During the winter, all life under the ice has to adapt to conditions that are strikingly different from those found in the summer. In the summer, our lakes are layered with the warmest water on top and the coldest on the bottom. As you descend, the temperature slowly decreases until you reach the thermocline where there is a sharp drop in temperature. This invisible line separates the more productive waters that contain microscopic plants called phytoplankton from the much colder, darker areas below.
Ice on a Northern Ontario lake.
The thermocline is just below the surface in the spring and fall and drops to twenty-five feet or more in midsummer. Lake trout, burbot and a few other species of fish spend almost all their time below the thermocline and just enter the warmer waters for short periods to feed. They move up and down depending on the thermocline and even come to the surface when the temperature of the lake water is the same everywhere in late fall and early spring.
In winter, the water beneath the ice is also layered, but it is the opposite of what is found in the summer. This, and the unusual properties of ice, is due to the bizarre chemistry of water. Water is the most dense at 39 Fahrenheit (4 Celsius) and consequently the water at the bottom of deep lakes is this temperature regardless of the season. This causes a topsy-turvey world where the warmest water in the winter is on the bottom (39 F.) and the coldest water is just below the ice (32 F.) at the top. Consequently, a lake trout swimming at a depth of 80 feet in the summer is in the coldest water in the lake and the same lake trout swimming at the same depth in winter is still at the same 39 temperature but is now in the warmest water in the lake.
Anyone who has forgotten to add sufficient antifreeze to a vehicle or didn’t drain the plumbing at their cabin is well aware that ice expands as it freezes. Water is highly unusual in expanding, rather than contracting, as it goes from a liquid to a solid. If it acted like most compounds, ice would sink as it formed, and ice would be continuously forming at the surface and dropping to the bottom. This would cause lakes to freeze from the bottom up. Much ice would form and sink to the bottom on a -35 night in January.
Virtually all our lakes, with the possible exception of the deepest, largest lakes, would be solid ice by March. This would obviously be devastating to life in our lakes. Since ice forms on the top of the water, it puts an insulating layer between the water and the colder air above. This greatly slows the formation of more ice. The accumulation of snow on top of the ice adds another, and better, insulating layer. Even with these insulating layers, we can still get over three feet of ice. This is a strong testimony to the severity and length of our winters.
Ice and snow protect from the cold, but they also greatly decrease the amount of light entering the water. The combination of low light levels and low temperatures causes photosynthesis to virtually stop in the winter. Phytoplankton levels drop dramatically and, therefore, the production of oxygen virtually comes to a halt. The amount of oxygen that is in the water at freeze-up has to last until the ice-cover melts in the spring.
Fortunately, the colder water is, the more oxygen it holds. Once again the chemistry of water works to the advantage of living things. Water obtains oxygen from two main sources: the photosynthetic organisms growing in it (phytoplankton, algae and photosynthetic bacteria), and from direct contact with the air. Both contact with the air and light levels are dramatically reduced at freeze-up and consequently very little oxygen is added to water during the winter. This means that for almost half the year, fish and other oxygen-using organisms, have to get by on the oxygen present when the ice forms.
Since photosynthesis virtually stops, no more food is being made and the amount of food under the ice dwindles during the winter. It’s no wonder that many organisms cope with these conditions by slowing their metabolism. Cold-blooded reptiles and amphibians that overwinter under the ice survive by hibernating in the mud at the bottom of ponds or shallow bays of lakes. A frog meet its oxygen requirements by simply breathing through its skin. The skin’s large surface area allows it to remain stationary and still take up enough oxygen so it can survive a sleep of half the year.
Turtles meet their oxygen needs in a different way. They are also found at the bottom of shallow bays or ponds, but take in oxygen through their cloaca, the opening to their reproductive and excretory systems. They can do this because it is lined with a rich network of blood vessels that functions as a gill. Since lungs are specialized organs for breathing air, organisms like frogs and turtles have adapted other ways of absorbing oxygen from water.
Many microscopic organisms, such protozoa and rotifers, form protective shells known as cysts and remain in an inactive state until the water warms up in the spring. Water fleas and other small crustaceans produce thick walled eggs in late autumn and these survive through the winter and hatch in the spring.
Many fish also slow their metabolism in the winter. Species that spend most of the time in the warmer parts of the lake in the summer, such as largemouth bass, smallmouth bass, and walleyes, become much less active in colder water. This significantly lowers their requirements for both food and oxygen. They won’t increase their activity until the water warms in the spring.
Fish that spend most of the summer in the cold water below the thermocline continue to be active throughout the winter. Lake trout are the best-known species of fish that stays active all winter. They thrive in cold water and travel freely throughout the lake in the winter. Their metabolism remains high and they have to continue to seek out and catch minnows and small fish all winter long. Although the cold does not bother lake trout, the decreasing amounts of both its prey and oxygen levels can be a problem.
It would be interesting to know how lake trout hunt in the depths of the lake where there is little or no light. The deeper you go in the water, the less light penetrates. Even in very clear water lakes, light only penetrates to fifty feet or so, and below that it slowly fades to pitch black. Lake trout are caught by people fishing at depths where virtually no light penetrates. Even at shallow depths in winter, they have to capture prey in the very low light that penetrates through the snow and ice. How they accomplish this is one of the many unknowns concerning the winter ecology of our lakes.
