It’s winter. The lake is frozen—or, at least, it’s partially frozen. Under the ice, it’s dark. Mostly quiet. Inhospitable. And, down in the murky depths, the aquatic flora and fauna of cottage country are tackling the cold season the best they can. It’s a challenging environment. But probably not for the reasons that you’d think. In the summer, aquatic plants are photosynthesizing, providing oxygen to the lake’s inhabitants. But—as with deciduous trees and shrubs—many aquatic plants go dormant in the winter; with ice and snow blocking most sunlight, they can’t photosynthesize. This is a problem. “Once a waterbody freezes, there’s only a set amount of oxygen inside it,” says Jeff Hathaway, a herpetologist and the founder of Scales Nature Park in Oro-Medonte, Ont. “And everything, from the bacteria to the fish to the turtles, is going to use that oxygen. For many species, if it gets used up, it’s game over,” he says. That’s why, like hibernating mammals or migratory birds, aquatic species have to adapt to the environment that has changed drastically since the summer. Here’s how.
Illustration by Mathias Ball
How the lake changes in the fall
To understand life under the ice in winter, you have to first understand how the lake has changed over the course of the fall. “People always think it’s cold in a lake in the winter, but down under the surface, especially in the sediment, it’s warmer,” says Kaleigh Mooney, a restoration technician with Yukon Seed & Restoration in Whitehorse. This is thanks to reverse stratification—the “fall turnover.” In the summer, a warm layer of water sits on top of a colder transitional layer, which sits on top of a third, even colder layer: the cold zone. (Cottagers experience this temperature difference when they swim from shallow water to deeper water in the summer.) In the fall, as the air temperature drops, the warm top layer of the lake also cools, eventually reaching the same temperature as the bottom layer. The lake exists in this unstratified state until ice formation means that the top layer of water has become the coldest (around 0°C) and the bottom layer has become the warmest (around 4°C). “A lot of bugs, a lot of amphibians, a lot of plants have adapted to seek out that warm layer,” says Mooney.
And they know what they’re doing. “One time, my prof and I were collecting some samples of starry stonewort bulbils along the edge of the lake, and he went through the ice,” she says. “He kind of sank into the sediment, up to his hips, almost. When he got out, I was like ‘Oh my God, you must be so cold!’ And he was like ‘I was much warmer down there than I was out here.’ ”
How the animals get by
Turtles (Blanding’s, map, painted, and snapping, for example) and frogs (such as leopard, green, and bullfrogs) survive winter through the process of brumation. It’s similar to hibernation in mammals. All their body functions slow down: their heart rates, metabolism, and digestion. (Frogs might tuck their limbs underneath themselves, and turtles might pull themselves into their shells.) This all helps to reduce their demands for oxygen.
Because, probably unsurprisingly, “most of our turtles and frogs in Ontario can’t thrive in a low-oxygen environment,” says Jeff Hathaway. “In some cases, to wait out winter, they go to a place where the oxygen is not going to run out. That generally means deeper water, or flowing water—both are less likely to freeze.”
For warmth and protection from predators, such as otters (who are active all winter; see below), some turtles and frogs will partially bury themselves in the silt of the lake bottom, or in the vegetation, in the case of a bog-dwelling turtle. Or not. If there’s nothing comfortable to snuggle into, they won’t attempt it. “If the bottom of the lake is gravel, the frog or turtle will stay on top of the gravel,” says Hathaway. “If you were to go swimming under the ice, you could spot them. They just kind of…sit in between two rocks.” It doesn’t sound comfortable, but this trade-off is usually for the best—the ability to bury oneself comes secondary to having enough oxygen. This makes a deep, gravel-bottom lake generally safer than a shallow, sandy-bottomed one.
What about the fish?
Unlike reptiles and amphibians, fish don’t brumate. But they do slow down in winter to varying degrees. “Fish are ectothermic—that means that their internal body temperature is regulated by the surrounding water,” says Adam Weir, a fisheries biologist with the Ontario Federation of Anglers and Hunters. “When surface waters freeze, their metabolic function decreases, and their heart rate slows,” he says. This saves energy and, therefore, decreases oxygen demand. Since food (insects, crustaceans, aquatic plants, etc.) availability is lower in winter, in part because it’s a harsher environment for almost everything in the lake, fish will also decrease their foraging behaviour to avoid spending energy that they can’t replenish.
But the degree to which fish behaviour changes varies with the species—different fish have different temperature preferences. It’s called their “preferred thermal range,” says Weir. Warmwater fish, such as largemouth and smallmouth bass, for example, become dramatically less active in the winter. Similar to a hibernating mammal, they’ve spent the fall bulking up on prey so that they can rely on stored energy reserves for the winter. Groups of bass will also congregate in overwintering grounds where the water is warmer.
