In the zone

Scientists like to name and classify things, so it should come as no surprise that they have names for different zones in our lakes. Mainly they talk about four of them.

The one we think most about, that we interact most with, is the littoral zone. This is the area from the shoreline out to the point where the water is deep (or murky) enough so that there is too little light at the bottom to support rooted plants.

The Minnesota DNR pegs this zone as from shore to a depth of about 15 feet, but that depth can vary a lot with water color and clarity. The width of the littoral zone also varies. Where the bottom slopes down steeply, it may be quite narrow. If the bottom slopes gradually, the littoral zone may extend far into the water. In fact, a shallow lake may be all littoral zone.

Life can be incredibly diverse in this zone. Most fish spend most of their time there (and as a consequence so do anglers). It’s a rich environment, with relatively warm water, plenty of light, and nutrient-rich bottom sediments.

All manner of plants grow here, from emergent species like bulrushes, cattails and arrowhead, to floating-leaf plants like water lily, spatterdock and watershield, to submerged vegetation like pondweeds, wild celery and milfoils. Algae are also abundant, some species clinging to the larger plants. The plants provide cover for young fish, which in turn attract larger predators. Frogs, muskrats, turtles, insects and other creatures populate this zone.

Out beyond the littoral zone lies the limnetic zone. This is the open-water world. It begins where the littoral zone ends; its depth again depends on how deep the light can penetrate. Fish move in and out of this zone, but for the most part its inhabitants are plankton – one-celled algae of various kinds (phytoplankton) and tiny creatures (zooplankton) that eat by filtering algae out of the water. Plankton are critical to a lake’s food web, and the phytoplankton are responsible for most of the photosynthesis (thus oxygen production) that occurs in the lake.

Below the limnetic zone lies the profundal zone. This zone may not exist in shallower lakes. It’s the deep water where light penetration is greatly limited. In essence, this is where dead matter from above goes to decay. It is relatively cold, dark and oxygen-poor. The primary life in this zone consists of heterotrophs – small creatures that eat dead material.

Finally, there is the benthic zone, essentially the lake bottom sediments. Residents include bacteria and fungi that break down organic matter, releasing and recycling nutrients. Life just under the sediment surface can be quite diverse. Most benthic zone organisms are invertebrates. The eggs and larval stages of insects like mayflies and midges can be found here, along with worms and small crustaceans.

Sometimes as you look out over your lake, try to picture these zones. You’ll understand a little bit better how your lake ecosystem functions.

Two Pair

As my canoe came clear of a shrubby patch on a small point here on Birch Lake, there came an explosion of wings.

Mergansers – two males and two females – shot out of the water and arrowed away. The contrast of colors surprised and delighted me. I’m used to identifying mergansers by the female’s slender shape and rusty crest. The male with his green head (when in mating plumage) can fool the unsophisticated, like me, into thinking he’s a mallard.

Those of us who spend time on our lakes in spring get to see a variety of ducks pass through on their migration north. According to Audubon, common mergansers (the kind I saw) breed mostly in Canada and winter mainly south of here, in a swath that includes Ohio, Indiana, Illinois, Iowa, Missouri, Nebraska and Kansas.

I’ve heard mergansers described as early arrivals in the northward migration, though I’ve noted other species on our lake sooner after ice-out. Until the sighting of the two pair several days ago, I had never seen more than two mergansers together.

Now and then I’ve had the chance to watch a female diving for fish, which is mainly what mergansers eat. They vanish faster and surface sooner than do loons – they seem to bring to their “fishing” a greater sense of urgency.

The female’s crest looks pleasantly unkempt. As for the male merganser, he’s pretty easily distinguished from a mallard. He’s similar in overall size but more slender. His green head (not crested) isn’t as bright as a mallard’s. He also lacks the mallard’s chestnut breast and white neck ring. The merganser’s bill is long and red; the mallard’s is yellow.

You’ll also easily distinguish the male mergansers by their sound. Mallards give out the “quack” of the stereotypical duck. Mergansers don’t say a lot but emit a low, harsh “croak.”  All that aside, while mallards carry the taint of park ponds and domestication, mergansers portray the essence of the wild.

If I take any lesson from this sighting, it’s that we can appreciate spring and migration more if we see more than “just ducks” passing through our lake country. While I’m nobody’s birdwatcher, I find a little time spent with binoculars and a field guide book reveals a rich diversity in visitors’ shapes, colors and behavior.

