Habitat, Spread and Impacts

Habitat

Eurasian watermilfoil can be found on every continent except Antarctica. It is native to Europe, Asia, and northern Africa. The details of its introduction to North America are unclear, but it may have been introduced near Maryland around 1940, possibly through the aquarium trade. It is problematic across the United States and many states have programs in place to prevent its establishment and spread. This exotic watermilfoil colonizes rivers, lakes, and other water bodies. It grows under a range of trophic conditions, but it is considered an indicator of eutrophic (low levels of dissolved oxygen, high levels of organic matter) conditions. It captures nutrients from both ambient water and sediments, but mostly from fine sediments containing 10-25% organic matter. Root anchoring may be impeded by sand, gravel, or flocculent (loose and fluffy) substrate textures. Growth is limited by light, preventing colonization of deep waters or water with high suspended particles. Optimum water depth for growth ranges from three to 13 feet (1 to 4 meters), and a maximum depth for growth is 39 feet (12 meters). Cold temperatures have little influence on growth except under reservoir drawdown conditions when plants are exposed to the air. Eurasian watermilfoil can use bicarbonate as a source of dissolved inorganic carbon, and high growth rates and dominance in hard, alkaline, high pH waters is common. It grows vigorously in salinities up to 10 ‰ (10 parts per thousand) and survives at 20 ‰ (20 parts per thousand) salinity (the concentration of brackish water). It can tolerate moving water, and water currents and wave action facilitate fragmentation.

Early detection is a priority for management of Eurasian watermilfoil in Montana, and predicting where it will colonize can focus surveys to susceptible water bodies. A Wisconsin study developed models to predict the likelihood of finding Eurasian watermilfoil based on its presence or absence in Wisconsin lakes. Variables associated with dissolved inorganic carbon were the most important factors predicting occurrence. These variables included alkalinity, bedrock, and forest cover. Lakes with a one percent increase in forest cover in their drainage were five to 50 times less likely to become infested than other lakes. Variables affecting Eurasian watermilfoil growth were better predictors of presence than variables indicating human activities. The presence of a public boat launch was the best human activity predictor, followed by the relative abundance of walleye and smallmouth bass. Lakes with a public boat launch were 21 to 28 times more likely to become infested than lakes without a boat launch.

Spread

Eurasian watermilfoil disperses primarily by stem fragments. Wind, waterfowl, water flow between water bodies, and human related activities are believed to be the main mechanisms of fragment dispersal. The most often cited vectors are motor boats and boat trailers.

Impacts

The dense weed beds formed by Eurasian watermilfoil have adverse effects on native aquatic vegetation that are important food sources for waterfowl and some mammals, and habitat for fish. The dense beds create habitat for disease carrying insects, including mosquitoes, and parasites that cause swimmer’s itch. The richness, diversity, and distribution of invertebrate species on lake bottoms are reduced where infestations occur. The function of water ecosystems are altered, including biomass turnover and nutrient cycling. Reduced dissolved oxygen and changes in water temperature are associated with infestations. Eurasian watermilfoil is a nuisance species to humans when it reduces open areas along lake shores, washes up on beaches, and curtails recreational activities. Populations reduce water flow thus interfering with industrial, agricultural, and municipal water systems. Irrigation ditches, canals, farm ponds, and irrigation equipment can be clogged by the weed. Management of Eurasian watermilfoil is difficult and expensive.

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Last Modified: 02/13/2009