zooplankton. main characters side by side: daphnia and a copepod


“No one cares about zooplankton.”

You’ll hear this often from ecologists of the freshwater variety, so it’s no surprise coming from Shelley Arnott. She teaches and explores lakes out of Queen’s University in Kingston, Ontario.

And she knows she’s exaggerating, a little. Not only is she smitten by these amazing critters, she recalls her dad’s first visit to her lab. A contractor who didn’t finish high school, he’d never had the opportunity to look into a microscope. “You can see their little heartbeat!” he exclaimed when he saw Daphnia in live focus for the first time. For some time afterwards he carried in his pocket a closeup photo of a Daphnia. “Probably to brag about his daughter, but also to show people,” says Arnott. 

Zooplankton simply are the most amazing aquatic critters you can barely see. Some graze algae, some scavenge, and some eat smaller zooplankton. (And while I haven’t checked, I’d put good money that there are smaller zooplankton that eat larger zooplankton. That’s one of nature’s enduring plot twists.)

They come in a bewildering array of shapes, sizes, colors, feeding times, patterns of movement, and habitat tastes. Copepods — there are more than 2,800 freshwater species — can look like mythical stylings of crayfish while Daphnia — a common cladoceran — look much like Roald Dahl’s Vermicious Knids, only cuter and with helmets.

It’s an aquatic explosion of biodiversity. “They’re so diverse, and most people are not even really aware of their existence,” she says. The key thing is how they’re linked to the rest of the food web. “They play such a huge role in the functioning of ecosystems.” 

Not only are zooplankton co-captains of life in a lake, they’re also fabulous tools for ecological experiments.You can use them in labs and in the wild, changing up their circumstances and listening to the feedback. “You can more reliably study community interactions,” says Arnott. And then you can start to make predictions.

Just a few lab tests sparked Arnott’s work with salt. Canadian water quality guidelines permit 120 milligrams per liter— seawater clocks in at 35 grams per liter — and Arnott was looking at lower levels, between 5 and 40 milligrams. “We were shocked, because the Daphnia were sensitive,” she says, showing reduced reproduction and increased mortality. “That’s way, way below the water quality guidelines,” says Arnott.

3 daphnia stacked up together. As written in the piece, Daphnia — a common cladoceran — look much like like Roald Dahl’s Vermicious Knids, only cuter and with helmets.
Three Daphnia mendotae, in queue, for reasons unknown.


Every ecosystem is organized by the flow of energy. Most gather energy from the sun. Where lakes are shallow, this includes plants, but in deeper water it’s algae. You know algae — also called phytoplankton — small plant-ish organisms that make lakes green. You can think of zooplankton as the middle men, only cladocerans (Daphnia among them) can reproduce asexually, so in a lake most are females making more female clones while also grazing down the algae and helping to keep the water clear. They’re also tasty to fish.

Once you learn that rising salt levels could depress zooplankton abundance and diversity, you can make a simple ecological prediction: with less grazing the lake is going to get greener. Fewer zooplankton also means less fish food, and fish populations will show this.

That’s a trophic cascade, a concept that’s interesting and complex enough that it should be part of our vocabulary when it comes to understanding environmental change. The effects of any environmental stressor — in this case salt — are first felt by the most sensitive species. Then those effects can echo up and down the food chain via the trophic cascade.

Ecosystems change as different ecological actors take the stage. Not every single zooplankton performs the same role. There are grazers, predators, and scavengers. Some live near the shore and others in open water. Many have overlapping niches, or interests.

That diversity of ecological roles, of ecological function — that biodiversity — fills different roles in an ecosystem. Interchangeable pieces help a system adapt to stressors. A living system adapts to environmental change by swapping out parts and pieces of ecological function. “They’re playing different but complementary roles, so that diversity is really important,” says Arnott.

Losing zooplankton changes species interactions all over. You don’t see it at first, the gradual greening of the lake. You’re not likely to notice what’s happening to the fish for a while. But things are changing and, at least in the short term, probably not for the better. 

