. 10 cm/s). Using a comprehensive assessment across multiple organism groups (macroinvertebrates, eukaryotic algae, fungi, parasites) and ecosystem functions (primary production, organic-matter decomposition), we show that flow velocity reduction has a pervasive impact, causing community shifts for almost all assessed organism groups (except fungi) and inhibiting organic-matter decomposition. Salinization affected only dynamic components of community assembly by enhancing invertebrate emigration via drift and reducing fungal reproduction. We caution that the comparatively small impact of salt in our study can be due to legacy effects from past salt pollution by coal mining activities >30 years ago. Nevertheless, our results suggest that urban stream management should prioritize the continuity of a minimum discharge to maintain ecosystem integrity. Our study exemplifies a holistic approach for the assessment of multiple-stressor impacts on streams, which is needed to inform the establishment of a salinity threshold above which mitigation actions must be taken.

Evaluating macroinvertebrate metrics for ecological assessment of large saline rivers (Argentina)

Benthic macroinvertebrates have been used around the world as indicators of the biological quality of freshwater habitats. Because of the intensive deterioration of waterbodies as a result of different land uses, indicators are used for environmental monitoring, control and remediation. The aim of this study was to assess (1) the sensitivity of taxonomical metrics and (2) functional traits to select the most appropriate for evaluating environmental impacts on rivers with high salinity and (3) to propose a multimetric index based on the selected metrics. Information from a preexisting database on twenty-eight sites in the Salado River basin (Argentina) was used. One hundred and twenty-three metrics were calculated to assess sensitivity to different land uses along the gradient of habitat condition, from low-disturbed (reference), to medium-disturbed (agricultural and industrial) and high-disturbed (agricultural, industrial and urban). This gradient was defined by available information in original articles and by quantifying the percentage of the different land uses. Filtering collectors (%), Gathering collectors (%), Ostracoda density, Limnodrilus hoffmeisteri/Total density, Naididae (%), Tubifex/Total density and Pristina/Total density were the metrics that distinguished the different land uses along the gradient of habitat condition. These metrics were used to propose a macroinvertebrate multimetric index in saline rivers: Index of Benthic Invertebrates in Saline Rivers (IBIS). Thus, this study provides a useful tool for management and monitoring of saline rivers and diagnoses of salinized environments.

Salt pulses effects on in-stream litter processing and recovery capacity depend on substrata quality

Human activities have greatly extended and intensified freshwater salinization, which threatens the structure and functioning of streams and rivers. Research on salt effects on in-stream processes has been strongly biased towards chronic salinization at constant levels. The aim of this study was to assess microbial mediated decomposition of two leaf species contrasting in quality (alder and oak) and associated descriptors, during salt-pulsed contamination (salinization period) and after cessation of salt additions (recovery period). Leaves were incubated in a mountain stream (central Portugal) longitudinally divided over 22 m. Half of the stream (salinized half) was subjected to daily short-term sharp salinity increases (conductivity up to ~48 mS cm^-1) during 7 days while the other half (control half) was used as control. During the salinization period, salt exposure negatively affected mass loss and microbial respiration rate of alder (high-quality resource) while effects on fungal sporulation rate were independent of leaf quality. Fungal biomass was not impacted. After the recovery period, mass loss and respiration rate in both leaf species were similar between experimental stream halves. Fungal biomass associated with oak was enhanced and sporulation rate of alder, maintained in the previously salinized half, remained depressed. These results point out that the effects of salt pulses may be more deleterious in streams exclusively lined by high (vs. low) quality riparian trees as a result of a less efficient microbial-mediated leaf processing, and a reduced contribution to the conidial pool, even beyond the salinization period.

Assessing the ecological risk of active principles used currently by freshwater fish farms