And I must say those mergansers that rocketed off Birch Lake – boy-girl, boy-girl – were among the best two pair I’ve ever been dealt.

Dragonfly riot

It’s mostly over now, but for a couple of weeks dragonflies were everywhere around our place on Birch Lake, and probably around your lake, too.

Maybe it was the happy coincidence of a dragonfly hatch with the emergence of late-May and early-June mosquitoes. All I know for sure is that the air was full of dragonflies, sweeping up mosquitoes like vacuums on wings.

Though routinely spotted over land, dragonflies are without question water insects – they come out of your lake after a long metamorphosis. The adult stage we see in the air lasts a couple of months, really just a sliver of the insect’s life.

Dragonflies mate while on the wing – no doubt you have seen this act above the water on your lake. The female lays her eggs on a water plant or directly into the water. When the eggs hatch in a couple of weeks, nymphs emerge. They really don’t look anything like dragonflies, but they have one thing in common with the adults: They’re voracious feeders.

Dragonfly nymphs eat all sorts of water insects and insect larvae, and yes, that includes mosquito larvae (call wigglers). So dragonflies are putting a dent in the skeeter population long before they can fly. The nymphs are also quite agile in the water. They swim fast and have a jet-propelled “hyperdrive,” ejecting water from the anal opening.

The nymph stage can last as long as a few years. The nymphs live in your lake’s calm water, amid reeds, cattails and other plants. As they grow, they shed their skin several times. Each in-between phase after the skin is shed is called an instar.

Finally, once fully grown, the nymph climbs up the stem of a plant and emerges from its skin as an adult dragonfly, leaving behind a skin called the exuvia. You may at times have seen one of these clinging to a reed in shallow water. 

And now the dragonfly is ready for serious eating. Dragonflies are so agile in the air that other insects, like gnats, midges, mayflies and, of course, mosquitoes, have no hope of escape. They use their legs like a basket to catch bugs on the wing. Then they feed their prey into their jaws (mandibles) and crush it before swallowing.

How much do they eat? Well, according to dragonfly-site.com, they can eat their own weight in bugs in about half an hour. So that’s why, in mosquito season, we can be thankful to see squadrons of brightly colored dragonflies, sweeping the air around and above our homes and piers.

Here’s to Hoover-Mouth

There are species of fish in your lake that you may rarely if ever see, yet are important to the health of the fishery. The white sucker is a classic example.

You may know this fish best as bait. Raised in ponds, it is sold in increasing sizes for walleyes, northern pike and muskies. If like me you grew up on a Lake Michigan tributary, you may have fished for suckers using a dip net hung by a rope from a bridge, in springtime when the fish migrate upriver from the lake to spawn.

If your lake contains suckers, why don’t you see them? Well, because they tend not to take what anglers offer. I’ve fished for more than 50 years and have caught just one white sucker from a lake on hook and line.

Suckers are mainly bottom-feeders and have mouths well adapted to that purpose. The leathery lips aim downward instead of straight ahead, so the fish can cruise along, dining in comfort, casually vacuuming up food like insect larvae, worms, small mollusks and crustaceans, plant matter and fish eggs from the sediment. In turn, suckers are a vital food for favored game fish; they may also be eaten by herons, loons, bald eagles and osprey.

White suckers live in almost any lake and stream here in Northern Wisconsin. In fact, they’re abundant throughout the Northeast and Midwest U.S. and in parts of the Northwest. They do fine in clear, clean waters but also tolerate relatively low dissolved oxygen and so can thrive in turbid urban waterways.

Suckers have fine scales. Sides are dark greenish with a metallic luster; the belly white, and hence the common name. Adults can grow up to 20 inches long and weigh two pounds or more; musky anglers are known to use those at the top of the size range for bait in the fall.

Spawning generally starts when the fish are about four years old (later in colder climates where they grow more slowly). Spawning season runs from April to early May. The fish move into streams or, in lakes, select bottoms of gravel or coarse sand. The actual spawning happens at night. Most often, two males mate with one female. With one male to each side, the female lays 20,000 to 50,000 eggs, which the males fertilize.

The fish do not make spawning nests and do not care for the eggs, which simply sink to the bottom. The eggs hatch in five to ten days, and a week or two later the fry leave the spawning area and disperse.

Thus are born swarms of fish on which your lake’s most prized species may depend for growth. So even if you never see suckers on your lake except in your bait bucket, be sure to assign them a little respect. Here’s to Hoover-Mouth!

