Aquatic Plants & Animals

Nature Watch
February 25, 2008

By Susan Benson,CNHM Director of Education


Water is the coolest thing! It is the only natural substance that can become a solid, liquid, or gas. Water is colorless, odorless, and tasteless. It is the most abundant molecule on the Earth, covering about 70% of the Earth's surface in liquid, solid, and gaseous states. Up to 60% of the human body is water - the brain is 70% water, our blood is about 83% water, and our lungs are nearly 90% water. Oceans contain 97% of surface water, glaciers and polar ice caps have 2.4%, and the remaining 0.6% is found in other land surface water such as rivers, lakes and ponds. It is no surprise that humans in the north woods end up spending so much time on our lakes and rivers. Even in the winter, how can we stay away? The glistening snow, the serenity of the view, and the life still below is almost magnetic for many of us.

What happens to the water in the winter? First let us rewind in time to summer, when warm water is found at the surface while cooler water sinks to the bottom. Autumn winds tend to mix up the water and the lake “turns,” so that winter waters are warmer near the bottom where the fish hang out, and colder water is nearer the surface where the lake freezes.

Let’s take a closer look at this phenomenon. As the lake water continues to cool, unlike other substances, it maintains a maximum density at 39 degrees Fahrenheit. After water reaches this temperature, it becomes lighter and stays on the top of the lake surface where it converts to ice. Once the first thin layer of ice is frozen, wind cannot disturb the deeper water, causing the temperature to stay at the same temperature throughout the winter. This ice layer becomes a cap on the ecosystem, limiting the amount of nutrients, energy, and oxygen that can enter the lake. After these events occur, the lake residents living on or under the ice function and survive within a very small range of temperature that brings about some interesting changes.

For cold-blooded fish and aquatic invertebrates that cannot control their body temperature, their bodies maintain the 39 degree temperature. Their metabolism slows, allowing them to conserve more oxygen and energy. Although still very sluggish, they can react to their surroundings. These adaptations have some benefits, but there are still dangers in winters when there is low snow cover and extended cold, causing the water begins to freeze to the bottom, or when the amount of oxygen becomes depleted.

Reptiles and amphibians that hibernate in the bottom mud use oxygen all winter long, and since they are heavier users of oxygen, the oxygen in a body of water is first depleted near the bottom. Local ice fishermen are probably aware of the fact that fish species rise in the water as the winter continues – as a reaction to this lack of oxygen in the lower areas of the lake. Aquatic plants also deplete the amounts of oxygen through respiration, but fortunately, there are some plants that can still photosynthesize in such extreme low light levels.

Semiaquatic animals also have some adjustments to make to the winter lake changes. Muskrats compensate for any heat loss from the cold water by continual grooming themselves with oily secretions. Beavers have a special means of reducing heat loss from their bare tails and feet. Other animals have special fur adaptations to deal with the cold water conditions.

In spring, the lake will “turn” again, causing the warm water to rise, and the annual cycle to keep going. Right now we can feel the daily sun getting warmer, or observe the snow melting daily off our roof even when air temperatures are not as warm. For us, spring might seem just around the corner. Another thaw and spring melt might bring smiles to our faces in upcoming weeks, but the lake’s sealed lid will be slow to release the aquatic ecosystem for some time to come.


Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M or on the web at www.cablemuseum.org to learn more about exhibits and programs.