Water velocity shapes fish movement behavior.

Stream and river ecosystems present fluvial fishes with a dynamic energy landscape because moving water generates heterogeneous flow fields that are rarely static in space and time. Fish movement behavior should be consistent with conserving energy in these dynamic flowing environments, but little evidence supporting this hypothesis exists. Here, we tested experimentally whether three general movement behaviors-against the current, with the current, or holding position (i.e., staying in one position and location)-were performed in a way consistent with minimizing the cost of swimming in a heterogeneous flow field. We tested the effects of water velocity on movement behavior across three age classes (0, 1, and 5 years) of two different fluvial specialist fishes, the pallid sturgeon (Scaphirhynchus albus) and shovelnose sturgeon (Scaphirhynchus platorynchus). Individuals from the three age classes were exposed to a continuous and dynamic velocity field ranging from 0.02 to 0.53 m s-1, which represented natural benthic flow regimes occupied by these species in rivers. Both sturgeon species exhibited the same pattern with regard to their tendency to hold position, move upstream, or move downstream. Moving downstream was positively associated with velocity for all age groups. Moving upstream was inversely related to velocity for young fish, but as the fish aged, moving upstream was not related to water velocity. The oldest fish (age 5) moved upstream more frequently compared to the younger age classes. Holding position within a water current was the most frequent behavior and occurred with similar probability across the range of experimental velocity for youngest fish (age 0), but was inversely related to velocity in older fish. Our experiment across age classes suggests that the suite of swimming behaviors exhibited by fluvial specialists might have evolved to mitigate the energetic costs of complex energy landscapes generated by moving water to ultimately maximize net energy gain.

Freshwater salinization reduces vertical movement rate and abundance of Daphnia: Interactions with predatory stress.

Contaminants in human-dominated landscapes are changing ecological interactions. The global increase in freshwater salinity is likely to change predator-prey interactions due to the potential interactive effects between predatory stress and salt stress. We conducted two experiments to assess the interactions between the non-consumptive effects of predation and elevated salinity on the abundance and vertical movement rate of a common lake zooplankton species (Daphnia mendotae). Our results revealed an antagonism rather than a synergism between predatory stress and salinity on zooplankton abundance. Elevated salinity and predator cues triggered a >50% reduction in abundance at salt concentrations of 230 and 860 mg Cl-/L, two thresholds designed to protect freshwater organisms from chronic and acute effects due to salt pollution. We found a masking effect between salinity and predation on vertical movement rate of zooplankton. Elevated salinity reduced zooplankton vertical movement rate by 22-47%. A longer exposure history only magnified the reduction in vertical movement rate when compared to naïve individuals (no prior salinity exposure). Downward movement rate under the influence of predatory stress in elevated salinity was similar to the control, which may enhance the energetic costs of predator avoidance in salinized ecosystems. Our results suggest antagonistic and masking effects between elevated salinity and predatory stress will have consequences for fish-zooplankton interactions in salinized lakes. Elevated salinity could impose additional energetic constraints on zooplankton predator avoidance behaviors and vertical migration, which may reduce zooplankton population size and community interactions supporting the functioning of lake ecosystems.

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.

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

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.

Nutrients influence the multi-trophic impacts of an invasive species unaffected by native competitors or predators.

Non-native species often lead to undesirable ecological and environmental impacts. Two hypotheses that predict establishment of non-native species are enemy release and biotic resistance. Support for these hypotheses in freshwater invasions is mixed. Experiments combined with field observations provide a complementary approach to understanding how interactions between native and non-native species lead to enemy release or biotic resistance. We tested experimentally whether these hypotheses provided insights into the invasion of the banded mystery snail (Viviparus georgianus), which has invaded the Great Lakes region and northeastern Unites States (US) from the southeastern US. Because freshwater systems vary widely in their nutrient concentrations due to natural and anthropogenic processes, we tested whether nutrient additions altered competitive and predatory interactions that regulate mechanisms of enemy release or biotic resistance. We evaluated the status of the mystery snail invasion in a 3-year field survey of Lake George (NY, US) to identify if field observations supported any experimental conclusions. The presence of the banded mystery snail led to a 14% and 27% reduction in biomass of a native competitor under low- and high-nutrient concentrations, respectively. The mystery snail also triggered a 29% biomass loss of a native snail predator, but only in low-nutrient concentrations. Field surveys indicated that the mystery snail dominated the snail community; of seven snail species, it comprised 77% of all snails. Results from the field surveys combined with experimental results indicate that neither competitors nor predators have likely suppressed the invasion of the banded mystery snail. This conclusion is consistent with competitive- and predatory-enemy release as we found no indication of biotic resistance via competition or predation from native species. Our results further highlight that the post-establishment impacts of invasive species are altered by the trophic state of freshwater ecosystems.

