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.