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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., Competing Interests: The authors declare no competing interest.

Calcium levels are declining in eastern North American and western European lakes. This widespread issue is affecting the composition of crustacean zooplankton communities, as the presence and abundance of several calcium-rich daphniid species are declining, while two other daphniids, D. catawba and D. ambigua, that apparently tolerate low calcium environments, are prospering. The physiological basis for low calcium tolerance of these daphniids is unknown. In this study the presence of one Ca-rich (D. pulicaria) and one Ca-poor (D. ambigua) daphniid species in Canadian Shield lakes is assessed in relation to lake water Ca levels. The occurrence of D. ambigua was independent of Ca levels in Ontario lakes, whereas D. pulicaria was more likely to occur in lakes with relatively more Ca. In the laboratory, D. ambigua maintained lower levels of hemolymph Ca 2+ across a range of low Ca levels (0.7 to 7 mg l -1 ) compared with D. pulicaria. The hemolymph pH remained steady across this Ca gradient in D. ambigua while it was significantly more acidic in D. pulicaria in the two lowest Ca treatments. While Ca 2+ uptake was observed adjacent to the surface of D. ambigua individuals, Ca 2+ loss was observed for D. pulicaria assayed under moderately high Ca levels. Based on these observations we propose that D. ambigua is able to survive in low Ca lakes by maintaining low free ionic Ca 2+ levels in the hemolymph which minimizes the Ca gradient across the body wall in low Ca water thus limiting overall Ca loss and facilitating Ca 2+ uptake., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Assessing biological recovery in damaged aquatic environments requires the consideration of multiple spatial and temporal scales. Past research has focused on assessing lake recovery from atmospheric or catchment disturbance at regional or catchment levels. Studies have also rarely considered the influences of adjacent terrestrial characteristics on within-lake habitats, such as subcatchment delta confluences. We used Hyalella azteca, a ubiquitous freshwater amphipod, as a sensitive indicator to assess the importance of local subcatchment scale factors in the context of multiscale lake recovery within the metal mining region of Sudbury, Canada following a period of major reductions in atmospheric pollution. At the regional scale, data from repeated surveys of 40 lakes showed higher probabilities of H. azteca occurrence with higher lake water conductivity, alkalinity, and pH and lower metal concentrations. The importance of metals decreased through time and the importance of higher conductivity, alkalinity, and pH increased. At the subcatchment scale, a subset of six lakes sampled across a colonization gradient revealed higher H. azteca abundances at subcatchment delta sites than non-delta sites in early colonization stages, and that abundance at delta sites was correlated with both within-lake habitat and terrestrial subcatchment characteristics. For example, wetland cover reduced the strength of positive associations between H. azteca abundance and macrophyte density. A single lake from this subset also revealed higher abundances at delta sites associated with higher concentrations of terrestrial organic matter and larger subcatchments. Our results demonstrate that factors affecting recovery can change with the scale of study, and that managing terrestrial-aquatic linkages is important for facilitating recovery processes within damaged lake ecosystems., (© 2017 by the Ecological Society of America.)

Salt pollution is a threat to freshwater ecosystems. Anthropogenic salt inputs increase lake and stream salinity, and consequently change aquatic ecosystem structure and function. Elevated salt concentrations impact species directly not only through osmoregulatory stress, but also through community‐level feedbacks that change the flow of energy and materials through food webs. Here, we discuss the implications of road salt pollution on freshwater rivers and lakes and how "one size fits all" ecotoxicity thresholds may not adequately protect aquatic organisms. This article is categorized under:Science of Water > Water QualityWater and Life > Nature of Freshwater EcosystemsWater and Life > Stresses and Pressures on Ecosystems [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Elevated lake chloride concentration has been observed in many regions, due to human activities such as mining, agriculture, and urbanisation. Meanwhile, lakes are also experiencing increasing frequency and intensity of heatwaves. The combination of elevated salinity and heatwaves has not been thoroughly studied in freshwater communities, limiting our ability to predict outcomes of future disturbances.We conducted a mesocosm experiment to investigate the individual and interactive effects of increased salinity and heatwaves on a freshwater zooplankton community. The combined effects of the two stressors were examined in two scenarios: when they occurred simultaneously and when a heatwave was preceded by an 8‐week increase in salinity. We expected to see a synergistic effect when the two stressors were applied simultaneously, as organisms might experience energy deficiency due to physiological changes caused by salinity stress and be overwhelmed by the heat treatment. When the two stressors were applied sequentially, we expected them to act independently as the two stressors trigger different physiological responses and physiological homeostasis may have already recovered from previous salt exposure and not influence an organism's response to a subsequent stressor.Individually, increased salinity and heatwave conditions both impaired zooplankton communities with largest effects on copepod nauplii and cladocerans. Together, these stressors caused antagonistic effects on total zooplankton abundance and biomass in both the simultaneous and sequential scenarios, with the combined effects being similar to the salt‐only effects.Our experiment illustrates the potential for heatwaves to have hidden effects when they occur in lakes experiencing salinisation. The findings suggested that the two stressors negatively impacted some zooplankton taxonomic groups, and at the community level, they acted antagonistically such that the occurrence of a 3‐day heatwave did not cause any additional loss of abundance or biomass regardless of whether the community was exposed to the sequential or simultaneous scenario. Our findings also illustrated that even when the two stressors were decoupled in time, the community could still be influenced by a previous stressor. [ABSTRACT FROM AUTHOR]

