Potamogeton diversifolius Raf.
Basis for Listing
Potamogeton diversifolius is a small, delicate aquatic plant that occurs in shallow water in small lakes and large ponds. There are currently only two records of this species from Minnesota. One is a herbarium specimen collected in Ramsey County in 1945 and the other is a herbarium specimen collected in Anoka County in 1992. Since 1990, over 1,500 lakes in central and northern Minnesota have been surveyed for aquatic plants but this species was not found. Because surveys have been less thorough in the southern part of the state and Minnesota is on the northern limit of this species range, there is still hope it will be found again. Threats to its aquatic, shallow water habitat continue to grow and include shoreline development, increased recreational activities, and general decline in water quality related to runoff from surrounding uplands. For these reasons, P. diversifolius was listed as an endangered species in Minnesota in 1996.
Potamogeton diversifolius closely resembles two other species of pondweeds that also occur in Minnesota, the rare P. bicupulatus (snailseed pondweed) and the more common P. spirillus (coiled pondweed). It is imperative that good specimens be used for reliable identification and that a specialist verifies the results. In this species, as in P. bicupulatus, there are 3 rows of sculpted ridges around the rim of the tiny, disk-shaped seed: 1 ridge on the rim and 2 smaller ridges, 1 on each side, of the rim. Potamogeton diversifolius also has extremely fine, narrow leaves but these are slightly wider than P. bicupulatus, typically about 0.5 mm (0.02 in.) wide. The measurement must be taken from leaves along typical segments of the underwater stem. Floating leaves, if present, are small and oval, at most about 2 cm (0.8 in) wide and 4 cm (1.6 in.) long, often smaller. Floating leaves are variable in size and are not useful for distinguishing this species (Reznicek and Bobette 1976).
There are not enough known locations of P. diversifolius to gain a good understanding of its preferred lake habitats. Preliminary indications are that it prefers clear lakes on the Anoka Sand Plain with relatively low levels of dissolved minerals (soft water). These are probably small lakes or large ponds in a gently sloping basin, or perhaps a small, quiet bay in a larger lake.
Biology / Life History
Potamogeton diversifolius in Minnesota is not well known at this time. It is a perennial herbaceous species that can overwinter and resprout from a buried rhizome. Apparently it does not produce turions, which are vegetative propagules sometimes called winter buds (Haynes and Hellquist 2000). It does produce seeds regularly, which are dispersed in water currents and possibly in the fur or feathers of aquatic animals. Owing to the delicate nature of this plant, it is generally found in calmer portions of larger lakes or in smaller lakes not subject to turbulent wave action.
Conservation / Management
Maintaining high water quality in selected lakes in Minnesota that support this species or other rare aquatic life is a primary challenge. Activities occurring throughout a watershed can affect the water quality of lakes. Runoff can be from adjacent sources, such as shoreline development, or from sources more distant in the watershed, such as industrial manufacturing. The cumulative impacts on lake water quality and on rare plants such as P. diversifolius are not fully known. Certain management activities within a lake itself also pose a possible threat. Aquatic herbicides are widely used, legally or illegally, to kill aquatic vegetation. The target of the herbicides is often invasive, non-native, aquatic plants such as Myriophyllum spicatum (Eurasian watermilfoil) or P. crispus (curly-leaf pondweed). Herbicides are also used to treat luxuriant growth of native aquatic plant species perceived by lakeshore owners as unsightly or as an obstacle to water access. It is not unusual for these chemicals to be applied with inadequate understanding or consideration of the potential negative impact to rare aquatic species.
Conservation Efforts in Minnesota
Targeted searches for rare aquatic plants by the Minnesota Biological Survey are currently underway. Results of this effort and of ongoing water quality monitoring efforts will be major contributions to identifying which of Minnesota's lakes are most imperiled and how to better conserve rare resources.
Haynes, R. R., and C. B. Hellquist. 2000. Potamogeton. Pages 48-70 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 22. Oxford University Press, New York, New York.
Reznicek, A. A., and R. S. W. Bobette. 1976. The taxonomy of Potamogeton subsection hybridi in North America. Rhodora 78:650-673.