Synonyms

Potamogeton diversifolius var. trichophyllus

  Basis for Listing

The status of this aquatic plant in Minnesota was unclear for many years because it was known only from a single lake in Anoka County where it was discovered in 1959. Attempts to locate Potamogeton bicupulatus in east-central Minnesota in the late 1980s and early 1990s resulted in discoveries in 4 additional lakes. These same 4 lakes also harbor other rare or uncommon species, possibly indicating that some unique combination of environmental factors is controlling this species' distribution. All of these lakes occur in areas where residential development or other activities are disturbing native vegetation in the surrounding uplands. These findings led to the listing of P. bicupulatus as a state endangered species in 1996.

Additional rare plant surveys were conducted in over 2,000 lakes throughout central and north-central Minnesota in the 1990s and early 2000s. Potamogeton bicupulatus was found in only 9 more lakes, where it was again associated with other rare or unusual aquatic plants. It tends to occur in lakes that have clear water and relatively low levels of dissolved minerals (soft water lakes). None of the lakes in which it occurs have their surrounding uplands protected, although one site is part of a private conservation area. Residential development of lakeshores continues to be one of the greatest threats to maintaining good water quality over the long-term.

  Description

A small delicate aquatic plant, P. bicupulatus closely resembles another rare species of pondweed that also occurs in Minnesota, P. diversifolius (diverse-leaved pondweed). All Minnesota sites of P. bicupulatus have had fruiting plants present. Fruits, though very small, appear to the naked eye to have a noticeably bumpy surface. This is due to their 3 rows of sculpted ridges around the rim of a tiny disk-shaped seed: 1 ridge on the rim and 2 smaller ridges, 1 on each side of the rim. These ridges are also found in P. diversifolius but distinct from most other narrow-leaved pondweeds that have a single ridge or no ridge. Potamogeton bicupulatus also has extremely fine hair-thin leaves, less than 0.3 mm (0.01 in.) thick, along typical segments of the underwater stem. If present, the floating leaves are small and oval, at most about 1 cm (0.4 in.) wide and 2 cm (0.8 in.) long. Floating leaves are variable in size and not definitive in distinguishing this species from other species (Reznicek and Bobette 1976).

  Habitat

The characteristics of lake sediments, water quality, and surrounding vegetation in places where P. bicupulatus has been found have not been fully analyzed. Preliminary indications are similar to what is observed in the field -- that this plant tends to occur in clear, soft water lakes. Typical aquatic plant associates include Brasenia schreberi (watershield), Eriocaulon aquaticum (pipewort), Myriophyllum farwellii (Farwell's watermilfoil), Myriophyllum tenellum (slender watermilfoil), Najas gracillima (slender naiad), and Nymphaea odorata (American white waterlily). Plants have been found rooted in pure sand and in silty sediments along the shores of shallow lakes, and rooted to the bog edge or stranded on low boggy hummocks by receding water.

  Biology / Life History

Potamogeton bicupulatus appears to be a perennial herbaceous species that survives winter as a buried rhizome. It apparently does not produce turions, which are vegetative reproductive structures sometimes called winter buds (Haynes and Hellquist 2000). It is known to produce abundant seeds, which are dispersed on 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.

The best time to search for P. bicupulatus is when the plant is in fruit during July and August.

  Conservation / Management

It is important to maintain high water quality in the select lakes in Minnesota supporting P. bicupulatus or assemblages of other rare aquatic plants. Activities occurring throughout a watershed can affect the water quality of lakes. Run-off 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. bicupulatus 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.

Potamogeton bicupulatus has been found consistently in clear-watered lakes with relatively low levels of dissolved minerals (soft water lakes). Therefore activities or actions that alter these characteristics would likely threaten existing plants or prohibit the establishment of new occurrences of the species.

  Conservation Efforts in Minnesota

Targeted searches by the Minnesota DNR County Biological Survey for rare aquatic plants in Minnesota are underway. Results of the survey and ongoing water quality monitoring will contribute to our understanding of the relationship between the condition of Minnesota's lakes and rare aquatic vegetation.

  References and Additional Information

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.