Myriophyllum heterophyllum

This species has very convoluted taxonomy. In some parts of NY (including the Finger Lakes) it is considered native; in some parts it is introduced and invasive (please see NYFA Atlas link). It is on the FLNPS FInger Lakes list based on observations by Robert Wesley (Cornell Botanic Gardens). Statewide regulations also prohibit the hybrid with M. laxum from sale and distribution although it is not clear the hybrid exists in NYS. It is fully aquatic (lakes & ponds as well as rivers & streams). It is capable of growing up to one inch per day and spreading rapidly by fragmentation, as well as by seed. Apparently spread by boats, and possibly dumped aquariums. Leaves under water are finely dissected; those above are much wider & entire. See Invasive.org for more info.