Rare Species Guide

  Basis for Listing

Since 1990, the DNR has conducted intensive statewide searches for rare aquatic plants. More than 2,000 lakes have been searched but Najas gracillima has been found in only about 200 of them. The discoveries are clustered in the north central counties, an area of approximately 29,526 sq. km (11,400 sq. mi.) - about 13% of the state. Historic occurrences in southern Minnesota are believed to have been extirpated by declining water quality, which seems to be the fate of N. gracillima across its range in eastern North America (Wentz and Stuckey 1971; Haynes 1979). Najas gracillima was listed as a special concern species in Minnesota in 1996.

  Description

Najas gracillima is a submerged aquatic plant (macrophyte). The stems are 4.5-48 cm (1.8-18.9 in.) long and branched on the distal portion. The internodes are 0.1-3.2 cm (0.04-1.26 in.) long and have no prickles. The leaves are 0.6-2.8 cm (0.2-1.1 in.) long, and the blade portion is 0.1-0.5 mm (0.004-0.020 in.) wide. The margins of the blade are minutely serrulate with 13-17 teeth per side, and the midvein has no prickles. There are 1-3 flowers in each leaf axil, and the male flowers and the female flowers are on the same plant. The light brown seeds are fusiform in shape, 2-3.2 mm (0.08-0.13 in.) long and 0.4-0.7 mm (0.016-0.028 in.) wide. The seed is not recurved and the style is situated off-center at the apex. The surface of the seeds is dull and pitted.

  Habitat

The lakes where N. gracillima has been found are generally clear, healthy, soft water lakes where impacts from shoreline development or agriculture have been minimal. Water chemistry data is available for 90 of the known lakes. They show a range of total alkalinity of 1.5-116 ppm (mean: 21 ppm), with 83 of the 90 having a total alkalinity less than 50 ppm. The pH of the lakes ranged from 4.8 to 9.2 (mean pH 7.2). The size of the lakes ranged from 2-214 ha (4.2-530 ac.), averaging 24 ha (60 ac.). Najas gracillima is most often found rooted in sand, but sometimes in silt, and in relatively shallow water, often less than 1 m (3.3 ft.). It is a delicate plant and not usually found where it might be exposed to significant wave action. The most common aquatic plant species associated with N. gracillima are Nymphaea odorata (white waterlily), Brasenia schreberi (water shield), Dulichium arundinaceum var. arundinaceum (three-way sedge), Potamogeton epihydrus (ribbon-leaved pondweed), Nuphar variegata (bullhead pond-lily), Sagittaria latifolia (broadleaf arrowhead), Utricularia vulgaris (common bladderwort), Potamogeton pusillus (small pondweed), and P. amplifolius (large-leaved pondweed).

  Biology / Life History

Najas gracillima is an annual, which reproduces largely, if not entirely, by seed. It is a prolific seed producer, and nearly all mature plants will produce seeds during the summer. Ducks are known to be a major predator of Najas seeds, and it is sometimes theorized that they may also spread the seeds from lake to lake during their fall migrations. It is also likely that seeds are dispersed within a watershed by natural waterways and flowages.

The best time to search for N. gracillima is from early July through August.

  Conservation / Management

Published reports indicate that N. gracillima may be more vulnerable to deteriorating water quality than most other aquatic plant species (Wentz and Stuckey 1971; Haynes 1979). This is probably due to the effects of turbidity, warming, or possibly sedimentation. In nearly all cases, this type of lake deterioration results from activities occurring somewhere in the watershed rather than in the lake itself. For this reason, the integrity of shoreline buffer vegetation is imperative in lakes where N. gracillima occurs, especially on shorelines directly adjacent to colonies of N. gracillima. Other factors such as pollution reaching a lake via streams, ditches, storm water pipes, and septic systems should not be ignored either.

Many resources and tools regarding shoreland management, including lakescaping and shoreland restoration, are available for landowners and land managers, and any opportunity to incorporate these concepts would benefit N. gracillima as well as many other aquatic species.

  References and Additional Information

Clausen, R. T. 1936. Studies in the genus Najas in the northern United States. Rhodora 38:333-345.

Haynes, R. R. 1979. Revision of North and Central American Najas (Najadaceae). Sida 8:34-56.

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.

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.

Stuckey, R. L. 1984. Distributional history of Najas flexilis, N. gracillima, and N. marina (Najadaceae) in eastern North America. The Ohio Journal of Science 84(2):4-5.

Wentz, W. A., and R. L. Stuckey. 1971. The changing distribution of the genus Najas (Najadaceae) in Ohio. The Ohio Journal of Science 71(5):292-302.