[Wikipedia] Environmental Impact: This pondweed is considered an invasive species in much of North America. It was introduced to the Great Lakes and inland lakes within that region. The plant thrives in conditions normally less habitable to native plant species. It is also able to survive beneath the surface of frozen waterways and resume its rapid vegetative growth before other water plants can regrow.[9] It competes with native plant life and sometimes displaces it. Curly pondweed may clog waterways, inhibiting aquatic recreation, and is considered a nuisance in some areas.[9] It has also been introduced to South America and New Zealand. (link added by Mary Ann Machi)