Sunday, October 12, 2014

Closing Time

I hope you were among the fortunate souls who spent last weekend at their lake homes or cabins. I met several such folks as I took a solo paddle, my last of the season, around the shoreline of Birch Lake, at Harshaw.

This was a prototype October Saturday afternoon, clear sky, temperature mid-50s, the softest of breezes, the lake’s surface smooth, oaks and birches still holding their colored leaves, the air scented like (to borrow a phrase from Garrison Keillor) fine brandy.

When traveling alone in our red Kevlar Old Town, I always assume the bow seat and paddle stern first; sitting farther amidships keeps the canoe flat instead of nose-up in the water. At this season there’s something appropriate about paddling “backwards”: The trip is more about looking back than forward.

You tend to think, as autumn closes down, on what was instead of what will be. My annual spring canoe reconnaissances are about watching for life in the shallows, spotting painted turtles released from hibernation, following smallmouth bass across the reef on the lake’s east end, spying on walleyes hunkered deep in sunken tangles of brush.

On this mid-October ride, there was of course little life to observe other than a somewhat heavier-than-usual clouding of green algae. The fish had gone deep. Several small ducks in a cluster skittered away and up well before I could get close enough for an identification.

I did encounter several lake neighbors enjoying the day in various ways: one man disassembling a pier, ratchet wrench periodically rasping; another enjoying a drink while seated atop a short stairway of timbers; a woman at the end of a pier with a small black dog that barked at me sharply; a man and wife prepping a pontoon boat for storage, two fishermen in boats working rocky points, presumably for muskies.

From here on there would be few days like this. It’s hard at such times not to regret the decline of the seasons and to long, far prematurely, for spring. It’s too soon to embrace the idea of November’s bleakness and then the long winter. So, while taking in the glory of the day, we tend to scan back over the good times of spring and summer past.

As I pulled the Old Town from the lake and tipped it over on shore, for the last time until next year, the couple from three lots down paddled by in their canoe, just two more lake country folks lucky enough to enjoy this day, around or on the water.

Saturday, October 4, 2014

This turnover isn’t for dessert

Right now many Northwoods lakes are going through (or soon will) something called the fall turnover. It’s a phenomenon as beneficial as it is interesting. 

Fall turnover is a restorative process, a bit like opening doors and window in a long-sealed, musty basement and letting lots of clean, fresh air course through.

A previous column in this space told how lakes stratify (form layers) in summer – warmer, lighter water above and colder, denser water below. At the height of the warm season, these layers don’t mix very much because the difference in density between surface water (at, say, 80 degrees F) and deep water (at, say, 40 or 45 degrees) is considerable.

So as the summer wears on, all kinds of materials sink from the surface water into that cold bottom layer. Plant parts, algae, fish carcasses, dead insects and more drift down and decompose, consuming oxygen. As a result, the oxygen down there can become quite depleted.

What would happen if your lake remained stratified all the time? Those deep waters would become largely lifeless, hospitable mainly to organisms that thrive in anaerobic (without oxygen) conditions.

But fortunately, along comes the fall turnover, generally sometime in late September or early October (likely on the early side this year because of all the chilly weather). In simple terms, what happens is that the surface water gradually cools, and the difference in density between the surface and deeper water decreases, so that eventually wind and wave action can mix the layers together. And that means the lake, from surface to bottom, becomes infused with oxygen.

This is great for all manner of lake creatures – especially fish that dwell in the depths – that need oxygen to make it through the winter.

How can you tell if your lake has turned over? Well, for one thing, the water suddenly becomes cloudier than usual because the mixing action brings up nutrients and debris from the bottom. You might even notice a hint of sulfur scent (like rotten eggs) as decomposing material comes to the surface. When the turnover is complete, the water becomes clear again, likely more so than in high summer.

Some anglers say fishing is tougher during the turnover because with oxygen available everywhere, the fish are more scattered.

Different lakes experience fall turnover in different ways. Deeper lakes take longer to turn over. Shallow lakes may not turn over at all because they never actually stratify in the first place – wave action keeps them well mixed all through summer. The turnover itself can play out in a few days in some lakes, or during a week or more in others.


So watch for signs of turnover in your lake. It’s another seasonal milestone, like ice-in and ice-out, that can be fun to track over the years.

The bounty of the benthos

Leaving an airport, you see signs that say Ground Transportation. After flying at 36,000 feet and a few hundred miles an hour, that travel mode seems quite unglamorous.

So it is with life on at the bottom of a lake, which the limnologists (freshwater biologists) call the benthos. Up above in the water column the fish are like the aircraft and birds of our dry-land world. Creatures less appreciated live on (an in) the “ground” below.

It’s appropriate at this season to think about the benthos, because that’s where a lot of lake life is heading as the water gets cold and winter comes on. The term “benthos” comes from a Greek word, “bathys,” which means “deep.” It’s a zone much richer in life than most of us appreciate.

Of course, crayfish live on the bottom, as do clams, mussels and snails. Aquatic insects like mayflies and damselflies also live on the bottom, or buried in sediment, at stages of their metamorphosis from egg, to nymph, to winged adult.

These creatures are important links in the lake food chain. They eat algae or sunken plant matter and in turn provide food for fish (as anyone who has ever caught bluegills with nymphs or perch with wigglers can attest). An assortment of worms can also be found in upper layers of bottom sand and muck.

Leopard frogs and bullfrogs become benthos dwellers in winter. They do not (as many believe) dig into the bottom – the sediment contains too little oxygen to get them through until spring. Instead, they lie on the bottom, or only partly bury themselves. Some may even swim around slowly from time to time.

Painted and snapping turtles, on the other hand, do burrow into soft lake bottom mud and hibernate. In that state, they need very little oxygen and can absorb it through exposed mucous membranes in the mouth and throat.

An important function of the small benthic creatures (the worms and inserts) is that they allow scientists to assess water quality in a lake (or stream). A researcher can take “grab samples” of the bottom sediment, sort out and identify the organisms it contains, and get a good idea how healthy the lake is.

One measure they use is species diversity. In general, the more different creatures they find, the better the water quality. Another criterion is pollution tolerance. If a bottom sample is rich in immature forms of mayflies and stoneflies, which are sensitive to pollutants, that indicates good water quality. But if only midges and worms are present, that signals polluted water.

So while we get ready to “hibernate” for the winter, it’s good to think about the importance of all those creatures spending the cold season on and under the benthic blanket.