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.