Alisma wahlenbergii (Holmb. Juz.) (Alismataceae)

Compiled by: Terhi Ryttäri, Finland

1. Description / Ecological demands / Autecology

Alisma wahlenbergii is a short-living perennial, aquatic plant which grows on soft bottoms (silt, clay, sand) in shallow waters (in depth between 5-45 cm, up to 1,5 m according to Tzvelev, 2000), most often in sheltered shores. The species demands high water quality. In its main distribution area in the Bothnian Bay the growing sites turn unsuitable in less than a decade due to land-upheaval, and new sites arise continuously. Reproduction by seeds is efficient and the plant seems to have a permanent seed bank. Fruits, which ripen in August-September, are spread by water currents, drifting icefloes and waterfowl. Populations fluctuate in size from a few to thousands of individuals.

2. Distribution

A. wahlenbergii is endemic in the Baltic Sea and in some adjacent lakes. The main population is situated on the Finnish coasts of the Bothnian Bay. Populations from the Quark and the Swedish coast on Western Gotland Basin have disappeared. A. wahlenbergii has populations also in the eastern part of the Gulf of Finland, where it is still found in Russia in the Leningrad region.

3. Importance

A. wahlenbergii is included in the Bern Convention and listed in Annexes II and IV of the Habitats Directive as a priority species. As it is endemic to the Baltic Sea region, conservation of this plant is also globally important.

4. Status of threat

A. wahlenbergii is classified as endangered in Sweden (Red List 2005), vulnerable in Finland (Red List 2000) and endangered in the Leningrad region (Red List 2000). A wahlenbergii is also listed as rare in the 1997 IUCN Red List of Threatened Plants.

5. Threat factors

Eutrofication of the sea followed by overgrowth of strong competitors such as reeds is a major threat for this species. The plant is favoured by cattle-grazing which keeps the growing sites open, whereas grazing has strongly declined during past decades which enhances overgrowth. As the species demands clean and transparent water it quickly dies out under conditions of high water turbidity and pollution. Reasons for local disappearances include also road construction, harbours, marinas and summer cottages as well as dredging of waterways.

6. Conservation measures needed

Water quality in the Baltic Sea and especially in the Leningrad region must be improved. Negatively affecting human activities must be controlled and restricted in and near the growing sites of A. wahlenbergii.

7. References:

Gärdenfors U. (ed.) 2005: Rödlistade arter i Sverige – The 2005 Red List of Swedish Species. Swedish Species Information Centre, Uppsala.

Rassi P., Alanen A., Kanerva T. & Mannerkoski I. (eds.) 2001: Suomen lajien uhanalaisuus 2000 (Threatened species in Finland 2000). – Ympäristöministeriö & Suomen ympäristökeskus, Helsinki.

Tzvelev N.N. (ed.) 2000: Red data Book of Nature of the Leningrad Region. Vol. 2 – Plants and Fungi. St. Petersburg. 672 p.

Ulvinen T. & Tzvelev N. 1998: Alisma wahlenbergii (Alismataceae). In Kotiranta, H., Uotila, P., Sulkava, S. & Peltonen, S-L. (eds.) Red Data Book of East Fennoscandia. Ministry of the Environment, Finnish Environment Institute & Botanical Museum, Finnish Museum of Natural History. Helsinki. 351 pp.