The ice abides: Who’s winning the war?

While those of us in the Northwoods have shivered our way through December, January and now most of February, a silent war has been playing out on your lake, and mine, and others.

Below-zero temperatures, day after day, want to make the ice thicker, while two feet or so of light, fluffy snow wants to act like the Pink Panther’s favorite product and insulate. So, in the face of this winter’s record-challenging cold, who is winning? Is the ice steadily building or is the insulation keeping its thickness close to normal?

It turns out the answer isn’t entirely clear. Today I asked a young man at Kurt’s Island Sport Shop in Minocqua (Wis.) what ice conditions are like on the local lakes. He told me that on average the ice is about 30 inches thick. That’s a lot of ice, but not unprecedented, and as cold as it has been I wouldn’t have been surprised if it had been much thicker. Last February here on Birch Lake (near Harshaw) I went ice fishing with a friend, and we had to power-auger through more than two feel of ice to find water. Of course, last year wasn’t as cold, but there was very little snow cover.

So this year it seems the snow’s R-value is having an effect. And the snow is having other effects too. The fellow at the sport shop said there’s a heavy layer of slush beneath the surface snow. Why? According to the folks at WinterTrekking.com, slush forms on top of lake ice from overflowing water: 

"As soon as it snows on top of ice, that creates a force pushing down on the floating ice.  All ice, including perfectly safe thick ice, naturally cracks day and night, expanding and contracting with changing air temperatures. When the ice cracks, water can rush up through the cracks on top of the ice but under the insulating snow, and form slush pockets.  These slush pockets can become very broad, sometimes covering entire lakes under the snow, and they are a hazard to travelers."

And what’s the consequence? A lot of ice anglers are getting their 4-wheelers stuck on the way to their shanties. They’re learning a lesson from trout fishing writer John Gierach: "Four-wheel drive doesn‘t mean you can go anywhere. It just means you can get stuck in worse places.” And I can’t think of many worse places to get stuck than hubcap-deep in slush in the middle of some lake snowscape. So if you’re going fishing these days, a snowmobile is the best way to get to your favorite spot (if it’s too far out to reach on snowshoes).

And just wait until (if ever) all that snow starts to melt. Then we’re going to see some messy conditions on our lakes.

The ice abides: How thick is it?

It has been cold. Very cold. For a long time. I am confident we have not seen a temperature above freezing since Thanksgiving, and now the forecast calls for 30 below zero on Sunday into Monday.

So, living on a lake as I do, I wonder: How thick is that ice? It’s not easy to find out unless you are friendly with ice fishermen. Two winters ago, when a friend came north in February to go fishing, we found the ice more than two feet thick. It’s been much colder this year, but it’s only January.

And of course there’s the matter of snow – a good insulator – on top of the ice. Just how much insulation does snow provide? It’s hard to get a good answer. One “fact” I’ve seen in several places on the Internet (though without a credible source cited) says 10 inches of fresh snow that is 7 percent water (and the rest presumably air) has about the same R-value (R-18) as a 6-inch layer of fiberglass insulation. That’s quite impressive, if true. We have at least 10 inches of snow on Birch Lake now.

Then there’s the matter of the ice itself. On the Internet I found statements (again unattributed) that ice a foot thick has an R-value of 9, “much higher than wood, newspaper, or rigid foam board.” I also found values (credible ones from university sources) for the thermal conductivity of ice. Without getting into a lot of scientific units, it turns out that ice is only about twice and thermally conductive as glass, which is a notoriously poor heat conductor.

So what does all that have to do with the thickness of this winter’s ice? Well, for the ice to get thicker, heat (such as it is) has to escape from the water immediately below the ice. To do that, it has to penetrate first the ice and then the snow. The thicker the ice, and the deeper the snow, the harder it is for that heat to escape. 

I am betting that 30 degrees below zero for a couple of days will meaningfully thicken the ice. But if you want to know how thick the ice is on your lake, you’ll likely have to talk to an ice angler. Or go out and bore a hole yourself.

The ice abides: The otter slides

Last winter for the first time, on one of my walks around the rim of Birch Lake, I saw otter tracks in the snow. I wasn’t certain that’s what I had seen until I did a little research – which also told me I wasn’t rightly interpreting what I had seen.

Along the frozen shore of your lake is a great place to look for otter tracks, especially before the snow gets too deep. Otters (technically river otters, scientific name Lontra canadensis) come out of their winter burrows, usually near water, to forage for food.

The tracks I saw had paw-print trails interrupted by long, slender depressions in the snow. I assumed those depressions were made by the otter’s belly dragging. That wasn’t quite accurate. Actually, according to an article from Wisconsin Natural Resources magazine, otters “conserve energy moving across the snow by taking a few bounds and then sliding on their tummies. From above, these tracks look a bit like Morse code, in a dot, dot, dash pattern.” Otters can travel quite fast, about 15 to 18 miles per hour, faster than most people can run.