The global aquaculture industry has grown exponentially in recent years using to control of infections and diseases, a variety of veterinary drugs (VMP) are used, including antibiotics, antifungals and antiparasitics, which have different routes of emission, environmental persistence and side effects to aquatic organisms, becoming one of the main concerns in its use of veterinary drugs (VMP) and its potential toxicological impact on the environment, in this context, Chile is considered one of the main salmon producers. Ecological risk assessment of active principles used infreshwater fish farms worldwide and in Chile were investigated. We recollect a physical - chemical properties of active principles used by fish farms and we could estimate the relative hazard a priori. Later active principles grouped as antibiotics (n = 6), antiparasitics (n = 5), anesthetics (n = 3), and disinfectants (n = 7) were assessed using a mass balance model based on fugacity was developed for each active principle under treatments via immersion and food administration in fish, while a volumetric model for disinfectants and sodium chloride was used for estimating the predicted environmental concentration (PEC), under a real smolt farming scenario in fish farms. Ecotoxicological data were collected from open literature to predict the no-effect concentration (PNEC). The ecological risk assessment was characterized using a risk quotient (RQ = PEC/PNEC) based in two assessment tiers. Results revealed that 12 active ingredients showed a high risk (RQ ≥ 1), thus indicating that adverse effects could occur and further investigation with measured concentrations in the field are required to reduce exposure in surface waters.

Evolutionary mismatch along salinity gradients in a Neotropical water strider

The evolution of local adaptation is crucial for the in situ persistence of populations in changing environments. However, selection along broad environmental gradients could render local adaptation difficult, and might even result in maladaptation. We address this issue by quantifying fitness trade-offs (via common garden experiments) along a salinity gradient in two populations of the Neotropical water strider Telmatometra withei-a species found in both fresh (FW) and brackish (BW) water environments across Panama. We found evidence for local adaptation in the FW population in its home FW environment. However, the BW population showed only partial adaptation to the BW environment, with a high magnitude of maladaptation along naturally occurring salinity gradients. Indeed, its overall fitness was ~60% lower than that of the ancestral FW population in its home environment, highlighting the role of phenotypic plasticity, rather than local adaptation, in high salinity environments. This suggests that populations seemingly persisting in high salinity environments might in fact be maladapted, following drastic changes in salinity. Thus, variable selection imposed by salinization could result in evolutionary mismatch, where the fitness of a population is displaced from its optimal environment. Understanding the fitness consequences of persisting in fluctuating salinity environments is crucial to predict the persistence of populations facing increasing salinization. It will also help develop evolutionarily informed management strategies in the context of global change.

Development and First Tests of a Lab-Scale Electric Field for Investigating Potential Effects of Electric Barriers on Aquatic Invasive Invertebrates

Canals and other connected waterway systems, including the Chicago Area Waterway System (CAWS), have often facilitated the spread of non-native species. Electric barriers have recently emerged as a method for preventing this spread and protecting uninvaded ecosystems from new invaders. The largest system of electric barriers in the world is in the CAWS and is operated primarily to prevent the spread of invasive Asian carp. It is not known whether these barriers are effective for other species, particularly invasive invertebrates. Here, we provide data regarding the efficacy of an electric field that operates at the same parameters as the electric barrier in the CAWS in affecting behaviors of two invertebrate species, the red swamp crayfish Procambarus clarkii and the amphipod Hyalella azteca. We constructed an electric field within a tank that operates at the same parameters as the existing CAWS barriers and determined the effects of the electric field on our test species. At the electric field parameters of the CAWS barriers, the vast majority of P. clarkii individuals showed altered movement with maintained equilibrium. For H. azteca, behavioral responses were less extreme than for P. clarkii, with a majority of individuals experiencing altered movement. By measuring the orientation of organisms to the electric field, we determined that the test organisms are affected by the electric field, especially at lower field strengths where they exhibited no or little other behavioral response. At lower field strengths, P. clarkii exhibited changes in orientation, but at higher field strengths, individuals were less able to orient themselves. H. azteca exhibited changes in orientation to the electric field at all field strengths. The results of this study suggest that the existing electric barriers may not slow or prevent spread of invasive invertebrates—including amphipods and crayfish—through passive movement attached boats/barges or through downstream drift, but that the barriers may prevent spread by active upstream movement. Overall, our work gives new data regarding the efficacy of electric fields in preventing the spread of invasive invertebrates and can inform management decisions regarding current and future electric barriers in the CAWS.

