In freshwater ecosystems, variation in light quantity is a determinant of production that causes bottom-up effects in food webs. Most research has only explored the effects of light on system metabolism and primary producers. In contrast, multitrophic effects of light availability – including fluctuations in light availability – remain understudied. Using outdoor mesocosms, we examined how changes in the average quantity and temporal fluctuations of light alter lake food webs. We hypothesized that reduced average light should decrease the quantity but increase the quality of primary production for consumers, thereby promoting secondary production. We also hypothesized that regular fluctuations in light should increase production by promoting species coexistence, whereas irregular fluctuations should have the opposite effect. To test these hypotheses, we constructed communities containing periphyton, phytoplankton, zooplankton, and snails and exposed these communities to three constant light treatments (90

Freshwater salinization is occurring around the world and impacting a wide variety of freshwater species that have evolved under low-salt conditions. Salt pollution reduces the survival of many freshwater taxa, but we know less about the effects of salt on individual traits. Moreover, we know even less about how salt pollution may affect phenotypically plastic traits that have evolved in response to natural stressors. In this study, we examined wood frog tadpoles (Rana sylvatica), which are a model system for predator-induced plasticity, and determined how their growth, behavior, and morphology changed in the presence of chemical cues from dragonflies (Anax junius) under four concentrations of NaCl (16, 250, 500, and 1000 mg Cl-/L). Early in the experiment, the tadpoles reduced their feeding activity in response to predator cues but did not respond to increasing salt concentrations. Tadpole mass increased with predator cues but decreased with increased salt concentrations. As expected, the predator cues induced relatively deeper tails and tail muscles, while inducing relatively shorter bodies and narrower mouths. However, we also discovered that salt induced relatively longer tails, longer bodies, and smaller eyes. Interestingly, the predator effects did not interact with salt effects for any of the traits. These results suggest that freshwater salinization has the potential to alter the traits of other freshwater species, but the effects may simply be additive. Future studies should examine salt-induced changes in a diversity of other freshwater species and investigate whether salt-induced changes in morphology have consequences to individual performance.

Abstract Harmful algal blooms (HABs) rapidly change and threaten aquatic ecosystems. HABs in oligotrophic lakes are becoming more commonly observed, despite the long‐held paradigm that algal blooms cannot happen in low‐nutrient systems. This raises the question of whether nutrient loading, understood as a major driver of HABs, plays an important role in the potential for HAB development in oligotrophic lakes. Using in‐lake mesocosms, we examined the effects of a gradient of nitrogen and phosphorus additions on the pelagic communities of an oligotrophic lake over 4 weeks. We hypothesized that increasing nutrients would result in an increased abundance of phytoplankton and an increased dominance by cyanobacteria. Although our nutrient additions caused an increase in the fluorescence of chlorophyll a (i.e., a proxy for total phytoplankton) and phycocyanin (i.e., a proxy for total cyanobacteria), these increases plateaued at low‐nutrient concentrations and the increases were short‐lived. The identification and enumeration of phytoplankton species confirmed that the composition of the phytoplankton also did not change with added nutrients, nor did the abundance of the dominant zooplankton group (i.e., cladocerans). Furthermore, our biweekly nutrient additions maintained elevated concentrations of total phosphorus and total nitrogen in the water column, with total dissolved phosphorus and nitrate both readily available throughout the experiment. Given that light is very abundant (Secchi depths of 9–10 m), this suggests that another nutrient, such as iron or carbon, may have limited phytoplankton growth. Our findings suggest that excess nutrients may not always drive HABs in oligotrophic lakes and further studies should examine the effects of other micronutrients using similar controlled seminatural mesocosms.

