William Hintz likes to think of the complex connections making up our environment as a series of dominoes.
“You push one domino and many other dominos fall — even though you didn't push them,” Hintz, an assistant professor in the University of Toledo’s Department of Environmental Sciences, tells Inverse.
In his recent research, Hintz shows just how far the dominoes can fall, linking the de-icing salt Americans use to keep our roads safe to the survival of freshwater ecosystems. Hint’s research offers a grim look into the ways human activity is already harming food webs in lake ecosystems. The findings were published Monday in the journal Proceedings of the National Academy of Sciences.
“Everything in the environment is interconnected,” Hintz says.
What’s new — Salinization — the buildup of salt, particularly in freshwater ecosystems — can result from humans’ agricultural and mining activities, de-icing salt operations, and even climate change.
Until now, researchers hadn’t thoroughly examined the effects of human-induced salinization across numerous freshwater lake ecosystems throughout North America.
Hintz’s findings reveal that human-induced salinization causes substantial die-off of zooplankton populations in lakes across North America. Zooplankton are tiny microorganisms that feed on bacteria and algae. Creatures higher up in the aquatic ecosystem food chains in turn eat the zooplankton.
Mortality increased even in lakes where chloride levels — indicators of salinization — are at or below acceptable thresholds according to U.S. and Canadian laws. Chloride is often combined with sodium to create the salt (NaCl) often used in deicing salts, so it is a good marker for determining how much salt is in water.
In more than 70 percent of the lake sites studied where significant zooplankton mortality occurred, chloride levels did not exceed the legal limit. For reference, the acceptable levels of chloride in the U.S., Canada are:
- U.S.: 230 milligrams of chloride per liter
- Canada: 120 milligrams of chloride per liter
Europe does not have standardized freshwater chloride limits, but Germany classifies water containing 50 to 200 milligrams as “slightly polluted by salt.” Drinking water limits are typically 250 milligrams per liter across EU nations.
The current chloride thresholds “across North America and Europe clearly do not adequately protect lake food webs” the researchers state.
The researchers also conclude that human-induced salinization of freshwater lakes could “trigger a substantial loss” in zooplankton diversity and, as a consequence, generate “shifts in lake food webs.”
The death of zooplankton causes cascading effects that ripple throughout lake ecosystems, including a large increase in phytoplankton. Phytoplankton are microscopic marine algae that live in the limnetic zone of freshwater ecosystems where sunlight shines.
Why it matters — Zooplankton are “critical to lake food webs,” according to the report. Notably, they help transfer energy from primary consumers — like the algae they eat — all the way up the food chain to the fish that wind up on our dinner plates.
“Almost all fish species consume zooplankton when they are young and need zooplankton to grow and eventually become bigger fish,” Hintz says.
“Everything in the environment is interconnected.”
If salinization kills off zooplankton, that process could trigger reductions in both fish growth and population. Less zooplankton also likely means more algae since the zooplankton aren’t feeding on them. More algae could potentially block sunlight for lake ecosystems and affect water quality.
Saltier lakes could potentially mean bad news for humans, too. We depend on freshwater ecosystems for our drinking water. The Great Lakes on the Canada-U.S. border provide drinking water for more than 40 million Americans.
“Once salts get into our freshwater supplies, it is difficult or in some cases impossible to get salt out, and high salt concentrations can persist for decades,” Hintz says.
Freshwater lakes are also a major source of fisheries, recreation, and tourism.
“We — as a society — need to recognize that salt pollution is a major ecological issue affecting our freshwater ecosystems,” Hintz says. “We all need fresh water.”
How they made the discovery — The researchers conducted experiments using water from lakes in the U.S., Canada, and Europe. These water samples contained microorganisms like zooplankton, allowing the researchers to test how the creatures responded to elevated salt levels.
Then, the scientists conducted 16 different experiments in mesocosms, which are large tanks filled with lake water. Mesocosms allow ecologists to isolate specific factors in a way they could not if they were studying water in the lake directly.
“With an experimental mesocosm approach, we can be more confident that we are indeed observing the effect of salt and not some other factor in the environment,” Hintz says.
Using statistical models, the researchers estimated the levels of chloride — an indicator of salt levels — that would cause a 50 percent reduction in zooplankton. Their experimental methods allowed the researchers to conclude that current regulations are insufficient to protect widespread salinization from harming lake ecosystems.
What’s next — The researchers are crystal clear when it comes to the implications of their study.
“Policymakers will need to craft environmental legislation that lowers the allowable chloride concentrations in many regions to protect freshwater ecosystems,” Hintz says.
Still, that’s easier said than done. After all, de-icing salts significantly reduce car accidents and injuries, so we can’t just give them up overnight. But Hintz stresses that if we better understand the sources of salt contamination, we can, in turn, work to protect freshwater lakes from salinization.
“This may require better environmental monitoring by scientists, and regulations aimed at reducing salt pollution from multiple human-caused sources,” Hintz says.
For his part, Hintz hopes other scientists will build on his findings “to more fully elucidate the ecological impacts of salt pollution.”