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Lake algae changes throughout the summer

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outdoors Detroit Lakes,Minnesota 56501
Detroit Lakes Online
Lake algae changes throughout the summer
Detroit Lakes Minnesota 511 Washington Avenue 56501

Have you ever wondered why sometimes during the year the lakes look "greener" than at other times? The concentration of algae and the types of algae in the lake change throughout the season. This pattern is called algal succession.


You can think of it like plants in a garden. All plants are there for the whole season, but different plants prefer different conditions and bloom at different times. Like terrestrial plants, aquatic plants and algae need light and nutrients to grow and photosynthesize.

In a lake, there are many different kinds of algae, but we can group them into 3 main types. Diatoms are the first to "bloom" in the spring and do best in cold water. They also give a last hurrah at the end of the season in September. They can deal with widely variable light levels that occur as the lake is mixing in the spring.

Diatoms are very small and brownish, so that is why lakes usually look fairly clear in the spring. If you were to filter lake water on filter paper in the spring, the residue will look brownish.

Once the water warms up and stops mixing, the green algae out-compete the diatoms. Green algae are a diverse group and include filamentous algae such as Spirogyra.

This is the type of algae that form long strings in the water in mid-summer. Green algae are larger than diatoms, and are green in color. That is why the lake looks greener in July and August than in May. If you were to filter lake water on filter paper in mid-summer, the residue will look green.

In late summer in lakes and ponds that have a lot of nutrients (phosphorus), blue-green algae (Cyanobacteria) take over. This is the type of algae that makes the water look like green paint.

They thrive in warm water, and sunny, calm conditions. Blue-green algae can be toxic at times, and so far scientists are unable to predict when the toxins form. When the water looks like green paint, avoid swimming and letting your dogs swim or drink from the lake.

You can track the different algae blooms by measuring water clarity with a secchi disk weekly throughout the summer. You will see the clarity go up and down throughout the season as the different types of algae bloom.

Until next week, enjoy the lakes!

(Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465,