Lake with snow.

Cindy Hagley, Great Lakes Environmental Quality Educator, Minnesota Sea Grant, (218) 726-8106

This year is the first in many where some parts of the state have had lots of snow. This is great for snowmobilers and skiers, but it can be tough on fish in shallow lakes. The long, snowy winters that cause many of us to head for Florida can be the death knell for “lake-locked” fish. Winter fish kills in shallow lakes are natural events, although, as with most things involving lakes, how we manage our shoreland can make the problem worse.

Understanding why this sometimes happens requires us to learn something about how lakes function. Fish, of course, need oxygen to survive, but many of us have never considered how oxygen makes its way into lakes. In summertime, there are several ways lakes become oxygenated. Wave action “stirs” it in, and long hours of sunlight allow algae and aquatic plants to generate plentiful quantities through photosynthesis. Highly oxygenated streams flowing into lakes are also a source.

Notice that a couple of things are required for oxygen to be generated in lakes: sunlight so plants can grow, and water in contact with the atmosphere. In winter, both of these routes can be shut down. Aquatic plants can continue growing until light levels are reduced to about 1 percent of levels at the surface. This doesn’t sound like a lot of light, but in years when heavy snows cover lake surfaces for long periods, light levels can drop to the point that algae and aquatic plants not only can’t produce oxygen, but large quantities die and decompose.

Not only does the bacteria-driven decomposition process consume large quantities of oxygen, but shallow lakes often have plants growing throughout the lake, meaning lots of oxygen consumption when plants die. This can cause oxygen to drop to lethal levels, resulting in die-offs of fish already stressed by winter conditions. Deeper lakes usually have much less vegetation relative to lake volume, so winter oxygen levels tend to remain more stable.

In years when snow cover is light, like in the picture below, sunlight continues to penetrate into the lake, allowing plants to keep on growing and generating oxygen. Of course, these favorable growth conditions also tend to result in lake managers getting lots of calls from shoreland property owners because “the plants are taking over.”

Lake without snow.

So what is the connection between how we manage our shoreland property and fish kills? The more algae and aquatic plants living in a lake when it freezes, the higher the likelihood that a fish kill may occur. In addition to sunlight, algae and aquatic plants need nutrients, so anything we do that adds nutrients can increase the likelihood of a fish kill. Sediment entering a lake from an eroding shoreline or construction project carries nutrients, as does runoff from fertilized lawns or leaky septic systems. Sediment also carries organic matter which, when decomposed by bacteria, further depletes oxygen. There are many simple steps you can take to prevent excess nutrients and sediments from entering your lake. Probably the simplest and most effective is to retain or restore natural vegetation along your shoreline. For more ideas of ways to keep your lake healthy, take a look at Protecting our Waters: Shoreland Best Management Practices.

Posted in Fish and Wildlife |