Aim: Inferences of the predominant processes that structure communities are commonly based on single 'snapshots' in time, which may miss temporally transient but important mechanisms. In this study, we compared key environmental and spatial drivers of zooplankton composition across multiple years to quantify shifts in their relative importance through time, and to identify any drivers of temporal change. Location: Southern Ontario, Canada. Taxon: Zooplankton. Methods: Zooplankton were collected from 29 lakes in southern Ontario, Canada over four years (2013–2016). Variation partitioning was used to quantify the relative importance of independent and covarying components of local environmental conditions, regional east‐west and north‐south compositional trends, and inter‐lake geographic distance in each year. Measured environmental metrics included aspects of lake morphology and chemistry, and spatial relationships were quantified using lake latitude/longitude coordinates and Moran's Eigenvector Maps (MEMs). Redundancy analyses (RDAs) were also used to compare the influence of individual environmental and spatial variables across years. Results: Most local‐scale and regional‐scale community processes were consistently important across all surveyed years, but some were less consistent. Specifically, geomorphology was always an important driver of local environmental and regional spatial community patterns. This occurred because local community composition was strongly affected by whether a lake was shallower versus deeper, and due to spatial clusters of shallow and deep lakes that produced negative spatial autocorrelation in community composition. Conversely, the individual influences of lake chemistry and spatial east‐west compositional trends were important in some years and not in others, potentially due to inter‐annual shifts in the predominant environmental variables and extreme weather events. Main conclusions: A single‐year community snapshot can provide insight into consistent or slowly changing community structuring processes, such as those driven by geomorphology, but may not completely capture temporally transient mechanisms. Furthermore, snapshots collected during anomalous seasons or years may misrepresent which mechanisms are predominantly determining community composition. Future efforts to understand local and regional community drivers would therefore benefit from considering which processes are likely temporally 'consistent' versus 'transient', and studies with more variable components would benefit from considering or controlling for temporal shifts in their importance. [ABSTRACT FROM AUTHOR]

Northern regions are expected to experience large environmental change over the next few decades. The response of biota will depend on changes in the local environment, regional processes that influence lake connectivity, and species interactions. In 2008, we surveyed 92 lakes and ponds across Wapusk National Park, located on the southwestern shore of Hudson Bay. At each site we assessed water chemistry and zooplankton community composition. In an effort to understand how the aquatic ecosystems will respond to future environmental change, we determined local characteristics (e.g., water chemistry), regional spatial factors (e.g., dispersal), and biotic interactions (e.g., species associations) influencing community composition. Important environmental variables included lake area, pH, ionic composition, total phosphorus, and chlorophyll a; however, spatial variables explained more variation than environmental variables, suggesting that dispersal is an important driver of zooplankton composition in this region. Additionally, species exhibited negative co-occurrence patterns, suggesting biotic interactions are important in structuring the zooplankton communities. As environmental conditions change and the distribution of habitat (i.e., coastal fen, interior peatland, and spruce forest) shifts, evidence that the park's zooplankton community is spatially structured coupled with our suspicion that zooplankton are likely to experience high dispersal levels in Wapusk leads us to suggest zooplankton may indeed be able to track changing environmental conditions within the park, although it remains unclear how species interactions will modify this expectation. [ABSTRACT FROM AUTHOR]