So knowing what I do now, I am even a little more inclined to be anthropomorphic about otters and say: Here’s an animal that knows how to enjoy life.

I followed the trail of last winter’s otter until I came to a hole in the ice up against a mucky shoreline interlaced with tree roots, near our lake’s outlet to a creek. I had followed him (or her) home. Go down to your lake soon and see if you can’t find an otter’s trail to follow across the snow. 

The ice abides: If Thoreau had a Secchi disc

Your lake is probably clearest at the time you’re least likely to be looking down into it – which is now, in winter, the water encased in ice and covered with snow.

Lakes clear up in winter for a variety of reasons. Cold water slows down the growth of algae (phytoplankton). In addition, the snow cover shuts out sunlight so that the algae cannot photosynthesize (make food) and so die back. Also, in essentially a closed vessel, the water is very still – no wave action from wind or boaters means bottom sediments are not stirred up and particles suspended in the water can settle out. There’s no rainfall runoff to wash soil and debris into the lake. Water still enters from springs, but that’s groundwater, essentially clear.

If you’re lucky enough to experience an early winter when the lake freezes solid enough to walk on, and a week or so goes by with no snow, you can enjoy a real spectacle. Even in fairly deep water, you can see all the way to the bottom and make out every detail.

Scenes like this remind me of a favorite passage from Henry David Thoreau’s Walden, telling about the clarity of the author’s beloved pond:

“Once, in the winter, many years ago, when I had been cutting holes through the ice in order to catch pickerel, as I stepped ashore I tossed my axe back on to the ice, but, as if some evil genius had directed it, it slid four or five rods directly into one of the holes, where the water was twenty-five feet deep.

“Out of curiosity, I lay down on the ice and looked through the hole, until I saw the axe a little on one side, standing on its head, with its helve erect and gently swaying to and fro with the pulse of the pond; and there it might have stood erect and swaying till in the course of time the handle rotted off, if I had not disturbed it. Making another hole directly over it with an ice chisel which I had, and cutting down the longest birch which I could find in the neighborhood with my knife, I made a slip-noose, which I attached to its end, and, letting it down carefully, passed it over the knob of the handle, and drew it by a line along the birch, and so pulled the axe out again.”

I wonder what a Secchi disc clarity measurement would have yielded in water that clear. Of course, Walden pond was naturally clear – Thoreau reported seeing the bottom 25 to 30 feet down while canoeing even in mid-summer. Your lake most likely is not that clear; my lake (Birch, near Harshaw, Wis.) certainly isn’t. Still, I long to experience an early December of clear ice. It almost happened this year. A couple of cold, still nights created a smooth skin, but then came snow. Now about a foot of snow lies on the ice; there will be no looking down into clear, cold water this year.

The ice abides: Turtles in winter

On my winter walks on the Birch Lake ice I come upon fallen logs where, in summer, painted turtles line up to sun themselves.

You may have asked, “Where do the turtles go in winter?” And that’s an easy answer, right? To the lake bottom, to hibernate. But then, we know turtles aren’t like fish. They spend a lot of time underwater, it’s true, but they don’t have gills. They need to surface to breathe; we’ve seen their snouts poke up in calm water, then disappear.

So, if in summer they need to surface every so often for a breath, how do they survive under the ice for, say, four to five months with no access to the air? It turns out they actually can breathe down there – though not with their lungs, by way of the snout. And they need very little oxygen to make it through the winter, because their metabolism slows to almost nothing. 

At the lake bottom, the winter water temperature hovers around 4 degrees C (39 degrees F). Since turtles are cold-blooded, that becomes their body temperature. They become extremely sluggish; if they crawl or swim at all, it’s very slowly. Their hearts slow down to as low as one beat every ten minutes. They eat very little, if at all.

Yet because their body functions don’t shut down completely, they need oxygen. They get it from the oxygen dissolved in the water. It enters their bodies through the linings in the mouth and throat, and through two small sacs near the anus with very thin skin, laced with numerous tiny blood vessels. They could never survive breathing this way in summer, when their metabolism is high, but in winter, it’s enough. ← Larry the Bullfrog speaks about living in Wisconsin

Ice Fishing Contest on the St Croix Flowage Saturday, January 19 →

So unless we go through a winter of truly epic proportions, there will be enough oxygen in the water for turtles to survive. And next summer we’ll see them again, sunning themselves on those same old fallen logs.

The ice abides: When winter kills

When winter starts this way – snowy and very cold from early December – we know we’re in for a long slog until spring. We may worry about the birds, about deer starving in their yards, and about the fish in our lakes, especially if those lakes are shallow. We’ve all heard of winterkill. Can a lake really freeze clear to the bottom?

