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.