In 2022, researchers found that salt levels within water-quality guidelines could still damage lake food webs; tiny algae-eating animals were vulnerable

A little salt can make an icy winter walkway safer, but once that salt reaches a freshwater lake, the consequences can be far harder to see. The most vulnerable creatures may not be the fish that people notice, but tiny zooplankton that sit near the foundation of the lake food web, as per a report by Queen’s University.

A major international study found that freshwater ecosystems could suffer serious ecological damage from salt concentrations that governments had considered safe, raising questions about whether existing water-quality guidelines were actually protecting lakes.

The findings were especially striking because the damage appeared at salt levels below, or around, established chloride thresholds in Canada, the United States and Europe.

How much salt is too much for a lake?

During winter, de-icing salt is widely used to keep roads, driveways and walkways safer. Agriculture fertilizers, mining operations and climate change can also contribute to rising salinity in freshwater ecosystems. The problem is that salt does not simply disappear after winter.

The international research led by Queen’s University researcher Shelley Arnott, from Biology, and Bill Hintz, from Ecology at the University of Toledo, examined the effects of increasing salinity on freshwater ecosystems.

The study involved 16 sites across four countries, with dozens of scientists from North America and Europe collaborating on the research. Queen’s students Danielle Greco, Brooke Rathie, Alex McClymont and Haley Richardson were also involved.

Their findings, published in the Proceedings of the National Academy of Sciences (PNAS), showed that existing water-quality guidelines were not necessarily preventing ecological harm, as per a report by Queen’s University.

In Canada, the chloride threshold is around 120 milligrams per litre. In the United States, it is around 230 milligrams per litre, while the threshold is generally much higher across Europe. Yet the researchers found that freshwater organisms could be affected at concentrations below these limits.

Why are zooplankton so important?

One of the clearest warning signs was what happened to zooplankton. These tiny organisms are an important food source for fish. When salt levels rise, researchers found that zooplankton can decline dramatically, while algae can increase. At 11 of the 16 study sites, sodium chloride concentrations that caused more than a 50 per cent reduction in zooplankton were either at or below the Canadian government's established chloride thresholds. That finding changes the way the problem looks.

The issue is not simply that extremely salty water can harm freshwater life. The study showed that ecological effects could occur within concentration ranges that governments had established as protective.



Dr Arnott explained why the loss of these tiny organisms could have consequences far beyond the zooplankton themselves.

"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," says Dr. Arnott. "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."

The chain reaction begins with something very small.

Fewer zooplankton can mean more algae. More algae can reduce water clarity. Changes in the lake can then affect organisms living at the bottom and potentially influence the broader functions and benefits that freshwater ecosystems provide.

Are current salt limits protecting freshwater ecosystems?

The researchers argued that the findings showed an urgent need to reconsider existing sodium chloride thresholds.

The concern is particularly significant because salt pollution is increasing as a result of human activities, including road de-icing.

Dr Hintz said the existing guidelines were not necessarily achieving the protection they were designed to provide.

"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 fresh waters aren’t doing their job," says Dr. Hintz. "Our study shows the ecological costs of salinization and illustrates the immediate need to reassess and reduce existing sodium chloride thresholds and to set sound guidelines in countries where they do not exist to protect lakes from salt pollution."

The researchers said future thresholds should take into account the vulnerability of ecological communities at local and regional scales.

That approach matters because the study found differences among the sites examined. Rather than relying only on a single broad threshold, the researchers suggested that ecological susceptibility should be considered when determining what salt concentrations freshwater systems can withstand.

The scale of the problem can also be difficult to appreciate. The supplied research notes that it can take less than a teaspoon of salt to pollute five gallons of water to a level harmful to many aquatic organisms.

What enters a lake may therefore seem insignificant when compared with the size of the water body, while still creating serious consequences for sensitive freshwater organisms.

The researchers ultimately pointed toward a difficult balance.

Communities depend on salt during winter to melt snow and ice and make roads and walkways safer. At the same time, the widespread use of de-icing salt contributes to freshwater salinization.

The proposed response is not simply to ignore human needs, but to reduce the ecological cost by cutting the amount of road salt used or finding alternatives.

The study's message is therefore focused on the gap between what regulations allow and what freshwater ecosystems can actually tolerate.

If zooplankton begin disappearing before salt concentrations reach the levels governments currently regard as harmful, then the consequences can move through the lake food web long before the water is considered unsafe under existing guidelines.

For the researchers, that makes reassessing those thresholds urgent.

The findings highlight how a substance used routinely during winter can become an environmental problem once it enters freshwater systems. What looks like a small amount of salt can affect organisms at the base of aquatic food webs, with possible consequences for algae, fish, water clarity, fisheries, drinking water quality and recreation.

FAQs

1. What does salt pollution do to lakes?
   It can reduce zooplankton and increase algae.
2. Where does freshwater salt pollution come from?
   Road salt, fertilizers, mining and climate change can contribute.