Potamogeton confervoides Reichenb.
Basis for Listing
Potamogeton confervoides (algae-like pondweed) is a submersed perennial aquatic, endemic to northeastern North America, occurring primarily along the Atlantic Coast from Labrador to South Carolina and westerly into Ontario, Michigan, and Wisconsin. Minnesota’s two known populations represent a western range extension of the species. Throughout its range, P. confervoides is found only in fairly acidic waters (Haynes and Hellquist 2000) and is generally considered to be most uncommon.
The first Minnesota population of this species was discovered on June 25, 2008, in a small peatland pond within the Superior National Forest’s Boundary Waters Canoe Area Wilderness in northern St. Louis County (Border Lakes Subsection). The population was restricted to a small area near shore, submersed in shallow (~50 cm [20 in.]) water and was just beginning to flower. Associated plants included Brasenia schreberi (watershield) and Utricularia spp. (bladderworts). The pond was completely surrounded by a floating mat of graminoid (northern) poor fen (MNDNR 2003).
In early September 2015, a very fine leaved sterile Potamogeton (pondweed) fragment was collected from another small peatland pond approximately 7.24 km (4.5 mi.) distant from the state’s first known location. A return visit, one week later, confirmed the state’s second population. After surveying the ponds’ entire shoreline, plants were only found in a small area along the southwest portion of the lake. Most of the plants were algae-covered sterile fragments, but several fertile fruiting plants were also observed. Associated plants included Potamogeton epihydrus (ribbon-leaved pondweed), Utricularia minor (lesser bladderwort), and U. intermedia (flat-leaved bladderwort). This pond was also completely surrounded by a floating mat of graminoid (northern) poor fen (MNDNR 2003).
Although over 2,000 lakes and ponds have been surveyed across Minnesota, Potamogeton confervoides has all but eluded botanical detection until recent years. This species is visually distinctive and would not be easily overlooked by a trained botanist. Given the documentation of only two populations, the species’ restrictive water chemistry requirements, and the vulnerability of so few populations to degradation or destruction, P. confervoides was designated an endangered species in 2013.
Potamogeton confervoides is a submerged aquatic plant. The stem is 10-80 cm (4-32 in.) long and grows from a buried rhizome. The leaves are flaccid, up to 20 cm (8 in.) long, but less than 0.5 mm (0.02 in) wide, and arranged in spiral fans. The leaves are so fine they appear hair-like or algae-like in the water. When the plant is removed from the water, the leaves collapse onto each other. There are no floating leaves. The inflorescence is found at the end of the stem and consists of an unbranched peduncle, 5-25 cm (2-10 in.) long. At the end of the peduncle is a single capitate spike, only 5-12 mm (0.2-0.5 in.) long. There are 2-3 individual fruits in each spike; each fruit contains a single seed. Each fruit is sessile, nearly round, and keeled on the back and sides. There is a short straight beak about 0.5 mm (0.02 in.) long on the fruit. Overwintering buds known as “turions” develop in the axils of the leaves.
The very narrow flaccid leaves of P. confervoides distinguish it from all other species of Potamogeton found in northern Minnesota.
Potamogeton confervoides occurs in acidic (soft) waters of lakes, slow moving streams, bog ponds, pools, and peaty depressions. Plants have been documented from waters having a pH as low as 4.2 (NatureServe 2015).
Biology / Life History
Potamogeton confervoides is a perennial that reappears every year from rhizomes buried in the substrate beneath the water. Sexual reproduction is accomplished by seeds, which likely develop underwater through the process of autogamy or self-fertilization. If flowers appear above the surface of the water, which happens in some species of Potamogeton but has not been reported to happen with P. confervoides, then insect-mediated cross-fertilization becomes possible. Vegetative reproduction is accomplished by a structure called a “turion” or “winter bud”. This is a tightly rolled structure composed primarily of undeveloped leaves. It drops to the substrate where it survives the winter. If successful, the turion will resume growth the following season and become a fully formed and sexually reproducing individual.
Conservation / Management
Potential threats to the species include habitat destruction and physical, chemical, or hydrological alteration of habitat conditions. Potamogeton confervoides requires acidic environments, and any activities that would result in eutrophication or increased alkalinity of its habitats would be detrimental. Such activities might include lakeshore development, peat mining, intensive timber harvest, herbicide treatments, and liming of ponds for fisheries management.
Small peatland lakes in the northeast portion of Minnesota are sometimes managed as brook trout fisheries. These small lakes may also be used for the commercial trapping of leeches. As a result, lakes that are otherwise quite isolated, and not known as recreation destinations, can rather suddenly become popular fishing destinations or a source for fishing bait.
Best Time to Search
The best time to search for this species is early July to late September, with mid-season likely being most ideal. Plants may become more difficult to observe later in the season as they begin to fragment and become covered with fine algae.
Conservation Efforts in Minnesota
Both of Minnesota’s known populations of P. confervoides are in rather remote areas administered by the Superior National Forest.
Lynden B. Gerdes and Webly Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Crow, G. E., and C. B. Hellquist. 2000. Aquatic and wetland plants of northeastern North America. Volume 1. Pteridophytes, Gymnosperms, and Angiosperms: Dicotyledons. University of Wisconsin Press, Madison. 448 pp.
Haynes, R. R., and C. B. Hellquist. 2000. Potamogetonaceae. Pages 47-74 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. 2015. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. <http://www.natureserve.org/explorer>. Accessed 20 November 2015.
Ogden, E. C. 1943. The broad-leaved species of Potamogeton of North America north of Mexico. Rhodora 45:57-105.
Schultz, J. 2003. Conservation assessment for algal pondweed (Potamogeton confervoides). United States Forest Service, Eastern Region, Milwaukee, Wisconsin. 18 pp.
Voss, E. G. 1972. Michigan Flora: Part I Gymnosperms and Monocots. Cranbrook Institute of Science Book 55, Bloomfield Hills, Michigan. 488 pp.
Voss, E. G., and A. A. Reznicek. 2012. Field manual of Michigan flora. University of Michigan Press, Ann Arbor, Michigan. 1008 pp.