“Under the surface, there’s a loss of biodiversity,” she says. “It’s tricky with aquatic systems, because you don’t see what’s going on, and because zooplankton are so small we just don’t see what’s happening, or how widespread the change is, until too late.”

Arnott’s first experiments suggested that we could already be losing sensitive species. From there, the effects could cascade. 

Graph showing rising salinity in Ontario's Lake Simcoe from 1971 to 2017.
Salinity is rising in Ontario’s Lake Simcoe. (Image source: Lake Simcoe Region Conservation Authority)


Salt pollution can be a little confusing. We know life thrives in water both salty and sweet, and some organisms can even handle both. On the inside, freshwater organisms are a little salty, reflecting the biochemistry of the early seas where life took hold. We need salt to survive.

This memory of the sea frames a chemistry problem for most living things. Freshwater organisms like Daphnia are always fighting to keep freshwater OUT of their bodies. They’re pumping ions across membranes, essentially trying to keep from being dissolved by the water that they live in. 

“If you increase salt in the water, shouldn’t that make their life better?” Arnott asks. But it doesn’t. Against intuition, it stresses their precisely tuned membrane chemistry, forcing creatures to use a lot of extra energy to maintain balance. They breed less often, and live shorter lives.

Water is our planet’s most incredible solvent, and it brings other chemistry into the picture.

By the 1970s, streams and lakes in eastern Canada and the northeast of the U.S. were becoming dangerously acidic, inhospitable to fish and other creatures. Sulfur was the culprit. The most iconic source was the colossal nickel smelter in Sudbury, Ontario, which in 1972 built a 1,250 foot smokestack to disperse the exhaust as broadly as possible. (The solution to pollution is dilution, remember?) 

Fixing the problem meant tweaking everything from coal plants to smelting processes to internal combustion. That happened through broadly bipartisan legislation, with international and regional cooperation. It spawned the first cap and trade market, using market-based incentives.

Arnott began her career studying the recovery of Canadian lakes from acid rain. Ontario is paradise for lake scientists, with more than 250,000 lakes and a disproportionateamount of the world’s fresh water. “Acid rain did so much damage,” says Arnott. “But I was looking at the recovery, so there was a lot of hope in that.”

That recovery is still incomplete, but now many lakes in Ontario are affected by salt pollution. Lake Simcoe — the largest lake in southern Ontario that is not also a Great Lake — shows alarming increases since measurements began in the 1970s.

Lakes harmed by acid rain are more vulnerable to salt pollution because of depleted calcium. The 5th most common metal in the earth’s crust, calcium in natural systems is gradually released as water flows through soil and over rock. Acid rain stripped those calcium reserves, lessening calcium flow into lakes today.

Calcium in antacids buffers your tummy, and can buffer lakes from a variety of pollutants, including salt. “In some ways, we created an even more sensitive landscape,” says Arnott. “It makes a big difference. They become much more sensitive as calcium declines.”

Shelley Arnott, turned away from the camera as she hauls in an unseen unseening device while sitting in a canoe.
Shelley Arnott, sampling from a canoe. (Image courtesy Shelley Arnott)


Lake science — called limnology — faces a lot of challenges because every lake is unique. While governed by the same rules, differences in size, shape, rainfall, and geology make every lake something of a unicorn. 

The Global Lake Ecological Observatory Network (GLEON) is an international grassroots collaborative that began meeting in 2005. More than 200 scientists attended the 2017 gathering at Mohonk Mountain House in New York. During loosely themed networking time some half dozen people — Arnott included — gathered in a circle of comfortable chairs to talk about salt.

They shared what they were working on and thinking about, including Arnott’s recent findings showing that some zooplankton were more sensitive than expected. Wouldn’t it be cool—not to mention incredibly useful—to run the same experiment in lakes thousands of miles from each other? “Lakes can vary a lot,” says Arnott. But how much?

