Quality improvement measures
On 1 January 2016, Agricultural Nature and Landscape Management (ANLb) was introduced with the aim of efficient agricultural nature management that benefits nature. A number of farmers in the surroundings of Utrecht use various ANLb management packages to manage their ditches.
For example, no dredged materials are applied to the ditch banks so as to reduce the amount of nutrients in the surface water. Another initiative is ‘ecological ditch cleaning’, which involves leaving some of the vegetation in place. Yet another measure is to establish a botanically valuable meadow border. This also allows the use of fewer chemical herbicides, thus reducing nutrient leaching into surface water. Furthermore, mowing only once a year and removing the clippings reduces the level of nutrients in the meadow border over the years.
The Red List species water soldier has been found in some ditches. Water soldier can be recognised by its rosette of elongated leaves, with a spine on each leaf tip. This plant is also the host plant of several species of dragonflies, including the protected green hawker. This dragonfly species only deposits its eggs on water soldier plants. Some farmers have therefore implemented measures in their ditches to encourage the growth of this aquatic plant. Others have changed the landscaping of their ditches by constructing nature-friendly banks (NFBs).
Figure 1. Ditches monitored in the HDSR management area for the purpose of this study [3]
Method
To analyse the impact of the various management and landscaping initiatives – ANLb management packages, water soldier policy and NFBs – volunteers and professionals monitored dozens of ditches in the area managed by De Stichtse Rijnlanden water board (HDSR) for riparian and aquatic plants from 2016 until 2021 [1] (see Figure 1). The impact was measured using several ‘impact variables’: the ecological quality ratios (EQR) defined under the Water Framework Directive (WFD), the cover of different groups of plants, biodiversity, and the floristic score. For the floristic score, the plants present were given a score linked to their botanical value according to the floristic point system in Vanuit de rand gezien (Seen from the edge) [2]. These value scores were added together. An overall floristic score of 25 or higher was considered ‘good’.
Examining multiple variables alongside the EQR score can provide additional ecological and in-depth information on quality and potential changes in ditch vegetation. In order to identify the changes in vegetation caused by any one of these ecological measures, reference ditches in which no special measures were taken were also included in the monitoring.
For each effect variable, it was examined whether there was 1) a significant improvement between the initial and final values per management and landscaping initiative, 2) a significant difference in final condition between management and landscaping initiatives, correcting for initial values and 3) a difference in final condition between management and landscaping initiatives.
WFD fact sheets from 2015 and 2021 for the water bodies near monitoring sites were consulted to obtain information on chemical water quality. These fact sheets contained details of general physical and chemical quality, specific pollutants, and ubiquitous and non-ubiquitous substances.
Results and discussion
EQR score
According to the EQR quality parameters, the ditches monitored – which were of the water types ‘peatland waterways and canals’ and ‘buffered peatland ditches’ identified in the WFD – were on average of insufficient overall ecological quality. Looking at the impact of management and landscaping measures, it was found that ecological quality increased slightly in the case of NFBs and, to a lesser extent, ditches containing water soldier. However, this increase was not found to be statistically significant for any of the management and landscaping initiatives (Figure 2). This suboptimal ecological quality is partly explained by the low numbers of specific groups of plants (see below).
Figure 2. Initial and final values for the EQR sub-score ‘Other aquatic flora per management and landscaping initiative’ [3]
Cover growth forms
Few large floating leaf plants (such as the yellow water-lily), submersed and emersed plants were found in the ditches. On average, these growth forms therefore did not yet provide optimal cover (according to WFD standards) in the ditches monitored. In particular, the cover of submerged plants and large floating leaf plants was not yet sufficient.
There had been no significant change in the abundance of thread algae, emersed plants and large floating plants over the years in ditches where management and landscaping measures had been implemented.
