Lake Mendota and Invasive Eurasian Water Milfoil

Eurasian water milfoil (Myriophyllum spicatum) is an invasive aquatic plant that poses significant threats to American lakes and waterways. Originating from Europe, Asia, and North Africa, it was first introduced to North America in the mid-20th century, likely through ballast water discharge from ships or aquarium trade.

I moved to Madison, WI a couple of years ago and I love the lakes! So last summer I was watching these interesting boats on Lake Mendota and wondering. What the heck are they doing? A little investigation and I found out they were the weed boats.

"Dane County manages an aquatic plant harvesting program that uses 12 mechanical weed cutters to harvest Eurasian water milfoil and other invasives.

Identification

Eurasian water milfoil can be identified by its:

• Feathery Leaves: These are arranged in whorls of four around the stem, with each leaf having 12-21 leaflet pairs.

• Flowering Spike: Emerges above the water surface, typically reddish or pinkish in color.

• Growth Pattern: Forms dense underwater mats that can reach the water’s surface.

Ecological Impact

Feral hogs, European starlings, kudzu vine, and the emerald ash borer. What do they have in common? They are all invasive species.

Invasive Species

Invasive species “are plants, animals or pathogens that are non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause harm.”

Milfoil meets this definition because it out-competes the native weeds in the lake. It forms dense underwater mats that can reach the water’s surface.Growing 15 feet tall it emerges above the water surface, typically reddish or pinkish in color.

The introduction and spread of Eurasian water milfoil have several detrimental effects:

Often confused with watermilfoil, the native coontail leaves are spiny and forked rather than feather-like. Many waterfowl species eat the shoots; it provides cover for young bluegills, perch, largemouth bass, and northern pike and supports insects that fish and ducklings eat.

1. Outcompetes Native Plants: It grows rapidly and forms dense mats that shade out native aquatic plants. Native plants are typically better suited to provide natural habitats and spawning ground for young fish fry

2. Alters Habitat: Not just fish. These dense mats alter habitats for waterfowl, muskrats and other wildlife, making it difficult for some species to feed, reproduce, and find shelter.

3. Degrades Water Quality: The plant can increase water temperature and reduce oxygen levels, negatively impacting fish and other aquatic organisms. Because Eurasian water milfoil grows so densely it causes problems when it decays and takes oxygen from the water. This decreases available oxygen for fish and other aquatic species.

4. Facilitates Algal Blooms: By outcompeting native plants, it can create conditions that favor harmful algal blooms.

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Economic Impact

Eurasian water milfoil also has significant economic consequences:

1. Recreational Activities: Dense growth interferes with boating, fishing, swimming, and other water-based recreation, reducing the appeal and usability of lakes and rivers.

2. Property Values: Properties adjacent to infested water bodies may experience decreased value due to reduced aesthetic and recreational appeal.

3. Management Costs: Controlling and managing the spread of Eurasian water milfoil is costly, involving mechanical harvesting and biological controls.

Weed boat unloading weeds at Mendota County Park. Weeds are taken to a local composting facility.

Control and Management

Managing Eurasian water milfoil requires a combination of strategies:

1. Mechanical Control: Harvesting and removing the plant can provide temporary relief but may inadvertently spread fragments that can regenerate. more on this below.

2. Chemical Control: Herbicides can be effective but must be used carefully to avoid harming native plants and animals. Unfortunately, this option can harm local wildlife.

3. Biological Control: Introducing natural predators, such as the milfoil weevil (Euhrychiopsis lecontei), can help control the plant population.

4. Preventative Measures: Public education and policies to prevent the spread, such as cleaning boats and equipment before moving between water bodies, are crucial.

5. Control Fertilizers: Phosphates are a big contributor to the growth of weeds. Eurasian milfoil thrives on nutrient-dense waters that are rich in phosphates. Currently Dane County banes lawn fertilizer however the watershed for Lake Mendota is 72,093 acres so a lot of agricultural land drains into this lake.

Weed Boats

Harvesting is done with specialized boats called weed boats. This is a common practice around the globe. The front of these boats are like a pitchfork and they scoop up the weeds from the bed of the lake. The weeds are then unloaded and put into a dump truck. You can check out the above video to see video I took of this process.

Problems with Weed Boats

What about the fish and invertebrates that live among the plants? What happens to them when the weed boat starts grabbing up the plants?

I called the Dane County Parks Department to ask that question. They told me that harvesters were trained to keep an eye out for fish so they could put them back in the water. They also said that while Eurasian water milfoil provides some habitat for fish and invertebrates, the quality of the habitat is not as good because it grows in high densities and forms a monoculture.

Conclusion

Eurasian water milfoil poses a significant threat to American lakes and waterways. Effective management and control require a multifaceted approach involving mechanical, chemical, biological, and preventative strategies. Public awareness and cooperation are essential to prevent its spread and protect aquatic ecosystems.

Author, Ame Vanorio, is the founder of Fox Run EEC, is an author and teacher.