How does your lake get its water?

You’ve read here about classifying lakes by trophic state – how poor or rich in nutrients they are. But that’s not the only way to categorize them. Another, just as interesting, is by how water gets in and out.

The number of lake types based on source of water depends partly on who is doing the defining. The Wisconsin Department of Natural Resources lists four types, but there is a fifth that many geologists mention. Here are five basic lake types found in Northern Wisconsin and Upper Michigan:

Drainage lakes. On these lakes, a stream brings water in, and a stream takes water out. That is, the lake has an inlet and an outlet. Some lakes may have more than one of each. The water level in these lakes tends to stay fairly constant. Think of a bowl into which you run a slow flow of water from the tap: An equal amount of water flows in and flows out. I live on a drainage lake and its level is largely self-regulating. In 30 years, through wet times and dry, there has been at most a foot of difference between the highest and lowest levels.

Spring lakes. These lakes have no inlet on the surface, but they do have an outlet. They get their water mainly from groundwater flowing in. Many streams originate in spring lakes, which are quite common in northern Wisconsin.

Seepage lakes. These lakes have no stream flowing in or out. Their water comes mainly from rainfall and runoff, sometimes supplemented by groundwater. Their water levels are therefore cyclical, rising and falling with wet and dry years and their effects on the water table.

Drained lakes. These lakes are like spring lakes in that they have an outlet but no surface inlet. They differ in that they are not fed by groundwater – they get their water almost solely from rainfall, snow and runoff. For that reason, their levels can fluctuate greatly. During long dry spells, the streams flowing out of these lakes may dry up. Drained lakes are uncommon here in northern Wisconsin.

Perched lakes. These lakes are truly landlocked. They have no inlet, no outlet, and no contribution from groundwater. In fact they sit on relatively high ground, above the water table, with dense bottom sediments that hold the water in. Water levels in perched lakes can drop dramatically during long dry spells.

If you want, you can add a sixth type of lake: Reservoirs. These of course are like drainage lakes in that they have a stream inlet and outlet. The difference is that they were created by humans – they wouldn’t exist if not for dams. Here in the northern Wisconsin we have the Willow, Rainbow, Turtle-Flambeau, Chippewa and other smaller flowages.  

Which type is your favorite lake? If you don’t already know, consider doing some investigating to find out.

Minnows? Are You Sure?

Soon after ice-out I do canoe reconnaissance: slow paddle around the shoreline to look for signs of life. When I did that recently here on Birch Lake (at Harshaw), I encountered huge schools of little fish at the far-in end of what we call Indian Bay.

My mind reflexively said, “Minnows!” But of course that was both non-specific and taxonomically incorrect. The vertical black stripes on these guys, anywhere from about 1.5 to 2.5 inches long, clearly labeled them as young yellow perch.

It amazes me how soon fish fry take on the markings of adults. Baby smallmouth bass, for example, have the signature black-tipped tails and red eyes. Largemouth bass have the black stripe along the side, northern pike the oblong oval spots. And so it goes.

The young fish seem to mimic adults in temperament, too. Little muskies, for example, are hyper-aggressive. Last summer, I caught a 4-inch musky that slashed at and grabbed a crappie minnow impaled on my hook.

But back to the matter of minnows: We tend to apply that label to any small fish, especially in schools. That’s probably because we refer to the baitfish we buy at the tackle shop as minnows (again not precise, but a well-accepted term).

Scientifically speaking, the term “minnow” applies to a family of fish defined not by size but by characteristics. Members of the minnow family have one brief dorsal fin with nine or fewer soft rays. They have smooth-feeling scales that may come off when the fish is handled. They do not have true spines in their fins. They have no teeth in the jaw but have rows of toothlike structures on the bony frame that supports the gill tissues. Their teeth are in the throat and help grind food.

Most minnows are in fact small, a few inches long. That’s true of the shiners we use for bait – they are in fact minnows. But the minnow family also includes carp that can grow to three or four feet or longer and can weigh 50 pounds or more.

Chances are the schools of fish you see beside your pier will not be minnows but small game fish or panfish. If you can net a few (not easy, I admit), you’ll get a clue to what’s breeding in your lake. The fish I saw in Indian Bay on my canoe ride assuredly were not minnows; from their numbers I can conclude that our lake’s perch of brought off some successful hatches.

