Synonyms

Najas guadalupensis var. olivacea, Najas olivacea

  Basis for Listing

Najas guadalupensis ssp. olivacea (olive-colored southern naiad) is usually considered endemic to the Great Lakes region. It appears to be rare, or at least poorly documented, throughout its range. The type locality is Norway Lake (Kandiyohi County), where specimens were first collected in 1932. A two decade long statewide botanical survey of 2,025 lakes discovered N. guadalupensis ssp. olivacea in just 108 lakes in the central part of the state. This is a decidedly rare plant in Minnesota. The greatest threat is the rapid development of lakeshore property and the deterioration of aquatic habitats that usually accompanies it. Given the relatively small number of documented populations despite targeted botanical surveys, the increasing statewide pressure on lakeshore habitats, and the importance of Minnesota’s populations to the global security of the species, Najas guadalupensis ssp. olivacea was designated special concern in 2013.

People with direct field experience with this plant have commented about its uniqueness. It seems likely that more in-depth research would reveal this plant to stand alone, taxonomically, as Najas olivacea.

  Description

Najas guadalupensis ssp. olivacea is a rooted submerged aquatic. The stems are profusely branched to 40 cm (16 in.) long and 1-2 mm (0.06-0.08 in.) thick. The leaves are opposite or whorled, spreading or ascending, 0.9-1.8 cm (0.4-0.8 in.) long, and 1.5-2.0 mm (0.06-0.08 in.) wide. Each margin of the leaves has 20-40 minute unicellular teeth (seen with magnification). The flowers are unisexual, solitary in the leaf axils, very small and inconspicuous, and have an apical style. The seeds are 1.2-2.5 mm (0.05-0.10 in.) long and have a pitted surface. The typical subspecies of N. guadalupensis (ssp. guadalupensis) also occurs in Minnesota and is more common and widespread than ssp. olivacea. It differs by having 50-100 teeth along each margin of the leaves and stems less than 1 mm (0.04 in.) thick (Haynes 2000).

For those without field experience identifying specimens of Najas, it might be helpful to know that the leaves and stems of N. guadalupensis ssp. olivacea are very turgid or firm and somewhat brittle when handled, whether in the water or out. Although this feature is present in all species of Najas, it is more pronounced in N. guadalupensis ssp. olivacea.

  Habitat

Najas guadalupensis ssp. olivacea generally occurs along the margins of lakes in 1-2 m (3-6.5 ft.) of water. The substrate is typically sand or silt. Najas guadalupensis ssp. olivacea prefers fairly alkaline lakes. Water analysis at one site showed the total alkalinity to be 136 ppm and a pH of 8.4. 

  Biology / Life History

Najas guadalupensis ssp. olivacea is quite rare. Reliable and well-documented reports of occurrences are few and far between, so very little is known about its life history. Although most species of Najas are annuals, N. guadalupensis ssp. olivacea is apparently a perennial, and it reportedly does not produce rhizomes. Flower structure indicates they are pollinated underwater, and pollen is carried on water currents.

  Conservation / Management

Based on available data, N. guadalupensis ssp. olivacea prefers fairly alkaline lakes. Total alkalinity is often in excess of 130 ppm and pH often 8.0 or higher, indicating that acidification of lake conditions could be particularly detrimental to this species. The near-shore and shallow habitat of this species could also be vulnerable to shoreline development and bank stabilization projects. In essentially all cases, the natural contours and a buffer of native shoreline vegetation should be retained or restored during any construction activity.

  Best Time to Search

In Minnesota, identifiable specimens of N. guadalupensis ssp. olivacea have been collected from June 13 to September 11. July seems to be the peak month.

  Authors/Revisions

Welby Smith (MNDNR). 2018

(Note: all content ©MNDNR)

  References and Additional Information

Haynes, R. R. 2000. Najadaceae. Pages 77-83 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 22. Oxford University Press, New York, New York.

NatureServe. 2009. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. <http://www.natureserve.org/explorer>. Accessed 26 June 2009.

Rosendahl, C. O. 1939. Additional notes on Najas in Minnesota. Rhodora 41:187-189.

Rosendahl, C. O., and F. K. Butters. 1935. The genus Najas in Minnesota. Rhodora 37:345-348.