Basis for Listing

Subularia aquatica ssp. americana (awlwort) is a small aquatic species that occurs in shallow lake margins. Although it has wide distribution in North America and Eurasia, it is uncommon or local over much of its range. It was first discovered in Minnesota in 1944 in Cook County by Butters and Abbe (1953), but that particular population now appears to be gone. An additional historical population is thought to have disappeared, and one population is in serious decline. When originally listed as state endangered in 1984, S. aquatica ssp. americana was known from only six sites. Since 1984, intensive surveys have discovered several additional populations. The majority of sites are in the Border Lakes Subsection, but the species has been documented as far southwest as Johnson Lake in Itasca County (St. Louis Moraines Subsection). Because it was found to be more common than formerly believed, S. aquatica ssp. americana was reclassified as a threatened species in 1996. State threatened status remains reasonable and necessary, because it is still among the rarest species in the state, and it occurs in vulnerable shoreline habitat.

  Description

Like all members of the mustard family, S. aquatica ssp. americana has 4 petals, 4 sepals, 6 stamens, and a 2-locular fruit (silicle), but the flowers are very small and rarely seen. It differs from other species in the family by being a dwarf stemless aquatic plant with minute white flowers, ovoid silicles with a broad partition, and simple awl-like leaves. At some submerged sites in north-central Minnesota, detritus or epiphytic algae covers the small leaves of S. aquatica ssp. americana, leaving only its slender stalk of several tiny seedpods visible. Viewed through dark water, seedpods appear pink to tan.

  Habitat

Subularia aquatica ssp. americana is an aquatic species that inhabits shallow water, typically 15-45 cm (6-18 in.), sometimes as deep as 90 cm (35 in.). The shallower habitats experience bottom freezing in the winter. In Minnesota, appropriate habitats have been found primarily in the littoral zone of the large sandy oligotrophic lakes in the northeast; there are a few occurrences from silty substrata in non-oligotrophic lakes in north-central Minnesota. During seasonal fluctuations in lake levels, S. aquatica ssp. americana may become stranded on beaches and produce fully developed flowers, or it may produce its several-flowered stalk below water level. All populations discovered since 1984 are associated with sandy, gravelly, or silty substrata and number in size from only a few individuals to over 100 plants. Associated species include Elatine minima (small waterwort), Eleocharis acicularis (least spikerush), Eriocaulon aquaticum (pipewort), Isoetes echinospora (Braun's quillwort), Lobelia dortmanna (water lobelia), Myriophyllum tenellum (slender water milfoil), Ranunculus repens (creeping buttercup), and the rare Littorella americana (American shore plantain) (United States Forest Service 1999).

  Biology / Life History

Subularia aquatica ssp. americana is a small aquatic perennial. It is known to flower during the first half of August and possibly earlier and can flower while stranded out of water. Apparently, pollination can occur both underwater and out of water. It usually self-pollinates by cleistogamy under water but is said to be capable of becoming chasmogamous and insect-pollinated in air (Nowak et al. 2010). Details of seed dispersion are unknown but likely involve water currents. Based on herbarium records, the plant is in fruit from the second week of August through mid-September.

  Conservation / Management

Subularia aquatica ssp. americana populations are threatened by recreational activities in both remote wilderness areas in northeastern Minnesota and in more urban areas in north-central Minnesota. This is because they inhabit the same sandy-gravelly shorelines that are often used by recreationists and wilderness travelers as boat landings, campsites, and portages. Indirect effects due to unnatural water-level fluctuations, acidification, and certain fisheries management practices may also pose some threat. At least two known populations have succumbed, and another population is in serious decline, though exact causes have not been determined. Subularia aquatica ssp. americana has been observed floating in the water at boat landings, possibly because of being uprooted by boat wakes. These plants may eventually wash up on the beach, but it is unlikely they will root and may likely die. Rooted plants that become stranded on the beach due to natural water-level fluctuations appear to do fine and may successfully flower and produce seed; in fact, this may be a necessary stage in their life cycle. But in high-use areas, they are vulnerable to disturbance and trampling damage. Shoreline alterations, location of boat ramps, aquatic plant control, and other activities that disturb the beach should attempt to avoid populations of S. aquatica ssp. americana.

  Best Time to Search

The best time to search for Subularia aquatica ssp. americana is from late July through mid-September, when the plant is most likely to be at the peak of its development.

  Conservation Efforts in Minnesota

Through the work of the Minnesota Biological Survey, several previously unknown populations of S. aquatica ssp. americana have been discovered. Searches are ongoing and it is likely that at least a few more populations will be discovered before the survey is completed.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Butters, F. K., and E. C. Abbe. 1953. A floristic study of Cook County, northeastern Minnesota. Rhodora 55:21-55.

Mulligan, G. A., and J. A. Calder. 1964. The genus Subularia (Cruciferae). Rhodora 66:127-135.

Nowak, J. S., J. Ono, and Q. C. B. Cronk. 2010. Anatomical study of an aquatic mustard: Subularia aquatica (Brassicaceae). Aquatic Botany 93(1):55-58.

United States Forest Service. 1999. Population viability assessment in forest plan revision. Statement of purpose and reason. Draft species data records: Subularia aquatica. United States Forest Service, Region 9, Milwaukee, Wisconsin.