What controls the self-cleaning capacity of ditches? How can rich plant and animal life be promoted in ditches and what is the best way to dredge and clean ditches? When can a ditch support a wide range of aquatic animals? An innovative study of the ecological performance of ditches is seeking the answers to these questions.

The Netherlands is the land of ditches: some 300,000 kilometres of ditches are of service to the agricultural sector, assist in water management, and are home to aquatic plants and animals. However, little is known about this wealth of species or, for example, about the mechanism responsible for the self-cleaning capacity of ditches. Moreover, water managers also lack the information they need for the effective, economical and sustainable maintenance of ditches. The PLONS project (Project Langjarig Onderzoek Nederlandse Sloten, or “Long-term study of Dutch ditches project”) will map the ecological performance of ditches (document in Dutch) with a view to changing this situation. PLONS is comprised of three subprojects:

  • Self-cleaning
  • Management & Restoration
  • Diversity

Self-cleaning capacity

The Self-cleaning subproject is examining the effect of ditch ecosystems (document in Dutch) on the self-cleaning performance of ditches. Ditch ecosystems are comprised of groups of plants, animals and microorganisms in a community. The intensive use of meadows next to ditches imposes a manure burden on the water in the ditch and the plants and animals that live in the water. Manure contains significant amounts of phosphorus and nitrogen. An excessive amount of these nutrients coming into contact with the ditch results in an abundant growth of duckweed and algae. That in turn results in a sharp decline in its ecological quality. At the same time, denitrification occurs in many types of manure, a process in which bacteria in the ditch convert nitrate in the manure into nitrogen gas. This gas removes the nitrogen from the water, as a result of which the ditch “cleans” itself. The researchers wish to determine the effect of a range of groups of aquatic plants and animals on the self-cleaning capacity of ditches in order to gain insight into their contribution to the ecological quality of ditches.

Influence of ditch management

Large numbers of plants usually grow in ditches. The accumulation of plant residues can result in the development of a layer of mud on the bed of the ditch which decreases the depth of the water. This in turn offers other species of aquatic plants an opportunity to grow. A ditch that is not cleaned or dredged will ultimately become clogged up and turn into land. Higher levels of nutrients in the water promote the growth of aquatic plants and, ultimately, can result in the ditch filling up. For this reason, management is necessary to prevent the ditch becoming clogged up. Once the ditch has been cleaned or dredged the vegetation begins a new cycle: pioneer species become established and are subsequently replaced by other species. The Management & Restoration subproject intends to provide an insight into the manner in which cleaning and dredging can increase the variation in and growth of aquatic plants in the ditch and the role ample amounts of nutrients play in this process.

Animals in ditches

Aquatic plants are of essential importance to the animals that live in ditches: they offer fish, amphibians and invertebrate freshwater animals a suitable habitat. They also provide food, exert an influence on the course of processes in ditch ecosystems, etc. An excessive amount of manure in ditches and incorrect management both result in the impoverishment of the continually increasing number of ditches that previously supported a rich aquatic plant life. The Wageningen researchers working on the Diversity subproject wish to determine the underlying mechanisms which result in an increased range of animal species in ditches and the range of animal species that are of functional importance to ditches.

Broad approach

The PLONS researchers are using a number of research methods for each subproject. They began by analysing all the available information about ditches and then examined eighty ditches in a large-scale field study. This yielded a comprehensive insight into the ditch variants in the Netherlands. Laboratory experiments are investigating a number of conditions and mechanisms in isolation. All studies are currently in full swing. The researchers are using existing and new methods in order to correctly interpret the results. The findings will contribute to the ability to predict the potential effects of interventions and promote an appropriately balanced management of ditches. The Dytiscus latissimus diving beetle, rare broad-leaf pondweeds, and caddis fly larvae will then be able to return to locations they were forced to leave earlier.