Coldwater fish, as any angler could tell you, are more variable in their winter whereabouts. Lake trout will hang out directly beneath the ice, or in areas of open water, or close to the lake bottom, says Weir. “Brook trout, on the other hand, are often found in shallow nearshore zones with woody cover, particularly where groundwater upwellings—colder, underground, nutrient-rich waters that move upwards to the surface—moderate temperature and create an ideal environment for them to live.”
For certain fish species, the cold lake is important for breeding and long-term survival. Burbot, for example, spawn in winter water that’s between 1°C and 4°C. (Most freshwater species spawn in spring or fall.) Cold conditions are also key to walleye reproductive success. “They rely on consistently prolonged winter periods to support proper gonadal development,” says Weir. “Elevated temperatures disrupt the process.” Too-warm water can lead to reduced egg size, higher egg mortality, and, ultimately, poor reproductive success. This has become increasingly relevant in the context of climate change, Weir points out. “Shorter and less severe winters favour warm-water adapted species—the so-called ‘climate change winners’—while disadvantaging cold-adapted species.”
Illustration by Mathias Ball
Where the mammals at?
Aquatic and semi-aquatic mammals, such as beavers, muskrats, and otters, are active in winter and adapt to the frozen lake environment by changing their behaviour—mostly to make sure that they don’t run out of food. Beavers fell and stockpile trees in the fall in preparation. Muskrats, on the other hand, need to forage for underwater vegetation all year. When the lake begins to freeze, they chew holes through the ice, then stuff the openings with plant material to make sure they don’t close up. These “push-ups” act as air holes. Otters are hunters, and they become nomadic—instead of sticking to one waterbody—travelling over land (see above) to find open areas in lakes where they can access their now sluggish or sleeping fish or amphibian prey.
Aquatic plants are busy
Like almost everybody else under the ice, most aquatic plants spend the winter conserving energy. Their green, leafy tissues of spring and summer have died and decomposed, but their roots are still alive, acting as energy storehouses.
“When plants are photosynthesizing in the growing season, they can either decide to put a ton of energy into producing green stuff that will allow them to capture the most amount of energy in the quickest amount of time and start growing right away, or they can decide to allocate energy into sustaining themselves for a longer period of time,” says Kaleigh Mooney.
That involves putting more energy into their roots. Water lilies do this. The lily pads and white blooms that you see floating on the lake’s surface are actually one plant, with the visible parts all connected to a large rhizome—a thick root that looks a little like a giraffe’s neck. It sits at the bottom of the lake. By the time winter arrives, like a bear who has fattened up ahead of hibernation, “it’s packed with hardy carbohydrates,” says Mooney. But unlike a bear, the plant isn’t starved for nutrients when spring rolls around. “It already has a store of food to allow shoots to start pumping when the ice clears and the temperatures get to that prime spot.”
Plants such as native milfoil and bladderwort take advantage of the lake bottom’s comforts. “When the aquatic environment changes in autumn, these plants will change their growth strategy to form little buds on their stalks,” says Mooney. “They’re called turions, and they’re packed with nutrients and chlorophyll.” Eventually, those buds pop off the plant, sink underwater, and land on the lake bottom. “It’s warm enough down in the silt to incubate the turions, so when spring arrives, they’re ready to grow.”
Of course, just like evergreen trees, not all aquatic plants go dormant for the winter—coontail, for example. It looks like a tiny pine tree, and, like an actual pine tree, it keeps its green foliage all year. “It’s basically free-floating, so it’s able to sink down and exists in a part of the water that isn’t freezing,” says Mooney. “It just hangs out there until spring.”
Even the bugs too
Insects that you spot in spring or summer at the cottage—including mayflies, dragonflies, caddisflies, or stoneflies—are winter’s unexpected warriors. Sort of: their larvae and nymphs spend winter in the water. (Like other underwater creatures, at that life stage they can breathe via adaptations such as gills or can absorb oxygen through their skin.) This is particularly true in groundwater-fed streams, where the constant water movement and the lack of snow and ice cover means that there’s always enough—you guessed it—oxygen to keep the youngsters alive. “But even in a lake, as long as it’s not frozen to the bottom, bugs can do okay,” says Hannah Bodmer, a PhD candidate in the Murray and McCauley labs at the University of Toronto Mississauga. “It seems counterintuitive, but you can find quite a few bugs under the water—and above the water—in the winter.”
When it comes to insects, the winter water world is still a little understudied, she admits. “Because it’s cold out, humans don’t want to be out. So there aren’t as many people who do winter research,” she says. “Plus, there’s this assumption that nothing’s going to be active, so you don’t look for it. And because you don’t look for it, you don’t see it,” she says. “But once you do get out there and look, it’s kind of amazing how much is still going on.”
Jackie Davis is a senior editor at Cottage Life. She wrote “Have a Nice Flight” in our Sept/Oct ’25 issue.
This story originally appeared in our Winter ’25 issue.
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