Given the expansion of aquatic plants (i.e., macrophytes) that can occur with eutrophication, lakes and ponds are often managed with herbicides to kill large areas of undesired plants. In doing so, the dead plants decompose and release large amounts of nutrients that can facilitate major algal blooms, which are subsequently managed by applying algicides. These rapid shifts in community structure might also encourage other shifts, such as the proliferation of invasive species. More understanding is needed about how herbicides and algicides affect entire food webs, including food webs containing invasive species. We employed outdoor mesocosm communities to explore the community-wide effects of a common herbicide (fluridone) and an algicide (copper sulphate; CuSO4). Communities included algae, zooplankton, Eurasian milfoil (Myriophyllum spicatum) and two native snails (Physa acuta and Helisoma trivolvis). We further manipulated the presence of the invasive Chinese mystery snail (Cipangopaludina chinensis) to examine how the same chemicals might alter its ability to invade a community. Surprisingly, we discovered that the herbicide caused a 40% reduction in phytoplankton and a decline in copepods, likely as an indirect effect. Adding CuSO4 caused a 60% reduction in phytoplankton but no reduction in periphyton, while also causing an increase in cladocerans and declines in copepods and the two native snail species. In contrast, the invasive Chinese mystery snail was unaffected by the algicide, while it drove down the abundance of periphyton and one of the native snails. Collectively, these results highlight the importance of examining multiple human impacts-alone and in combination-to more effectively assess the planned and inadvertent effects of human activities on aquatic ecosystems.

Wetland communities are increasingly threatened by multiple stressors simultaneously, such as pesticides and salinization. We examined the effects of ecologically-relevant exposures to broad-spectrum insecticides and salinization on amphibian neurodevelopment, which is strongly linked to how organisms respond behaviorally to environmental change. Prior research showed that exposure to trace concentrations of an organophosphate pesticide (chlorpyrifos) altered the brain shape and behavior of larval and metamorphic amphibians. It is unknown whether brain shape is altered by additional pesticides and road salt. Using outdoor mesocosms, we tested whether salt (NaCl) and representatives from three pesticide families (organophosphates, pyrethroids, and neonicotinoids) altered tadpole (Lithobates pipiens) brain shape. Of the two organophosphates, chlorpyrifos induced relatively longer telencephalon lengths relative to body mass, consistent with previous studies, but malathion had no effect on brain shape. Of the two pyrethroids, permethrin, but not cypermethrin, increased telencephalon length. For the neonicotinoids, there were marginally significant effects of imidacloprid and thiamethoxam on telencephalon length. Thus, the impacts of pesticides on brain shape was not dictated by pesticide family. Exposure to relatively high concentrations of salt resulted in brains that were less wide but had longer optic tecta. Although we failed to find strong interactive effect of salt with pesticides, there was some weak, nonsignificant, evidence that exposure to salt masked responses to pesticides. Together, our results indicate that environmentally realistic levels of pesticides and salinization can alter larval brain shape. Our study highlights the importance of studying the impacts of naturally-occurring levels of pesticides and salinization on vertebrate neural development.

Harmful algal blooms (HABs) are common in many eutrophic lakes and frequently associated with nutrient excesses, warm waters, and calm conditions. While HABs can also occur in oligotrophic waterbodies, bloom-stimulating factors remain elusive for these ecosystems. Here, using a high-frequency sensor platform supported by hydrodynamic modeling, we document a clearly linked sequence of three hydrodynamic events that immediately preceded each HAB event on Skaneateles Lake, NY during 2018. HABs were first preceded by interactions between internal waves in the thermal structure of the lake with the northern slope of the lakebed. These interactions disturbed sediments resulting in upwelling of nutrient-rich sediment and possibly cyanobacterial cells into the lower water column. Next, moderate winds stimulated Langmuir circulation which facilitated the transport of material from the lower water column to well-lit surface waters. Finally, the sequence culminated with a calm wind period, which is often classically associated with surface HAB formation. While individual steps in this sequence occurred independently and repeatedly throughout the year, the complete sequence occurred only eight times, immediately preceding each of the eight documented HABs. We empirically derive thresholds for and timing of the sequence to independently predict HAB occurrence and elevated cyanobacteria levels in the lake. The discovery of this specific and repeated event sequence and the identified thresholds provides the potential for forecasting of HABs days in advance. Ongoing climate change impacts, including increasing water column thermal stratification and changing wind speeds may alter the frequency, timing, and duration of this sequence and consequent HAB formation.

