Johnsen, T.J., Larsen, S.E. & Baattrup-Pedersen, A. 2021. Evidensbaseret og omkostningseffektiv grødeskæring i små danske vandløb – dataopsummering 2020. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 40 s. - Teknisk rapport nr. 209. http://dce2.au.dk/pub/TR209.pdf
The majority of Danish streams are small and located in agricultural regions, which places heavy demands on the streams, because they have to be able to divert excess water from the fields, while also having to live up to the environmental objective of good ecological status. These two things are not easy to combine. Even though weed cutting is performed frequently in many streams, this may not guarantee that the water flow will be sufficient to ensure the diversion of surplus water from the fields, but the frequent weed cutting may, on the other hand, have major consequences for the biological communities of the streams.
The purpose of this project is to gain knowledge about how new methods and timing of weed cutting affect the ability of small streams to divert water, while at the same time investigating the consequences of the different methods and timing for the environmental state. In this way, the project will hopefully create the basis for more cost-efficient and environmentally friendly stream management in the future.
The project includes 65 stretches of small streams in the Danish municipality of Assens. Annual surveys are conducted of the physical conditions and water flow of the streams, as well as the composition of the species of plants, fish and invertebrates during a four-year period (2019-2023), in order to document the effects of the new weed cutting methods and timing. Two weed cutting methods are tested. One is selective weed cutting where selected helophytes, that are highly resistant to water flow, are removed from the stream. The other is bank cutting, where the same helophytes are removed from the stream, but where the vegetation 2 metres up the bank is also cut, so that the light conditions for the plant communities in the stream are improved to the benefit of plants, invertebrates and fish. In addition, the cutting is carried out earlier on some stretches and later on other stretches than hitherto.
Analyses of data collected in 2019 show that there was no difference in species composition among the stream stretches, other than what can be expected from variability in slope and profile of the stretches. This means that the changes that may occur in species composition will most likely be attributable to the new methods and timing of cutting.
Analyses of data collected in 2020 show that there is a coarser substrate and greater depth variation in the streams after just one year with changed weed cutting practice, and there is also an increased incidence of mayflies, stoneflies sand caddisflies and a better ecological staus assessed according to the Danish Stream Fauna Index (DVFI) in the streams. However, at the moment we cannot observe any effects on either the plant communities or on the fish communities. It will be interesting to see whether changes will occur after a longer period of time with changed weed cutting practice and whether there is a method and/or a timing that is preferable to another in relation to achieving the environmental objective of good ecological status in the streams.
