Eurasian milfoil can have strongly negative inpacts on native plant diversity and cover, especially in lakes with moderate to high nutrient levels (Nichols and Mori 1971, Madsen 1999). Established colonies can easily expand into native plant communities, forming dense beds (> 50% cover) and suppressing the native community in only a few years (Nichols and Mori 1971, Madsen et al. 1991). Eurasian milfoil is one of only a handful of species found in North America that is capable of shading out the also-invasive curly-leaved pondweed, Potamogeton crispus L. (Aiken et al. 1979). Stands of Eurasian milfoil can become so dense at the surface that they support the weight of frogs and wading birds (Aiken et al. 1979). Eurasian milfoil is somewhat unusual compared to native aquatics in that it can establish in water as deep as 10 m (33 ft), and reach the surface from a depth of 5 m (16 ft) (Aiken et al. 1979).
In addition to their effects on aquatic plants, dense beds of Eurasian milfoil can also adversely impact animal populations. Abundance and diversity of aquatic insects and other macroinvertebrates was greatly reduced in milfoil beds as compared to nearby native communities, in an Ontario lake (Keast 1984). During their daytime feeding period, Keast (1984) found 3-4 times as many fish feeding in areas with the native plant community as in the milfoil patches. A radiotracer study in an Oklahoma reservoir (Toetz 1997) supports these findings - almost no carbon from Eurasian milfoil had made its way up the food chain to the reservoir's invertebrates and fish. Dense stands of milfoil can also alter predator-prey relationships, leading to increases in forage fish and decreases in larger fish (Lillie and Budd 1992, Valley and Bremigan 2002).
Eurasian milfoil is considered low-quality food for waterfowl. By reducing or eliminating other species that waterfowl rely upon, milfoil also reduces waterfowl food indirectly (Elser 1969 in Aiken et al. 1979).
The dense canopy often formed by Eurasian milfoil colonies strongly affects pH and temperature below the canopy. Dissolved oxygen levels are lowered significantly, due to attenuation of light to algae and other aquatic plants below, restriction of water movement and resulting gas exchange with the atmosphere, and biochemical oxygen demand caused by decomposing vegetation in the fall (Honnell et al. 1993, sources in Getsinger et al. 2002a). By extracting nutrients (especially phosphorus) from the sediments, and releasing them into the water column during fragmentation and fall senescence, Eurasian milfoil can also contribute significantly to eutrophication (nutrient enrichment) of lakes and ponds (Carpenter 1980b, Smith and Adams 1986).
In addition to its ecological effects, the dense canopy formed by Eurasian milfoil often interferes with recreational activities such as swimming and boating. Populations have become dense enough in some cases to obstruct industrial and power plant water intakes (Hoffman and Kearns 1997).
Status in Area
Eurasian water-milfoil is locally well-established in the upper Great Lakes region. It is considered "ecologically invasive" in Wisconsin, "highly invasive" in Upper Michigan, and is listed as "prohibited exotic species" in Minnesota (WIS 2003, MIPC 2003, MDNR 2003).