Freshwater salinization increases survival of Escherichia coli and risk of bacterial impairment

Elevated levels of Escherichia coli (E. coli) are responsible for more designated freshwater stream impairments than any other contaminant in the United States. E. coli are intentionally used as a sentinel of fecal contamination for freshwaters because previous research indicates that salt concentrations in brackish or marine waters reduce E. coli survival, rendering it a less effective indicator of public health risks. Given increasing evidence of freshwater salinization associated with upland anthropogenic land-use, understanding the effects on fecal indicators is critical; however, changes in E. coli survival along the freshwater salinity range (≤ 1500 µS cm^-1) have not been previously examined. Through a series of controlled mesocosm experiments, we provide direct evidence that salinization causes E. coli survival rates in freshwater to increase at conductivities as low as 350 µS cm^-1 and peak at 1500 µS cm^-1, revealing a subsidy-stress response across the freshwater-marine continuum. Furthermore, specific base cations affect E. coli survival differently, with Mg²⁺ increasing E. coli survival rates relative to other chloride salts. Further investigation of the mechanisms by which freshwater salinization increases susceptibility to or exacerbates bacterial water quality impairments is recommended. Addressing salinization with nuanced approaches that consider salt sources and chemistry could assist in prioritizing and addressing bacterial water quality management.

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.

Impacts to water quality and biota persist in mining-influenced Appalachian streams

Elevated dissolved major ions (salinization) from surface coal mining are a common impact to central Appalachian headwater streams. Salinization is associated with alterations of benthic macroinvertebrate communities, as many organisms are adapted to the naturally dilute streams of the region. These geochemical and biological alterations have been observed in streams decades after mining, but it remains unclear whether and at what rate water quality and aquatic biota recover after mining. To address this issue, we analyzed temporal trends in specific conductance (SC), ion matrix ratios, and benthic macroinvertebrate communities over an eight-year period in 23 headwater streams, including 18 salinized by surface coal mining. We found strong, negative correlations between SC and diversity of benthic macroinvertebrate communities. Temporal trend analysis demonstrated limited recovery of water chemistry to natural background conditions. Five of the 18 mining-influenced streams exhibited declining SC; however, annual rates of decline in these streams ranged from 1.9% to 3.7% of mean annual SC, suggesting long time periods will be required to reach established benchmark values (ca. 25 years) or values observed in our five reference study streams (ca. 40 years). Similarly, there was limited evidence for recovery of macroinvertebrate community metrics, even in the few mining-influenced streams with decreasing SC. These findings indicate that salinization and its biological effects persist, likely for decades, in central Appalachian headwater streams. Our work also highlights the value of long-term monitoring data for assessing recovery potential of salinized freshwaters, as well as the need for improved understanding of water quality and biological recovery processes and time frames.

Flow-Mediated Vulnerability of Source Waters to Elevated TDS in an Appalachian River Basin

Widespread salinization—and, in a broader sense, an increase in all total dissolved solids (TDS)—is threatening freshwater ecosystems and the services they provide (e.g., drinking water provision). We used a mixed modeling approach to characterize long-term (2010–2018) spatio-temporal variability in TDS within the Monongahela River basin and used this information to assess the extent and drivers of vulnerability. The West Fork River was predicted to exceed 500 mg/L a total of 133 days. Occurrence and duration (maximum = 28 days) of—and thus vulnerability to—exceedances within the West Fork River were driven by low flows. Projected decreases in mean daily discharge by ≤10 cfs resulted in an additional 34 days exceeding 500 mg/L. Consistently low TDS within the Tygart Valley and Cheat Rivers reduced vulnerability of the receiving Monongahela River to elevated TDS which was neither observed (maximum = 419 mg/L) nor predicted (341 mg/L) to exceed the secondary drinking water standard of 500 mg/L. Potential changes in future land use and/or severity of low-flow conditions could increase vulnerability of the Monongahela River to elevated TDS. Management should include efforts to increase assimilative capacity by identifying and decreasing sources of TDS. Upstream reservoirs could be managed toward low-flow thresholds; however, further study is needed to ensure all authorized reservoir purposes could be maintained.

