Your lake just took a deep breath

The last post talked about thermal stratification in your lake. I mentioned that in winter there’s a layer (a thin one) of cold water directly under the ice, and beneath that the slightly warmer – yet denser – water that makes up most of the lake’s volume.

Well, a week or two ago, the ice melted, and the wind’s action stirred that surface water in with the rest, so that the water temperature was fairly uniform from top to bottom. This condition is important because it allows the lake to, in essence, take a deep and restorative breath.

In this time when the lake is not yet stratified (with warmer water on top and colder water below, as in summer), wind and wave action can mix oxygen into the water. So if you imagine putting a drop of blue food coloring into a bowl and stirring, that in a sense is what happens with the oxygen in your lake – it is thoroughly and evenly mixed in. This is called the spring turnover.

Of course, oxygen is what the fish, snails, crayfish, plankton and all manner of creatures need to survive. That includes the microscopic creatures in the lake sediments that break down dead material that sinks to the bottom.

As the weather gets warmer, your lake is starting the process of stratifying for the summer – warmer water on top, cooler water below. Later on we’ll explore what this stratification means to the life under your lake’s surface. 

Your lake has layers

Your lake has layers

A year ago at the Wisconsin Lakes convention, a featured speaker asked for a show of hands: “How many know what a thermocline is?” Only about half the hands went up, and this in a room filled with lake lovers – lake residents, lake association people, anglers, DNR staffers, watershed managers.

So that tells me if I say your lake has layers, not all of you will say, “I knew that!” Yes, your lake has layers – and it’s for the simple physical reason that a less-dense liquid floats on a more-dense one, as oil floats on water. OK, that’s not a pretty image, but it makes the point. Warm water is less dense than cold water and will float on top of it.

What happens as spring moves into summer, and warm air and sunlight pump heat into the lake, is that the deep water stays cold, while the water nearer the surface gets warm. So in summer, your lake has a layer (a thick layer, mind you) of warmer water floating on cold. Between them there’s a zone where the temperature changes rapidly.

The technical term for this condition is thermal stratification. And again in technical terms, the warm upper layer is the epilimnion, the cool lower layer is the hypolimnion, and the zone in between is the thermocline.

On most lakes in Wisconsin, where I live, it’s easy to experience the thermocline – and you may well have done so without knowing it. Next time you go swimming on a nice summer day, get out where the water is fairly deep, say 15 feet or so. The surface water you’re in will likely be comfortably (or at least reasonably) warm. Now, do a feet-first surface dive. With an upstroke of your arms, propel yourself down. Soon your feet will feel a sudden change to much cooler. That means they have hit and passed through the thermocline. And with this simple test you can determine roughly how thick that epilimnion is.

This thermal stratification is a seasonal thing – it actually less reverses itself in winter, when the colder water is on top and the (only slightly) warmer water is on the bottom. This happens because water has the unique property of actually becoming less dense as its temperature falls below about 39 degrees F and heads toward the freezing point (32).

Stratification makes all sorts of interesting things happen in your lake – about which, more in later posts. For now, just be glad that if you’re asked amid a roomful of people what a thermocline is, you’ll be able to raise your hand. And that you now have something new and interesting to tell your grandkids next time they visit your lake.

Last ice, first fish

Have you been fishing yet? What does your lake hold? If it has walleyes, and if the ice has gone out, those walleyes in all likelihood are moving into the shallows to spawn. Their timing is dictated not by the calendar but by the water's temperature, and three very warm days since ice-out have kicked that up quickly here on Birch Lake. Putting in the pier two days ago, I was surprised that the water didn't chill me through the hip boots.

When I bought walleye minnows (fatheads) yesterday at the bait store in Lake Tomahawk, the attendant advised me to fish no more than 10 feet from shore, in water as shallow as a foot. I shoved off from the pier at about 7 p.m. Being a little hog-tied by habit, I first tried a deeper-water brush pile where I have caught many early-season walleyes. I found none this time and so took the counsel I'd been given.

A great thing about fishing your lake this time of year is the quiet. If it's not a weekend, chances are good you'll be alone on the water, or nearly so. The only other angler I saw was standing in waders, about up to his waist in the water, a couple dozen yards in front of his cabin. He told me he had seen no walleyes but had caught a nice smallmouth bass. He also, as it turned out, could make a very credible loon call, blowing into his cupped hands. (I guess I'd have to say not credible to the loons, who declined to play call-and-answer with him.)

I moved along the lake's southeast shore (where our cabin stands), tossing a minnow on a chartreuse jig into shallow, gravelly areas -- along the edge of a rock bar, next to a white pine that toppled into the water last summer, and finally off a point of land where logs and branches lay in the water. And here I felt the season's first twitch of life transmitted up the line, through the rod, to my right hand. A quick snap of the wrist and I had a fish on. Right away it felt too heavy for a Birch Lake walleye (they don't run too big here), so it had to be a smallmouth bass. And so it was -- about 17 inches, a female plump with spawn. I unhooked and released her.

I hadn't really expected to catch anything -- just wanted to get out on the water, since it was a perfect warm and overcast evening and the forecast called for a cold front overnight. So, bringing in a fish was a bonus. Here's hoping your first outing of this new season is fruitful. Try the shallows for walleyes.

Bottoms up: The lake unveiled

Has the ice gone out on your lake? Were you there to observe it? I had always wanted to see the thaw happen, and this year, at last, I did. It wasn't (as I expected) a matter of observing slow changes over a number of days. In fact, it was sudden, much of the process unfolding in little more than an hour.

Did you know (I didn't until I recently did some reading) that your lake ice thaws from the bottom up? First the snow melts off the surface. Then the sun penetrates the ice and warms the water underneath. Warm air above the ice accelerates the thaw, of course, but it's the warming water below that really does the trick.

When we arrived here on Sunday, April 28, Birch Lake (southeast of Minocqua) was still frozen stiff. That day and the next two days were in the 70s to 80s. Then came three more days of below-freezing temperatures, rain, snow and sleet, before winter's grip finally broke, on Saturday, May 4. At that point, the lake ice still looked solid (we heard reports from other lakes of remaining ice up to two feet thick). Who knew how long it would be until our lake opened up?

Then Saturday saw highs in the 70s, as did Sunday, and Monday, the day it finally happened. I suspect that on those warm-to-hot last days of April, the ice clear of snow, the sun had pumped enough calories into the water to keep the bottom-up thaw going, even as winter came back for a spell.

When I visited the lake's shore Monday morning, ice still covered all except a small area on the far north side. There had been little change (that one could see) when I left for town about 2 p.m. But when I returned at about 5 p.m., about 40 percent of our lobe of the lake had cleared, the remaining ice forming an irregular pattern, like continents in an ocean.

Then, at about 6 p.m., a wind brewed up from the east and began pushing the ice away, at more than glacial speed. Sitting on our deck, I could mark with my eye a feature on an ice sheet, note its distance from the trunk of a tree, and verify its progress. It was a bit like watching the minute hand on a clock, the motion barely perceptible, yet unmistakable.

Within about an hour, all the ice had blown off to the west, the stirring action of wind-driven wavelets surely speeding up the thaw at the same time. Just like that, our entire end of the lake, some 90-or-so acres of it, lay fully open. By morning, the entire lake had cleared, and loons plied the water, crying out with joy.

After the longest winter or the worst spring I remember in my 60 years, a new season has arrived. And all I can say to that is: Bottoms up!