Another fascinating cold-water species is the burbot. This unusual looking fish, also known as eelpout or lawyer, is a fresh water relative of the codfish. Burbot have been caught, along with lake trout, in nets at depths of over 400 feet in Lake Superior. They are active all winter and are often caught by people ice-fishing. Their eel-like shape, elongated fins and smooth skin probably account for their being discarded on the ice in spite of their excellent taste.
Whitefish and their smaller relative, ciscoes, are other fish that are active under the ice. Both are usually found below the thermocline in the summer, but often enter warmer water to feed. Ciscoes can be seen surfacing on still days in the summer, but can also be found at more than a hundred feet. Their wide-ranging travels are responsible for their being reported to be important summer food sources for both lake trout and burbot in deep water and loons near the surface.
The conditions faced by organisms beneath the ice is similar in many ways to those faced by organisms at the bottom of the snow pack, in the pukak. The temperatures are constantly just a few degrees from the freezing point, it is either pitch black or with dim, filtered light, and the food source is constantly dwindling. Just like organisms in the pukak, they have met these conditions in diverse and inovative ways.
It is a real challenge for cold blooded animals like fish, reptiles and amphibians to survive the cold conditions under the ice, but there are also a few mammals that enter the water under the ice. A thick coat is obviously needed to survive our winters and animals that enter the water need coats that are waterproof as well as exceptionally warm. Water siphons heat from the body much more rapidly than air and it is extremely difficult for mammals to maintain their body temperature in cold water.
Beavers and muskrats spend most of the winter in their lodges huddled together for warmth. The insides of beaver lodges, insulated with mud and sticks, is many degrees warmer than the outside air. They spend much time grooming and keeping their fur well oiled. They have to enter the water to retrieve food that they have stored in a food cache next to their lodge. Even short immersions in the icy-cold water are enough to put a strain on their systems.
Beaver and muskrat have an unusual metabolic tactic that increases the time that they can spend in the water under the ice. They elevate their body temperature slightly just prior to entering the water and this gives them a few more minutes before their body temperature drops too far. Researchers have also shown that muskrats obtain oxygen from air bubbles trapped under the ice and presumably beavers do the same. Beaver and muskrat also have physiological adaptions that decrease the amount of heat loss when they are in cold water. In their feet and at the base of their tails they have a network of capillaries where the arterial blood flowing toward the extremities passes in close contact with the blood in the veins returning from the extremities. The arteries give up heat to the colder blood returning from the feet and tail. This warms the blood returning to the body and cools the blood going to the extremities. This allows them to maintain a higher core body temperature and their cooler extremeties lose less heat to the water.
Land animals, such as lynx and bobcat, also have a heat exchange system in their feet. This allows them to lose less heat to the cold snow they stand and walk on. They carefully avoid, however, getting their feet wet in the winter. Most other mammals also avoid getting wet, since wet fur loses most of its insulating value. There are exceptions: both mink and otter enter the water in the winter to hunt their prey. When I snowshoe along the French River in the northeast corner of Quetico, I often see otter tracks along the river. Their tracks clearly show that they swim for stretches in the open water and come out and travel along the shore where the river is iced over. I am amazed that an animal can come out of the water in sub-freezing temperatures and travel over land in a wet fur coat. Two special adaptions enable them to do so. They have extremely dense and oily underfur that is both warm and sheds water. They also have outer guard hairs that are hollow for additional insulation, and that interlock with each other to protect the underfur.
Otters have a streamlined body that, with extremely short ears, short legs and heavily furred tail, is also designed to minimize heat loss. Because of their short legs, they have to plow through the snow on land and leave a distinctive trough in their wake. It seems appropriate that this toboggan-shaped animal sides on its belly whenever possible.
The late biologist Olaus Murie, in his interesting and informative book “A Field Guide to Animal Tracks” recalled that he “…was snowshoeing up a small stream when I spied movement in the snowy stream bank up ahead. I realized that it was an otter, and the next moment it slid down the bank. Another one appeared, clambered up the bank and slid down. A third appeared from the hole in the ice, and for several moments I watched these frolicsome animals, climbing, sliding, climbing, sliding, over and over again – until all disappeared under the ice. Their playtime was over, and they all went on their way beneath the ice, as so often they do.”
I have never observed otters playing in the winter but I have seen otter slides in various places in Quetico. There is usually one into the water below the rapids from Quetico to Beaverhouse Lake. Otter slide down the hill adjacent to the portage directly into the fast water that remains ice-free year around.
The docks at both the Canada Customs and the Ranger Station at Prairie Portage were always covered with otter droppings when we arrived in the spring the years we were rangers at Prairie Portage. The open water below the rapids is a prime location for otter to hunt fish and crayfish in the winter. They obviously found the docks a convenient place to come out of the water, bask in the sun and relieve themselves. Beaver and muskrat huddle together for warmth in insulated lodges when they return from the water. Otters, on the other hand, are mainly solitary creatures in the winter and don’t have a primary lodge to return to. They evidently commonly use old beaver lodges for dwellings. They must consume a great deal of food in the winter in order to maintain their active lifestyle and stay warm.
To do this they seek out prey both in open water and under the ice all winter long. They have successfully adapted to our extreme winter conditions both in and out of the water by taking an extremely active and aggressive approach to winter.