Rare Species Guide

  Basis for Listing

Potamogeton pulcher (spotted pondweed) is a submerged aquatic plant, which occurs in lakes with good water clarity. As of 2015, botanical surveys of more than 2,000 Minnesota lakes have been completed by botanists working for the Minnesota Biological Survey. During that work, P. pulcher was found in only three lakes: Colby Lake in Chisago County, Allen Lake in Clearwater County, and Egg Lake in Cass County (all in the Laurentian Mixed Forest Province). This reveals P. pulcher to be one of the rarest plants in the state; hence, its designation as an endangered species in 2013.  It is also rare across the northern portion of its range. It is considered endangered in Wisconsin, imperiled in Michigan, possibly extirpated in Ontario, and absent from Iowa and the Dakotas. It is more common in the southeastern U.S.

  Description

Potamogeton pulcher is an aquatic species, with both submerged leaves and floating leaves. The submerged leaves have petioles 0.5-4.5 cm (0.2-1.8 in.) long, and linear-lanceolate to lanceolate arcuate blades 3.5-14 cm (1.4-5.5 in.) long and 6-16 cm (2.4-6.3 in.) wide. The blade margins are entire, and the tip is acutely pointed. The floating leaves have petioles 1-16 cm (0.4-16.3 in.) long, and the blades are lanceolate to round-ovate, 2.5-8.5 cm (1.0-1.7 in.) long, and 1.1-4.4 cm (0.4-1.8 in.) wide. The base of the floating blade is rounded or cordate, and the tip is pointed or rounded. The inflorescence is a terminal emergent spike, 1.7-3.6 cm (0.7-1.4 in.) long.

Potamogeton pulcher is most similar to the common P. amplifolius (large-leaved pondweed), except the leaf blades have more veins (7-19 vs. 19-49 for the submerged blades; 15-19 vs. 27-49 for floating blades). Another feature that may be useful is the stems of P. pulcher are reported to have dark purple spots or mottling; this should help distinguish it from Potamogeton natans (floating pondweed) (Haynes and Hellquist 2000; Reznicek et al. 2011; Ogden 1943).

Most people (non-specialists) will probably need to refer to the technical literature to confirm the identification of P. pulcher. The key and descriptions in Flora of North America (Haynes and Hellquist 2000) are highly recommended.

  Habitat

Our knowledge of the habitat requirements of P. pulcher in Minnesota is rather meager and does not reveal why P. pulcher is so rare in the state (Pip 1987). We do know it has been found in water less than 2 m (6.5 ft.) deep, rooted in substrates of deep silt and fine organic sediments. Additionally, all three lakes where it has been found have good water clarity.

Colby Lake (where P. pulcher has been known to occur since 1921) has a dense marshy / boggy shoreline. Associated species include Nymphaea odorata subsp. tuberosa (American white waterlily) and Potamogeton obtusifolius (blunt-leaved pondweed). 

Allen Lake (in Itasca State Park) is the only lake with water chemistry data: 15 ppm total alkalinity; ph 6.42. This puts it in the category of soft water lakes. 

There are no data available for Egg Lake.

  Biology / Life History

Potamogeton pulcher is a perennial aquatic plant that remains rooted in the substrate at the bottom of the lake (not free-floating). It is a fairly large and robust species and, under ideal habitat conditions, it can form sizeable patches a few or several meters across. This is the result of rhizomatous growth in the substrate. Except for the flowering spike, which rises a few cm above the surface of the water, the entire plant is underwater or, occasionally, the uppermost leaves will float on the surface. Pollination is accomplished by small flying insects. The seeds are dispersed by water currents or by aquatic animals that feed on the fruits.

Normally, the dispersal of seeds is the only way P. pulcher spreads from one place to another. Leaf or stem fragments do not normally regenerate rhizomes or roots.

  Conservation / Management

The most important goal is to maintain high quality habitat for P. pulcher at sites where it is known to occur. This is accomplished by maintaining shoreline vegetation and aquatic vegetation in an undisturbed condition, to serve as a nutrient buffer for water quality protection.

To achieve this goal, a number of actions must be considered, where appropriate: 

1)  Minimize removal of any shoreline or aquatic vegetation, either by mechanical or chemical methods.

2)  Avoid alteration of the littoral zone, except for efforts that will restore or improve the natural function of the system.

3)  Restrict and limit pier placement, bank grading, boat ramps, sand blankets, dredging, retaining walls, or filling.

4)  Maintain a no-wake zone to protect vegetation and prevent the sediments from becoming re-suspended; this will help maintain water clarity.

  Best Time to Search

Potamogeton pulcher can be expected to produce fruits during the months of July and August. This is the best time to search for the species.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  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.

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

Ogden, E. C. 1943. The broad-leaved species of Potamogeton of North America north of Mexico. Rhodora 45:57-105.

Pip, E. 1987. The ecology of Potamogeton species in central North America. Hydrobiologia 153:203-216.

Reznicek, A. A., E. G. Voss, and B. S. Walters. 2011. Michigan Flora Online. University of Michigan. <http://www.michiganflora.net/species.aspx?id=2336>. Accessed 13 March 2013.