Geographical origin determines responses to salinity of Mediterranean caddisflies

Many freshwater ecosystems worldwide, and particularly Mediterranean ones, show increasing levels of salinity. These changes in water conditions could affect abundance and distribution of inhabiting species as well as the provision of ecosystem services. In this study we conduct laboratory experiments using the macroinvertebrate Smicridea annulicornis as a model organism. Our factorial experiments were designed to evaluate the effects of geographical origin of organisms and salinity levels on survival and behavioral responses of caddisflies. The experimental organisms were captured from rivers belonging to three hydrological basins along a 450 Km latitudinal gradient in the Mediterranean region of Chile. Animals were exposed to three conductivity levels, from 180 to 1400 μS/cm, close to the historical averages of the source rivers. We measured the behavioral responses to experimental stimuli and the survival time. Our results showed that geographical origin shaped the behavioral and survival responses to salinity. In particular, survival and activity decreased more strongly with increasing salinity in organisms coming from more dilute waters. This suggests local adaptation to be determinant for salinity responses in this benthic invertebrate species. In the current scenario of fast temporal and spatial changes in water levels and salt concentration, the conservation of geographic intra-specific variation of aquatic species is crucial for lowering the risk of salinity-driven biodiversity loss.

Response of the mollusc communities to environmental factors along an anthropogenic salinity gradient

Anthropogenic salinisation of freshwater ecosystems is frequent across the world. The scale of this phenomenon remains unrecognised, and therefore, monitoring and management of such ecosystems is very important. We conducted a study on the mollusc communities in inland anthropogenic ponds covering a large gradient of salinity located in an area of underground coal mining activity. A total of 14 gastropod and 6 bivalve species were noted. No molluscs were found in waters with total dissolved solids (TDS) higher than 17.1 g L^-1. The share of alien species in the communities was very high in waters with elevated salinity and significantly lower in the freshwaters. Canonical correspondence analysis (CCA) showed that TDS, pH, alkalinity, nitrate nitrogen, ammonium nitrogen, iron, the content of organic matter in sediments, the type of substrate and the content of sand and gravel in sediments were the variables that were significantly associated with the distribution of molluscs. The regression analysis revealed that total mollusc density was positively related to alkalinity and negatively related to nitrate nitrogen. The taxa richness was negatively related to TDS, which is consistent with previous studies which indicated that a high salinity level is a significant threat to freshwater malacofauna, causing a loss of biodiversity and contributing to the colonisation and establishment of alien species in aquatic ecosystems.

Leaf litter microbial decomposition in salinized streams under intermittency

Human-induced salinization of freshwaters constitutes a growing global problem, whose consequences on streams functioning are largely unknown. Climate change projections predict enhanced evaporation, as well as an increase in extreme events and in variability of precipitation. This will result in more frequent, extended and severe drought periods that may aggravate water salinization of streams and rivers. In this study we conducted a microcosm experiment to assess the combined effects of three drought regimes - abrupt (AD), slow (SD) and very slow transition to dryness (VSD) - and three levels of salinization (0, 4, 6 g L^-1 NaCl) on microbial-mediated oak leaf decomposition over ten weeks. Salinization did not affect mass loss and associated microbial respiration of colonized oak leaves but significantly reduced the biomass and eliminated the sporulating capacity of fungi. Desiccation negatively affected leaf decomposition regardless of regime. Even though microbial respiration did not react to the different treatments, lower fungal biomass, diversity, and conidial production were observed under AD; for fungal biomass these effects were amplified at higher salt concentrations (particularly at 6 g L^-1). Our results indicate that effects of leaf litter desiccation depend on the rate of transition between wet and dry conditions and on the level of salt in the water. The two factors jointly affect decomposer survival and activity and, by extension, the dynamics of detrital food webs in streams.

Are fungal strains from salinized streams adapted to salt-rich conditions?

Anthropogenic salinization of freshwater is a global problem with largely unknown consequences for stream functions. We compared the effects of salt addition (6 g l^-1 NaCl) in microcosms on leaf mass loss and microbial parameters in single- and multispecies assemblages of fungal strains (Heliscus lugdunensis, HELU; Tetracladium marchalianum, TEMA; Flagellospora curta, FLCU) isolated from a reference (R) or salinized (S) stream. Fungal growth and interactions were also assessed. Salinization inhibited leaf decomposition and fungal biomass, but no differences were observed between species, strains or species combinations. Sporulation rates in monocultures were not affected by added salt, but differed among species (FLCU > HELU > TEMA), with S strains releasing more conidia. Fungal assemblages did not differ significantly in total conidia production (either between strains or medium salt concentration). HELU was the dominant species, which also had highest growth and most pronounced antagonistic behaviour. Fungal species, irrespective of origin, largely maintained their function in salinized streams. Strains from salt-contaminated streams did not trade-off conidial production for vegetative growth at high salt levels. The expected reduction of fungal diversity and potential changes in nutritional litter quality owing to salinization may impact leaf incorporation into secondary production in streams.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Do all roads lead to Rome? Exploring community trajectories in response to anthropogenic salinization and dilution of rivers

