Sammendrag
The purpose of this project has been to determine the distribution and abundance of the invasive species Elodea canadensis in lakes in the Nitelva catchment, give an overview of earlier findings in both lakes and river localities in the watercourse, and discuss possible actions to avoid further spread. The focus has been Elodea canadensis, only, no other aquatic macrophytes are surveyed.
Elodea was first recorded in the Nitelva catchment at the end of 1970s and is now spread to several lakes and river localities in the catchment.
In 2019, most of the littoral zone in the lakes Mylla, Svea, Yttersvea, Viggeren and Belteren were surveyed, while only a few selected areas in Harestuvatnet, Strykenvatnet, Strekan and Storfløyta were included in the survey.
Elodea was recorded in most lakes, except Viggeren, Strykenvatnet og Strekan. However, in another survey, also conducted in 2019, we recorded Elodea also in Strykenvatnet. At the same time, small stands of Elodea were also recorded in several areas in Harestuvatnet. In the northern part of Harestuvatnet, the stands were approximately as extensive as in 1979. In Belteren, the plants were short and had not developed into stands. In Mylla, the species was more common, but the plants were small and had developed into large stands only at a few locations. In Yttersvea, Elodea had developed one stand at the inlet, but were otherwise not observed in the lake. The presence of Elodea in these lakes will not reduce the ecological status according to the trophic index TIc.
Large stands of Elodea were recorded in several areas in Svea. As a result, the ecological status for aquatic macrophytes cannot be assessed as good and will be reduced to moderate.
It may be possible to remove the small Elodea population in Yttersvea, while reduced nutrient load may reduce the population in Svea. We recommend monitoring the Elodea population in Mylla.
It is difficult to prevent downstream dispersal in a river catchment. Dispersal to other dispersal watercourses can be limited. The main risk for dispersal is likely by people (boating, angling). We therefore recommend following the published action plan for Elodea, with special focus on information about dispersal and how to clean boats and fishing equipment when moving between watercourses. In addition, restrictions for boat traffic and fishing in Elodea-lakes should be considered. Also, a general reduction in nutrient availability in both water and sediment could lead to a reduction of the biomass.
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