Tuesday, January 13, 2009

Science of the seasons: Many factors affect lake ice formation, thickness

As air temperatures descended this fall, most lakes became covered with a layer of ice.

Shallow lakes, with lots of surface area compared to the volume, froze first, and some very shallow lakes or ponds freeze all the way to the bottom. At the other end of the size spectrum, those lakes with large volumes and great depths, like Kenai, Skilak, Hidden and Tustumena lakes, are the last to freeze.

The reason for slower ice formation is due to the huge amount of heat loss that must occur before ice is formed. A basic property of water is that it takes a lot of heat loss — one calorie per gram of water — to cause a reduction of 1 degree Celsius. Then it takes even more heat loss — 80 calories per gram of water — to get ice to form after the water has already reached zero degrees Celsius.

In order for a lake to freeze, the entire water body, top to bottom, needs to drop down to 40 C. Another pivotal property of water that comes into play here is that water is most dense at 40 C, and thus it sinks to the bottom when it reaches that temperature. Water that is colder (or warmer) will be less dense and will remain above the deeper, 40 C water. Eventually, the entire lake will be at 40 C.

As the cold winter air on the surface causes the top layer of lake water to get colder yet, it becomes less dense and stays on top. Once it has gotten cold enough to freeze, it is 10 percent less dense than liquid water and, as we all know, ice floats. At that point, all the water underneath the ice is going to be 4 C or colder.

Initially, the formation of ice insulates the underlying lake water from further heat loss. However, heat is continually being lost from the ice. As ice loses heat, the underlying water freezes and ice forms on the bottom of the ice layer. It is not uncommon for ice to grow to 30 inches or more in lakes on the peninsula.

Lakes that have continued inflow of water after an ice cover has formed may have thinner ice cover in areas where groundwater seeps in or where stream water enters. Inflowing water will probably not be as cold as the lake water, so it will rise and possibly melt some of the overlaying ice. Even if the incoming water is colder than 40 C, it will stay on the top of the lake and may still cause some thinning of the overlaying ice. As careful ice skaters have known for a very long time, it is good to avoid those areas where water is still entering a lake.

As water enters and leaves a lake through normal input or drainage patterns, it can have an impact on the ice surface. Imagine a situation where water continues to enter the lake but the shallow outflow is blocked off with ice. The water level will rise imperceptibly and will push up on the ice. Since all lake ice covers have cracks, the rising pressure of extra water underneath can push liquid water through the cracks.

Water leaking through the fissures in the ice is referred to as overflow. When overflow occurs, unfrozen water sits on top of the ice. At times, overflow can be many inches deep. If there is no snow cover, the air will rapidly cool and freeze the newly exposed water. However, a thick snow cover acts as an efficient insulator and keeps the water from contact with the much colder air. Because of this insulating ability of snow cover, lake overflow may remain liquid for weeks at a time.

Ice cover on a lake can decrease the exchange of oxygen from the air into the water. Snow cover can reduce the amount of light reaching the lake bottom to virtually nothing, and very little oxygen-producing photosynthesis can occur.

Bacterial breakdown of dead plant materials on the lake bottom uses up much of the limited oxygen. By the end of winter, there can be very little oxygen left in the water. In some cases, there is so much oxygen depletion that the fish die. Shallower lakes are most often the ones with winter fish kills. Generally, lakes that are more than 15 feet deep can retain enough oxygen in the water to prevent overwinter fish kills.

Because there are so many variables — such as lake size, snow cover, incoming waters and variable air temperatures — that impact how fast ice forms, always check ice thickness before venturing onto the surface. This fall when prospecting for a lake to do some early ice fishing, I found one lake with a mere 4 inches of ice, and a lake less than half a mile away had more than 9 inches.

Have fun on the ice, but be careful the ice is thick enough for your intended use.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.

No comments: