Subularia aquatica ssp. americana Mulligan & Calder
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Subularia aquatica var. americana
Basis for Listing
Subularia aquatica ssp. americana is a small, aquatic species that occurs in shallow lake margins. Although it has wide distribution in North America, it is uncommon or local over most 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. At least two other historical populations are 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. Between 1984 and 2008, 11 additional populations were discovered. The majority of sites are in the Border Lakes area, but the species has been documented as far southwest as Johnson Lake in Itasca County. 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.
Like all members of the mustard family, S. aquatica ssp. americana has 4 petals, 4 sepals, 6 stamens, and a 2-locular fruit (silicle). It differs from related species in being a dwarf, stemless, aquatic plant with minute white flowers, ovoid silicles with a broad partition, and simple, subulate basal leaves. At some submerged sites in north-central Minnesota, detritus 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.
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.). In Minnesota, it has 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 between 1984 and 2008 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 watermilfoil), Ranunculus repens (creeping buttercup), and the rare Littorella americana (American shore plantain) (U.S. 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, but the exact mechanism of pollination is not known. Details of seed dispersion are also unknown. 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, although exact causes have not been determined. Subularia aquatica ssp. americana has been observed floating in the water at boat landings, possibly as a result 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.
Conservation Efforts in Minnesota
Through the work of the DNR Minnesota Biological Survey, several previously unknown populations of S. aquatica ssp. americana were discovered between 1990 and 2008. Searches are ongoing and it is likely that at least a few more populations will be discovered before the survey is completed.