Human impacts on ecological communities are pervasive and species must either move or adapt to changing environmental conditions. For environments polluted by contaminants, researchers have found hundreds of target pest species evolving increased tolerance, but we have substantially fewer cases of evolved tolerance in non-target species. When species do evolve increased tolerance, inducible tolerance can provide immediate protection and favor the evolution of increased tolerance over generations via genetic assimilation. Using a model larval amphibian (wood frogs, Rana sylvatica), we examined the tolerance of 15 populations from western Pennsylvania and eastern New York (USA), when first exposed to no pesticide or sublethal concentrations and subsequently exposed to lethal concentrations of three common insecticides (carbaryl, chlorpyrifos, and diazinon). We found high variation in naïve tolerance among the populations for all three insecticides. We also discovered that nearly half of the populations exhibited inducible tolerance, though the degree of inducible tolerance (magnitude of tolerance plasticity; MoTP) varied. We observed a cross-tolerance pattern of the populations between chlorpyrifos and diazinon, but no pattern of similar MoTP among the pesticides. With populations combined from two regions, increased tolerance was not associated with proximity to agricultural fields, but there were correlations between proximity to agriculture and MoTP. Collectively, these results suggests that amphibian populations possess a wide range of naïve tolerance to common pesticides, with many also being able to rapidly induce increased tolerance. Future research should examine inducible tolerance in a wide variety of other taxa and contaminants to determine the ubiquity of these responses to anthropogenic factors.

The amount of genetic diversity within a population can affect ecological processes at population, community, and ecosystem levels. However, the magnitude, consistency, and scope of these effects are largely unknown. To investigate these issues, we conducted two experiments manipulating the amount of genetic diversity and environmental factors in larval amphibians. The first experiment manipulated wood frog genetic diversity, the presence or absence of caged predators, and competition from leopard frogs to test whether these factors affected survival, growth, and morphology of wood frogs and leopard frogs. The second experiment manipulated wood frog genetic diversity, the presence or absence of uncaged predators, and resource abundance to test whether these factors affected wood frog traits (survival, morphology, growth, development, and behavior) and other components of the ecological community (zooplankton abundance, phytoplankton, periphyton, and bacterial community structure). Genetic diversity did not affect wood frog survival, growth, and development in either experiment. However, genetic diversity did affect the mean morphology of wood frog tadpoles in the first experiment and the abundance and distribution of zooplankton in the second experiment. It did not affect phytoplankton abundance, periphyton abundance, or bacterial community structure. While effect sizes (Cohen's d) of genetic diversity were approximately half those of environment treatments, the greatest effect sizes were for interaction effects between genetic diversity and environment. Our results indicate that genetic diversity can have a large effect on ecological processes, but the direction of those effects is highly dependent upon environmental conditions, and not easily predicted from simple measures of traits.

Understanding spatial and temporal heterogeneity in ecosystems is essential to forecasting the effects of environmental changes. Freshwater microbes, including cyanobacteria, play a crucial role in food-web structures and biochemical processes, yet can exhibit substantial heterogeneity through space and time. They also act as powerful indicators of natural and human-induced stress due to their high metabolic and rapid response to environmental change. The formation of cyanobacteria blooms can be particularly important due to the potential production of toxins that are harmful to humans and wildlife. While high water temperatures and high nutrients are largely recognized as triggers of cyanobacterial bloom formation, there is growing evidence of the role of its associated microbiome in bloom formation. The inability to accurately forecast cyanobacteria blooms is challenged by uncertainty in the degree to which microbial diversity, and bloom forming taxa in particular, exhibit spatial heterogeneity and how spatial heterogeneity varies seasonally or between lakes spanning the trophic gradient. Here, we used long-read sequencing of the 16S rRNA gene to quantify variations in microbial spatiotemporal dynamics over the course of an ice-free season between two lakes that varied substantially in trophic status. Our results showed that the microbial community composition of eutrophic Chautauqua Lake was seasonally and spatially structured; however, during bloom events we observed lower diversity and a homogeneous community dominated by Microcystis and enriched with Gammaproteobacteria. In oligotrophic Lake George, seasonality rather than the basin of origin played a major role in structuring the microbial community; however, there was a significant difference between basins when controlling for the temporal effect and was linked to a South-to-North anthropogenic gradient. This study provides a solid foundation for exploiting long-read sequencing of prokaryotes and couples sequencing with traditional water quality monitoring to assess microbial dynamics (e.g., cyanobacteria bloom microbiome) and the effect of local and global stressors.

