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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.

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.

Human activities and climate change cause abiotic factors to fluctuate through time, sometimes passing thresholds for organismal reproduction and survival. Multiple stressors can independently or interactively impact organisms; however, few studies have examined how they interact when they overlap spatially but occur asynchronously. Fluctuations in salinity have been found in freshwater habitats worldwide. Meanwhile, heatwaves have become more frequent and extreme. High salinity pulses and heatwaves are often decoupled in time but can still collectively impact freshwater zooplankton. The time intervals between them, during which population growth and community recovery could happen, can influence combined effects, but no one has examined these effects. We conducted a mesocosm experiment to examine how different recovery times (0‐, 3‐, 6‐week) between salt treatment and heatwave exposure influence their combined effects. We hypothesized that antagonistic effects would appear when having short recovery time, because previous study found that similar species were affected by the two stressors, but effects would become additive with longer recovery time since fully recovered communities would respond to heatwave similar to undisturbed communities. Our findings showed that, when combined, the two‐stressor joint impacts changed from antagonistic to additive with increased recovery time between stressors. Surprisingly, full compositional recovery was not achieved despite a recovery period that was long enough for population growth, suggesting legacy effects from earlier treatment. The recovery was mainly driven by small organisms, such as rotifers and small cladocerans. As a result, communities recovering from previous salt exposure responded differently to heatwaves than undisturbed communities, leading to similar zooplankton communities regardless of the recovery time between stressors. Our research bolsters the understanding and management of multiple‐stressor issues by revealing that prior exposure to one stressor has long‐lasting impacts on community recovery that can lead to unexpected joint effects of multiple stressors. [ABSTRACT FROM AUTHOR]

Concern about salt levels in freshwater habitats receiving road de-icer runoff has inspired the development of "eco-friendly" formulations that are intended to be less toxic to aquatic organisms, but few experiments have determined that these products are environmentally benign. Mesocosms containing lake water were established for 6 weeks to compare traditional road salt with two newer de-icers, one an inorganic mixture of chloride salts and the second of beet extract and brine. Amplicon sequencing and algal blocking sequences facilitated the identification of differentially impacted bacterial taxa. Ironically, although there was only a minor effect on bacterial structure at high road-salt concentrations, there was an increased relative abundance of salt-resistant genera in the mixed-salt formulation. After amendment with the beet brine de-icer, there was a turnover of taxa coincident with a 68-fold decrease in dissolved oxygen, with decreased diversity and displacement by anaerobic genera indicating a shift across a threshold to a new, apparently stable state, suggesting mesocosm recovery was unlikely. Overall, although we applaud the sentiment behind the formulation of less-damaging "eco-friendly" de-icers, they appear to have more negative environmental impacts than the traditional road salt that they were made to replace. [ABSTRACT FROM AUTHOR]

The North American invasion of a non-native mysid, Hemimysis anomala, has been expanding since 2006, with the first inland lake invasions detected for Oneida Lake in 2009 and Seneca Lake in 2010. Although we know that Hemimysis primarily consumes zooplankton, our ability to predict the community-level impact of an invasion is hampered by a lack of information on taxon-specific effects. To investigate the effects of Hemimysis on native zooplankton, we conducted two mesocosm experiments that compared composition between communities with and without Hemimysis and studied how the effects of this predator on zooplankton species composition varied across a natural gradient of low to high invader densities (0.010.1 individuals x [L.sup.-1]). Our first experiment found that Hemimysis primarily affected cladocerans, and particularly Daphnia, shifting communities towards dominance by copepods. Our second experiment showed that Hemimysis invasions may do little to suppress Daphnia abundances until between 0.067-0.11 individuals x [L.sup.-1] or higher. Cladocerans are important links in freshwater trophic transfer and the nutrient cycle, and disruption of these linkages following Hemimysis invasion could result in both bottom-up and top-down impacts in nearshore food webs. However, Hemimysis can also fill a similar trophic role as the zooplankton they consume, and longer-term experiments are required to better assess their eventual impacts on native communities. Resume : L'invasion nord-americaine d'un mysidace non indigene, Hemimysis anomala, prend de l'ampleur depuis 2006, les premieres invasions de lacs interieurs ayant ete detectees dans les lacs Oneida, en 2009, et Seneca, en 2010. S'il est etabli qu'Hemimysis consomme principalement du zooplancton, la capacite de predire l'impact d'une invasion a l'echelle de la communaute est limitee par le manque d'information sur les effets a l'echelle des taxons. Afin d'etudier les effets d'Hemimysis sur le zooplancton indigene, nous avons mene deux experiences en mesocosme qui comparaient la composition de communautes avec et sans Hemimysis et etudie comment les effets de ce predateur sur la composition specifique du zooplancton variaient le long d'un gradient naturel de densite d'envahisseurs (de 0,01 a 0,1 individus x [L.sup.-1]). La premiere experience a etabli qu'Hemimysis avait principalement un impact sur les cladoceres, et plus particulierement Daphnia, exercant une transition vers une predominance de copepodes. La deuxieme experience a demontre que les invasions d'Hemimysis pourraient ne pas reduire l'abondance de Daphnia jusqu'a des densites entre 0,067 et 0,11 individus x [L.sup.-1] ou superieures. Les cladoceres sont des liens importants dans le transfert trophique en eau douce et le cycle des nutriments, et la perturbation de ces liens a la suite d'une invasion d'Hemimysis pourrait se traduire par des impacts ascendants et descendants dans les reseaux trophiques pres des rives. Hemimysis peut toutefois jouer un role trophique semblable a celui du zooplancton qu'il consomme, et des experiences de plus longue duree sont requises pour mieux evaluer ses impacts futurs sur les communautes indigenes. [Traduit par la Redaction], Introduction Hemimysis anomala (the 'bloody-red shrimp', hereinafter referred to as Hemimysis) is a non-native Ponto-Caspian mysid that was first detected in Lake Ontario and Lake Michigan in 2006 (Pothoven et [...]

