Posted by Trena Phenix on February 3, 2005

Q: We live on Great Bear Lake in the Northwest Territories, Canada. Our lake holds trout, jackfish, greyling, whitefish and we want to know how the fish get oxygen when the lake is frozen. Our lake freezes in October and doesn't thaw until the summer (June). THe ice is so strong that it is used for a road in the winter. How do fish get oxygen when the surface is ice?

A: Good question! Although a lake may be sealed over with ice, the water does have a certain amount of oxygen dissolved in it that is available to aquatic life. I'm not familar with Great Bear Lake but if there are lots of streams feeding this lake, these will bring in oxygen too although maybe not enough to sustain life throughout the entire lake in winter. As you would expect however, over time as organisms use up oxygen in an "ice-sealed" lake, the oxygen level will slowly drop over time if it can't be replenished. If the oxygen level does drop too low, then there will be a die-off of organisms. But, since some organisms can tolerate lower levels of oxygen than others, a die-off hits those that require higher levels for survival first - trout are especially susceptible to low oxygen levels and probably are one of the first to be adversely affected.

There are many variables at work in your lake that affect whether there will be enough oxygen for life under the ice throughout the winter. One important variable is the ratio of water volume to animal life present - the greater the water volume and the fewer organisms present, the more oxygen is available. Another variable is whether there is a lot of plant life in the lake - dead plants in winter are broken down by bacteria which use oxygen so if there are lots of decomposing plants, there will be less and less oxygen available for animals. Keep in mind too that in the cold water, all of the organisms have a slower metabolism and therefore need less oxygen to survive so that works in their favor.

With all that said, probably the worst setting for adequate oxygen levels throughout the winter would be a shallow lake with lots of plants and animals that has been completely sealed over with ice. Conversely, the best setting for high oxygen levels would be a large, deep lake with little plant and animal life that is sealed over. Hope this helps.

John Wiessinger ( February 4, 2005)