During the winter months, your favorite New Hampshire lake or pond might be sealed off from the outside world by a layer of ice.
Have you ever wondered why the ice floats on top of the water?
In late fall, the water at the surface of a lake begins to cool. When the surface layer of water cools to 39 degrees Fahrenheit, it reaches its maximum density. This is when water at the surface is at its heaviest, and it sinks to the bottom of the lake. Warmer less dense water replaces this layer at the lake surface. This process continues until all the water in the lake has cooled to 39 degrees.
Then, something rather unique happens–as the water at the lake surface continues to cool, the water molecules spread apart and the surface layer becomes less dense. (Usually, as things cool in nature, they become more dense, not less dense.) When the surface water cools to 32 degrees, it freezes into ice and floats on the denser, slightly warmer water below.
In New Hampshire, we have been fortunate that our lakes and ponds typically become covered with ice during the winter, allowing for another season of recreation. Many of us enjoy ice skating, cross-country skiing, and ice fishing, and some of us may even enjoy ice sailing!
However, climate change is making the ice cover on our lakes unpredictable. ‘Ice-in’ on our lakes occurs later in the fall, and ‘ice-out’ happens earlier in the spring. Research suggests that over the past approximately 100 years, the period of ice cover on New Hampshire’s lakes has decreased by one to two weeks.
A shorter period of ice cover on our lakes means less opportunity to recreate safely on the ice. It also negatively impacts lake health. Less ice on the lake for less time means a longer growing season for plants (including invasive plants), algae, and potentially toxic cyanobacteria.
But what happens beneath the ice during the winter?
Since the ice seals the lake off from the atmosphere, no additional oxygen is mixed into the lake during the winter. When snow covers the ice, sunlight is not able to penetrate into the water so algae and plants are unable to make additional oxygen through photosynthesis. As the winter progresses, the amount of oxygen, which is vital to the survival of the organisms living in the lake, is reduced. If the snow and ice cover remain for a lengthy period, it is possible that all the oxygen in the lake will be used up before the ice melts causing organisms—particularly fish—to perish.
Luckily, when the ice cover melts in March or April, the lake mixes with the atmosphere, bringing more oxygen and essential nutrients for aquatic life back into the lake!
by Ale Consulting