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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. [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]

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.