That of course is not the same as successfully reproducing: Hatched fry do not a large or stable population make. Those little perch have a tough gauntlet to run before they reach adult size. All I can do is wish them well.

Waiting for Water

Just before hamburgers were served to son Todd and me at Birch Lake Bar a week ago, co-owner Ed stopped by our table and lamented the lake’s condition.

I’d been enjoying the extended thaw – greatly, I might add – but to Ed the lake’s surface of deep slush meant the end of snowmobile season, the end of ice fishing, and so a tough time for business. I can certainly sympathize: an early thaw means different things to different people.

Now the question of the day is: When will our lakes open up? That depends on how the weather behaves from here on, though the past two weeks of well-above-average temperatures have given the thaw a nice head start.

Last year and the year before, the ice went out here on Birch Lake a few days after the official fishing opener (first Saturday in May). It went out a great deal earlier in 2010, the first year we had our land here – I remember wading in the lake, quite comfortably, in mid-April.

2011 was a different story. On April 16, when our family held a ceremonial groundbreaking for the cabin that has since become our year-round home, the lake was still frozen solid, the day cold, wind-blown snow stinging our faces. We drank our champagne huddled inside the RV trailer that served as our first shelter.

As for 2012, I have written evidence of an early ice-out. An entry elsewhere on this blog says I put the pier in on April 7, the Saturday before Easter. Are we due for another early open-water season? Signs point that way, but we can’t forget what April and May were like last year: Cold, cold, and more cold, with a couple of April blizzards tossed in.

Right now, as I write, on Sunday, March 15, it’s pleasantly mild, about 50 degrees, and the forecast, if it can be trusted, calls for highs well into the 40s for the next several days. The snow has melted off the metal components of our pier, arranged neatly on shore, and off the cedar pier board sections I stacked and covered with a tarp last November.

If you’re like me, you’re aching for the ice to be gone and for the start of whichever open-water recreation you prefer. It’s a wondrous time – the days getting longer, the clock sprung ahead, loons on their way north, the long months of spring, summer and fall awaiting, full of promise. We could do worse than to have an early ice-out bring that promise forth sooner.

Magic on ice

Assuming our cold snap goes away and stays gone, something almost magical will soon happen to your lake’s ice. It’s called candling, and it reveals a property of ice that’s hidden from us most of the time. It’s fascinating, but it also leads to a significant hazard for those venturing out for late-season ice fishing or other adventures.

As the thaw sets in, lake ice changes from what we know as a strong, monolithic structure to a matrix of crystals, arranged (if imagined from above) as hexagons, like the cells in a bee’s honeycomb, though by no means as perfect. These crystals align vertically, from the top of the ice to the bottom; they are shaped somewhat like candles.

In its candled state, ice is often called “rotten.” You can see how weak it is in this video. A man (wearing a life vest, over shallow water) walks on candled ice and repeatedly breaks through, even though the ice is 13.5 inches thick and if intact would support a 9,000-pound vehicle with a 3:1 margin of safety.

How and why does this happen? The best explanation I got came from Dan Heim, an old friend, an Arizona resident, and author of the Sky Lights blog about astronomy, meteorology, and earth science.

As ice forms, he tells me, the mostly hexagonal crystals grow from the surface down. In the dead of winter, the crystals are strongly fused (frozen) together so that the ice appears monolithic.

“Ice expands as it warms, up to the point where it melts,” says Dan. “As the thaw approaches, the ice goes through many cooling and warming cycles, and that’s where the stress to form cracks begins to build. When you look at images of candled ice, you see that most but not all candles are hexagonal. Because of impurities in the water, the fractures are sometimes non-hexagonal. As things warm up, the ice preferentially cracks along the crystal boundaries.

“Once the cracks form, that’s where additional melting happens, further separating the candles. The load-bearing capacity of the ice, which is proportional to the square of its thickness, starts dropping as soon as the microscopic cracks form.” And dropping quite fast, one might add. So for safety’s sake, stay off of candled ice.

Another thing about candled ice: It can be almost musical. If you were to find a thick sheet of such ice driven by wind up onto shore, and if you were to tap at it, candled crystals would tumble off, making a soothing sound a bit like a set of wind chimes. Magical indeed!

A place for lake lovers

What are you doing April 23-25? If you love your lake, you might want to consider attending the 2015 Wisconsin Lakes Partnership Convention, or at least a part of it.

