Shallow Lakes: an important yet vulnerable habitat

Over the summer I have touched on the concept of shallow lakes in other columns, but this week I will focus on them. Shallow lakes behave differently and have different dynamics than deep lakes. Before we go any further, let’s define “shallow”. Shallow lakes are lakes where the sunlight can reach the bottom. Generally, this corresponds to 10-15 feet deep or less. Since the sunlight can reach the bottom, plants are able to grow there.

There are over 5,000 shallow lakes in Minnesota that are over 50 acres in size. These lakes are a very valuable habitat for wildlife, and are also very vulnerable to human impact.

A couple weeks ago I talked about lake layers (stratification) and mixing. 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.

There are two general types of shallow lakes, plant-dominated lakes and algae-dominated lakes. Plant-dominated shallow lakes have fairly clear water, dense plant growth and are excellent habitat for fish, ducks, bugs and other wildlife. The plants in these shallow lakes lock up a lot of the nutrients in their tissues so that there is not dense algae growth, and they produce oxygen throughout the water as a byproduct of photosynthesis. These plants also keep the sediments stable 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).

Algae-dominated shallow lakes are the ones that look like pea soup. These shallow 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 good of habitat for fish and wildlife. 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.

Unfortunately, a clear-water plant-dominated shallow lake can turn into an algae dominated lake if certain things occur. If 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.

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.

If carp get into the lake, they can churn up the sediments as they search for food. Churning up the sediments will cause the water to become cloudy and nutrients to get suspended which favors algae growth.

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.

To help keep shallow lakes healthy, work to preserve the vegetation in the lake that ties up the phosphorus. Restore your shoreline to native grasses and plants so they can soak up and filter runoff. Also, prevent more phosphorus from entering the lake by keeping your yard free of pet waste and unbagged piles of leaves. Keep your septic system maintained and pumped properly. Avoid applying fertilizer to steep slopes where it can run off more easily. Think twice about paving more of your yard and increasing your impervious surface.

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