The Wonder of Lakes
by Don Meredith © 2004
(first published in the March 2004 Alberta Outdoorsmen
Crossing lake ice can be nerve racking if you let your imagination get the best of you — even when you're crossing a lake with ice ten or more feet in thickness. That's what I was doing many years ago on the Boothia Peninsula in the Canadian High Arctic. My Inuk assistant, David Nanook, and I were following fresh caribou tracks across the lake. Although it was May, the temperature was somewhere around -20 degrees Celsius and we were crossing ridges of wind-sculpted, hard packed snow (called "mapsuq" by the Inuit) on top of the ice.
David and I were trying to find the calving ground of the caribou near the northern-most tip of the peninsula (also the northern-most point of North America) for a biological study we were doing for some petroleum companies planning the route of a proposed gas pipeline. David knew where the calving grounds should be, and I needed to document them. He was also teaching me a lot about this barren world and the life it contained.
On seeing the tracks of a group of animals crossing the lake, David suggested we leave our snowmobiles on the shore and follow the tracks on foot. He explained the animals were not too far ahead and the machines might scare them. Also, the mapsuq was a rough ride for snowmobiles.
Suddenly, a rifle shot rang from one side of the lake to the other and echoed off the low hills to the north. I froze in position, looking around to see from where it might have come. But it was hard to focus on its origins as the sound seemed to have exploded from below and then rang through the air. David looked at me with a mischievous grin and then broke into a laugh. There was no one within hundreds of kilometres of us.
"It's the ice!" He pointed to the sun now arching high in the late morning sky. "The sun is getting hotter and the ice is waking — stretching, trying to break free."
We continued walking, and it wasn't long before we heard another in a series of these explosive cracks that increased in frequency as the day wore on. We found the caribou later that day and eventually their calving grounds. And within a few weeks, the ice on all the lakes in that region did melt and arctic char swam into some of them from the Arctic Ocean to spawn before the ice formed again.
Lakes are interesting ecosystems. For a biologist, they are fairly easy to define with inlets, outlets, and distinct limits of depth and dimension. Most of what goes on in a lake stays in the lake. Like most ecosystems, sunlight provides the energy. It's trapped in the water by algae and other green plants and passed on to insects, worms and a host of other invertebrates that eat vegetation. These are eaten by small fish that are in turn eaten by larger fish. Only we anglers and few other terrestrial predators occasionally remove fish from the ecosystem.
Of course, water is a major limiting factor. Each species requires so much in which to live. However, in the winter, there is a problem with water — it freezes when it drops to zero degrees Celsius and below. Most fish can't survive freezing and we wouldn't have any fish in our northern lakes if it weren't for an important property of water — it is at its densest at four degrees C. In other words, a litre of water at four degrees is heavier than another litre of water gathered at say 10 or 1 degree C. Why is this important? As lake water cools in the fall, loosing more heat to the night air than it gains from the sun in the daytime, it approaches four degrees near the surface of the lake. Being heavier, this four degree water drops below the warmer water and into the depth of the lake. This forces the warmer water to the surface where it in turn cools to four degrees and drops, causing the lake water to "turnover." Soon, the whole lake is at four degrees and the water at the surface is now beginning to cool past those four degrees. However, this water is lighter than the warmer water below it and stays at the surface. It eventually reaches zero degrees and forms ice, while the heavier water settles at the bottom of the lake, remaining near four degrees. If the lake is deep enough, it does not freeze to the bottom all winter, allowing fish and invertebrates to continue with their livelihoods.
Another property of water is its ability to hold dissolved oxygen. In the summer oxygen in the air freely enters the lake water at the surface and with the water coming from inlet streams. Some is produced by the algae and other green plants. As ice forms across the lake surface in the fall, much of the oxygen from the air is cut off, and that from plants is greatly reduced. Some oxygen does penetrate the ice through cracks and the crystal lattice work, but it is a much slower process. So, life below the ice is largely dependent on the amount of oxygen the lake water possesses at freeze up. If this oxygen runs out, especially in smaller lakes, large die-offs of fish and other animals can occur in what is called a "winterkill." In large lakes, winterkill may not occur throughout the lake but only in shallow bays where fish may be unable to escape to deeper waters.
As spring approaches and the sun gets higher in the sky, more of its light penetrates the ice and begins warming the lake bottom as well as the ice itself. The temperature of the ice begins to rise and its crystal structure changes, causing cracks to suddenly occur with loud reports. Warmer water rising from the depth of the lake begins to melt the ice from below, much as the sun is melting it from above. The ice soon melts and once again the lake water is at a uniform temperature from top to bottom. This is the spring turnover.
However, soon the water at the top of the lake warms well above four degrees, and is lighter than the colder water at depth. Often this difference in temperature can be quite large, causing a "summer stagnation" where the surface water does not mix with the deeper water. As a result, less oxygen reaches the deeper water, and sometimes there is a "summer-kill."
In our northern lakes, this stagnation doesn't last long and the lake water is once again cooling to begin the cycle all over again.
The next time you find yourself sitting over an ice hole wondering when the next fish will bite, or you hear the ice loudly crack in the early spring sun, consider how the properties of water and ice keep the lake alive and a wonder to behold.