Arnott took the lead in organizing the collaboration, recruiting 20 sites. And it was truly grass roots, devoid of dedicated funding.

A lot of science is directed by the agencies and businesses that pay the bills. Science costs money, and for the most part what’s left for ecological research comes from deep in the couch cushions — the coins sticky with mysterious residue and grit. 

“I’m still blown away by the fact that we didn’t have any funding for this,” Arnott. “It was just a group of people saying ‘This would be fun to do.’” And useful. “It’s so powerful to ask the same question using the same methods in multiple systems. Context matters, right?” 

The final analysis compared 16 sites in Canada, Spain, Sweden, and the U.S.. Model ecosystems called mesocosms were filled with local water and local zooplankton and algae. These organisms had not previously been exposed to salt, but were hit with a range of salt concentrations on the lower end of current guidelines.

There were definitely differences — California zooplankton were not very sensitive, while those in one lake in Quebec “almost died when they looked at them.” Half of the sites experienced the green cascade: zooplankton declined and algae surged. Of the 16 sites, 12 showed a significant decline in cladoceran — that’s what Daphnia are — biodiversity. 

Despite scouring the data for common variables like water hardness and nutrients that might confound the results, no other common thread emerged.

Comparing lakes often generates contradictions. A study in one lake gets one result, while the next lake over drops a different result. “What I like about ecology is all this complexity,” she says. “I love that things are so different, that you get different responses in different places.”

This complexity isn’t as popular with voters and policy makers, so having the same result across so many lakes is powerful, a strong indicator that salt is a major freshwater threat. “I think we were all surprised that most of the zooplankton were super sensitive, with huge losses in abundance below the water quality guidelines.”

Shelley Arnott using a net in a small boat on a lake in Ontario; trees in the distance.
Shelley Arnott wields a sampling net. (Image courtesy Shelley Arnott.)


Acid rain is a reminder that we can solve complex environmental problems. We identified a significant and systemic pollutant and came together to reduce acid rain by an order of magnitude. Government did its job, then nature set about repairs. 

“It’s not just studying the doom and gloom,” Arnott says. “It’s an exercise in hope.”

Salt action, she hopes, is gaining traction. Salt sheds in Canada now must be covered. Numerous improvements in technology and training have led to reductions in salt application. The rising cost of salt has helped nudge the process into a higher gear.

“I think we’re doing so much better on roads,” Arnott says. Parking lots? Not so much.

Meanwhile, she’s helping develop regional and local strategies. For example, lakes on the Canadian Shield are very dilute — not just low in salt, but poor in nutrients and ions of any kind. And the evidence that she and her colleagues have uncovered so far is that zooplankton here are extremely sensitive to salt pollution.

Arnott and others are working with the government of Ontario to better understand the relationship between water hardness and salt toxicity. She believes that lakes on the Canadian Shield may need a substantively lower salt guideline — 40 or even 20 milligrams per liter — instead of the one-size-fits-all standard of 120 milligrams per liter that governs Canada.

Frankly, it’s a different calculus than we typically use for salt. If you’ve ever been on the highway and happy to see a salt truck, you know what a difference salt can make. Unequivocally, salt saves human lives.

We’re not as experienced at accounting for the value of things like zooplankton biodiversity. But there is little question that it matters. 

“It is difficult to imagine a salt-free future for global freshwaters,” Arnott wrote last year with colleague Hilary Dugan of the University of Wisconsin — Madison. “To turn off the tap of salt requires a societal shift towards recognizing and prioritizing freshwater quality and eliminating our dilution mindset.”

They concluded: “Only when we reduce or eliminate salt inputs will we be able to confront the legacy of salt pollution that has accumulated in the terrestrial landscape, and be able to predict and protect the future of freshwater.”

“I’m hoping that the work that we’re doing really highlights the negative consequences,” adds Arnott. “It is damaging systems that are really important to us. Hopefully, that pushes us to other solutions.”


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