Trophic Status – One Way to Classify Lakes

There a various ways, scientific and otherwise, to classify lakes. So, what categories include your lake?

Large versus small? Shallow versus deep? Clear water or stained? How does your lake get its water? From groundwater (seepage lake)? From a stream (drainage lake)? From rain and snow only (perched lake)?

Lakes come in many varieties, but one form of classification matters perhaps more than the others: Trophic status. That is, how rich is your lake in nutrients that support life? Typically, more nutrients – chiefly nitrogen and phosphorus – mean greater growth of algae and plants, and often by extension more fish, insects, mollusks and other life.

Scientists typically place lakes into three trophic states: oligotrophic, mesotrophic, and eutrophic. Generally speaking, it’s not hard to tell where a given lake falls on the scale.

* Oligotrophic lakes (“oligo” means “few) are poor in nutrients. They tend to be relatively deep with sandy or rocky shorelines. The water is clear (these lakes can be great for snorkeling). Weed growth is very limited. If deep and cold enough, these lakes may hold cold-water fish like lake trout and cisco. Think Crystal Lake in Vilas County, or Lake Superior.

* Eutrophic lakes, on the other end of the scale, tend to be shallower with mucky bottoms. They may become choked with weeds in summer, and the water may be murky from floating algae, sometimes the noxious blue-green type. They’re likely to hold warm-water fish like northern pike, bass and bluegills, along with bullheads and carp that tolerate low oxygen. Think Lake Erie, or Madison’s Lake Mendota.

* Mesotrophic lakes basically fall between these extremes. Many of Northern Wisconsin’s lakes are mesotrophic. The lake where I live (Birch, at Harshaw) falls quite squarely in the meso camp, at least by my reckoning.

In reality, not all lakes neatly fit one category or another; sometimes the lines get blurred. Vilas County’s Trout Lake, for example, falls by experts’ reckoning on the borderline between oligo and meso.

It’s common to think of eutrophic lakes as polluted or impaired. That’s not always so. While some lakes can be made eutrophic through runoff of farm manure, lawn fertilizer or other nutrient sources, some lakes are naturally eutrophic.

It’s also tempting to think of clearer, lower-trophic lakes as “better” than others – but that’s a value judgment. It all depends on how you want to use the lake. Some eutrophic lakes (think Winnebago) are terrific fisheries. Others, partly surrounded by marshes, are great spots for duck hunting or wildlife observation.

Trophic status is a fascinating and complex subject. It will be worth exploring in more detail in future columns. For now, think of your lake. Where does it fit? Chances are you already know enough about it to make a good stab at choosing the right category.

Travels etched in snow

All spring, summer and fall, animals come and go across our woodland and lakefront properties, but we barely notice because they leave little evidence.

In winter, though we easily see their tracks in the snow. Your snow-covered lake is a great place to track wildlife: The trails traverse open space instead of weaving among trees and brush.

The only trouble with winter tracking is that it can be hard to identify the actual prints. The animals’ footfalls don’t leave clear impressions in powdery snow the way they would in mud or soft sand. You need to go by clues such as the track pattern, the sizes of the impressions, and the spacing of the prints.

The New Hampshire Fish and Game Department offers a free “Pocket Guide to Animal Tracks” (http://www.wildlife.state.nh.us/Wildlife/Wildlife_PDFs/Track_Card.pdf). Its detailed images of the various prints won’t help you as much in winter as in other seasons, but the guide does include the types of track pattern and the typical print sizes.

Anyway, an essential step in tracking winter wildlife is knowing who is out and about. For example, you won’t find bear tracks in snow, since the bears are denned up until spring. Beavers don’t hibernate but typically store enough food underwater to get them through the winter, so they’re not seen very often.

So how can you identify those trails in the snow? The New Hampshire guide identifies four basic track patterns. First are the hoppers, chiefly squirrels and rabbits. Squirrels leave roughly box-shaped sets of tracks, a larger pair (the hind paws) toward the front in the direction of travel. At each hop, the front paws land first, and the rear paws leapfrog past them. Rabbit track sets are similar except that the front paws fall one behind the other instead of side by side.

Then there are tracks that proceed in a nearly straight line. Foxes, coyotes, bobcats and deer share this pattern. Deer hooves commonly exert enough pressure to leave well-defined cloven marks in the snow. As for foxes and coyotes, absent clear paw impressions, track spacing can help you tell the difference: 14 to 16 inches for red foxes, 19 to 21 inches for coyotes.

Raccoons, porcupines, opossums, skunks and muskrats leave pair of tracks, one behind the other. The sizes can help you differentiate. Otters, fishers, minks and weasels leave pairs of prints side by side (and otters, as mentioned last week, leave their unmistakable slide marks).

The recent thaw has eliminated much of the tracking snow on the lakes, but more snow will come. Consider heading out (when convinced that the ice is safe) and trying to determine just who made all those trails in the snow.

Just you and the otter

If you live on a lake, one of winter’s pleasures is walking the snow-covered ice, until the snow gets too deep, after which you can walk it on snowshoes.

You soon find you’re not the only one who takes these walks – animals will have left their tracks before you. Imagine about six inches of snow on the ice and a soft snow falling, a couple of inches of new powder already down, as you embark in your insulated boots.

