Shallow Lakes and plants vs. algae

Have you ever wondered why shallow lakes have such dense aquatic plant growth? Having aquatic plants established in a shallow lake is a good thing, because the alternative is not so desirable. Today I’ll explain the natural state and importance of shallow lakes, and explain what happens if this natural state is not protected.

Before we go any further, let’s define “shallow”. Shallow lakes are lakes where the sunlight can reach the bottom.


Generally, this corresponds to 15 feet deep or less. Since the sunlight can reach the bottom, plants are able to grow there. Examples of shallow lakes in the Detroit Lakes area include Little Pelican, Rock, Rice, Rossman, Marshall, and Shell Lakes.

Shallow lakes provide some of the most important wildlife habitat. Aquatic plants in and around the lake are home to nearly all aquatic animals such as waterfowl, muskrats, otters, fish, insects, frogs and turtles. These lakes are also important resting areas for migrating waterfowl. There are over 5,000 shallow lakes in Minnesota that are over 50 acres in size.

Shallow lakes behave differently and have different dynamics than deep lakes. I often have explained how deep lakes separate into layers (stratification) in the summer. Deep lakes only mix in spring and fall, and the bottom of deep lakes stays cold and dark because light cannot reach the bottom. Shallow lakes, in contrast, mix all summer because light reaches the bottom of the lake and warms the whole water column.

A healthy shallow lake has clear water and dense aquatic plant growth. Many shallow lakes have large stands of bulrush and/or wild rice. The plants in these shallow lakes lock up a lot of the nutrients in their tissues so that there is not as much algae growth, and they produce oxygen throughout the water as a byproduct of photosynthesis. These plants also keep the sediments stable at the bottom of the lake and not mixed up into the water column. Tiny invertebrates called zooplankton eat algae and use plants as a hiding place from their predators (perch, sunfish and crappies).

Unfortunately, if a shallow lake isn’t taken care of, it can turn into pea soup. If large areas of plants are removed by pulling them out, cutting them with a weed roller or with a boat motor, the sediments can get churned up and nutrients are released. If there are fewer plants to use the nutrients, the algae will use them and multiply. In addition, bulrushes and wild rice are protected by the DNR, and a DNR permit is necessary to remove them.

Once the water is “green” with dense algae, these lakes have mostly muck on the bottom instead of plants because the sunlight can’t get through the dense algae to the bottom of the lake. Algae-dominated shallow lakes are also not as high of quality habitat for fish and wildlife. If the plants are gone there is no place for aquatic animals to hide. In addition, the oxygen at the bottom of these shallow lakes is usually depleted because of all the decomposition of dead algae that sinks to the bottom.

If there are fewer plants, the zooplankton have nowhere to hide and are eaten up by small fish. With the zooplankton gone, there is nothing to eat the algae and keep it in check. The lake just continues to support more algae.

All these factors are like a positive feed back loop that just keeps pushing the lake towards more and more algae, cloudier (turbid) water, and less plants and wildlife.

If you live on a shallow lake, keep in mind that the natural state of the lake is to have abundant aquatic vegetation. Enjoy the excellent bird and wildlife viewing available on these lakes.

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