European Frogbit (Hydrocharis morsus-ranae)
European frogbit is an annual free-floating macrophyte. It has long roots, up to 50cm, that are typically suspended in the water column rather than attached to the sediment beds. As a stoloniferous plant with steam like extensions producing new growth, an individual can range from .1-1.5m across with distinct rosettes or plantlets varying between 1 and 30cm. The rosettes contain entire, heart-shaped leaves, 1.2-6cm long and 1.3-6.3cm wide. Green marks the topside of the leaves, while a purplish-red spongy underside is characteristic of European frogbit. Most but not all populations are considered dioecious, displaying emergent, small, white solitary female and clustered male flowers. However, some argue European frogbit is actually monoecious. It is very difficult to examine the entire plant because the stolons tangled with one another and commonly break upon inspection.
native range and distribution
As it's name suggests, H. morsus-ranae is native to Europe, but it can occupy and sometimes dominate a wide range of habitats. It prefers low wave energy areas like swamps and marshes; backwaters; bays, sheltered, coves, and shorelines of lakes, rivers, or streams. Its generalist habitat preferences, as well as human intervention through intentional planting and aquatic plant hitchhiking, has enabled H. morsus-ranae to successfully establish throughout the world and invade the northeastern United States, Washington, and southeastern Canada. Since 1932, when it was intentionally introduced for the aquarium trade from Arboretum of Central Experimental Farm in Ottawa, Canada and found growing wild in Dows Lake, it has spread throughout the Great Lakes Basin and United States at 15.6km/year. This spread is largely due to intentional or accidental planting, but within a waterway European frogbit can spread quickly because it is free-floating. One plantlet can become detached from the parent and drift to another portion of the waterway where it can continue growth and turion maturation. In New York the Oswegatchie River (the first infested water body) and Lake Champlain have supported European frogbit populations since 1974 and 1993, respectively. It is now common in in counties surrounding the Adirondack Park and has been sighted in the Grass River of the western Adirondacks and a small private pond in Essex County.
threat and impacts
The establishment of European frogbit comes with deleterious effects on human based activities and the environment. Commercial industry (i.e. clogging intake pumps, water traffic) and recreational activities (i.e. swimming, fishing, boating) can be inhibited by European frogbit's thick, vexitious growths. As for the virtue of the environment, H. morsus-ranae can become dominant or codominant in local aquatic ecosystems within 5 years of its establishment. Through rapid vegetative propagation, dense plots can form monocultures and diminish growth of native submersed plants by limiting light and competitively occupy habitats. Also native, aquatic plant beds support a greater diversity of associated fauna like macroinvertebrates than European frogbit, and consequently an invasive establishment of European frogbit can reduce native flora and fauna biodiversity. Furthermore, nuisance growths in Oneida Lake, NY have been observed with over 500 plantlets/m2 and dissolved oxygen levels as low as 1.9mg/L underneath the plants. Other changes in oxygen could occur in autumn when plants die and decomposition begins. The large amount of biomass produced by European frogbit may largely increase the dissolved oxygen depletion which can be stressful or fatal for fish and other aquatic organisms.
Management has been sought to control established populations and condense the distribution of European frogbit because of its damaging impacts on the ecosystem and water based industry. Studies predicting the success of invasions and propagule pressures of European frogbit are vital tools for allocating limited resources and determining management goals and methods. Control options include:
- Mechanical Harvesting
- Most widely used and should be done prior to turion development
- Proven successful in eradication if populations are caught early
- Provided temporary relief in established populations and subsequent years of harvesting required