Rare Species Guide

  Basis for Listing

Botanists have been studying the lake plants of Minnesota for well over a century, yet Potamogeton oakesianus (Oakes’ pondweed) has proved very elusive. Between the years 1995 and 2015, a total of 2,025 lakes were surveyed by highly trained DNR botanists, and only 22 of those lakes were found to have P. oakesianus (Laurentian Mixed Forest Province). This clearly ranks P. oakesianus among the rarest aquatic plant species in the state.

The lakes where P. oakesianus has been found are rather small, making P. oakesianus particularly vulnerable to local extirpation caused by declining water quality, contact with herbicide, nutrient enrichment, or other types of pollution. For these reasons, P. oakesianus was listed as an endangered species in 2013.

  Description

Potamogeton oakesianus is an aquatic species, with both submerged leaves and floating leaves. The submerged leaves are 5-16 cm (2-6 in.) long and no more than 1 mm (0.04 in.) wide. The floating leaves have a petiole, 3.2-7.5 cm (1.3-3.0 in.) long; the blade is lanceolate to elliptic or ovate, 2-4 cm (0.8-1.6 in.) long, and 1-2 cm (0.4-0.8 in.) wide, with an acutely pointed tip. The inflorescence is a terminal spike, 2.5-8.0 cm (1-3 in.) long that rises to the surface of the water (Haynes and Hellquist 2000; Ogden 1943).

In Minnesota, P. oakesianus is most likely to be confused with P. natans (floating pondweed), which is a common and widely known species. By way of comparison, P. oakesianus is a more delicate species in every way. The stems of P. oakesianus (at least when dry) are 0.4–0.9 mm (0.02-0.04 in.) thick, while those of P. natans are usually 1 mm or more (0.7–2.5 mm [0.03-0.10 in.]). The petioles in P. oakesianus seem rarely, if ever, to develop the distinct whitish band at the summit, which is usually characteristic of P. natans (Reznicek et al. 2011).

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

  Habitat

Potamogeton oakesianus is an aquatic species that occurs in quiet waters of ponds and lakes (Haynes and Hellquist 2000). It is currently known to occur in about 20 water bodies in Minnesota, 10 of which have water chemistry data available. The total alkalinity of the lakes ranges from 1-126 ppm, with an average of 40.7 ppm, which is very low. These are commonly called “soft-water lakes”. The pH ranges from 5.9-8.8, which is a rather wide range and is difficult to interpret.

  Biology / Life History

In general, P. oakesianus occurs entirely underwater. However, the inflorescence is designed to appear above water to be cross-pollinated by flying insects. Some species of Potamogeton (pondweeds) have inflorescences that are designed to remain underwater and be self-pollinated. In other words, pollen is not transferred from one flower to another but remains within the flower of origin (Philbrick and Anderson 1987).

The fruits of P. oakesianus are considered drupaceous and are likely spread by water currents and, possibly, by water animals that feed on the fruits or foliage.

This species is reported to be perennial by rhizomes (Haynes and Hellquist 2000). Rhizomes are underground stems, which survive from year to year and continue to produce new shoots and roots. Rhizomes are not normally considered reproductive structures, since they are unable to create a new individual plant. However, it is possible that a rhizome, or fragment of a rhizome, with viable buds could be “uprooted” from the substrate and carried by water currents to a new location, where it could take root. Fragments of leaves and stems apparently do not survive long and do not regenerate roots or rhizomes.

  Conservation / Management

The only way to reasonably assure the continuation of a population of P. oakesianus is to safeguard the habitat in which it occurs. The simplicity of this statement may seem obvious, but it is a good place to start a conversation about habitat management.

With the goal of maintaining or improving the ecological conditions of lakes where P. oakesianus is known to occur, certain actions need to be considered:

1) Establish a slow no-wake zone. This will prevent disturbance of the sediments on the floor of the lake and help maintain the health of the aquatic plant community.

 2) Maintain, protect, or restore, when necessary, a natural vegetation buffer at the shoreline. Studies have shown that a natural shoreline, versus a developed shoreline, can directly benefit the aquatic plant community in the littoral zone of the lake.

 3) Carefully maintain the septic systems of lakeshore homes. Nutrient enrichment is perhaps the greatest threat to water quality in a lake.

 4) Consider designating certain shoreline and aquatic habitats as restricted use zones, taking advantage of state, county, and local land use regulations.

 5) Establish a monitoring program, which can serve the dual purpose of tracking the health of the P.  oakesianus population and provide an advance warning of the appearance of invasive non-native species.

  Best Time to Search

The window of time for effectively searching for P. oakesianus is during the months of July and August.

  Authors/Revisions

Welby Smith (MN DNR), 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.

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

Philbrick, C. T., and G. J. Anderson. 1987. Implications of pollen/ovule ratios and pollen size for the reproductive biology of Potamogeton and autogamy in aquatic angiosperms. Systematic Botany 12(1):98-105.

Reznicek, A. A., E. G. Voss, and B. S. Walters. 2011. Michigan Flora Online. University of Michigan. <http://www.michiganflora.net/species.aspx?id=2332>. Accessed 13 March 2013.