Author: Erin Mastine

A Look Under the Ice: Winter Lake Ecology in New Hampshire

Lakes in winter: frozen recreation wonderlands or dynamic aquatic worlds? That’s a trick question–they’re both! While winter offers a season of ice fishing and skating on our frozen lakes, life beneath the ice is just as fascinating and vital to the health of our lakes.

We told you about how lakes freeze in winter here. But we didn’t tell you what happens under the ice and about all the living creatures there. What happens to the fish, plants, and algae under the ice? Do the streams still flow into the lake or freeze solid? Wonder no more, and read on. 

A hidden world under the ice:

Algae

Some algae, like diatoms, continue growing under the ice. Under the microscope, diatoms look like little glass houses because they’re made of silica (found in materials like sand). Many diatom species adapt to cold temperatures and bloom in winter. Winter algae, such as our native water flea, are critical food sources for plankton and remain active below the ice. Plankton are just as crucial as algae; they are a primary food source for other lake dwellers, like fish. Surprisingly, winter algae and plankton growth can sometimes rival the growth that happens in the summer, setting the stage for spring blooms and supporting the entire aquatic food web.

Diatoms (Image Credit: Mark Edlund, Planktonic community of Uro)

Fish 

Fish species in New Hampshire have unique adaptations to survive under the ice. Coldwater fish, like lake trout and salmon, remain active throughout the winter, moving closer to the frozen lake’s surface. Prime targets for ice anglers! They primarily prey on smaller fish during winter. Warmwater species, such as bass and perch, seek out the deepest and, consequently, warmest water they can find, where their metabolism slows to conserve energy until spring when the ice melts and the sun warms the water again. Some species, like chain pickerel, can even tolerate the lower oxygen levels under ice.

Aquatic Plants

Much like perennial plants in your garden, our native aquatic plants die back in winter. Most aquatic plants rely on specialized systems (rhizomes, turions, and tubers, oh my!) to regrow when enough sun shines through the ice and the water temperature rises. For example, bladderwort forms a winter bud, or turion, that survives through the winter and sprouts into a new plant in the spring. Invasive aquatic plants don’t play by these rules. Some invasive plants, like hydrilla or Eurasian watermilfoil, can survive the winter and may not even die back. Yes, you may still find them happy and green under the ice!

Bladderwort forms a winter bud, or turion, that survives through the winter and sprouts into a new plant in the spring.

Do streams still flow in winter?

Yes! Streams continue to flow and flush water and nutrients, including oxygen, into the lake throughout the winter. Streamflow helps sustain aquatic life under the ice during the winter months. Groundwater seeps, or areas under the lake’s surface where groundwater leaks into, provide pockets of slightly warmer water that some fish and plankton seek out for refuge. This water inflow is significant for deep lakes and the creatures in them.

The future of winter lakes:

Our northern lakes and the life within them depend on the reset that winter brings to help regulate temperature and biological processes. Without a consistent freeze-thaw cycle, lakes may experience disruptions in nutrient cycling, which can lead to excessive plant, algae, and even cyanobacteria growth in the warmer months.

Scientists are closely monitoring ice trends in New Hampshire, and the data is clear: our lakes are losing their winter ice. As you can see from this graph created by the New Hampshire Department of Environmental Services, ice coverage has decreased, and ice-out dates have occurred earlier in the spring over the last century.

Ice coverage has decreased, and ice-out dates have occurred earlier in the spring over the last century. Graph provided by The New Hampshire Department of Environmental Services.

What can you do to help our lakes in winter?

  • Use dock bubblers or agitators responsibly. Set them on timers and temperature gauges to minimize unnecessary open water. Learn more about dock de-icers.
  • Reduce winter road salt usage. Road salt and runoff water containing road salt can lower the freezing point of lake water and contribute to delayed ice-in and poor ice stability. Learn more about salt and our lakes.
    • Use only the necessary amount of salt and consider preventative maintenance with brine or shoveling early and often during winter storms.
    • Sweep up excess salt on your driveway or walkways after storms to prevent it from washing into the lake and increase the longevity of your supply.
    • Advocate for reduced salt use areas and roadways in municipal and state road maintenance.
    • Educate your community that white, salted roads do not necessarily mean safer roads.
  • Maintain shoreline and stream buffers. Vegetated buffers filter pollutants, like road salt and sand, stabilize banks and protect aquatic habitats. 
  • Reduce polluted runoff water. Polluted runoff water brings nutrients into our lakes. Excess nutrients in our lakes can increase plant and algae growth, impacting ice formation and winter oxygen levels. 
    • Plant native vegetation along the stream bank and lakeshore. Learn more about creating a lake-friendly shoreline buffer.
    • Maintain your septic system or sewer connection to prevent sewage from leaching into the soil and groundwater. 
    • Limit, if not avoid, lawn fertilizers, especially near lakes and streams.

Winter may seem like a time of dormancy, but life thrives in remarkable ways beneath the ice. By understanding and protecting these processes, we can help ensure the health of our lakes for generations to come.

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