According to an international study led by Rensselaer Polytechnic Institute experts, current water quality guidelines aren’t safeguarding freshwater habitats from growing salt contamination caused by road de-icing salts, agriculture fertilizers, and mining operations.
The study, which was published in the Proceedings of the National Academy of Sciences (PNAS) today (February 21, 2022), shows that freshwater salinization causes a massive loss of zooplankton and an increase in algae even when levels are below the lowest thresholds established in Canada, the United States, and Europe.
“It’s clear that salt pollution in freshwater lakes, streams, and wetlands, even when constrained to levels specifically chosen to protect the environment, threatens the biodiversity and overall function of freshwater ecosystems. This is a global problem that has the potential to impact ecosystems and human health,” said study co-author Rick Relyea, an expert in the impacts of road salt on freshwater ecosystems, and director of Rensselaer’s Darrin Fresh Water Institute.
“The good news, as we’ve seen in our own region, is that communities are learning how to apply road salts in smarter ways while still providing safe roads and saving considerable money in snow and ice removal.”
Dr. Relyea, director of the Jefferson Project at Lake George and a member of the Rensselaer Center for Biotechnology and Interdisciplinary Studies, has done considerable study on the effects of road salt on aquatic habitats. His research has contributed to the discovery that road salt masculinizes young frogs and disrupts zooplankton’s circadian cycle.
Dr. Relyea has recently worked with a network of 16 experimental sites in four countries spanning North America and Europe. Dr. Relyea and that network published experimental data earlier this year, led by Canadian scientist Marie-Pier Hébert, showing that lake salinization reduces zooplankton number and diversity.
Salt pollution occurring from human activities such as the use of road de-icing salts is increasing the salinity of freshwater ecosystems to the point that the guidelines designed to protect freshwaters aren’t doing their job. Our study shows the ecological costs of salinization and illustrates the immediate need to reassess and reduce existing chloride thresholds and to set sound guidelines in countries where they do not exist to protect lakes from salt pollution.Bill Hintz
The University of Toledo and Queen’s University in Kingston led the PNAS study, which found that even at salt concentrations below those deemed safe and protective of freshwater creatures by government regulators, considerable harm to freshwater lakes occurs.
Rising salinity threatens zooplankton, a crucial food supply for many young fish, and changes in nutrient cycling, water quality, and clarity, as well as growth and population decreases in commercially significant fish species, could be influenced by rising salinity.
Researchers say the findings point to a major threat to freshwater ecosystem biodiversity and function, as well as the need for governments to reassess current threshold concentrations to protect lakes from salinization caused by sodium chloride, one of the most common salt types that cause freshwater lake salinization.
“Salt pollution occurring from human activities such as the use of road de-icing salts is increasing the salinity of freshwater ecosystems to the point that the guidelines designed to protect freshwaters aren’t doing their job,” said Bill Hintz, assistant professor of ecology at The University of Toledo, author, and co-leader of the project. “Our study shows the ecological costs of salinization and illustrates the immediate need to reassess and reduce existing chloride thresholds and to set sound guidelines in countries where they do not exist to protect lakes from salt pollution.”
The Environmental Protection Agency has set a minimum chloride concentration level of 230 milligrams per liter in the United States. It’s 120 milligrams of chloride per liter in Canada. Thresholds are generally higher in Europe.
It only takes a teaspoon of salt to poison five liters of water to the point that many aquatic organisms are harmed.
Concentrations of chloride between 50 and 200 milligrams per liter are defined as “slightly polluted by salts” in other nations, such as Germany, and concentrations between 200 and 400 milligrams per liter are classified as “moderately polluted by salts.” Throughout much of Europe, the safe drinking water guideline is 250 milligrams per liter.
However, as the study demonstrates, detrimental consequences can occur much below those limitations. Chloride concentration thresholds that caused a more than 50% decline in zooplankton were at or below the governments’ defined chloride thresholds at roughly three-quarters of the study sites.
At nearly half of the study sites, the loss of zooplankton generated a cascading effect, resulting in an increase in phytoplankton biomass, or microscopic freshwater algae.
“More algae in the water could lead to a reduction in water clarity, which could affect organisms living on the bottom of lakes as well,” said Shelley Arnott, professor of aquatic ecology at Queen’s University and co-leader of the project and paper.
“The loss of zooplankton leading to more algae has the potential to alter lake ecosystems in ways that might change the services lakes provide, namely recreational opportunities, drinking water quality, and fisheries.”
The researchers chose to study zooplankton communities in natural habitats rather than single-species laboratory studies because this approach encompasses a greater diversity of species as well as naturally occurring predator-prey and competitive interactions over a six to seven-week timeframe within the zooplankton community.
The goal of the study was to see how the chloride thresholds would fare in a more natural ecological situation.
They wanted to see if current chloride-based water-quality criteria preserve lake creatures in varied geographies, water chemistry, land-use patterns, and species pools.
“Many salt-contaminated lakes with chloride concentrations near or above thresholds established throughout North America and Europe might have already experienced food web shifts,” Dr. Hintz said.
“This applies to lakes across the globe, not only among the study sites. And the variability in our experimental results demonstrates how new thresholds should integrate the susceptibility of ecological communities at the local and regional scale. While the government guidelines may protect freshwater organisms in some regions, that’s not the case for many regions in the U.S., Canada, and Europe.”
Finding strategies to strike a careful balance between human use of salt and ecological implications, such as lowering the quantity of road salt used to melt winter snow and ice to keep people safe and traffic flowing, are potentially possible solutions. Best management strategies were suggested in a prior study done by Dr. Hintz.