Algae in winter?

Did you know that algae are alive under lake ice in the winter? If you enjoy ice fishing, you have maybe seen this occur.

Some species of algae are adapted to survive in cold temperatures, such as the 39º F water under the ice. The growth of these winter algae mainly depends on how much light and nutrients are available to them.

The light available depends on how much snow we have on top of the ice. At this time last year, we had a lot of ice, but not much snow, so light was able to shine through the ice and be used by algae and plants for photosynthesis. This winter, we have had a fairly thick snow cover over the ice, so the amount of light able to get through the ice is probably much less.

Algae grow best in the winter in eutrophic lakes because they have more nutrients (phosphorus and nitrogen) available for them to use as food.

So why does it matter that algae can grow in the winter? When they photosynthesize, they give off oxygen as a byproduct. This oxygen is then available for fish, aquatic insects and other aquatic animals to breathe.

This dynamic is especially important in shallow, nutrient-rich lakes. In these lakes, there is a lot of decomposition going on at the bottom of the lake, which uses a lot of oxygen. During periods of prolonged ice cover, the lake is sealed off from the atmosphere and cannot be recharged with oxygenated air. The decomposition process and all the fish and aquatic organisms in the lake use up the oxygen, and when it does not get replenished, oxygen levels can get too low for fish to survive.

If the conditions are just right – crystal clear ice and enough nutrients available – you can even see the algae growing under the ice. Next time you’re out ice fishing or walking on the lake, take a look and remember not all algae is bad!