Worldwide, freshwater zooplankton communities have been subjected to multiple environmental stressors including acidification, invasive species introductions, habitat alteration and climate change. Understanding the factors that affect zooplankton community responses in the face of environmental change is an important goal if ecologists expect to make predictions regarding the impact of current and future stressors. The acidification and recovery of lakes provides a well-studied example of the impacts of environmental change., For this study, we conducted a field enclosure experiment using zooplankton communities that are recovering from the effects of acidification to determine whether dispersal levels, Allee effects and biotic interactions could influence comˋmunity responses as pH levels increase. The experiment involved the introduction of four acid-sensitive colonist species ( Daphnia retrocurva, Skistodiaptomus oregonensis, Epischura lacustris and Tropocyclops extensus) into enclosures stocked with recipient communities. The experiment was a factorial design with colonists introduced at three starting densities (low, medium and high) and enclosures being stocked with either diverse communities typical of neutral lakes or relatively depauperate communities dominated by the acid-tolerant copepod Leptodiaptomus minutus. Per capita growth rates ( r) for colonist species were measured by sampling communities in enclosures over a 10-week period. Factorial anovas were conducted to determine whether r differed according to starting densities, recipient community type or their interaction., Our results indicated that r for the copepods E. lacustris and S. oregonensis were greater when introduced at high initial densities, suggesting an Allee effect based on mate limitation. In addition, r for S. oregonensis was lower when introduced to acid communities, suggesting biotic interactions with the resident community were important for this species. Per capita growth rates for T. extensus and D. retrocurva did not differ among treatments., Synthesis and applications. These results suggest that community response to environmental change can be influenced by dispersal, Allee effects and community interactions. Studies evaluating the ongoing recovery of zooplankton communities from acidification, or their response to other stressors, should take these factors into account. If rapid community responses are desired, dispersal rates for sexual zooplankton species (copepods) may need to be artificially increased. [ABSTRACT FROM AUTHOR]

Summary 1. Acidification has damaged biota in thousands of lakes and streams throughout eastern North America. Fortunately, reduced emissions of sulphur dioxide and nitrogen oxides beginning in the 1960s have allowed pH levels in many affected systems to increase. Determining the extent of biological and pH recovery in these systems is necessary to assess the success of emissions reductions programmes. 2. Although there have been promising signs of biological recovery in many systems, recovery has occurred more slowly than expected for some taxa. Past studies with crustacean zooplankton indicate that a mixture of local abiotic variables, biotic variables and dispersal processes may influence the structure of recovering communities. However, most studies have been unable to determine the relative importance of these three groups of variables. 3. We assessed chemical and biological recovery of acid-damaged lakes in Killarney Park, Ontario. In addition, we assessed the relative importance of local abiotic variables, biotic variables and dispersal processes for structuring recovering communities. We collected zooplankton community data, abiotic and biotic data from 45 Killarney Park lakes. To assess the recovery of zooplankton communities, we compared zooplankton data collected in 2005 to a survey conducted for the same lakes in 1972-73 using several univariate measures of community structure, as well as multivariate methods based on relative species abundances. To determine the factors influencing the structure of recovering zooplankton communities, we used hierarchical partitioning for univariate measures and spatial modelling and variation partitioning techniques for multivariate analyses. 4. Our survey revealed significant pH increases for the majority of sampled lakes but univariate measures of community structure, such as species richness and diversity, indicated that only minor changes have occurred in many acid-damaged lakes. Hierarchical partitioning identified several variables that may influence our univariate measures of recovery, including pH, dissolved organic carbon (DOC) levels, fish presence/absence, lake surface area and lake elevation. 5. Multivariate methods revealed a shift in communities through time towards a structure more typical of neutral lakes, providing some evidence for recovery. Variation partitioning suggested that the structure of recovering copepod communities was influenced most by dispersal processes and abiotic variables, while biotic ( Chaoborus densities, fish presence/absence) and abiotic variables were more important for cladoceran zooplankton. 6. Our results indicate that the recovery of zooplankton communities in Killarney Park is not yet complete, despite decades of emission reductions. The importance of variables related to acidification, such as pH and DOC, indicates that further chemical recovery may be necessary. The differing importance of abiotic, biotic and dispersal processes for structuring copepod versus cladoceran zooplankton might indicate that different management approaches and expectations for recovery are needed for these groups. [ABSTRACT FROM AUTHOR]

