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Ice-out: What Could Happen to your Ponds and Lakes?

The thawing of a lake or pond is called ice-out. It is often slow and gradual with a widening band of water showing at the shoreline, but sometimes it is rapid with ice piled on the shore in the wake of a strong spring storm. In the Chicago area ice-out may happen multiple times, with open water occurring for a brief period during a warm spell in winter, followed by a deep freeze and a return of ice-on. There have been many observations that the ice-on period has been getting shorter due to climate change. Depending on where you live, ice out may have occurred already or is about to happen.

What can happen when ice-out occurs?

1. Discovery of a winter fish kill. A heavy blanket of snow covering the ice prevents sunlight from reaching the water and causes algae and plants to die. This depletes oxygen in the water and stresses fish. Although the metabolism of fish slows dramatically in cold water, they still need oxygen, and if the level gets very low (< 1 ppm) then fish die. Winterkill is observed at ice-out when many dead fish float on the surface.

How can winterkill be prevented? Winterkill is strongly dependent on ice and heavy snow, and is more likely to occur in shallow lakes and ponds (average depth < 5 feet) and in nutrient-rich waters. It can be prevented by:

  • Plow snow off a section of ice to allow sunlight to penetrate the water and help aquatic plants and algae to grow and produce oxygen.
  • Running aeration systems could prevent winterkill, but it will also attract geese to the open water and more nutrient loading with their droppings.
  • Reduce the amount of oxygen depletion caused by decomposing organic matter such as dead plants and algae. Excessive aquatic growth is caused by nutrients entering the lake, which can be reduced by using no-phosphorus fertilizers on nearby lawns; planting native plants along the shoreline, and reducing the goose population (see blog ‘The geese are coming”).
  • Deepen the lake or pond by removing nutrient-rich sediment.

2. Shoreline erosion. Ice heave can damage shorelines and piers, especially if heavy wind accompanies ice-out. I have witnessed shorelines that have had ice heave by as much as three feet, severely damaging the shore.

Install native plants along the shore to minimize erosion. Native plants have very deep roots that can usually withstand ice heave, where lawn grasses cannot. If ice-heave is a continuing problem and the shoreline is subject to severe erosion then armoring the shoreline with rock or boulders from the low water level to several feet above normal water  level is recommended. Maximize the shoreline protection by adding a buffer of native plants above the rock.

3. Algae-blooms. When the lake turns over the colder water at the surface warms and sinks mixing the water and allowing nutrients that were at the bottom to become  distributed throughout the water. Algae feed off nutrients in the water, so an algae bloom could occur shortly after ice-out. What can be done to prevent algae blooms?

  • Reduce nutrient loading into the lake by reducing the use of phosphorus containing fertilizers in the watershed.
  •  Perform an alum treatment to bind and sink phosphorus to the bottom sediment.
  • Plant a native buffer shoreline to absorb nutrients from runoff before it enters the lake.
  • Remove nutrient-rich sediment which contributes to nutrient-loading in the lake when dissolved oxygen levels are very low.

Provided by Integrated Lakes Management

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