As I write this morning, it’s 5 below and the forecast is calling for wind chills of -30 or more in a couple of days. I do love winter, but am glad that I have a house for shelter. For wild creatures, winter is a tough time, especially days like these. Those that don’t migrate must find ways to conserve their energy and body heat. Some hunker down and hibernate through the coldest weeks of the year. The rest have found ways to cope with the cold and ice.

The creatures and plants found in lakes and ponds are among those that have ways to cope. Life continues under the ice in winter, but just like in the frozen forests above, lake denizens have to adapt to the changed conditions. As I started reading up on winter under the ice, I found that our understanding of winter life in lakes is beginning to change.

What We Know About Winter In Northern Lakes And Ponds

Ice-covered lake

Ice takes many forms on winter lakes. Photo: Vincent Foret,

It’s not easy to get data during the winter. Lakes don’t always have good ice cover to travel on and boating in winter is a treacherous thing. I sampled Flathead Lake (Montana) during the winter, years ago now, and remember skiing out over the ice to the middle of the lake. We’d drill a hole and then try to get water samples before our instruments and hands froze. Sometimes we got what we came for, other times not. It’s not surprising that we don’t have a good understanding of freshwater aquatic ecosystems in winter.

We do know lakes can stratify in winter as well as in summer. This is called inverse or winter stratification. It has important implications for nutrient chemistry and the food web. Water becomes less dense when temperatures fall below 39 degrees F (4 degrees C). Warmer water forms layers at the bottom of the lake with progressively cooler water on top. Winter stratification can occur without ice cover, but ice cover doesn’t always mean a lake is stratified. Currents can prevent stratification under the ice.

Winter stratification can lead to significant chemical changes at the bottom of the lake, just like summer stratification does. Oxygen at the bottom of the lake can drop, triggering internal loading of phosphorus. Severe episodes of low oxygen can lead to fish kills under the ice. Water carrying dissolved materials is denser and sinks to the bottom. This means that some things, like dissolved deicing salts can build to high concentrations near the bottom. That can harm the organisms hunkered down there for the winter.

Photosynthetic organisms have numerous ways to ride out the winter. Light for photosynthesis dwindles as the days get shorter, so many go dormant. Aquatic plants that drop their leaves each fall often use rhizomes, tubers, and turions to store carbohydrate food for use in winter and a jump start in spring. Species that keep their leaves typically have specialized winter versions that are tougher and more densely crowded. Fruits and seeds produced by annual aquatic plants are also tough enough to make it through the winter period. Algae produce overwintering cysts with tough covering and stored carbohydrates. These act like seeds, lying dormant on the bottom until conditions improved. Some algae do remain active in winter, including diatoms and cyanobacteria. Some cyanobacteria can even bloom under the ice.

Zooplankton and rotifers also slow down in winter. Most Cladocerans, like Daphnia, produce overwintering eggs (ephippia) that lie dormant til spring. Some overwinter as adults, though there are many fewer individuals than would be found in summer. Copepods do periodically enter a stage of reduced biological activity called diapause, but it’s not always related to winter conditions. Many copepod species are active during the winter and rely on food sources other than phytoplankton to survive.

Fish enter a state of torpor under winter conditions. They locate a spot with good circulation and the warmest water they can find to hunker down. Heart and metabolic rates slow. They need little food or oxygen. Coldwater species are a little more active than warm water ones, but they too slow down. Some fish may burrow down into the mud, but most spend their torpor period in large schools.

What We Don’t Know

In the non-polar parts of the world, we don’t know as much about winter freshwater lake biology as we’d like. It’s long been assumed that there isn’t much biological activity in the winter. Many body functions slow down when temperatures drop. With less sun to support photosynthesis, there is less plant growth and therefore less energy entering the food web. We expect lakes in winter to be quiet places, with most living things dormant or moving as little as possible to conserve their energy until spring.

This is not the case in the polar marine waters, Those areas team with life under the ice (see BBC’s Rich Marine Life Under Frozen Ice). The same temperature limitations on body functions occur there, so life thrives at a slower pace. In the last decade or so, scientists in other parts of the world have begun to use equipment developed for polar waters in the non-polar parts of the world. They are finding that there is more happening under the ice than we knew.

Diatioms in sea ice.

Diatoms growing in sea ice. Photo: Krebbs and Deming,

Most of the scientific literature suggests phytoplankton productivity is low under the ice, but there is relatively little hard data. Algae grow very well attached to winter sea ice at the poles, but we don’t have a good understanding of how much surface area freshwater ice provides for winter algae in northern climates. Just how much winter phytoplankton contribute to the overall energy processes of a lake remains to be uncovered.

– We have assumed that the winter food web in lakes is similar to the summer one. This doesn’t hold true under the sea ice and may not be true in freshwater lakes either. With fewer phytoplankton, what are zooplankton to eat? Recent studies suggest they eat more detritus or bacteria. Fish may rely more on benthic food sources. That all indicates that the winter food web in lakes, especially ice-covered ones, may be very different from the summer food web.

– Ice cover changes the air-gas exchange processes of a lake. Gasses are essentially trapped until the ice melts. We don’t know much about these processes. We also need to learn more about oxygen cycling in winter months. Higher nitrogen loading to lakes seems to affect the intensity of oxygen depletion under the ice, something scientists are trying better to understand. Low oxygen also leads to nutrient release from the sediment during the winter and we need to better understand what that means for our lakes in summer.

It’s Changing

Graphic showing the change in ice thaw data.

Change in ice thaw dates, showing the decrease in days of ice cover 1905-2019., see the original.

With climate change, winters are changing rapidly. Lakes are experiencing less ice cover (see the figure for examples from EPA). That may mean more evaporation and lower water levels. Light can penetrate deeper into the water without ice, stimulating phytoplankton growth. More heat from the sun is held by the water, limiting ice formation even more. Less ice cover and warming winter waters affects summer conditions too. Lakes start in the spring with warmer waters, leading to increased late summer temperatures, They stay warmer longer in the fall – all of which means more growing time for algae and plants. Scientists are rushing to gather more information about conditions now so we can better understand what these changes may mean for our lakes in the future.

Keep reading about winter under the ice with these resources: