Blog # 368 Winter Waters

Water temperature in Cayuga Lake in the winter, modified from geoearth.com. 

The wind has been howling all day across the lake and snow squalls shrouded us in  snow up until a half-hour ago. As the sky clears, the east shore slowly comes into view.  What is it like living in the depths below Cayuga’s cold and stormy surface? 

At the beginning of winter water temperatures in Cayuga Lake are a uniform 39ºF.  Plunging temperatures and winds chill the water surface until it is 32ºF causing it to freeze. Ice is about 9% less dense than water—one of water’s stranger properties— causing the ice to float. These thermal stratifications or layers remain throughout the  winter. If water were most dense as a solid, lakes would freeze from the bottom up,  eventually freezing solid. In that case, little or nothing would survive in the lake. Winter  stratification persists because ice and snow cover prevents blowing winds from mixing  the water and acts to insulate the water below.  

Finger Lake winters aren't long or cold enough to completely freeze the surface of the  deepest lakes—Cayuga and Seneca—but they are deep enough to stratify, or form  "layers" of water with different temperatures. This thermal stratification occurs in the  summer and winter because of the large differences in density (weight) between warm 

and cold waters. (Density depends on temperature: water is most dense or heaviest at  about 39ºF, and less dense or lighter at temperatures warmer and colder than 39ºF.) In  the spring and fall, wind and the changing temperatures cause these layers to mix or  turnover. The seasonal mixing and layering of Cayuga’s waters, as illustrated below, dictates the life cycles of organisms living within its bounds. The turnover process in the  spring and fall are critically important to life in the lake as it infuses and distributes  oxygen and nutrients throughout the entire water column. In spring, increasing solar  energy brings the plants and animals out of dormancy and enables aquatic life to again  thrive. 

Diagram of seasonal turnover of a deep lake like Cayuga Lake, adapted from Encyclopedia Britannica. 

Fish have several adaptions to survive winter below the lake ice. First, they are cold blooded meaning their body temperature matches their environment. This means colder temperatures reduce their metabolic processes, such as respiration, digestion, and  activity level. Fish are often classified as cold-water, cool-water, and warm-water  species based on their preferred temperature zone within the lake. Warm-water species  will seek out the warmest water they can, which is often found at the bottom of the lake.  They will also stay away from areas with strong currents to save energy. Fish hunker down in these areas and enter a state of torpor, reducing their respiration and using as  little energy as possible. The cold-water species, such as trout and salmon, stay active  throughout the water column and continue preying on other organisms.

Sunfish swimming below the ice, courtesy of nhfishnews.com. 

When ice forms on the surface of a lake, it seals off the water from atmospheric oxygen  exchange and blocks much of the light. While Cayuga rarely freezes over completely,  

Ice begins to form along the edges of the lake, courtesy of Sherri McWhirter.

ice covers the shallower areas for months at a time. This ice blocks the sunlight  necessary for aquatic plants and phytoplankton to produce oxygen within the lake.  Therefore, oxygen levels in the lake slowly decline throughout the winter. This presents  a serious challenge to fish inhabiting the shallow zones; if the lake stays frozen for too  long, oxygen levels can become low enough to kill them. 

What happens in the winter to the other aquatic organisms that call the lake home?  Phytoplankton, small single-celled photosynthetic organisms, produce thick-walled  resistant cells that settle to the bottom of the lake where they overwinter. 

There's a whole world alive under the ice throughout the winter, courtesy of Julia Volk. 

Zooplankton, the small, nearly microscopic animals that feed on phytoplankton, use a  similar strategy. They produce a special resting stage that allows them to overwinter in  the relatively warm lake sediments. In the spring when the lake turns over again, and  light returns, these resting stages become active. Many aquatic plants die back, but  their root systems remain intact and store energy to produce new vegetation in the  spring. Some phytoplankton remains productive throughout the winter by utilizing the  little bit of light that makes it through the ice and snow. Many amphibians will seek  protected areas and enter a state of hibernation, often burrowing down into the  sediment along the lake bottom. 

The combination of the unique properties of water and the diverse adaptions to cold  allow almost all of the plants and animals that live in our lakes to survive the winter. The  is far more happening under the water than meets the eye. 

Eyes to the sky! 

Candace

Candace E. Cornell 

Friends ofSaltPoint 

Lansing, NY 

Cayuga Lake Osprey Network 

cec222@gmail.com 

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