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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]

Throughout much of the 20th century, unprecedented industrial emissions have led to widespread acidification of regions in North America and Europe and, as lake water pH dropped, aquatic ecosystems have experienced dramatic declines in biodiversity. International emission‐control agreements have led to sweeping increases in lake pH, however acid‐structured zooplankton communities still persist in many lakes. Concomitantly, calcium concentrations have been declining as a legacy of acidification and are approaching or have reached concentrations that could represent a barrier to the re‐establishment of zooplankton communities similar to those in non‐acidified or circumneutral reference lakes. To understand how declining calcium may influence the re‐establishment of zooplankton in acid‐damaged lakes we manipulated calcium and pH using a factorial in‐lake mesocosm experiment and assessed their individual and combined effects on a regionally diverse zooplankton assemblage. We found that the impacts of low calcium on zooplankton species were similar to those of acidification and, consequently, may prevent the recovery of acid‐structured communities. Abundance of the larger bodied and acid‐sensitive Daphnia pulex/pulicaria increased in high pH treatments, albeit nonsignificantly yet, by the end of our experiment, only two individuals were sampled among our 10 low calcium enclosures. In contrast, small acid‐tolerant cladocerans, such as Daphnia catawba, Daphnia ambigua, and eubosminids maintained significantly higher abundances in low calcium treatments relative to all other treatment combinations. Although we did not detect an effect of calcium on mean body size, the disproportionately high abundance of small cladocerans in low calcium treatments resulted in low calcium communities with higher overall abundance and lower cladoceran evenness. Our results, along with a landscape comparison demonstrating parallel changes in zooplankton relative abundance from 34 historically acidified lakes, suggests that declining calcium will be an important, on‐going factor that may limit the recovery of zooplankton, despite regional improvements in lake pH. [ABSTRACT FROM AUTHOR]

Stressors can interact to affect animal fitness, but we have limited knowledge about how temporal variation in stressors may impact their combined effect. This limits our ability to predict the outcomes of pollutants and future dynamic environmental changes. Elevated salinity in freshwater ecosystems has been observed worldwide. Meanwhile, heatwaves have become more frequent and intensified as an outcome of climate change. These two stressors can jointly affect organisms; however, their interaction has rarely been explored in the context of freshwater ecosystems. We conducted lab experiments using Daphnia pulicaria , a key species in lakes, to investigate how elevated salinity and heatwave conditions collectively affect freshwater organisms. We also monitored the impacts of various recovery times between the two stressors. Daphnia physiological conditions (metabolic rate, Na+-K+-ATPase (NKA) activity, and lipid peroxidation level) and life history traits (survival, fecundity, and growth) in response to salt stress as well as mortality in heat treatment were examined. We found that Daphnia responded to elevated salinity by upregulating NKA activity and increasing metabolic rate, causing a high lipid peroxidation level. Survival, fecundity, and growth were all negatively affected by this stressor. These impacts on physiological conditions and life history traits persisted for a few days after the end of the exposure. Heat treatments caused mortality in Daphnia , which increased with rising temperature. Results also showed that individuals that experienced salt exposure were more susceptible to subsequent heat stress, but this effect decreased with increasing recovery time between stressors. Findings from this work suggest that the legacy effects from a previous stressor can reduce individual resistance to a subsequent stressor, adding great difficulties to the prediction of outcomes of multiple stressors. Our work also demonstrates that cross-tolerance/susceptibility and the associated mechanisms remain unclear, necessitating further investigation. [Display omitted] • We lack knowledge of how temporal variation in stressors affects their interactions. • Daphnia were used to examine the interaction between salinity stress and heatwaves. • Salt caused physiological responses that persisted after the termination of stress. • Daphnia previously treated with salt were more susceptible to subsequent heatwaves. • Legacy effects of previous stress reduced individual resistance to a novel stressor. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

1. Interactions between multiple anthropogenic stressors can have unexpected synergistic or antagonistic effects, making it difficult to predict their combined effects using single-stressor studies. The interaction between invasive consumers and nutrient enrichment is particularly important as both stressors frequently co-occur, and their respective bottom-up and top-down effects have the potential to interact across multiple trophic levels. 2. We conducted a mesocosm experiment that crossed an increasing nutrient addition gradient against an increasing zebra mussel invasion gradient. Native zooplankton communities were added to the mesocosms, and after 3 months, we examined how the single-stressor effects on available resources and the zooplankton community were altered by their multiple-stressor interaction. 3. Added nutrients had no effect on small phytoplankton, but increased the abundance and dominance of copepods and reduced the density of large phytoplankton, probably due to increased topdown predation pressure. Zebra mussels reduced large phytoplankton concentration by about 80%, rotifer abundance by about 75%, and shifted communities towards dominance by cladocerans and adult/juvenile copepods. 4. When combined, the top-down control exerted by the mussels interacted antagonistically to prevent any bottom-up influence of nutrient enrichment on the zooplankton community. These results provide insight into the potential outcomes of nutrient and invasive consumer stressor interactions, and illustrate the need for researchers to consider environmental change in a multiple-stressor context. [ABSTRACT FROM AUTHOR]