A White Christmas: No Wish for a Fish

So how do fish fare when lakes take on their coat of winter ice?  It depends on a few critical conditions.

Take this year, for example.  (Yes, please do, cries the Nordic skier in me, yearning for snow.)

With ice thinner than normal and snow cover minimal across large areas of Minnesota, light penetrates lakes much deeper than normal.  From a fish point of view, that’s a very good thing.

Light triggers photosynthesis by plants, enabling them to grow, but more important to fish, it also releases oxygen.  What a joyous winter this has been for fish.  Oxygen aplenty!

Will it last?  Who knows.

Photo by Ryan Rodgers

With 16 inches of clear ice about 70 percent of sunlight penetrates through to the water below.  Add a few inches of “snow ice” and light penetration drops precipitously.  (See more photos by Ryan Rodgers)

Those infamous state basketball tournament dumpings of fifteen to twenty inches of sloppy wet snow that persists can devastate a fish population.  The snow dump cuts off light penetration.  Little light equals little photosynthesis, which leads to very little production of oxygen for fish and other aquatic life.

Bacteria in the lake use the lake’s oxygen to decompose dead plant and animal material, reducing the supply available for fish.  In worst cases those microbes can consume essentially all the oxygen.  This can lead to massive fish die-off—winterkill.

Are all lakes equally vulnerable to winterkill?  See the question of the week for insight.

Question of the Week:

What features of a lake make it more vulnerable to winterkills of fish? (Submit your ideas and look for the answer next week.)

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