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.

Freshwater salinization from anthropogenic activities-including the application of road deicing salts-is a global environmental concern, harming aquatic biota and drinking water quality. However, the relative sensitivity of zooplankton communities to salinization across large scales remains largely unknown. Performing experiments in parallel across North America and Europe, we show that there is substantial variation in the sensitivity of different zooplankton taxa to salinization, but that chloride sensitivity is more common and pronounced in crustaceans. This study demonstrates that the abundance and diversity of the communities can be reduced at chloride levels below common water quality guidelines (120-250 mg Cl À1 L À1 , depending on the study country). Our results suggest that freshwater biodiversity might be reduced by chloride contamination at a global scale.

Not all populations of a species have the same sensitivity to contaminants, yet no studies have assessed this variation for multiple species across large geographic regions. Our study is the first to quantify within-species variation in salt tolerance (chloride, Cl À ) by conducting experiments at 16 environmentally diverse locations and compiling published tolerances from laboratory studies. Across our study sites, we found that Cl À sensitivity varied up to 4.2x AE 3.0 SD within species. This variation was related to the species they co-existed with, suggesting that species interactions can modulate Cl À tolerance, making it difficult to predict how individual communities respond to Cl À increases. To adequately protect freshwater zooplankton from harm, water quality guidelines should be based on multiple populations and communities to incorporate variation in sensitivity.

Scientific Significance StatementCurrent water quality guidelines for chloride may not protect aquatic life against rising lake salinity in cold regions where de-icing salts are applied to paved surfaces. Although chloride tolerance of Daphnia pulex  Daphnia pulicaria can increase with food availability, it is unclear how nutrient-and salt-driven changes in phytoplankton and protist composition will impact zooplankton sensitivity to chloride. We exposed freshwater zooplankton communities to a chloride gradient under either ambient nutrients (mesotrophic) or high nutrients (meso-eutrophic) and found that while additional nutrients increased available prey, this did not increase zooplankton tolerance, resulting in large declines in zooplankton abundance and biomass at the current Canadian Water Quality Guideline limit for chloride.

Spring ephemerals take advantage of the high light levels available in the spring by completing the aboveground portion of their lifecycle before the canopy develops and while few other understory plant species are growing. The spring is marked by high resource availability, yet spring ephemerals are variably abundant throughout forests. Research indicates that canopy conditions can influence the growth of spring ephemerals; consequently, we tested whether the variation in canopy conditions predicted variation in the abundance of Erythronium americanum Ker Gawl. across 50 forest plots. We also tested whether the specific leaf area (SLA) of E. americanum in plots was predicted by variation in plot-level canopy conditions, reflecting E. americanum‘s ability to adapt to different canopy conditions. The abundance of E. americanum was significantly lower in the plots with greater hard canopy closure (i.e., permanent cover: tree architecture + evergreen leaf cover), and significantly higher under canopies that reached full development earlier. Canopies with greater hard canopy cover at the start of the growing season were associated with significantly higher SLA, quantifying local adaptation by E. americanum to variable canopy conditions. Erythronium americanum takes advantage of the high light levels available in the spring. It is unclear at this time why higher abundance of E. americanum is associated with canopies that close earlier.