Ask Lyn: 3 Things to Know About Your Lake in Winter

Dear Lyn,

With the recent cold, my lake is freezing over! What happens beneath that icy surface? And what will it mean for aquatic plants and animals this summer?

Thanks!

Frozen Fish

_____

Dear Frozen Fish,

What a great and complex question! 

Cold weather transforms our lakes in visible and invisible ways, and those changes definitely affect aquatic ecosystems. How much? Well, that depends on a few factors, including how cold the water gets and for how long. 

Here are three key things to know about your lake in winter.

1. Water freezes from the top down.

As air temperatures drop, the temperature of your lake drops too, starting at the surface. Once water at the lake’s surface drops below 39 degrees Fahrenheit, it becomes less dense than slightly warmer, deeper layers of water. At about 32 degrees, liquid water becomes solid–in other words, it freezes.

Now, most solids are denser than their liquid forms, but water’s amazing properties make it unique. When you add ice to a glass of water, the ice always floats, right? It’s the same concept in a lake. The solid water (ice) at the surface of your lake is less dense than the liquid water below, so it floats. If that weren’t the case, ice would sink, and the lake would freeze from the bottom up! 

As the weather gets colder, that floating ice may thicken. At a certain point, it actually provides insulation, keeping cold air from reaching deeper layers of lake water. Those layers can stay liquid even when it’s crazy cold, like during a polar vortex. 

That’s why your lake won’t freeze all the way through–good news for plants and animals!

2. Life under the ice slows down dramatically.

When a lake freezes over, sunlight, air, and water can’t mingle. That reduces the amount of dissolved oxygen in the water, which fish and other aquatic creatures need to live. Snow-covered ice also blocks light, which aquatic plants need to grow. 

To survive winter, plants and animals adapt. They get together, get cozy, and rest. (Like us!)

Plants take a break from photosynthesis and go dormant or die back, storing energy in their root systems for spring. That means fish and other aquatic creatures can’t rely on them for oxygen or nutrition. Cold-blooded organisms often snuggle in groups near the bottom of the lake, where water is warmer in winter. Some species, like bullfrogs, hibernate in or on the muck, essentially sleeping away the season. Other species, like fish, just gather in the deepest water and go into a state called “torpor.” Because they don’t move much, their heartbeats slow, and they need less food and oxygen–a smart strategy when resources are scarce.

3. Winter lasts just long enough–usually.

Here in northern Indiana, winters can be harsh, but typically, our lakes aren’t frozen solid for more than a few months. Our plants and animals (and people!) know how to outlast the cold, even with limited resources. For example, thanks to fall turnover, there’s usually enough dissolved oxygen in the deepest parts of our lakes to sustain life until spring comes. As the months go by, that oxygen decreases, so if the ice hangs around too long, it can spell trouble for fish and other creatures. But that’s uncommon.

This finely-tuned dance of chemistry and biology means that unless we have a truly extraordinary winter, we’ll see our usual plants and creatures springing back to life when warmer weather comes!

Isn’t life under the lake’s surface amazing?

Lyn