Evolved tolerance to freshwater salinization in zooplankton: life-history trade-offs, cross-tolerance and reducing cascading effects.

Recent discoveries have documented evolutionary responses to freshwater salinization. We investigated if evolutionary responses to salinization exhibit life-history trade-offs or if they can mitigate ecological impacts such as cascading effects through mechanisms of tolerance and cross-tolerance. We conducted an outdoor mesocosm experiment using populations of Daphnia pulex-a ubiquitous algal grazer-that were either naive or had previously experienced selection to become more tolerant to sodium chloride (NaCl). During the initial phase of population growth, we discovered that evolved tolerance comes at the cost of slower population growth in the absence of salt. We found evolved Daphnia populations maintained a tolerance to NaCl approximately 30 generations after the initial discovery. Evolved tolerance to NaCl also conferred cross-tolerance to a high concentration of CaCl2 (3559 µS cm-1) and a moderate concentration of MgCl2 (967 µS cm-1). A higher concentration of MgCl2 (2188 µS cm-1) overwhelmed the cross-tolerance and killed all Daphnia Tolerance to NaCl did not mitigate NaCl-induced cascades leading to phytoplankton blooms, but cross-tolerance at moderate concentrations of MgCl2 and high concentrations of CaCl2 mitigated such cascading effects caused by these two salts. These discoveries highlight the important interplay between ecology and evolution in understanding the full impacts of freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Regulations are needed to protect freshwater ecosystems from salinization.

Anthropogenic activities such as mining, agriculture and industrial wastes have increased the rate of salinization of freshwater ecosystems around the world. Despite the known and probable consequences of freshwater salinization, few consequential regulatory standards and management procedures exist. Current regulations are generally inadequate because they are regionally inconsistent, lack legal consequences and have few ion-specific standards. The lack of ion-specific standards is problematic, because each anthropogenic source of freshwater salinization is associated with a distinct set of ions that can present unique social and economic costs. Additionally, the environmental and toxicological consequences of freshwater salinization are often dependent on the occurrence, concentration and ratios of specific ions. Therefore, to protect fresh waters from continued salinization, discrete, ion-specific management and regulatory strategies should be considered for each source of freshwater salinization, using data from standardized, ion-specific monitoring practices. To develop comprehensive monitoring, regulatory, and management guidelines, we recommend the use of co-adaptive, multi-stakeholder approaches that balance environmental, social, and economic costs and benefits associated with freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Timing and frequency of sublethal exposure modifies the induction and retention of increased insecticide tolerance in wood frogs (Lithobates sylvaticus).

Although the paradigm for increased tolerance to pesticides has been by selection on constitutive (naïve) traits, recent research has shown it can also occur through phenotypic plasticity. However, the time period in which induction can occur, the duration of induced tolerance, and the influence of multiple induction events remain unknown. We hypothesized that the induction of increased pesticide tolerance is limited to early sensitive periods, the magnitude of induced tolerance depends on the number of exposures, and the retention of induced tolerance depends on the time elapsed after an exposure and the number of exposures. To test these hypotheses, we exposed wood frog tadpoles to either a no-carbaryl control (water) or 0.5 mg/L carbaryl at 4 time periods, and later tested their tolerance to carbaryl using time-to-death assays. We discovered that tadpoles induced increased tolerance early and midway but not late in our experiment and their constitutive tolerance increased with age. We found no difference in the magnitude of induced tolerance after a single or 2 exposures. Finally, induced pesticide tolerance was reversed within 6 d but was retained only when tadpoles experienced all 4 consecutive exposures. Phenotypic plasticity provides an immediate response for sensitive amphibian larvae to early pesticide exposures and reduces phenotypic mismatches in aquatic environments contaminated by agrochemicals. Environ Toxicol Chem 2018;37:2188-2197. © 2018 SETAC.

