By Spencer McGrew
Aquatic Invasive Species Specialist
Otter Tail County

We are kind of between seasons here in Otter Tail County. We’ve got snow on the ground, but the ground isn’t frozen, and we are below zero, yet the lakes are still open. This has important implications for aquatic invasive species.

One invasive species that take advantage of the seasons in Minnesota is Curly Leaf Pondweed. Most species of aquatic plants grow in the summer and die back in the wintertime. Curly Leaf Pondweed flips this script by growing in the winter under the ice. This gives it an advantage over its competition because it’s the only game in town—it has more sunlight, nutrients, and space. While other plants are going dormant, the curly leaf is growing, photosynthesizing, and producing its reproductive structures, known as turions. These turions sprout under the ice in late fall-winter and are ready to thrive when spring rolls around. This head start doesn’t last forever, though—as summer heats up, the Curly Leaf dies back, consuming oxygen during decomposition and potentially contributing to algae blooms due to its nutrients cycling back into the water column. The consumption of oxygen during decomposition can also stress game fish, such as walleye, which are very sensitive to dissolved oxygen concentrations.

I mentioned our early cold spell this November. Another dimension of Curly Leaf biology is its sensitivity to snowfall. Since it is dealing with already limited winter sunlight, the amount of snowfall–and when–plays a major role in how invasive a curly leaf pondweed infestation can be on a year-to-year basis. Early snow suppresses sunlight and can diminish the plant’s acreage. Light snowfall, conversely, produces more.

Recent studies done by the Minnesota Pollution Control and Minnesota DNR show how the state has one of the fastest-changing climates in the United States. Shortening ice seasons, warming summers— all pose challenges to Minnesota’s lake ecosystems. 

Sadly, it is often invasive species, like CurlyLeaf Pondweed, that are better adapted to changes in climate. 

Curly-leaf pondweed, which has infested Lake Lida and others in the area, has these impacts:
• Dense mats at the water’s surface inhibit water recreationists.
• Overtakes habitat and outcompetes native aquatic plants, potentially lowering diversity.
• Provides unsuitable shelter, food, and nesting habitat for native animals.
• Midsummer die-offs can litter the shoreline with dead plants.