On Thin Ice: Trends and Implications for Great Lakes Ice Cover

Whether you skate on it, ice fish through it, or you simply take in its sights and sounds, ice is worth celebrating. For those who live in the Georgian Bay Biosphere, ice cover is important to our economy, recreation, as well as to native species and their habitats. But what happens if we start to lose that ice cover on our lakes? This month, join us in exploring the frozen world around us.

Back to Basics

Let’s start by talking about the basics – why ice floats. It’s something most of us don’t give any thought to, but the explanation behind why a thick sheet of ice covering a lake actually floats, rather than sinking to the bottom is pretty neat.

As the days and nights get colder, so does the water, until it reaches 4°C, at which point it reaches its maximum density. Cold water is more dense than warm water so it sinks. However, as it gets even colder, instead of getting denser and sinking, like almost every other liquid in the universe that we know about, it starts expanding, becomes less dense, and remains at the surface until, at 0°C, it freezes and is even lighter than the water around it. In fact, ice is about 9% less dense than liquid water. In other words, ice takes up about 9% more space than water, so a liter of ice weighs less than a liter of water.

Why is ice less dense than water? It has to do with water’s molecular structure and polarity. Each water molecule consists of two positively-charged hydrogen atoms and one negatively-charged oxygen atom. When water cools below 4°C, the hydrogen bonds adjust to hold the negatively-charged oxygen atoms apart creating a hexagonal crystalline structure. This structure takes up more space than water molecules jumbled randomly together, which is the case for liquid water.

But imagine for a moment if ice sank. Lakes would freeze from the bottom up. With both water and ice being good insulators, they might never totally unfreeze, and would remain very cold. Bottom-dwelling life could not exist in these areas, and life in the cold water above would be quite different, as would our climate. Instead, the ice on our lakes and rivers insulates the water beneath, maintaining it at a stable 4°C until break-up in the spring. As long as oxygen remains, this is a pretty good habitat for aquatic life.

The Protective Properties of Ice

Winter storms on the Great Lakes can be fierce. A solid layer of ice along a shoreline helps protect it from powerful waves stirred up by strong winds that would otherwise be capable of causing erosion and damage to infrastructure. In essence, a thick, solid cover of ice acts as a breakwall protecting the shore.

In addition, ice cover also helps to protect fish eggs from that same wave action stirred up by winter storms. Lake whitefish (Coregonus clupeaformis), for example, spawn on shallow reefs in the fall. Their eggs have to survive throughout the winter before hatching in the spring. Without ice cover protecting the eggs, they can be damaged by waves or dislodged and washed away from ideal habitat.

Light penetration is also affected by ice cover. The greater the ice cover on a lake, the less light penetrates through to the water below. Low light conditions under the ice limits the growth of aquatic plants. Aquatic plants native to our lakes are adapted to these conditions and go dormant during the winter months. Reduced ice cover could allow colonization of cold-tolerant invasive species that are not adapted to go dormant on the same schedule as our native species. This would give the invasive species a big advantage over our native species.

The amount of light penetration could also be important for fish species. Researchers at the University of Vermont are studying how light impacts cisco (Coregonus spp.) egg development and survival after hatching. Like lake whitefish, cisco eggs incubate during the winter under the ice and hatch in the spring as the ice melts. Read more about the research here: https://ijc.org/en/more-light-and-less-ice-cover-could-mean-fewer-cisco-great-lakes.


Burbot (Lota lota), also commonly called ling, are the only freshwater fish in Canada that spawn under the ice! As if that wasn’t cool enough, they have drumming muscles on their swim bladder that they contract in order to produce calls or vocalizations under the ice. It is believed that burbot produce calls as part of their mating system because of the limited light under the ice which would reduce the effectiveness of visual cues.

Troubling Trends in Great Lakes Ice Cover

There has been a lot of media coverage this winter about ice cover on the Great Lakes. From a slow start to winter, to a cold snap in February that saw the amount of ice cover on the Great Lakes more than triple in just over two weeks, ice has been a hot topic. The bigger story here, however, is the long-term decline of ice cover in the Great Lakes as a direct result of climate change.

The Great Lakes region has seen wintertime warming resulting in a declining trend in ice cover and duration of the ice season. There is considerable variability in ice cover year-to-year, so we may still see years with a high percentage of ice cover. On the whole though, the trend is one of less ice. Annual maximum ice coverage on the Great Lakes is, on average, 22% lower than it was 50 years ago. Looking at Lake Huron in particular, annual lake ice coverage decreased on average about 2% per year between 1973-2010.

Recently published research shows that close to 5,700 lakes in the Northern Hemisphere could permanently lose ice cover this century if greenhouse gas emissions are not mitigated. Lake Superior could become permanently ice free by 2055 (Sharma et al., 2020).

This is troubling information because of the potentially numerous and far-reaching impacts associated with less ice cover. As we have already discussed, ice plays a crucial role in protecting shorelines, infrastructure, and fish eggs from damaging waves. From an economic perspective, ice cover is an important part of the Great Lakes economy through tourism and recreation. Less ice means water temperatures will rise faster in the spring and summer which could lead to a whole host of water quality concerns. Warmer water could also have consequences for cold water fish species and present new opportunities for the colonization of invasive species. Click here for more information about ice cover and warming water temperatures in Lake Huron.

We all have a responsibility to help stop the global problem of climate change from getting worse. There are many ways we can reduce our carbon footprint, save energy and money, and reduce impacts on future generations. Visit the GBB’s Climate Change page to learn more!


The greatest ice coverage recorded on Lake Huron occurred in 1994 with almost 98% coverage. On the other hand, the least ice coverage recorded was in 2012 with only 24% coverage.

Get Involved!

IceWatch is a citizen science program that allows volunteers to contribute information about freeze-thaw cycles of Canadian lakes and rivers. By reporting ice on and ice off dates, citizen scientists contribute to an understanding of the effects of climate change on Canadian ecosystems. The program is open to anyone by simply registering with NatureWatch. Observations are especially important in areas with little geographic coverage.

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