You stay close to shore, because it’s a bit too early in the season to trust the open ice, but also because this is where you’ll find most of the hoof and paw prints. Now and then an animal will shortcut across a bay, or across the lake proper, but mostly the tracks hug the shoreline, food and cover close by.

Not far on your walk, you come upon sausage-shaped depressions in the snow, each six to eight feet long, paw prints between. These are the slide marks of otters. You know they’re fresh because they remain well defined, the edges not even slightly softened by the falling snow.

You may not like assigning human qualities to animals, but when it comes to otters, you can’t helping thinking that here are creatures who know how to have fun. They don’t walk or trot along – they run and slide. Yes, they take a few running steps, then flop on their bellies and glide over the snow. A few more steps and glide again.

And so it goes, the tracks continuing as you walk along. The paw prints’ orientation shows you and the otter are heading in the same direction. You keep your eyes forward, hoping to catch a glimpse, since these marks can’t be more than a few minutes old. Here and there the trail heads up into the woods, then emerges again on the ice.

You never see the otter, just follow its trail halfway around the lake to where it finally enters and stays in the woods. On this day, the new snow has cleared the lake’s slate; the only tracks in evidence are yours and the otter’s. You’re glad to have shared the moment.

The lid goes on

If you wonder what happens in your lake after the ice forms, the answer is: Not a great deal. Sure, fish still bite, some more readily than others (bass being among the reluctant).

But in general, things get quiet, still and dark down there under that translucent, snow-covered sheet. The three inputs that make your lake so very much alive in high summer – light, heat and oxygen – are much less abundant.

Only cold-blooded creatures spend winter in the water (though foraging otters may come and go through near-shore holes in the ice). In temperatures not much above freezing, fish move around sluggishly; reptiles and amphibians stay mostly still or outright hibernate. Aquatic insects winter in the bottom sediments.

Except to the extent that it receives inflows from a stream or groundwater springs, your lake becomes essentially a sealed container. Very little oxygen gets in. The deeper the snow cover, the less light can penetrate, and the less oxygen plants produce from photosynthesis.

And vegetative life itself is limited. The rooted aquatic plants (weeds if you will) have long since died back. The populations of plankton – the tiny critters and one-celled algae that form the base of the lake food chain – have plummeted. Whatever oxygen was dissolved in your lake’s water at the time ice formed steadily declines through the winter.

If you’re able to look through clear ice to the bottom, you may see places where occasional bubbles of gas issue from the muck and rise until they meet the ice cover. But biochemical activity and life in general slow to a crawl. For fish and other lake creatures, it becomes a question of survival until spring.

Imagine what it’s like down there, under the ice. There’s barely a sound. Maybe the noise of a roaring wind penetrates sometimes. But there’s no sound of wave action. No splashing as eagles strike carrion fish on the surface. No swirling noises as loons dive down to fish. No whine of outboard engines. Just unbroken silence.

On windless days and nights, before the snowmobile trails open, it’s a lot like that up here on the surface, too. It’s a time to treasure the quiet, to feel life’s pace slow down, to enjoy a sort of suspended animation that lasts until spring.

If it feels miraculous to see the earth burst forth with life as the weather finally turns warm, how much more so to ponder the way lake life blooms again when at long last the ice recedes.

The loons: Still here

A week ago I saw them, through the living room window, in a frame of white pine boughs and trunk, far out on the lake, in a perfect row, four white spots on deep blue.

 

I had my suspicion but reached for the binoculars to confirm, steadying by pressing one barrel against the glass. Yes, loons, even at long distance, their shapes unmistakable, slowly swimming toward me, white breast feathers lit by a low sun.

 

So, they were still here. Or maybe these were not Birch Lake’s resident loons but migrants coming south from Canada. I was surprised to see them after all the cold, in winter plumage for sure (though so far off that even at 8X magnification I couldn’t discern the colors clearly).

 

I worried for them a little, the lake’s southwest lobe largely iced over and a crust on the main lake starting to push out from shore. I have heard stories of loons getting iced in, though it does seem somehow they know enough to leave before it’s too late.

 

The fact remains, loons need a lot of space in which to take off. Just as a jet plane is marooned at an airport with a too-short runway, loons are stuck if there isn’t enough water on which to run and flap up to takeoff speed. The qualities that makes loons adept divers and hunters – short wings for streamlining underwater, and bodies less buoyant than those of other birds (solid bones instead of hollow) – are handicaps when it’s time to get airborne.

 

If loons live on your lake, you surely know the sound they make as they take flight. It’s that sound Fred Flintstone’s feet made as he ran his stone-wheeled car up to travel speed: Pat-a-pat-a-pat-a-pat-a-pat-a...And not just a few pat-a’s. Loons have to beat their webbed feet over a long distance to lift clear of the water.

 

Ducks? Startle them and they seem to leap right up, airborne in an instant. Loons, on a calm day, might need to skim 600 to 700 feet along the surface. They need less room if able to take off into a wind, which provides lift, and yes, they do aim themselves upwind if they can, without the benefit of the wind sock human pilots use. Once in the air, they fly fast, some 50 miles per hour, though their flight is energy-intensive. Soaring is out of the question; the wings must beat every second.

 

So there they were out on the lake in the middle of an Arctic cold front, in all likelihood gone by the next morning or maybe even that same evening. Anyway, I will assume so. It looks like a long, long time before they come back.

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.

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.