The Ridges Sanctuary

Nature Notes: The Impact of Ice

By Anna Foster

For Door County, the new year signals a return to ice in Green Bay and the cold days of January and February (and March, although we don’t want to admit it to ourselves just yet). The frozen landscape that the ice creates brings ice fishing, skating, cross country skiing, and snowmobiling to the forefront of outdoor activities. However, to the people who live here, the ice also indicates what the next summer season will bring.

Ice cover on the great lakes has varied greatly over the past 50 years. In 2021, the maximum ice cover was 33.3%, occurring during a polar front in mid-February. Variations in ice coverage can have both positive and negative consequences for residents of the great lakes and the ecosystems in which they live. For example, a polar vortex occurring in 2014 caused less water to evaporate over the winter and more significant snowmelt, which consequently led to water levels rising. In the summer of 2020, Lake Michigan’s shoreline reached a record  high water level at 582 feet, 3 feet higher than its yearly average of 579 feet.

Maximum Ice Cover Percentage by Year for Lake Michigan Source: NOAA Great Lakes Environmental Resource Laboratory.

When the maximum ice coverage is low during the winter, more lake water evaporation occurs, decreasing the water level. Many factors contribute to changing  ice cover and lake levels, and science suggests that these variations have become more severe over time. While the range of water levels remains the same in the hundred years over which they’ve been recorded, the cycling of water between high and low levels has become much more rapid over time. Scientists believe this trend of extremes will continue, although it seems that one thing is certain: Lake Michigan’s water levels are unpredictable.

How does ice affect The Ridges?

The swales of The Ridges are fed by groundwater and precipitation. While colder temperatures cause the swales to freeze for longer periods of time, Lake Michigan’s freeze percentage is not directly related to the swales. Perhaps the most impact that freezing water has is at the shoreline of The Ridges; the place where the landscape forms.

Ice is a key agent of erosion. The most obvious example of this is the effect that glaciers have had on Wisconsin’s landscape. Glaciers have the power to transport materials, as well as carve the landscape beneath them. The freezing of Lake Michigan’s surface also erodes the landscape, albeit at a smaller scale.

As water levels in Lake Michigan increased to record breaking numbers in 2020, newly formed ridges along the shoreline washed away. Along with the ridges themselves, newly growing plant species were also lost. The dynamic nature of the ridges and swales is vulnerable to severe weather events and high and low lake levels.

Ridges that have taken a hundred years to form can be washed away in a single summer of storms. These events serve as a reminder that this fragile landscape is ever-changing. With Lake Michigan’s water cycles becoming more severe in recent years, it’s hard to say what new ridges and swales will look like, and how water levels will impact the boreal forest ecosystem.

Animal Adaptations on the Ice

A frozen swale at The Ridges Sanctuary.

The inland swales of The Ridges freeze over in the winter, usually once the temperature drops below freezing for two or more consecutive days. Frozen swales force the sanctuary animals to adapt their behavior to survive in the cold weather.

Some hibernating animals, like turtles, remain underground or under the ice. The Painted turtle can survive in hypoxic (low oxygen) environments for months at a time. They survive by lowering their body temperature and metabolic activity. To survive in the hypoxic environment of an ice-covered swale, turtles process a chemical called glycogen. This allows them to survive but creates lactic acid as a byproduct. Painted turtles change their body chemistry to counteract the lactic acid and survive until they can absorb oxygen again.

The Wood frog doesn’t burrow in ponds like some other frog species. Instead, they chose to bury themselves in leaf litter and allow their bodies to freeze. They do this by producing a chemical similar to antifreeze in their cells, then push water out of their cells to survive. While the water freezes, their cells do not expand and burst, and they are able to thaw and hop away to vernal pools in the spring.

Muskrats, one of which we frequently see off our swale overlook by the entrance to Hidden Brook Boardwalk, will stay in their dens and venture underneath the ice until the surface of the swale thaws. In contrast, the river otter family that lives near our Hidden Brook Bridge frequently comes out of their den and explores the snowy ridges.

Animal Tracks on a frozen swale at The Ridges Sanctuary.

Frozen swales also provide highways of travel for mammals of the sanctuary. Fresh snowfall is the perfect canvas for animal tracks. We frequently see snowshoe hare, deer, coyote, and otter tracks scampering across the ice in the sanctuary.

The best way to view wildlife on the frozen swales is via the snowshoe trail that leaves from our Cook-Fuller Nature Center. After a lovely hike along Sandy Swale, you can explore the quiet, rustic trails of the sanctuary. Snowshoes are available for rent from our nature center for $5 Wednesday through Sunday. To get an in-depth experience, join one of our winter guided hikes on Fridays at 1:30pm and Saturdays at 10:30am and 1:30pm this winter.

Sources:

Great Lakes Integrated Sciences and Assessments. “Lake Levels.” GLISA: A NOAA RISA Team. NOAA, University of Michigan, Michigan State University, n.d. https://glisa.umich.edu/resources-tools/climate-impacts/lake-levels/.

Kelly, Mary Louise.  “Water Levels In The Great Lakes Approach A Record High.” NPR, June 16, 2020, sec. National. https://www.npr.org/2020/06/16/878852945/water-levels-in-the-great-lakes-approach-a-record-high.

US Department of Commerce, NOAA. “Ice Cover.” Accessed January 3, 2022. https://www.glerl.noaa.gov/data/ice/#historical.

“Water Level Data.” Accessed January 3, 2022. https://www.lre.usace.army.mil/Missions/Great-Lakes-Information/Great-Lakes-Information-2/Water-Level-Data/.