Well, not likely. But that doesn’t mean a long, cold winter can’t kill fish. It can, and it does. Only it’s not the ice that kills them – not directly anyway. They die from lack of oxygen, which is to say, from suffocation. The ice seals the lake, cutting off the supply of fresh air. The water’s oxygen level then has only one way to go, and that’s down. The fish themselves, and the decay of organic matter, use up oxygen. If the ice cover remains for too long, there’s so little oxygen left that the fish can’t breathe.

Fish need a certain level of dissolved oxygen in the water – at least 2 parts per million. An oxygen concentration below 1 part per million is lethal to many if it persists. Fish that are the most vulnerable are those in shallow lakes with lots of vegetation and mucky bottoms rich in organic matter.

“Winterkill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish, which bloat as the water warms in early spring,” says an article on the Michigan Department of Natural Resources website. Of course, some fish tolerate low oxygen better than others. Bass, walleyes and bluegills are fairly tolerant; perch and northern pike are more so. Bullheads can withstand severe oxygen depletion.

“February is usually a critical period and is the best time to check the oxygen content of lakes prone to winterkill,” says the Michigan DNR article. “A good mid-winter thaw about then often recharges the lake’s oxygen supply by means of photosynthesis and melt water. Conversely, a prolonged winter, with continuous snow cover and late ice-out increases the chance of winterkill.”

People on many small lakes protect them against winterkill by pumping in air with motor-driven aerators. It’s effective, though not a permanent solution. The real answer is to reduce the amount of nutrients entering the lake – nutrients that feed an abundance of plants that ultimately die, decompose, and deplete the winter oxygen supply.

Property owners can help by avoiding lawn fertilizers and making sure their septic systems are functioning properly, instead of seeping nutrient-rich water into the lake. Of course, some lakes are just naturally rich in nutrients, and in such cases there isn’t much lake residents can do, short of an extremely costly process of dredging out sediment. Winterkill in such cases is just part of the natural cycle. 

What happens to lakes that go through winterkill? Well, seldom do all the fish die. Enough usually make it through to reproduce. If the lake has an inlet stream, fish may come in that way and repopulate it faster. Nature takes its course; things heal. Some even maintain that a minor or moderate fish kill can be good for a lake: The fish populations are thinned out so there is more food for the survivors, which then prosper and grow.

Still, a winter die-off is not something to look forward to. If you live on a shallow lake, or if you have such a lake as a favorite fishing spot, a long and harsh winter can be a legitimate cause for worry.

The ice abides: In cold blood

What’s it like to live under the ice? We’ll never know the sensations, because after all, we’re not fish – and we can never relate to their experience because our metabolism is radically different from theirs.

We are warm-blooded; fish (along with frogs, crayfish, turtles and other water creatures) are cold-blooded. Our bodies regulate our temperature; much of the energy we consume as food goes to feed the inner furnace that keeps us at 98.6 (or so) degrees F. Cold-blooded creatures’ temperatures rise and fall with the temperature of their environment, which means right now the fish in your lake are at somewhere around 40 degrees F, just like the water.

What does that mean in a practical sense? It means the fish are quite sluggish. Their muscle movements rely on complex chemical reactions that proceed rapidly when warm and slowly when cold.

Imagine what it would be like to live in cold water as the warm-blooded mammals we are. Staying warm would be impossible. Even at 70 degrees, water pulls heat out of our bodies dramatically faster than does air at a similar temperature. At 40 degrees water temperature, our bodies simply could not keep up; we would be uncomfortable to say the least and would die soon from hypothermia. Think about the mammals that live in water (like whales) or spend a lot of time in it (seals, sea otters, walruses). Their adaptations tend to include heavy fur, a thick layer of insulating blubber, or both.

Cold-blooded creatures are perfectly fine in cold water. They don’t have to heat themselves, which means they don’t use a lot of energy. They don’t have to eat a great deal in winter because the cold tamps down their metabolism. Ice anglers can see evidence of this in the fish they catch and clean – the stomachs are often full of food. Prey that might digest in a day in summer may take a week in winter.

You may have wondered, if you ice fish, why you catch certain species in winter more so than others. It’s because fish react differently to the cold. Walleye and northern pike eat plenty as ice forms, weeds die back, and prey fish become more exposed. Bass and muskies, on the other hand, don’t move around much and eat just enough to sustain basic functions.

Where do fish go in winter? Some stay relatively near the surface where the water contains more oxygen. Some hunker down in the depths. Bullheads bury themselves in the bottom, not actually hibernating but moving very little.Many fish hang around the same kinds of places they haunted in warmer times – weed beds, brush piles, manmade cribs and other cover. Bluegills, for example, crowd around cover for protection. The flips side is that their presence in turn attracts predators, like northern pike.

Right now, the air temperature over my lake (Birch, near Harshaw, Wis.) hovers in the single digits, and a wind whips over the snow. Talk about wind chill all you want, but being in the water below the ice would feel much colder. This warm-blooded creature is glad to have a heated cabin and a bowl of soup to come to after a walk.