One of the lakes on our campus has the evocative name of Inkpot. Unlike the rest of them, which are clear, pristine, eutrophic kettle lakes, take a canoe out on Inkpot in the summer and you’ll discover that it’s surprisingly murky and full of aquatic vegetation. Part of the reason for this, according to one of the science teachers here, is that it’s at a slightly lower elevation than the other lakes; another reason is that a sluggish stream flows through it from a nearby spring, meaning it’s the only lake on campus with any current. There’s even an old beaver dam at the outflow, although it doesn’t appear to be actively maintained by beavers anymore.
The current means the ice is thinner on Inkpot than on the true kettle lakes, and I’d heard that last weekend someone saw otters there, coming and going through a small patch of open water. Today a friend and I skied out that way, and while we didn’t see otters or any open water, we did see otter tracks of varying ages criss-crossing the surface of the ice. At one point an older set of tracks appeared to vanish at the lake’s edge, and I managed to push my skis through a snowdrift to get a closer look.
So yeah, today I learned that otters make snow tunnels just like mice and squirrels do!
One other small point of interest: getting out to this lake, at the far western end of the property, required skiing a much longer, hillier trail than I’d done before, and actually getting to the edge of the lake required leaving the groomed trail and going down a slope through a couple feet of fluffy, unpacked snow. And then back up it when we were done. On skis. There was a lot of screaming and falling. Leanna, the friend who went with me (and the person in the panorama above), composed a haiku about the experience that she wanted me to share with you.
On an adventure Otter tunnels in the snow Sp-lat on my back
She is adamant that “sp-lat” should be pronounced as two syllables.
As the days grow colder, the lakes are getting ready to turn over. This phenomenon, which happens every fall, is related to the unique chemistry of water.
In summer, the water in a lake is typically separated into two distinct zones, separated by a boundary called the thermocline. Near the surface, closer to air and sunlight, the water is relatively warm and oxygenated. Deeper, below the thermocline, it’s colder and contains less oxygen. Because of the different densities of water at different temperatures, these two zones don’t mix much – think of oil and water. If you’ve ever gone swimming in a lake you might have even felt where the water abruptly becomes colder at a certain depth.
As winter approaches, the surface water cools along with the air. When it gets cold enough and dense enough, it sinks. The two distinct zones break up and all the water in the lake circulates and mixes freely. This is the “turnover.”
Here’s what’s really cool. Almost any other liquid becomes denser and denser as it cools and gets densest of all as a solid. Water is densest at 4°C or 39°F, several degrees above its freezing point. This is why ice floats – and why lakes freeze from the surface down than from the bottom up. The water under the ice, freshly re-oxygenated from the fall turnover, remains at 4°C all winter long, allowing the fish and other aquatic live to survive until the spring thaw.
There’s nothing like walking over the frozen surface of a lake in the dead of winter and imagining all the sleeping life sealed beneath your feet. This morning we woke up to another dusting of snow… it won’t be long.
This is a satellite photo of the area in Wisconsin where I live, courtesy of Google Maps. You can see why it’s called “Land O’ Lakes” (no relation to the butter, though we sometimes call the abandoned lumber mill on the edge of town “the butter factory” to confuse people). We also have a lot of bogs. These two facts are not unrelated.
These are kettle lakes that were formed by retreating glaciers 10,000 years ago. Many of them have Sphagnum moss growing around their edges. Sphagnum is amazingly absorbent, holding up to twenty times its dry weight in water, and the mats it forms eventually grow thick enough that they act as a substrate for a whole community of other plants we associate with bogs – leatherleaf, wild blueberry and cranberry, orchids, carnivorous plants like pitcher plant and sundew, and many more.
Slowly the lake fills with sediment, and the sphagnum eats away more and more at its edges, growing thicker and absorbing more water as it does. As the lower layers of spagnum die, they decompose very slowly due to anaerobic (oxygen-poor), acidic conditions, so that over time you get rich, moist deposits of peat. Below is an old lake on this property that’s been almost completely bog-ified, with only a small area of open water left in the middle.
Eventually the sphagnum and peat build up to the point where they can even support trees, mostly tamarack and black spruce.
This is all just one more example of the powerful process of ecological succession – one natural community transforming into another through time.