Cindy Hagley, Great Lakes Environmental Quality Educator, University of Minnesota Sea Grant

One of the biggest questions resource managers are getting this summer is, “Why are there so many aquatic plants on my lake this year?” Callers want to know what has changed about their lake and wonder whether they need to invest in an aquatic plant management program.

HOLD ON A MINUTE! Don’t buy that mechanical harvester or invest in herbicides yet. Let’s take a look at some of the factors that determine aquatic plant and algae populations in a lake and how they change from year to year. Understanding these factors requires understanding a little bit about your lake’s productivity. Primary productivity is the rate at which algae and aquatic plants convert light energy into plant matter. When we talk about a lake’s primary productivity, we are talking about how prolific a population of plants is and the algae it supports. A lake choked with plants or algae is highly productive, while a pristine, clear lake, like many of our northern Minnesota lakes, is considered unproductive.

Lake productivity, and the values and beauty we ascribe to the lake, are largely determined by the supply of nutrients the lake receives. The lakes and bays where aquatic plants or algae can become dense and troublesome are usually nutrient-rich. They are also often relatively shallow.

Although aquatic plants can grow in much lower light conditions than plants on land (up to 95 percent less light), lack of light still limits their maximum depth. If you live on a lake that receives a lot of nutrients from its watershed either because of how the land surrounding the lake is managed or because of natural sources, and your lake or bay is relatively shallow it is at risk of becoming dominated by either algal blooms or aquatic plants.

This Secchi disk is barely visible just a few feet below the surface because of the dense algae growth in the water.

So what should you do if your lake is choked with “weeds” this year? The first thing to consider is what factors might be influencing the natural variability of the plants, such as climate and lake levels. Determine if the climate over the last year been different than normal. For example, low snowfall winters can allow light to penetrate the lake throughout the winter, letting many species of aquatic plants continue to photosynthesize. This process gives them a head start the following summer. Are lake levels significantly lower or higher than average? For example, low lake levels can allow aquatic plants to colonize deeper areas of the lake than normal.

The second thing to consider is whether anything significant has changed in terms of land uses around the lake. For example, is there significant construction taking place in the watershed that could be introducing large quantities of nutrients to the lake?

Determining whether the changes in plant density are natural fluctuations, or whether they are caused by changes in lake management, will help determine what to do. Often the best answer is to simply wait it out. If you do consider aquatic plant removal by one means or another, it is important to understand the possible consequences of your choice.

Shallow, productive lakes or bays are often delicately balanced ecosystems. Research has shown the balance between aquatic plant dominance and algae dominance can shift very quickly and once a lake shifts to an algae dominated state, it is very difficult to return it to aquatic plants. If given a choice, most people would prefer their cabin to be located on a lake with healthy aquatic plant populations rather than a lake where dense blooms of algae have turned the lake pea soup green and has shaded out the plants.

Lakes dominated by aquatic plants tend to have clear water and low algal populations.

What can make a lake shift from dominance by plants to dominance by algae? Research shows a clear relationship between increased nutrients and increased algae in the water and, at first, increased plant growth. Soon the plants disappear through leaving the system dominated by algae. This complicated relationship will have to be the subject of a future article, BUT you should know this: Research shows other factors can cause lakes to shift to algal dominance, such as: mechanical harvester or boat damage to plants, herbicides, pesticides or grazing on plants by exotic fish, such as carp*. In other words, the very choices you might make to control plants might drive the lake to a much less desirable state – algal soup!

What does all this mean to you, as you struggle to find a path for your boat through the plants in your lake? Before you treat the symptoms (which are quite likely temporary), CONSIDER AND CONTROL the underlying causes (excess nutrients entering your lake).

*Moss, B., J. Madgwick, and G. Phillips. 1996. A guide to the restoration of nutrient-enriched shallow lakes. WW Hawes, UK. 180 pp.

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