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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]

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]

Anthropogenic acidification has affected biota in thousands of lakes in eastern North America and Europe. To measure the degree and extent of biological recovery following pH recovery in acidified lakes, many studies have assessed changes occurring in acid-damaged zooplankton communities. In this review we synthesize studies of zooplankton recovery from regions severely affected by acidification. In doing so, we provide a critical overview of: (1) the design of studies used to detect recovery; (2) the status of communities in acidified regions; and (3) our current understanding of the factors that limit recovery. The design of most studies assessing zooplankton recovery fall into three categories based on their selection of data to be used for recovery benchmarks: (1) historical; (2) reference-lakes; and (3) temporal. Within these study designs, the most commonly used metrics include species richness, indicator species, and relative species abundances. Many studies have used species richness as the sole indicator of recovery; however, we argue that additional metrics should be considered in analyses to make conclusions more robust. Studies conducted in eastern North America and Northern Europe have demonstrated significant, though often incomplete, recovery of zooplankton communities in lakes that reach a pH > 6.0. Data collected in central Europe indicate little recovery in the heavily affected Bohemian Forest lakes, but complete recovery of species richness in the moderately acidified Tatra Mountain lakes. Factors limiting biological recovery, including slow chemical recovery, dispersal limitation, and community resistance, vary in importance among and within regions, suggesting that region- and lake-specific management approaches may be required. [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.