The cover of submerged plants did see an average increase in ANLb ditches, but not in the case of the other management and landscaping initiatives. The cover of thread algae and duckweed was mostly low in the ditches monitored. This is in line with the desired situation, as duckweed can dominate the water surface under eutrophic and hypertrophic conditions and cut it off from light and oxygen. In the ditches monitored, there was no indication that duckweed has a negative impact on other aquatic plants. The coverage rate of floating algae beds is generally low for all management and landscaping initiatives.
Biodiversity
Along a 100-metre stretch of a ditch, volunteers found an average of 40 different species of riparian plants, with as many as 85 different riparian plants at one location. Common species (such as reed sweet-grass, marsh-bedstraw and greater duckweed) occurred most frequently and the dominant species present, such as reed sweet-grass, soft rush and great willowherb, indicated nutrient-rich conditions. This was in line with the low floristic score. In addition to common species, a number of rare species were found. The very rare red pondweed was also found in one ditch.
It was notable that the overall biodiversity of plants in the riparian zone increased over the study period for both the ANLb, water soldier and reference ditches, possibly indicating an improvement due to general management or site quality.
There was also an evident peak in the biodiversity of plants in the riparian zone in 2017 (see Figure 2). This could be due to the very wet summer in 2017, which may have widened the riparian zone towards the plot. There was no evidence yet of an increase in the number of species.
As in other research [3], we therefore saw an improvement in biodiversity mainly in the riparian zone and not in the water zone. Management (and construction of NFBs) focuses on the riparian zone, while in the watercourse the aquifer function is important. In order to encourage and preserve underwater vegetation, it is important that ditches do not become overgrown and landlocked. Since ditches are inherently narrow and shallow, this can happen quickly. As a result, tall riparian plants can block the light needed for submerged plant growth even if there is open water in the middle of the ditch. To increase biodiversity in the water zone, it is important that the riparian vegetation does not block out light. Phased mowing in the riparian zone is therefore recommended to encourage different growth forms. It is also important to create and maintain space for underwater plants during construction and management.
Figure 3. Biodiversity of plants in the riparian zone by year. The dotted lines are the individual measurement points and the bold line is the average, with the bars representing the standard error in that year [3]
Floristic score
The average floristic score was low, at five points. We only saw a statistically significant mean increase in floristic score among ditches with ANLb management. However, the average final measurement was nowhere near 25 points (good score).
Ecological and chemical water quality
These results show that management has a limited impact on the ecological quality of ditches. One of the reasons for this is the large variety of impact variables between years and few consistently monitored long measurement series, which means that there is currently little insight into long-term trends. This type of study requires multiple long, consistent monitoring series.
The study did not assess the impact of chemical water quality on ditch vegetation. According to the 2015 and 2021 WFD fact sheets, the overall physical and chemical water quality of water bodies near monitoring sites was ‘not bad’ and had even improved in some cases [4]. For example, the overall measured concentrations of phosphorus and nitrogen were ‘moderate’ or ‘good’. However, specific pollutants (such as ammonium) did not meet the standard in both 2015 and 2021. Ubiquitous substances (such as mercury) and non-ubiquitous substances (such as fluoranthene) generally also failed to meet the standard. For ubiquitous substances, it is often difficult to improve water quality because concentrations of these substances are slow to fall. For non-ubiquitous substances, concentrations can be rapidly reduced through efforts such as limiting discharges from industry or taking measures to reduce emissions.
Improvement of both management and landscaping (phased mowing in the riparian zone, creating and preserving space for submerged plants), improvement of other habitat conditions – such as water and soil quality – and, where relevant, discouragement of crayfish grazing (not included in this study) are probably needed to achieve verifiable good ecological quality.
From 2016 until 2021, dozens of ditches in the agricultural area around Utrecht were monitored to analyse the impact on ecological quality of ecological management, water soldier management and the construction of nature-friendly banks. The average scores of the ditches monitored were still below standard, and ecological management and landscaping measures were shown to have a limited impact on ecological quality in the period from 2016 until 2021. One of the reasons for this was the limited cover of submerged plants and large floating leaf plants. The biodiversity of plants in the riparian zone increased, but additional measures appear necessary to also increase biodiversity in the water zone.