Underwater mazes of pondweed habitats

Pondweeds (Potamogeton spp. and Stuckenia spp.) have a bottom-dwelling rhizome and loose shoots, which can form a dense jungle below the water’s surface. The maze-like growths provide shelter and food for many invertebrates.

Pondweeds are found in a variety of habitats. On exposed sandy bottoms, pondweeds typically remain short, while in sheltered bays they grow taller. The species diversity of pondweeds is large: a total of 16 species of pondweeds are known from the Finnish coast, two of which are hybrids.

Species vary according to environmental conditions

The most common species of pondweeds on the Finnish coast are the sago pondweed or fennel pondweed, and sometimes called ribbon weed (Stuckenia pectinata) and the claspingleaf pondweed, perfoliate pondweed or redhead grass (Potamogeton perfoliatus). They are flexible in their habitat, and P. perfoliatus in particular tolerates even fertile and turbid waters. Therefore, these two species usually dominate the vascular vegetation of pondweed habitats.

The other pondweed species of pondweed habitats vary depending on the place of growth and the salinity of the water. S. filiformis thrives best on exposed bottoms and in fairly salty water. On the other hand, the various-leaved pondweed, variableleaf pondweed, grass-leaved pondweed or grassy pondweed (P. gramineus) and the small pondweed (P. berchtoldii) prefer low-salinity waters. The large pondweed group also includes species that are sensitive to eutrophication and water turbidity.

Other vascular plants are also commonly found on bottoms dominated by pondweeds, such as watermilfoil (Myriophyllum spp.), horned pondweed (Zannichellia spp.) and buttercups (Ranunculus spp.)

Common and least concerned habitat

Pondweed habitats are common along the entire coast, and their number is considered to have remained stable or increased – although there is no actual monitoring data on the habitat. Pondweed habitat is assessed as least concerned, i.e. the habitat is not endangered or at risk of becoming endangered.

As eutrophication progresses, groups that are more tolerant of eutrophication, such as watermilfoils (Myriophyllum ssp.) and hornwort (a common name shared with the unrelated Anthocerotophyta), rigid hornwort, coontail, or coon’s tail (Ceratophyllum demersum), become more common in the habitat. They can withstand long periods of turbid water better than any other pondweed species.

Species of pondweed habitats

Perfoliate pondweed or redhead grass (Potamogeton perfoliatus)

Sago or fennel pondweed (Stuckenia pectinata)

Stuckenia filiformis

Grassy pondweed (P. gramineus)

Small pondweed (P. berchtoldii)

Watermilfoils (Myriophyllum spp.)

Horned pondweeds (Zannichellia spp.)

Buttercups (Ranunculus spp.)

Hornwort or coon’s tail (Ceratophyllum demersum)