Current water quality guidelines across North America and Europe do not protect lakes from salinization

Abstract

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.

Keywords: biodiversity; climate change; environmental policy; land use; water quality.

Fig. 1.

GAMs and 95% CIs (shaded regions) for cladoceran zooplankton abundance (black line) and chlorophyll a concentration (i.e., phytoplankton biomass; green line) among 16 experimental sites throughout North America and Europe. Cladocerans are featured as they are generally regarded as the largest, highest-biomass filter-feeding zooplankton grazers in our study systems. The vertical red dashed line indicates the lethal concentration value (LC₅₀) representing the Cl⁻ concentration at which there was a 50% reduction in abundance of each zooplankton taxa for each site with the associated 95% CIs (red shaded region around the dashed line). Graphs for calanoid and cyclopoid copepods and rotifers are shown in SI Appendix, Appendix 4, Figs. 4.2–4.4. No cladocerans were found at the Tavernoles study site in Spain.

Fig. 2.

LC₅₀ representing the Cl⁻ concentration at which there was a 50% reduction in abundance of each zooplankton taxa across experimental sites (A) Cladocerans, (B) Calanoids, (C) Cyclopoids, and (D) Rotifers. The horizontal red lines highlight the lowest global Cl⁻ thresholds for protecting aquatic life, which are set by governments in Canada (solid line, 120 mg Cl⁻/L) and the United States (dashed line, 230 mg Cl⁻/L). Vertical bars represent 95% CIs. Points and 95% CIs are red when they overlap with or were below the US threshold of 230 mg Cl⁻/L. When a zooplankton taxon is not found in the mesocosm experiment, it is noted as “NA.” One experiment did not count rotifers, denoted with an asterisk. LC₅₀ values were not estimable when model fits were unreliable due to a rapid decline in abundance to zero along the Cl⁻ gradient, there was no change, or an increase in abundance with Cl⁻ (for missing values, see Materials and Methods).

Fig. 3.

Mean (±1 SE) of the estimated percent change in zooplankton abundances for each taxa at the Canadian (120 mg Cl⁻/L) and US (230 mg Cl⁻/L) chronic threshold for Cl⁻. Percent changes were estimated using GAM models to compare the predicted abundances at each threshold to the predicted abundance in control conditions.

Fig. 4.

Map of study sites across North America and Europe where we investigated the impacts of chloride (Cl⁻) concentration—a strong indicator of human-induced salinization—on freshwater lake food webs. (A–D) Study site location across North America and Europe. (E) The regression-style design used to test how an environmentally relevant gradient of Cl⁻ concentration affected lake food webs among the study sites. Guidelines for Canada, United States, and Europe are indicated on the gradient.