Hydrilla and Brazilian Elodea - Exotics
Hydrilla is present in the nearby states of Maine, Massachusetts, Connecticut, and Pennsylvania, and Brazilian elodea is present in Nuts Pond in Manchester. Hydrilla is the most invasive submergent plant in the United States
Comparison of Hydrilla, Brazilian Elodea and Native Water Weed https://www.lhprism.org/species-spotlight-hydrilla
Brazilian Elodea (Source: Ontario's Invading Species Awareness Program)
Hydrilla has narrow leaves whorled around the main stem. Leaves are conspicuously toothed along the margins of the leaves, and may also be toothed along the rib on the underside of the leaf. The leaves are in whorls of four to six, and are approximately one-third to two-thirds inches long. The stem can grow to 20 feet or more. A small white flower emerges in July, often detaching from the plant and drifting. Perennial.
Hydrilla is often confused with Brazilian elodea but can be differentiated by its rough texture and larger teeth on its leaf margins. The presence of tubers on hydrilla can also be a good distinction between the plants. Hydrilla and Brazilian elodea can also be confused with Native Water Weed (elodea condensis). Native Water Weed, however, only has whorls of 3 leaves.
Hydrilla is the most invasive submergent plant in the United States, and can even outcompete invasive water-milfoil species by canopying over the surface. It has been observed to grow up to a half-inch per day in optimum conditions. It will only be outcompeted by floating invasive species like water hyacinth. (Excerpted from Aquatic Plants & Algae of New Hampshire's Lakes and Ponds).