Fish killed by too MUCH oxygen in lakes

As reported in the StarTribune:

Les Hassler, 65, who lives on the lake, strapped on ice skates last week to take advantage of the clear, smooth ice and was shocked to discover dead fish just below the surface.

“There were hundreds and hundreds of panfish, and some nice largemouth bass, walleyes and muskies,’’ he said Monday. “One of the muskies was 48 inches, and I saw eight others almost that size. It’s a shame.’’

Water samples taken over the weekend and on Monday showed oxygen levels of up to 14 parts per million, “which is extremely high,’’ Schrader said. “And that was after the event, so it may have been higher than that.’’

Here’s what the DNR thinks happened: Vegetation and algae in the lake produce oxygen, and when the lake quickly froze, excess oxygen was trapped beneath the ice. The lack of snow meant sunlight penetrated the clear ice, allowing plants to continue the photosynthesis process. Oxygen levels rose too high, killing some fish, mostly in shallow water.

Normally, fish die-offs occur late winter in shallow lakes because of a lack of oxygen. Snow and ice limit the amount of sunlight reaching vegetation, and oxygen is depleted.

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