Anthropogenic stressors including acid deposition, invasive species, and calcium (Ca) decline have produced widespread damage to Canadian Shield lakes, especially to their zooplankton communities. Here, we review current knowledge on the individual effects on zooplankton by the non-indigenous predator Bythotrephes longimanus and Ca decline; we identify knowledge gaps in this literature and examine the likely interactive impacts of Bythotrephes invasions and Ca decline on zooplankton. The negative impacts of Bythotrephes longimanus on zooplankton communities are well known, whereas current understanding of the effects of declining Ca on zooplankton is restricted to Daphnia spp.; hence, there is a large knowledge gap on how declining Ca may affect zooplankton communities in general. The co-occurring impacts of Bythotrephes and declining Ca have rarely been studied at the species level, and we expect daphniids, particularly Daphnia retrocurva and Daphnia pulicaria, to be the most sensitive to both stressors. We also expect a synergistic negative interaction on cladocerans in lakes with both stressors, leaving a community dominated by Holopedium glacialis and (or) copepods. Our predictions form testable hypotheses but since species and ecosystem response to multiple stressors are difficult to predict, we may actually see ecological surprises in Canadian Shield lakes as Bythotrephes continues to spread and Ca levels continue to fall. Key words: Cladocera, zooplankton, daphniids, Bythotrephes, multiple stressors, calcium decline. Les agents stressants anthropogenes incluant la deposition acide, les especes envahissantes et le declin du calcium generent des dommages largement repandus dans les lacs du Bouclier Canadien, surtout dans les communautes zooplanctoniques. Les auteurs passent en revue la connaissance des effets individuels sur le zooplancton du predateur non indigene Bythrotrephes longimanus et le declin du Ca sur le zooplancton; les auteurs identifient les manques de connaissances dans cette litterature et examinent les impacts interactifs probables des invasions du Bythotrephes et du declin du Ca sur le zooplancton. Les impacts negatifs du Bythotrephes longimanus sur les communautes de zooplancton sont bien connues, alors que la comprehension actuelle des effets du declin en Ca se limitent aux Daphnia spp.; par consequent, il existe un important manque de connaissance sur la facon selon laquelle le declin du Ca peut affecter les communautes de zooplancton en general. On a rarement etudie les impacts conjoints des Bythotrephes et du declin du Ca a l'echelle des especes, et l'on s'attend a ce que les daphnies, en particulier le Daphnia retrocurva et le Daphnia pulicaria soient les plus sensibles aux deux agents stressants. On s'attend egalement fi observer une interaction negative synergique sur les cladoceres dans les lacs avec les deux agents stressants conduisant a une communaute dominee par l'Holopedium glacialis et/ou les copepodes. Les predictions des auteurs constituent des hypotheses verifiables, mais puisque les reactions des especes et des ecosystemes fi de multiples agents stressants sont difficiles fi prevoir, on pourrait dans les faits rencontrer des surprises ecologiques dans le Bouclier Canadien, alors que les Bythotrephes continuent de s'etendre et les teneurs en Ca de chuter. [Traduit par la Redaction] Mots-cles : cladoceres, zooplancton, daphnies, Bythotrephes, agents stressants multiples, declin du calcium., Introduction Freshwater ecosystems cover only ~0.8% of the Earth's surface (Dudgeon et al. 2006), but they are hotspots of biodiversity and provide vital resources for humans (Strayer and Dudgeon 2010). [...]

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]

Across the Northern hemisphere, chloride concentrations in lakes are increasing from decades of road salt use, threatening aquatic ecosystems. Zooplankton are sensitive to increasing chloride but few studies have considered how additional stressors, such as soft water or increasing lake temperatures interact with the negative effects of chloride. To determine the effects of increasing chloride and temperature on freshwater zooplankton and phytoplankton communities, we conducted a mesocosm experiment using 30 concentrations of chloride from ambient levels to 1500 mg L−1, ambient or warmed (+2.7°C) temperatures, and plankton from a softwater lake. We compared the results with current Canadian Water Quality Guidelines (CWQG) for chronic exposure to chloride. Although warming elicited little response, increasing chloride concentrations led to reduced zooplankton biomass and richness, even at concentrations below the CWQG, and an increase in cyanobacteria abundance, suggesting that policies regulating road salt application should be reevaluated to better protect aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Increasing chloride concentrations threaten freshwater zooplankton. We questioned the protectiveness of the Canadian Water Quality Guideline for chloride because it is based on individual species studied under laboratory conditions and does not account for potential interactive factors, such as nutrient concentration. We exposed plankton communities to 30 chloride concentration increments for 6 weeks, crossed with either ambient or high nutrient treatments. Total zooplankton abundance, biomass, and richness declined with increasing chloride, with losses observed below the Canadian Water Quality Guideline. Nutrients did not affect the impact of chloride on zooplankton. Phytoplankton and protist responses varied by nutrient level. Under low nutrients, phytoplankton and protist abundance, biomass, and richness increased with chloride. Under high nutrients, phytoplankton and protist abundance and biomass were unaffected while richness decreased with chloride. These results indicate that current water quality guidelines do not sufficiently protect plankton and that nutrient context may alter phytoplankton and protist response. [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]