Abiotic stress shapes how communities assemble and support ecological functions. However, it remains unclear whether artificially increasing or decreasing stress levels would lead to communities assembling predictably along a single axis of variation or along multiple context-dependent trajectories of change. In response to stress intensity alterations, we hypothesize that a single trajectory of change occurs when trait-based assembly prevails, while multiple trajectories of change arise when dispersal-related processes modify colonization and trait-filtering dynamics. Here, we tested these hypotheses using aquatic macroinvertebrates from rivers exposed to gradients of natural salinity and artificially diluted or salinized ion contents. Our results showed that trait-filtering was important in driving community assembly in natural and diluted rivers, while dispersal-related processes seemed to play a relevant role in response to salinization. Salinized rivers showed novel communities with different trait composition, while natural and diluted communities exhibited similar taxonomic and trait compositional patterns along the conductivity gradient. Our findings suggest that the artificial modification of chemical stressors can result in different biological communities, depending on the direction of the change (salinization or dilution), with trait-filtering, and organism dispersal and colonization dynamics having differential roles in community assembly. The approach presented here provides both empirical and conceptual insights that can help in anticipating the ecological effects of global change, especially for those stressors with both natural and anthropogenic origins.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Biological interactions mediate context and species-specific sensitivities to salinity

Toxicants have both sub-lethal and lethal effects on aquatic biota, influencing organism fitness and community composition. However, toxicant effects within ecosystems may be altered by interactions with abiotic and biotic ecosystem components, including biological interactions. Collectively, this generates the potential for toxicant sensitivity to be highly context dependent, with significantly different outcomes in ecosystems than laboratory toxicity tests predict. We experimentally manipulated stream macroinvertebrate communities in 32 mesocosms to examine how communities from a low-salinity site were influenced by interactions with those from a high-salinity site along a gradient of salinity. Relative to those from the low-salinity site, organisms from the high-salinity site were expected to have greater tolerance and fitness at higher salinities. This created the potential for both salinity and tolerant-sensitive organism interactions to influence communities. We found that community composition was influenced by both direct toxicity and tolerant-sensitive organism interactions. Taxon and context-dependent responses included: (i) direct toxicity effects, irrespective of biotic interactions; (ii) effects that were owing to the addition of tolerant taxa, irrespective of salinity; (iii) toxicity dependent on sensitive-tolerant taxa interactions; and (iv) toxic effects that were increased by interactions. Our results reinforce that ecological processes require consideration when examining toxicant effects within ecosystems.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Predicting current and future background ion concentrations in German surface water under climate change

Salinization of surface waters is a global environmental issue that can pose a regional risk to freshwater organisms, potentially leading to high environmental and economic costs. Global environmental change including climate and land use change can increase the transport of ions into surface waters. We fit both multiple linear regression (LR) and random forest (RF) models on a large spatial dataset to predict Ca²⁺ (266 sites), Mg²⁺ (266 sites), and [Formula: see text] (357 sites) ion concentrations as well as electrical conductivity (EC-a proxy for total dissolved solids with 410 sites) in German running water bodies. Predictions in both types of models were driven by the major factors controlling salinity including geologic and soil properties, climate, vegetation and topography. The predictive power of the two types of models was very similar, with RF explaining 71-76% of the spatial variation in ion concentrations and LR explaining 70-75% of the variance. Mean squared errors for predictions were all smaller than 0.06. The factors most strongly associated with stream ion concentrations varied among models but rock chemistry and climate were the most dominant. The RF model was subsequently used to forecast the changes in EC that were likely to occur for the period of 2070 to 2100 in response to just climate change-i.e. no additional effects of other anthropogenic activities. The future forecasting shows approximately 10% and 15% increases in mean EC for representative concentration pathways 2.6 and 8.5 (RCP2.6 and RCP8.5) scenarios, respectively.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Effects of secondary salinisation on macroinvertebrate functional traits in surface mining-contaminated streams, and recovery potential