AbstractThe increasing salinization of freshwaters is a growing environmental issue as a result of mining, agriculture, climate change, and the application of de‐icing salts in regions that experience ice and snow. Due to narrow osmotic limits, many freshwater species are particularly susceptible to salinization, but it is possible that repeated exposures over time could favor the evolution of increased salt tolerance. Using collected nine populations of larval wood frogs (Rana sylvatica) as eggs from ponds and wetlands with close proximity to roads and spanning a wide gradient of salt concentrations. In the first experiment, we used a time‐to‐death experiment to examine the salt tolerance. In a second experiment, we examined whether population differences in salt tolerance were associated with trade‐offs in growth, development, or behavior in the presence of control water or a sublethal salt concentration. We found that populations collected from ponds with low and intermediate salt concentrations exhibited similar tolerance curves over a 96‐h exposure. However, the population from a pond with the highest salt concentration exhibited a much higher tolerance. We also found population differences in growth, development, and activity level among the populations, but these were not associated with population differences in tolerance. In addition, the sublethal concentration of salt had no impact on growth and development, but it did cause a reduction in tadpole activity across the populations. Collectively, these results provide further evidence that some species of freshwater organisms can evolve tolerance to increasing salinization, although it may only occur under relatively high concentrations and without trade‐offs in growth, development, or behavior.

Food webs provide context to understand how ecological communities will respond to environmental change, but revealing their structure typically relies upon time-intensive sampling and analysis of species' diets. As a result, all food web models require some unavoidable simplifications because of limited data availability, whether temporally, spatially, or taxonomically. Large databases of published trophic interactions have made this process somewhat easier, but knowledge gaps persist. We combine the use of databases with extensive field surveys, including gut-content analysis, to generate a food web for Lake George, NY. Including aquatic plants, phytoplankton, zooplankton, macroinvertebrates, and fish, our analysis identified 279 genera in the lake involved in 1910 interactions. After removing genera with no identified interactions or improbable interactions and grouping some genera into higher categories, the food web included 49 nodes with 484 interactions among them. The network structure of the inferred Lake George food web exhibits several common patterns such as relatively few trophic levels and the prevalence of tritrophic chains. Our results suggest that constructing food webs from databases provides a useful first step to determine topology. However, in situ sampling allowed us to account for additional interactions, as only 50 of the 106 directly observed interactions between fish and their prey were also found in published databases. Finally, we highlight the need to focus on developing a better understanding of herbivory in lakes, as species interactions among the diverse plankton and macroinvertebrate populations are not well known.

Ecotoxicological studies using single test populations may miss the inherent variation of natural systems and limit our understanding of how contaminants affect focal species. Though population-level variation in pesticide tolerance is commonly observed in host taxa, few studies have assessed population-level differences in the tolerance of parasites to different contaminants. We investigated population-level variation in insecticide tolerance of three Echinostoma trivolvis life stages (egg, miracidium, and cercaria) to three insecticides (carbaryl, chlorpyrifos, and diazinon). We tested two relevant metrics of insecticide tolerance-baseline and induced-across up to eight different parasite populations per life stage. Across all life stages, the insecticide treatments tended to reduce survival, but the magnitude of their effects often varied significantly among populations. Surprisingly, we found that exposure to chlorpyrifos increased the hatching success of echinostome eggs relative to the control treatment in three of six tested populations. We also found that cercariae shed from snails previously exposed to a sublethal concentration of chlorpyrifos had a significantly lower mortality rate when subsequently exposed to a lethal concentration of chlorpyrifos relative to individuals from snails that were not previously exposed; this suggests inducible tolerance in cercariae. We found no evidence that insecticide tolerance is correlated across parasite life stages within a population. Together the findings of our study demonstrate that single-population toxicity assays may greatly over- or underestimate the effects of pesticides on the survival of free-living parasite stages, insecticide tolerance levels may not be predictable from one parasite life stage to the next, and insecticides can have both expected and counterintuitive effects on non-target taxa.

