Special species: Nuttall’s waterweed – an invasive species

Waterweeds (Elodea spp.) are species of aquatic plants that originate in North America. Now, because of human interference, two different waterweed species are spreading globally, causing havoc as they outcompete local flora. Canadian waterweed, Elodea canadensis, is no new acquaintance here, as this species was first found in both Finland and Sweden in the 1870s. Since then it has become an established (although unwanted) part of the local flora and is found in both freshwater and brackish water systems throughout the countries. Another newer alien species is Nuttall’s waterweed, or western waterweed, Elodea nuttallii, that was found in Sweden for the first time in 1991, but still has not been found in Finland.

Canadian waterweed is bright green in color and has tongue-formed leaves. (Photo by Petra Pohjola, County Administrative Board of Norrbotten)
Nuttall’s waterweed is paler in color and has narrow and pointy leaves, often screw-formed. (Photo by Petra Pohjola, County Administrative Board of Norrbotten)

The two species of waterweed can be difficult to tell apart, at least for the untrained eye. Both species build dense green stands of upright stems with whorls of leaves. These leaves are the best way of telling these species apart. Canadian waterweed has leaves with a rounded tip and arching downward, resembling a tongue. Nuttall’s waterweed has thinner leaves with a sharp tip and can be twisted along its center. Also, the color of the plants can differ, as Canadian waterweed is often a deeper and cleaner shade of green. These species live in similar habitats, in shallow soft bottoms, and thrive in nutrient rich waters with a salinity under 2,5psu. Out of these invasive species, Nuttall’s waterweed has been seen to outcompete its relative and proven to be a better competitor, making it an even more threatening newcomer. This new waterweed now threatens local biodiversity through shading and outcompeting local plants as well as potentially changing its surroundings nutrient cycle and water quality.

The origin of these species is uncertain, but they are believed to have been spread from aquariums and botanical gardens. Once introduced to a new environment, the plants can spread shorter distances with currents, aquatic birds, and boat traffic. Only a small piece of the plants stem is enough for a successful invasion in a new environment. In Sweden Nuttall’s waterweed has already spread from fresh water systems in the south to the brackish water seashore in the Bothnian bay. The SEAmBOTH – area is now the center of attention as this is where the eastern edge of the distribution of Nuttall’s waterweed is situated. During field work last summer the species was commonly found in dense stands in the western end of the SEAmBOTH-area, with fewer findings to the east. Here the most eastern sighting of this species was made in Haparanda, just 5 km from the border to Finland!

 

In July-August Nuttall’s waterweed grows tiny flowers with long stems. Interestingly only female flowers have been noted here, and therefore this waterweed only reproduces asexually in our waters. A small fragment of the stem is enough to grow a new plant.
(Photo by Petra Pohjola, County Administrative Board of Norrbotten)

According to a recently published risk classification of invasive species in Sweden, both waterweed species can have a severe impact on local biodiversity as they both have the potential to have a large ecological effect and can colonize over large areas. They belong to the same risk class as Lupinus polyphyllus, the large-leaved lupine.

Now, as Nuttall’s waterweed is spread all the way to the Finnish border, is it just a matter of time before this invasive species is found on the other side? A possible explanation to why the species has not yet spread to the east is that the paths of migratory birds mostly follow a south-northerly line and so do not transport any plants eastward. Also boat traffic across the border is minimal and large-scale currents flow westward in the northern Bothnian Bay. Still these explanations seem like something that would slow down the spreading, not stop it. In a recently published action plan regarding invasive species in Finland it is stated that there is no natural barrier to stop the spreading of Nuttall’s waterweed. The spreading should be monitored, so that appropriate actions can be made to stop the invasion, when needed. Still, no effective way to stop this waterweed has yet been found.

Perfoliate pondweed (in the centre of picture) surounded by Nuttall’s waterweed.
(Photo by Petra Pohjola, County Administrative Board of Norrbotten)

Written by Petra Pohjola, County Administrative Board of Norrbotten

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