Horned pondweeds and widgeonweeds form diverse plant communities

Zannichellia and Ruppia are unpretentious-looking submerged plants. They have narrow, delicate leaves, and their shoots only rise a few tens of centimeters from the bottom, if at all. Despite their fragile appearance, pondweeds and widgeonweeds are tenacious little plants that survive in the unstable conditions of shallow sandy bottoms.

Like other aquatic plant communities, Zannichellia and Ruppia habitats provide shelter and food for many invertebrates. Benthic animals attached to the substrate are rarely seen in the vegetation, but grazing gastropods are common, as are crustaceans and insect larvae.

Multi-species meadows

Horned pondweeds and widgeonweeds are particularly prevalent in open areas where they remain very short. The same habitats are home to, for example, rough stonewort (Chara aspera), Stuckenia filiformis and eelgrass (Zostera marina). In sheltered bays, Zannichellia and Ruppia grow larger and alternate with, for example, pondweeds Stuckenia spp. and water-starwort Callitriche spp.

The species composition of Zannichellia and Ruppia varies both temporally and locally. In the outer archipelago and other particularly open habitats, several species of horned pondweeds and widgeonweeds are common, while in more sheltered areas the most abundant species are Zannichellia palustris and Ruppia maritima. Zannichellia ssp. are found along the Finnish coast, and Ruppia ssp. are found everywhere except in the Bay of Bothnia.

Near threatened habitat

Horned pondweed and widgeonweed bottoms– i.e. plant communities where Zannichellia ssp. and Ruppia ssp. make up at least half of the vegetation – are quite common along the entire coast of Finland. However, the habitat has declined over the past 50 years, which is why it is classified as near threatened.

The decline in Zannichellia and Ruppia habitats is estimated to be mainly due to eutrophication and the resulting turbidity of the water. Boating may also damage these habitats in some places.

Z. major, which thrives in the outer archipelago, is particularly sensitive to eutrophication and turbidity. As eutrophication progresses, the growth areas of Zannichellia and Ruppia are being taken over by, among others, watermilfoils (Myriophyllum spp.) and pondweeds (Potamogeton spp., Stuckenia spp.), which cope better with the competition. Epiphytic or overgrowing filamentous algae and loose drifting filamentous algae rafts can also suffocate Zannichellia and Ruppia  vegetation.

Species of Zannichellia and Ruppia habitat

Horned pondweed (Zan­nichellia palustris)

Greater horned pondweed (Zannichellia major)

Beaked tasselweed or widgeon grass (Ruppia maritima)

Rough stonewort (Chara aspera)

Stuckenia filiformis

Perfoliate pondweed (Potamogeton perfoliatus)

Sago or fennel pondweed (Stuckenia pectinata)

Eelgrass (Zostera marina)