I’ll attend this year for the fifth time, and I wish I had started long ago. It’s an inspiring event. You spend a couple of days surrounded by scientists, communicators, lake association leaders, advocacy group representatives and others all interested in one thing: making lakes better.

For a few days you shed your political affiliation, forget what you do for a living and just learn, in hands-on workshops, field trips, lecture sessions, poster presentations, an exhibit hall, and casual break-time and lunchtime conversations.

Sessions cover all manner of subjects: aquatic invasive species, wetland protection, fishery surveys, nutrients and algae, shoreland zoning and other government policies and, perhaps most important, how to get involved in improving the lakes you care most about.

My all-time favorite session, during my first trip to the convention, was a half-day workshop on aquatic plants. There were slide presentations followed by hands-on exercises examining specimens of common and less common plants and using what’s called a taxonomic key to identify them by name. I’m a fisherman by avocation, but now when I’m out on the water, I am much more attuned to the greenery below and on the surface – it’s no longer just “weeds.”

Also of note are the plenary (whole-group) sessions, which generally feature inspiring speakers. This year, the keynote speaker is Marion Stoddart, a citizen leader and grassroots organizer who is largely responsible for the conversion of New England’s Nashua River, once among the nation’s most polluted rivers, into a candidate for the National Wild and Scenic Rivers System.

I can’t wait to hear her talk. I know I haven’t done as much as I could for lakes, including my own, and her words may help nudge me into more action.

The convention will be at the Holiday Inn Convention Center in Stevens Point. That’s not so far away, and the registration fees are affordable. There’s a good chance that one or more leaders of your lake group plans to attend. If you’re interested in doing more for your lake, you might want to consider going along.

I know I appreciate lakes more deeply, and feel better qualified too advocate for them, because I’ve gone to this event. 

Eutrophic lakes: It’s a process

Mention a “eutrophic lake” and many people will picture a stagnant pool, matted with algae, murky, bad smelling, and generally unpleasant to be around.

It really isn’t that simple. A eutrophic lake by definition is at a fairly advanced stage of a process called eutrophication, whereby the lake accumulates high levels of plant nutrients, chiefly nitrogen and phosphorus. But the mere fact of being eutrophic does not mean a lake is “dirty” or “polluted” or otherwise undesirable – although that can be and often is the case.

Nitrogen and phosphorus are necessary for plant growth. Your lake, whether eutrophic, oligotrophic (few nutrients) or mesotrophic (in between), contains these nutrients. Otherwise there would be no lily pads, no fish-attracting cabbage weeds, and no tiny algae that form the base of the food chain.

The problem comes when the amounts of nitrogen and phosphorus become excessive. Blame for that often gets placed on human sources – uncontrolled stormwater runoff from city yards and streets, runoff from over-fertilized farmland, poorly maintained septic systems, and others.

But nutrients also come from natural sources as, for example, when a shallow lake is surrounded by and receives runoff from land with fertile soils and abundant organic matter. That is to say, some lakes are naturally eutrophic, and no amount of water-quality regulation or watershed management will change that.

Of the two main nutrients, phosphorus is the one that – here in Northern Wisconsin and in most regions of inland lakes – controls the pace of eutrophication. Some of the nitrogen in lake water exists as nitrate – an atom of nitrogen and three atoms of oxygen (NO₃). Over time, biological processes convert this nitrate to nitrogen gas (N₂), which then escapes to the atmosphere. So there is to some extent a natural “brake” on the buildup of nitrogen in lakes.

It’s different with phosphorus – it accumulates in lakes, and when present in excess it can cause explosive growth of algae. Darby Nelson, in his brilliant book, “For Love of Lakes,” explains with great clarity how this works.

He first describes the ingredients in his wife’s blueberry muffins and how, if she happens to have only two teaspoons of baking powder, she can only make one batch – no matter how much flour and sugar and how many eggs she may have on hand. Then:

“In lakes, except in unique circumstances, the ‘tin’ of phosphorus usually empties first. Compared to demand, it is phosphorus that is available in least supply, the bottleneck to alchemy. Little phosphorus in lake water begets few cyanobacteria, algae and aquatic plants. Lots of phosphorus begets lots of blue-green algae, or aquatic plants, or both.”

So if we want to forestall eutrophication in our lakes, the best thing we can do is take measures to keep phosphorus out.

The Goldilocks of trophic states

I’ve been writing about lakes as classified by trophic state: Oligotrophic (few nutrients), eutrophic (abundant or excessive nutrients) and mesotrophic (those in between).

