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.