Underwater life under ice

The cold weather has left an ice cover on many lakes, ponds and streams. You might wonder how fish survive under these rigid conditions.

Fish and other aquatic animals such as frogs, snakes and insects are cold-blooded; their body temperature adapts to the surrounding conditions. When water temperatures drop, their body temperature and metabolism decrease as well so they can withstand the colder water and do not need as much oxygen or food.

Fish also feed more aggressively in autumn to build up a fat layer that helps them get through winter. Many fish will continue to feed on smaller fish or insect life beneath the ice. Certain fish, such as trout, pike, crappie, walleye, bass, perch and bluegill, have adaptations that help boost their metabolism as long as they continue feeding. This gives them an advantage over slower-moving fish. Some species, such as catfish, bury in the mud and wait for spring.

Besides food, fish need to breathe dissolved oxygen in the water. Cold water can hold more dissolved oxygen than warm water. Most bodies of water receive about 20 percent of their oxygen from the surrounding atmosphere and 80 percent from the green plants living in the water. As long as these plants continue to produce oxygen under the ice, the fish life will remain healthy.

The problem arises when aquatic plants are not able to get enough sunlight to manufacture their energy and produce oxygen for the fish. This usually occurs when heavy snow covers the ice or the water is dirty from sediments, algae blooms or other materials. In the absence of sunlight, these life-sustaining plants may die, further compounding the problem because they use oxygen when they decompose. If the oxygen deficit is severe enough, it results in winter fish kill -- the fish suffocate from a lack of dissolved oxygen in the water below the ice.

How water freezes

Most bodies of water do not completely freeze from top to bottom because water is heaviest at 39 degrees and lighter at temperatures above and below this point.

Once the surface water cools to around 39 degrees, it begins to sink to the bottom of the lake and the waters of the lake begin to mix together in a process known as fall turnover. This turnover recycles nutrients and oxygen throughout the lake.

As winter begins and the water continues to cool, the colder water rises to the surface and freezes, while the water cooled to 39 degrees sinks to the bottom. This is known as winter stratification. As such, during winter, the warmer temperatures are at the bottom of the lake and ice does not form there.

Article by FCFCDB member

Nature Notes for 2/27/2011