No one of these trophic states is inherently “better” than the other. It’s to some extent a matter of personal preference, except that an extremely eutrophic (hypereutrophic) lake likely has serious water-quality issues. However, if I can be allowed an editorial opinion, I prefer to live on a mesotrophic lake, like our own Birch Lake at Harshaw.

Why? Because in many respects it mesotrophic is the best of all worlds – it is “just right.” A mesotrophic lake never gets seriously choked with weeds, nor does it typically see obnoxious late-summer algae blooms that cloud the water of eutrophic lakes. It is not as crystal clear as an oligotrophic lake, but it is reasonably clear, enough so to allow decent snorkeling, especially in June and July.

In general, mesotrophic lakes support more diverse plant, fish and other aquatic life than lakes in the other two trophic states. You won’t find cold-water fish like lake trout in mesotrophic lakes because the deep, cold water gets depleted of oxygen by late summer. However, these lakes can support excellent fisheries with panfish, largemouth and smallmouth bass, walleyes, northern pike and muskies (in varying proportions that depend on a host of other factors).

The trick with mesotrophic lakes is keeping them that way – that is, making sure that excessive nutrients (nitrogen and especially phosphorus) don’t get in and start tipping the scale toward the eutrophic side.

Nutrients get into lakes in various ways, and that in itself is not a bad thing. A creek flowing into your lake (as in the case of Birch Lake) almost certainly carries nutrients from decaying plants in the forests, marshes and fields through which it flows. That’s part of nature. The thing to avoid is needless nutrient enrichment from human sources.

Our mesotrophic lakes here in the north are typically surrounded by homes and cottages. Ideally, the property owners don’t dump fertilizers on their lawns and landscapes in excess amounts that run off into the water. And any fertilizers used should be phosphorus-free. So should any laundry or dish soaps the get discharged into septic tanks and ultimately dispersed through the soil.

Speaking of septic systems, they should be inspected regularly (a requirement here in Oneida County) to make sure they are functioning properly and not sending nutrients into ground or surface waters. Once excessive phosphorus gets into a lake, it is not readily flushed from the system. And then that lake is started on the path toward the eutrophic state. About which, more in a future column.

Where it all begins

Conventional wisdom has it that from the time any lake forms it is slowly dying. It receives nutrients that feed algae and plants that die and decompose; it steadily accumulates more nutrients until it gets choked with weeds and slowly fills in.

That’s an overly simple description of a process called eutrophication, in which lakes proceed from oligotrophic (few nutrients) to eutrophic (rich in nutrients). The reality is that most lakes here in our Northwoods started life as oligotrophic: They were formed from glaciers and were surrounded by infertile land, so nutrient inputs were severely limited.

However, not all lakes become eutrophic – or at least in some the process is exceedingly slow. Some of our area lakes remain in an oligotrophic condition. You can make a pretty good assessment on whether a lake is oligotrophic just from some simple observations.

From a distance, oligotrophic lakes appear a rich blue-green. That’s because the clear water allows blue wavelengths of light to penetrate deep. On these lakes you can see the bottom at a considerable depth – anglers often refer to them as “gin clear.”

They are tough to fish, partly because the fish can easily see their pursuers, and partly because there are not so many fish to be had. Lack of nutrients means the food chain is rather sparse. Although algae in such lakes tend to be diverse, their numbers are low. Since algae form the base of the food chain, there isn’t much nutrition to translate into fish flesh (although populations of large fish may be present).

The shorelines of oligotrophic lakes tend to be steep and rocky. The bottoms usually consist of clean rocks, gravel or sand, low in organic matter and also low in sediment-dwelling organisms. Rooted plants are scarce. You tend not to see big expanses of water lilies or deep beds of cabbage weeds, as you would on lakes more rich in nutrients.

Since plant life is limited, there is little organic matter to decompose and consume oxygen. That means these lakes can be rich in dissolved oxygen from the surface to the bottom all year long. As a result, if deep and cold enough, these lakes can support species like lake trout that depend on well oxygenated water.

Oligotrophic lakes are undeniably beautiful. For one thing, Realtor surveys show that water clarity ranks high among lake features that property buyers consider attractive. And if you are a snorkeler or scuba diver, a clear-water oligotrophic lake can be a paradise. But if fishing action is what you crave, a lake higher on the nutrient scale may be more to your liking.

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.

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.