Wintertime Lakes

Have you ever wondered how fish survive below the ice? Fish and other aquatic animals like frogs, snakes, and insects are cold blooded— their body temperature adapts to the surrounding conditions. When water temperatures drop, the fish’s body temperature and metabolism decrease, as well, so that they can withstand the colder water and do not need as much oxygen to breathe or food to eat. Fish feed more aggressively in the fall to build up a fat layer that helps them get through the winter. Many fish will continue to feed on smaller fish or insect life beneath the ice. There are certain fish like trout, pike, crappie, walleye, bass, perch, and bluegill that have adaptations that help them boost their metabolism, as long as they continue feeding. This gives them an advantage over slower moving fish. Some species, such as catfish, bury themselves in the mud and wait for the spring to come around.

Credit: frederick.forestryboard.org - Mike Kay

In addition to food, fish need to breathe dissolved oxygen in the water to survive. Colder water can hold more dissolved oxygen than warmer water can. For example, when water temperatures are 39 degrees Fahrenheit, water can retain 13 ppm of dissolved oxygen; but at a temperature of 70 degrees, the maximum dissolved oxygen water can retain is only 7 ppm. So as temperatures decrease, the water is able to store more oxygen for the fish. Most water bodies receive about 20% of their oxygen from the surrounding atmosphere, and 80% 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 life. 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, which further compounds the problem, because they use oxygen when they decompose. If the oxygen deficit is severe enough, it will result in winter fish kill, the fish suffocating from lack of dissolved oxygen in the water below the ice.

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. With diminishing fall temperatures, 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.” Therefore, during the winter, the warmer water is at the bottom of the lake, and ice does not form there.

Article by FCFCDB

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