Human activities are causing global change around the world including habitat destruction, invasive species in non-native ecosystems, overexploitation, pollution, and global climate change. While traditional monitoring has long been used to quantify and aid mitigation of global change, in-situ autonomous sensors are being increasingly used for environmental monitoring. Sensors and sensor platforms that can be deployed in developed and remote areas and allow high-frequency data collection, which is critical for parameters that exhibit important short-term dynamics on the scale of days, hours, or minutes. In this article, we discuss the benefits of in-situ autonomous sensors in aquatic ecosystems as well as the many challenges that we have experienced over many years of working with these technologies. These challenges include decisions on sensor locations, sensor types, analytical specification, sensor calibration, sensor drift, the role of environmental conditions, sensor fouling, service intervals, cost of ownership, and data QA/QC. These challenges result in important tradeoffs when making decisions regarding which sensors to deploy, particularly when a network of sensors is desired to cover a large area. We also review recent advances in designing and building chemical-sensor platforms that are allowing researchers to develop the next-generation of autonomous sensors and the power of integrating multiple sensors into a network that provides increased insight into the dynamics of water quality over space and time. In the coming years, there will be an exponential growth in data related to aquatic sensing, which will be an essential part of global efforts to monitor and mitigate global change and its adverse impacts on society.

Abstract The salinization of freshwaters is a global threat to aquatic biodiversity. We quantified variation in chloride (Cl−) tolerance of 19 freshwater zooplankton species in four countries to answer three questions: (1) How much variation in Cl− tolerance is present among populations? (2) What factors predict intraspecific variation in Cl− tolerance? (3) Must we account for intraspecific variation to accurately predict community Cl− tolerance? We conducted field mesocosm experiments at 16 sites and compiled acute LC50s from published laboratory studies. We found high variation in LC50s for Cl− tolerance in multiple species, which, in the experiment, was only explained by zooplankton community composition. Variation in species‐LC50 was high enough that at 45% of lakes, community response was not predictable based on species tolerances measured at other sites. This suggests that water quality guidelines should be based on multiple populations and communities to account for large intraspecific variation in Cl− tolerance.

The dynamics of populations in lake food webs are driven by a combination of spatial, environmental, and trophic processes. Differences among taxa in their ability to disperse and respond to the environment impact the food web and will be reflected in their abundances over time at different sites within a lake. The level of synchrony among lake sub-basins (within and across populations in a lake) is critical for determining the importance of spatial resolution when sampling and developing models. Using 7 years of survey data in Lake George, New York State (USA), we provide a high-level overview of changes in the densities of key food web groups over time (phytoplankton, zooplankton, and macroinvertebrates) and the synchrony of their dynamics among sub-basins. Phytoplankton biomass (measured as chlorophyll a ) and zooplankton densities showed strong seasonal and multi-year trends that were synchronous across space within each group. Several macroinvertebrate groups showed significant non-linear multi-year trends, but synchrony was lower. For phytoplankton and some macroinvertebrate groups, we found that densities were highest in the South Basin, likely reflecting the gradient of decreasing nutrients in the lake from south to north. Collectively, these results suggest that modeling the food web using sub-basins is a useful scale for better understanding species dynamics in large lakes.

Abstract Human‐induced salinization increasingly threatens inland waters; yet we know little about the multifaceted response of lake communities to salt contamination. By conducting a coordinated mesocosm experiment of lake salinization across 16 sites in North America and Europe, we quantified the response of zooplankton abundance and (taxonomic and functional) community structure to a broad gradient of environmentally relevant chloride concentrations, ranging from 4 to ca. 1400 mg Cl− L−1. We found that crustaceans were distinctly more sensitive to elevated chloride than rotifers; yet, rotifers did not show compensatory abundance increases in response to crustacean declines. For crustaceans, our among‐site comparisons indicate: (1) highly consistent decreases in abundance and taxon richness with salinity; (2) widespread chloride sensitivity across major taxonomic groups (Cladocera, Cyclopoida, and Calanoida); and (3) weaker loss of functional than taxonomic diversity. Overall, our study demonstrates that aggregate properties of zooplankton communities can be adversely affected at chloride concentrations relevant to anthropogenic salinization in lakes.