Inducible Tolerance to Agrochemicals Was Paved by Evolutionary Responses to Predators.

Recent research has reported increased tolerance to agrochemicals in target and nontarget organisms following acute physiological changes induced through phenotypic plasticity. Moreover, the most inducible populations are those from more pristine locations, far from agrochemical use. We asked why do populations with no known history of pesticide exposure have the ability to induce adaptive responses to novel agrochemicals? We hypothesized that increased pesticide tolerance results from a generalized stressor response in organisms, and would be induced following sublethal exposure to natural and anthropogenic stressors. We exposed larval wood frogs (Lithobates sylvaticus) to one of seven natural or anthropogenic stressors (predator cue (Anax spp.), 0.5 or 1.0 mg carbaryl/L, road salt (200 or 1000 mg Cl-/L), ethanol-vehicle control, or no-stressor control) and subsequently tested their tolerance to a lethal carbaryl concentration using time-to-death assays. We observed induced carbaryl tolerance in tadpoles exposed to 0.5 mg/L carbaryl and also in tadpoles exposed to predator cues. Our results suggest that the ability to induce pesticide tolerance likely arose through evolved antipredator responses. Given that antipredator responses are widespread among species, many animals might possess inducible pesticide tolerance, buffering them from agrochemical exposure.

A salty landscape of fear: responses of fish and zooplankton to freshwater salinization and predatory stress.

Predator-prey relationships are altered by anthropogenic contaminants. Road salt is a widespread contaminant among freshwater ecosystems, yet a relatively understudied subject in community ecology. Unknown is whether road salt salinization interacts with predatory stress to influence the growth, behavior, or reproduction of freshwater organisms. Using rainbow trout (Oncorhynchus mykiss) and zooplankton (Daphnia pulex), we exposed them to variable levels of road salt (NaCl) crossed with the presence or absence of alarm cues or kairomones. Alarm cue reduced trout activity and aggression and increased shoaling behavior. Road salt reduced trout growth in the high compared to moderate salt concentration, but neither concentration was different from the control. There was no interaction between alarm cues and salt for trout. Road salt and predatory stress had an additive effect on Daphnia abundance. Predatory stress decreased Daphnia abundance by 11%. Compared to the control, salt decreased Daphnia abundance by 40% in 860 mg Cl-/L and 79% in 1300 mg Cl-/L, and by the final day abundance was reduced by 85% in 1300 mg Cl-/L. Road salt and predatory stress had an interactive effect on Daphnia reproduction. Predatory stress in control water and moderate salt levels (230 mg Cl-/L) increased sexual reproduction of Daphnia, but these responses disappeared at high salt concentrations. Thus, road salt could limit reproductive adaptations to natural and anthropogenic stressors in Daphnia. Our results indicate road salt salinization could alter zooplankton population dynamics directly and by interacting with predatory stress, which might affect energy flow through freshwater food webs.

Impacts of road deicing salts on the early-life growth and development of a stream salmonid: Salt type matters.

