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Evidence-based and cost-efficient weed cutting in small Danish streams

The project

The majority of Danish streams are small and located in agricultural land, and this places great demands on their water flow capacity. At the same time, the streams must meet the environmental target, which is a good ecological status. In this project, we focus on investigating how different weed cutting methods and timings affects the water flow capacity and the environmental condition in 65 small streams located in Assens municipality. We are conducting a controlled study that will run for a minimum of four years (2019-2022). Such a long project period is necessary in order for us to be able to document how the weed cutting method and timing of cutting affect water flow capacity, water level changes, biological conditions (including the ecological state) and the morphometric changes that can arise as a result of the cutting.

Status 2021

A summary was compiled of the profile development in the experimental streams – see here

A note was prepared describing the results of the biological studies for the first two years of work

  • Evidence-based and cost-efficient weed cutting in small Danish streams  – data summary 2020 – Read the summary here

  • The aquatic plant communities were characterized in all stream sections to be able to assess the ecological status applying the Danish Stream Plant Index, DVPI) just before the first weed cutting was carried out.
  • Samples of small macroinvertebrates were collected and processed in all stream sections to be able to assess the ecological status applying the Danish Stream Fauna Index (DSFI).
  • Fish surveys were carried out in all of stream sections to be able to assess the ecological status applying the Danish trout index ((DFFVø).
  • Studies of the physical conditions were carried out in all stream sections (DFI) in both spring and autumn.
  • Water samples were collected from all sections in May, June and September. All samples were analysed in the laboratory.
  • A meeting was held with the advisory group on 9 September 2021
  • A meeting was held with the landowners on 4 November 2021, including a presentation of scientific results and discussions. See the three presentations from the meeting (presentations and images are reproduced with permission from the presenters):

Status 2020

  • The aquatic plant communities were characterized in all stream sections to be able to assess the ecological status applying the Danish Stream Plant Index, DVPI) just before the first weed cutting was carried out.
  • Samples of small macroinvertebrates were collected and processed in all stream sections to be able to assess the ecological status applying the Danish Stream Fauna Index (DSFI).
  • Fish surveys were carried out in all of stream sections to be able to assess the ecological status applying the Danish trout index (DFFVø).
  • Studies of the physical conditions were carried out in all stream sections (DFI) in both spring and autumn.
  • Water samples were collected from all sections in May, June and September. All samples were analysed in the laboratory.
  • A planning meeting was held in the project group (19 May 2020)
  • The landowners' meeting in November was cancelled due to coronavirus, but a written update for the landowners is being prepared.

Status 2019

The following activities were carried out in 2019:

  • A total of 65 study sections were identified by visual inspection, including contacting the relevant agricultural associations and landowners.
  • All 65 stream sections were mapped using GPS, and the profile dimensions were described in 5 transects in all sections.
  • The Danish Physical Index (DFI) was determined for the 65 stream sections.
  • Water samples were collected from all sections in May, June and September. All samples were analysed in the laboratory.
  • Hydrometric monitoring was initiated, i.e.  50 water-level stations and 15 water-transfer stations were established.
  • Two planning meetings were held in the project group (6 June and 15 August 2019, respectively).
  • Stakeholders were invited to participate in the advisory group and an invitation to the advisory group meeting (16/1-2020) was submitted
  • 'Stream Inspection' (24 June 2019) took place with the project group and employees from HedeDanmark, who carried out the weed cutting.
  • The DVPI (the Danish Aquatic Plant Index) was determined just prior to the first weed cutting in all stream sections.
  • 1st and 2nd weed cutting were completed.
  • Electro-fishing was completed to be determined by DFFV (Danish fish index For watercourse)

Assens municipality wrote about the project

Agriculture and public authorities requested knowledge – also internationally

The project group expects that the studies on weed cutting will contribute with knowledge in the course of the first couple of years. The results of the studies will include scientific measurements of water flow capacity – water levels and water flow - and measurements of the ecological status of the stream, including plants, small animals and fish, for which specific environmental targets have been set. The agricultural sector has been requesting this knowledge for may years in order to be able to utilise farmland optimally around the small streams. The authorities are also requesting knowledge to be able to meet the environmental targets that have been adopted for streams.

The knowledge generated by the project will benefit both agriculture and the authorities. The municipalities will look towards the municipality of Assens, and there will also be international interest in the results. We are the first  to document effects of different weed cutting methods, and it is amazing for us that we are in front of such a groundbreaking project. “I hope that the agricultural sector will support the project”, says Dan Gørtz, committee chair for Environment, Technology and Planning in the municipality of Assens.

In close dialogue with landowners, farmers and local agricultural associations, Assens municipality has identified 65 suitable sections for the project. GPS measurements were conducted in the winter of 2018-2019, and water level loggers were installed in all sections in the spring of 2019. Online water level data is available here: http://www.hydrometri.dk/hyd/.

Samples of small animals were also collected and data on the physical condition was collected. In late summer, electrical fishing was conducted at all sections in order to collect data on fish stocks.
The changed weed cutting practice was initiated from week 26, 2019.

Facts – weed cutting

Weed cutting is applying to remove plant biomass in streams to reduce the water level and to improve the ability of the streams to divert excess water, primarily from agricultural areas, but also from other types of land.

Read more on Assens municipality's website.

Study design

A total of 65 streams are included in the study. In five sections, the municipality's weed-cutting people continue to cut as usual, while at 30 sections cutting is exclusively done on the large aquatic plants in the stream (selective cutting of aquatic plants). At other 30 sections, cutting is done exclusively at the riverbank. In this way, we examine whether it is possible to achieve a better water flow capacity in the streams by cutting the plants that cause the greatest resistance to the water flow. At the same time, we will investigate whether there is an increase in underwater plant biomass in the streams due to improved light conditions and whether this will improve the physical conditions as well and, thus, the living conditions for small animals and fish.

In addition to changing the method, we also change the time of cutting. At a total of 15 sections, we cut twice, as today, while at another 15 sections we also cut twice, but later in the season than today. In this way, we will examine whether the water flow capacity in the streams is better in late summer/autumn, when field work is carried out.

We know that the results we obtain may depend on the gradient of the stream and on how physically affected the streams are. Often, the water flow capacity and the physical conditions are better in streams with a gradient. For this reason, we are conducting the experiment in both streams with and without gradients. The table below summarizes our study design, including the number of sections within the different types of streams.