In the event of an environmental disturbance, dispersal of native taxa may provide species and genetic diversity to ecosystems, increasing the likelihood that there will be species and genotypes present that are less vulnerable to the disturbance. This may allow communities to maintain functioning during a disturbance and may be particularly important when the perturbation is novel to the system, such as the establishment of an invasive species. We examined how dispersal of native species may influence crustacean zooplankton communities in freshwater lakes invaded by the invertebrate predator, Bythotrephes longimanus. Using large enclosures, we experimentally tested the effect of dispersal on zooplankton community abundance, richness, and composition in (1) a community invaded by Bythotrephes, (2) the same community with the invader removed, and (3) a community that was never invaded. Dispersal increased zooplankton community abundance and richness; however, these effects were usually only significant in the invader-removed treatment. Dispersal tended to make the invader-removed communities more similar to never-invaded communities in abundance, richness, and composition. Dispersal had little effect on zooplankton abundance in the invaded community; however, richness significantly increased, and the community composition changed to resemble a never-invaded community by the end of the experiment. Our results have implications for understanding the role of dispersal during transitory states in communities. Dispersal of native taxa may be particularly important during the period between the arrival and broad-scale establishment of Bythotrephes, as dispersal through space or time (i.e., from resting eggs) may rapidly increase zooplankton abundance when the, invader is absent or in low abundances. Overall, our results suggest that communities with strong local predatory and competitive interactions may be closed to immigration from colonists, but that invasive species may alter the conditions under which species can establish. These results have implications for the interaction of native and invasive species across broad spatial scales, as regional dispersal of native taxa may forestall the local extirpation of native species. In particular, transient phases that result from variable persistence of invaders within habitats or across a region may permit native colonists to successfully establish, and thus increase local and regional resistance to future disturbance. [ABSTRACT FROM AUTHOR]

Analyses of temporal patterns of diversity across a wide range of taxa have found that more diverse communities often show smaller compositional changes over time. This generality indicates that high diversity is associated with greater temporal stability in species composition. We examined patterns of diversity and community stability in zooplankton time series data from 36 lakes sampled over a combined 483 years. The species–time relationship was flatter in more species-rich lakes in the temperate zone. However, high-latitude lakes had both low richness and low turnover. These patterns were consistent for turnover both within and among years. Daily, annual and long-term richness were all higher in large lakes while turnover was unaffected by the surface area. Richness on all time scales, as well as turnover within and among years, all declined at high latitude. Species–area relations and latitudinal gradients in richness therefore reflect different temporal components of diversity. Our results suggest that diversity shows strong associations with compositional stability that vary qualitatively across biogeographical provinces. Community stability increases with diversity among lakes in the temperate zone; however, the two are negatively correlated across latitudinal gradients. These patterns indicate that either the direct effects of diversity on stability or their covariance with environmental fluctuations vary with latitude. [ABSTRACT FROM AUTHOR]

The article investigates the roles of local environmental conditions and dispersal limitation in crustacean zooplankton recovery from acidification in Swan Lake, Ontario, a historically acidified, metal-contaminated lake. Mesocosm and emergence trap experiments were conducted. The preindustrial pH of the lake was estimated to be between 5.6 and 6.1 but sulphur dioxide emissions from nearby smelters caused a reduction in pH to 4.0 in 1977. Three colonist species-C. b. thomasi and S. ore-gonensis and, to a lesser extent,. M. edax-were not affected by the pH treatment in the enclosure experiment but were reduced in abundance by the resident zooplankton community indicating an effect of local biotic conditions. The results suggests that local and regional processes are instrumental in zooplankton community assembly. This indicates that a multifaceted management solution will be required for complete restoration of acidified ecosystems. Recovery of some - taxa, such as Ceriodaphnia and Daphnia, will be affected without further reductions in acid deposition and subsequent increases in take pH.

Invasive species introductions into freshwater ecosystems have had a multitude of effects on aquatic communities. Few studies, however, have directly compared the impact of an invader on communities with contrasting structure. Historically high levels and subsequent reductions of acid deposition have produced landscapes of lakes of varying acidity and zooplankton community structure. We conducted a 30-day enclosure experiment in Killarney Provincial Park, Ontario, Canada, to test the effects of Bythotrephes longimanus, an invasive invertebrate predator, on two contrasting zooplankton communities at different stages of recovery from acidification: recovered and acid damaged. Bythotrephes significantly decreased zooplankton biomass and abundance in both communities but had a greater negative effect on the abundance of zooplankton in the recovered community. Bythotrephes reduced species diversity of the recovered zooplankton community but not of the acid-damaged community. Species richness of both community types was unaffected by Bythotrephes predation. The effect of Bythotrephes on small cladocerans, a preferred prey type, differed between the community types and appeared to be related to density-dependent predation by Bythotrephes. Both community- and species-level results suggest that recovered and acid-damaged zooplankton assemblages may be negatively affected by an invasion of Bythotrephes but that the specific response is dependent on the original community structure. [ABSTRACT FROM AUTHOR]