Secondary salinisation has become a hot spot internationally due to its adverse effects on freshwater ecosystems. Although its effects on ecosystem patterns has been broadly studied, its potential effect on ecosystem functions, in particular on the functional traits of freshwater organisms, and functional trait recovery are largely unknown. In this study, we conducted a field investigation at 405 sample sites from May 2009 to July 2016 in surface mining-contaminated streams, in order to evaluate the influence of secondary salinisation on macroinvertebrate functional traits and the recovery potential of dominant functional traits. Results of univariate models showed that sensitive, very tolerant, gill-breathers, cutaneous-breathers, shredders, predators and gatherers were the most responsive indicators to enhanced specific conductivity and sulfate loadings with sensitive, gill-breathers, shredders and predators demonstrating a reduction in abundance, whereas cutaneous-breathers and gatherers exhibiting an increase. Complicated relationships among different species indicated that co-exclusions would not occur because all macroinvertebrate taxa exhibited positive correlations. Results of relative recovery potential showed that omnivores and gatherers recovered quickly following improvements in water quality, whereas gill-breathers, pneumostome-breathers, filterers and scrapers would be expected to recover slowly due to their sensitivity to both specific conductivity and sulfate and low drift propensity. Overall, secondary salinisation has posed severely ecological risks to macroinvertebrate functional attributes in surface mining-contaminated streams, and their effects should be considered in future conservation plans.

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.

Sensitivity ranking for freshwater invertebrates towards hydrocarbon contaminants

Hydrocarbons have an utmost economical importance but may also cause substantial ecological impacts due to accidents or inadequate transportation and use. Currently, freshwater biomonitoring methods lack an indicator that can unequivocally reflect the impacts caused by hydrocarbons while being independent from effects of other stressors. The aim of the present study was to develop a sensitivity ranking for freshwater invertebrates towards hydrocarbon contaminants, which can be used in hydrocarbon-specific bioindicators. We employed the Relative Sensitivity method and developed the sensitivity ranking S _hydrocarbons based on literature ecotoxicological data supplemented with rapid and mesocosm test results. A first validation of the sensitivity ranking based on an earlier field study has been conducted and revealed the S _hydrocarbons ranking to be promising for application in sensitivity based indicators. Thus, the first results indicate that the ranking can serve as the core component of future hydrocarbon-specific and sensitivity trait based bioindicators.

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

Anthropogenic salinization of rivers is an emerging issue of global concern, with significant adverse effects on biodiversity and ecosystem functioning. Impacts of freshwater salinization on biota are strongly mediated by evolutionary history, as this is a major factor determining species physiological salinity tolerance. Freshwater insects dominate most flowing waters, and the common lotic insect orders Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) are particularly salt-sensitive. Tolerances of existing taxa, rapid adaption, colonization by novel taxa (from naturally saline environments) and interactions between species will be key drivers of assemblages in saline lotic systems. Here we outline a conceptual framework predicting how communities may change in salinizing rivers. We envision that a relatively small number of taxa will be saline-tolerant and able to colonize salinized rivers (e.g. most naturally saline habitats are lentic; thus potential colonizers would need to adapt to lotic environments), leading to depauperate communities in these environments.

Effects of repeated salt pulses on ecosystem structure and functions in a stream mesocosm

Rivers and streams affected by mining activities often receive short-term sharp salinity increases due to water-soluble stockpiled materials being washed into receiving water bodies. We conducted a mesocosm study to explore the response of structural (diatom and stream invertebrate communities) and functional descriptors (chlorophyll a concentration, fungal biomass and leaf decomposition) to repeated short salinity pulses (3h of duration, with nominal electrical conductivities of 5, 10 and 15 mS cm(-1)), mimicking the exposure pattern occurring at salt-mine affected rivers. The experiment was conducted in 12 artificial flow-through stream systems over 16 days. The effect of the salt pulses on the ecosystem structure and functioning did not fully match most of our initial hypotheses, with the community response being weaker than predicted. The diatom community was, however, dominated by salt-tolerant species throughout the experiment, showing no consistent response to the treatment. The invertebrate response was associated with statistically significant changes in community structure (i.e. abundance of the different taxa) but no statistically significant changes in taxa richness. The salt pulses affected some functional descriptors of the ecosystem: fungal biomass exhibited a unimodal response to treatment magnitude, algal growth (i.e. chl a biomass) was hampered with increasing conductivity and leaf decomposition was significantly reduced in the high treatment.