Ice-in and ice-out
What ice cover on our lakes and ponds can help us understand
If you live on or near a lake shore, you may have observed that lakes are freezing later in the season and thawing earlier than they did in the past. These changes in ice cover can impact the physical, chemical and biological characteristics of lakes and ponds.
The timing of ice cover on lakes in the winter, and its disappearance in the spring, are important indicators of climate change and can affect summer water conditions, including algae growth. This timing depends on air temperature, and to a lesser degree, cloud cover and wind. Heavy rains and snowmelt also can have an effect on the length of time a lake remains frozen.
The process of a lake forming surface ice is quite amazing. First, the entire water column, from top to bottom, must reach 39.2° F. During this natural cooling process, or fall overturn, the surface water slowly cools, becoming more dense than the warmer water below and sinking to the bottom. This is an important phenomenon because the water turbulence created by this exchanges causes nutrients to mix throughout the water column, thus redistributed them and making them available to plankton and the entire food web throughout the lake. Once the process of overturn is complete, surface water begins to cool below 39.2° F, at which point it becomes less dense, and eventually freezes. Small ponds tend to ice over more quickly because there is less water to cool.
When ice forms across the lake, it seals off the water from the atmosphere and thus eliminates the oxygen exchanges and limits much of the light that aquatic plants depend upon. Throughout the winter, oxygen levels decline slowly, which can present a challenge to the insects, amphibians and fish that live in the water. However, these creatures have adaptations that enable them to survive periods of low dissolved oxygen. Examples of such adaptations are the ability to slow their metabolism or seeking out the coolest parts of a water body where there is more oxygen available.
In many places in Maine and elsewhere, people are recording increased algae growth in lakes and ponds, and cyanobacteria in some locations. This is tied in part to the timing of ice coverage. The earlier the ice breaks up, the more time there is for the temperature of the water to rise, which favors the development of algae and bacteria. Likewise, the organisms have a longer season during which to grow and spread.
Tracking the dates of ice-in and ice-out on Maine Lakes is a very useful tool to help us understand conditions in our lakes. If you live on or near a water body, we would love for you to share your observations of when ice forms and disappears.
For standardization of what is considered ice-in and ice-out in Maine, we refer to Lake Stewards of Maine: “Ice-out is determined when the lake becomes clear of ice in the spring. However, the method of determination often varies from one lake to the next. Observers for some lakes will declare ice-out only when every last vestige of ice is gone, others will wait until the main part of the waterbody is free of ice, and some base it on a rough percentage of how much ice is gone (80% of ice is off the lake). The most important thing to remember in determining ice-out is to follow the method that has been used for your lake historically. If your lake doesn’t have a record of ice-out, we recommend that ice-out be declared when the majority of the lake surface is navigable.
“Ice-in can be even more difficult to determine than ice-out, and most lakes don’t have a historic record of these dates. Ice-in occurs when the lake is completely or nearly completely covered with ice. However, lakes can thaw after an early ice-in, and remain open for weeks until the ice finally closes in for the winter. Because lakes can freeze and thaw multiple times, we track the first instance of ice-in as well as the point in time when the lake is covered until the ice goes out in the spring. It should be noted that skim ice should not be counted as an ice-in.”
If you make an observation of either date for one of our local water bodies, please submit directly to Lake Stewards of Maine, using their form on this page.
