These pondweeds lack floating leaves and have thin, "grassy" like leaves. and resemble several other aquatic plants with thin leaves and delicate stems. Because the narrow leaved pondweeds look so much alike, close attention to minute details is necessary to distinguish between them. When uncertain it is easiest to identify them simply as thin-leaf pondweeds.
Small Pondweed (Potamogeton pusillus)
This species has numerous lacunae on either side of the leaf midrib (tissues with air cells). The submersed leaves are very narrow and thread-like with one to three veins. This pondweed has a grassy-like growth. (Excerpted from "Aquatic Plants and Algae of New Hampshire’s Lakes and Ponds").
Photo by Maria Gross
Leafy Pondweed (Potamogeton foliosus)
Leafy pondweed grows from a dense, mat-forming rhizome that anchors in wet substrate. It produces a thin, compressed, multi-branched stem growing to a maximum length around 30 inches. The delicate, hairlike leaves are up to 4 inches long. They are pale green to olive green or reddish in color. (Wikipedia)
Blunt-Leaved Pondweed (Potamogeton obtusifolius)
This is a branching plant with slender, flattened stems. The submerged leaves are long, rather grass-like, translucent, and are 2-4 inches long and 1.5–3.5 mm wide. The leaves are pale green, often with a very marked reddish or brownish tinge. There are 1-2 lateral veins either side of the midrib. As the name suggests, the leaf tips are rather blunt. (Excerpted from Wikipedia)
Robbins Pondweed (Potamogeton robbinsii)
A large field of Robbins Pondweed was found by Brad Gaudreault around the southern edges of the Thorne Islands in Zone 5. It was deep enough in the water that Cliff also dove to help identify it. A sample was sent to Amy Smagula for confirmation. A fragment was also collected by Maria Gross in Zone 7.
Leaves are alternately attached on opposite sides of the stem, ascending to spreading and often arching, usually closely spaced but may be more widely spaced on flowering stems.
Blades are firm and somewhat stiff, dark green to reddish, lance-linear, ¾ to 2¾ inches long, 1/8 to 1/3 inch wide, pointed at the tip and minutely toothed around the edges, at least near the tip. The midvein is conspicuous yellow, flanked by 20 to 60 lateral veins, about 5 of which are prominent and the rest faint.
Stems are round, without spots, few-branched on the lower plant, branching frequently in the upper plant when flowering. Colonies are often formed from creeping rhizomes. (Excerpted from MinnesotaWildflowers.info/aquatic/robbins-pondweed).