The use of road deicing salts in regions that experience cold winters is increasing the salinity of freshwater ecosystems, which threatens freshwater resources. Yet, the impacts of environmentally relevant road salt concentrations on freshwater organisms are not well understood, particularly in stream ecosystems where salinization is most severe. We tested the impacts of deicing salts-sodium chloride (NaCl), magnesium chloride (MgCl2), and calcium chloride (CaCl2)-on the growth and development of newly hatched rainbow trout (Oncorhynchus mykiss). We exposed rainbow trout to a wide range of environmentally relevant chloride concentrations (25, 230, 860, 1500, and 3000 mg Cl- L-1) over an ecologically relevant time period (25 d). We found that the deicing salts studied had distinct effects. MgCl2 did not affect rainbow trout growth at any concentration. NaCl had no effects at the lowest three concentrations, but rainbow trout length was reduced by 9% and mass by 27% at 3000 mg Cl- L-1. CaCl2 affected rainbow trout growth at 860 mg Cl- L-1 (5% reduced length; 16% reduced mass) and these effects became larger at higher concentrations (11% reduced length; 31% reduced mass). None of the deicing salts affected rainbow trout development. At sub-lethal and environmentally relevant concentrations, our results do not support the paradigm that MgCl2 is the most toxic deicing salt to fish, perhaps due to hydration effects on the Mg2+ cation. Our results do suggest different pathways for lethal and sub-lethal effects of road salts. Scaled to the population level, the reduced growth caused by NaCl and CaCl2 at critical early-life stages has the potential to negatively affect salmonid recruitment and population dynamics. Our findings have implications for environmental policy and management strategies that aim to reduce the impacts of salinization on freshwater organisms.

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.

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.

Investigation of road salts and biotic stressors on freshwater wetland communities.

The application of road deicing salts has led to the salinization of freshwater ecosystems in northern regions worldwide. Increased chloride concentrations in lakes, streams, ponds, and wetlands may negatively affect freshwater biota, potentially threatening ecosystem services. In an effort to reduce the effects of road salt, operators have increased the use of salt alternatives, yet we lack an understanding of how these deicers affect aquatic communities. We examined the direct and indirect effects of the most commonly used road salt (NaCl) and a proprietary salt mixture (NaCl, KCl, MgCl2), at three environmentally relevant concentrations (150, 470, and 780 mg Cl-/L) on freshwater wetland communities in combination with one of three biotic stressors (control, predator cues, and competitors). The communities contained periphyton, phytoplankton, zooplankton, and two tadpole species (American toads, Anaxyrus americanus; wood frogs, Lithobates sylvaticus). Overall, we found the two road salts did not interact with the natural stressors. Both salts decreased pH and reduced zooplankton abundance. The strong decrease in zooplankton abundance in the highest NaCl concentration caused a trophic cascade that resulted in increased phytoplankton abundance. The highest NaCl concentration also reduced toad activity. For the biotic stressors, predatory stress decreased whereas competitive stress increased the activity of both tadpole species. Wood frog survival, time to metamorphosis, and mass at metamorphosis all decreased under competitive stress whereas toad time to metamorphosis increased and mass at metamorphosis decreased. Road salts and biotic stressors can both affect freshwater communities, but their effects are not interactive.

Combined effects of road salt and an insecticide on wetland communities.

As the numbers of chemical contaminants in freshwater ecosystems increase, it is important to understand whether contaminants interact in ecologically important ways. The present study investigated the independent and interactive effects of 2 contaminants that frequently co-occur in freshwater environments among higher latitudes, including a commonly applied insecticide (carbaryl) and road salt (NaCl). The hypothesis was that the addition of either contaminant would result in a decline in zooplankton, an algal bloom, and the subsequent decline of both periphyton and periphyton consumers. Another hypothesis was that combining the contaminants would result in synergistic effects on community responses. Outdoor mesocosms were used with communities that included phytoplankton, periphyton, zooplankton, amphipods, clams, snails, and tadpoles. Communities were exposed to 4 environmentally relevant concentrations of salt (27 mg Cl- L-1 , 77 mg Cl- L-1 , 277 mg Cl- L-1 , and 727 mg Cl- L-1 ) fully crossed with 4 carbaryl treatments (ethanol, 0 µg L-1 , 5 µg L-1 , and 50 µg L-1 ) over 57 d. Contaminants induced declines in rotifer and cladoceran zooplankton, but only carbaryl induced an algal bloom. Consumers exhibited both positive and negative responses to contaminants, which were likely the result of both indirect community interactions and direct toxicity. In contrast to the hypothesis, no synergistic effects were found, although copepod densities declined when high concentrations of both chemicals were combined. The results suggest that low concentrations of salt and carbaryl are likely to have mostly independent effects on aquatic communities. Environ Toxicol Chem 2017;36:771-779. © 2016 SETAC.