Contamination of aquatic ecosystems is pervasive around the world and there has been a growing interest in understanding the ecological and evolutionary impacts. For contaminants such as pesticides, researchers are discovering widespread evolution of increased tolerance in target and non-target species and the role of phenotypic plasticity in facilitating this evolution. In contrast, we know much less about the evolution of tolerance in response to the increasing problem of freshwater salinization. In amphibians, recent studies have discovered that some populations from ponds with high salt pollution (from deicing road salts) have evolved higher tolerance. In this study, we examined whether populations of wood frog tadpoles (Rana sylvatica) possess rapid, inducible tolerance to salinity in a manner similar to their inducible tolerance to pesticides. Using newly hatched tadpoles from nine populations, we discovered that eight of the populations were able to alter their tolerance to salt. However, seven of the eight inducible populations experienced a higher sensitivity to salt while the eighth population experienced a higher tolerance to salt. Such inducible responses likely reflect the interplay of salt dynamics in the ponds, combined with the available genetic variation and selection intensity of each pond. This appears to be the first example of inducible salt tolerance in any animal and future studies should examine the generality of the response and how it may affect the evolution of tolerance to the global issue of freshwater salinization.

There is a growing effort to understand how sexual selection varies over space and time under different ecological conditions and how this can maintain variation in sexual traits. An interesting ecological condition is population density, which can either increase or decrease sexual selection depending on the natural history of the species. We examined sexual and natural selection in an undescribed amphipod species (Hyalella sp.) using surveys of two natural populations that increase in density from spring to summer. We also conducted an experiment that directly manipulated density to assess the effects on sexual selection. In the field during spring (low density) and summer (high density), we documented sexual selection on male traits, including body size, gnathopods and antennae. We found that the magnitude and direction of this selection changed from spring to summer and that selection patterns differed between populations. In the experiment, we found no sexual selection occurred at low density, but found significant positive selection on all three male traits at medium and high densities. These results underline the importance of isolating individual ecological factors to determine their role in sexual selection while also documenting patterns in natural populations to understand how selection varies over space and time.

Freshwater ecosystems are being exposed to increasing salinisation, often because of pollution from road deicing salts, which is becoming more widely acknowledged. To address this issue, municipalities are turning towards the sodium salt alternatives of CaCl2 and MgCl2, which are marketed as being safer for the environment. However, research into the actual safety of these salts on aquatic plants is lacking. We investigated the effects of the most common road salt (NaCl) and two alternatives (MgCl2 and CaCl2) on the productivity of a common freshwater plant (i.e., Elodea canadensis) under three salt concentrations (control, 250, and 1,000 mg Cl-/L). Light-bottle/dark-bottle trials were performed to quantify net primary productivity, respiration, and gross primary productivity. These responses were tracked over time (1 vs. 3 weeks) to assess plant acclimation and lag effects under different levels of the three salts. We discovered that NaCl and CaCl2 altered these measures of plant metabolism, but MgCl2 had no effects. We also observed instances of acclimation (i.e. salt effects after 1 week that disappeared after 3 weeks) and lag effects (i.e. no salt effect after 1 week, but salt effects after 3 weeks). These impacts are likely to be the results of plant responses to salt at the cell and molecular levels, including short- and long-term changes in photosynthetic pigments. Therefore, the plant responses were salt-specific, with instances of plant acclimation and lag effects. This appears to be the first study of net primary productivity, respiration, and gross primary productivity in freshwater plants across a range of different salts, and it highlights how freshwater salinisation can have substantial effects on plant productivity. These effects will probably have an impact on the growth of macrophytes, which play key ecological roles in aquatic ecosystems.1.2.3.4.

Significance The salinity of freshwater ecosystems is increasing worldwide. Given that most freshwater organisms have no recent evolutionary history with high salinity, we expect them to have a low tolerance to elevated salinity caused by road deicing salts, agricultural practices, mining operations, and climate change. Leveraging the results from a network of experiments conducted across North America and Europe, we showed that salt pollution triggers a massive loss of important zooplankton taxa, which led to increased phytoplankton biomass at many study sites. We conclude that current water quality guidelines established by governments in North America and Europe do not adequately protect lake food webs, indicating an immediate need to establish guidelines where they do not exist and to reassess existing guidelines.