"The increased occurrence of macrophytes is often perceived as a form of degradation of water bodies. Yet the opposite is often the case here in Europe at present: macrophytes had disappeared for decades due to excessive nutrient inputs and are now growing again with improved water quality and lower nutrient inputs," explains Sabine Hilt, who conducts research on aquatic plants at IGB. Macrophytes should therefore not be confused with free-floating algae (phytoplankton), which often dominate water bodies when nutrient loads are high and can also be problematic for humans and animals with their so-called "algal blooms," Sabine Hilt explains.
The important role of aquatic plants in the ecosystem
"Aquatic plants are an important part of our water bodies. They influence material cycles and interact with other aquatic organisms. During growth, they bind carbon dioxide, which can thus be stored in the sediment for longer periods. Macrophytes take excess nutrients such as phosphorus and nitrogen out of the water and release oxygen through their metabolism, which improves the aeration of the water and its sediments," adds Jan Köhler, who also conducts research on macrophytes at IGB. The aquatic plants also retain turbidity and prevent sediment from being stirred up. In streams, backwater effects from macrophytes can lead to higher water levels in the river and groundwater, contributing to water retention in adjacent areas. "In terms of climate impact adaptation, this is a particularly positive and important effect; in addition, excessive erosion of banks and the river bed can be slowed down," explains Jan Köhler.
In the IGB Fact Sheet, the researchers emphasize that macrophyte stands also promote biodiversity due to their diverse structure: A species-rich growth of algae and bacteria can develop on their surface, which in turn provides habitat and food for small animals (zoobenthos). Macrophytes offer small animals protection from predators and are themselves food for various water birds. In rivers, macrophyte stands increase the diversity of habitats with different flow velocities. Aquatic plant stands are also valuable spawning and hunting grounds for fish and refugia for their larvae and juveniles.
Is there an "overabundance" of aquatic plants? Societal use interests
From a freshwater ecology point of view, the advantages for nature clearly outweigh the disadvantages in most mass developments of macrophytes, the researchers note. However, our water bodies are also always subject to certain human objectives, mostly related to their use. For example, too many aquatic plants can interfere with boat traffic and other water sports, hinder anglers or be unpleasant for swimmers. Accordingly, water users often desire plant-free surfaces and water bodies.
Indirect use or safety interests also play a role, especially for flowing waters. A so-called "herbaceous jam" can slow down runoff and thus raise the water table on adjacent land; during floods, local flooding can also be intensified by a high macrophyte density in the water body. While more water in the landscape is beneficial from an ecological perspective, it is not always welcomed from an agricultural and flood control perspective for residential areas.
"Authorities and water management associations are responsible for preventing negative backwater effects and flooding as part of watercourse maintenance. However, at the same time, the ecological status is relevant for water management – among other things due to the European Water Framework Directive – as well as the conservation status of the habitat from a nature conservation perspective. So it can be very challenging for these stakeholders to deal with these conflicting objectives between protection or establishment of good ecological status on the one hand and various human use interests on the other, and to develop approaches that take into account and balance all objectives," explains Jan Köhler.
Measures against macrophytes: What are the effects of weeding water bodies?
If those involved in water management decide to take measures against macrophytes, weeding – i.e. mowing or removing aquatic plants – is the most common measure. It often only serves to improve the quality of use for certain interest groups and does not lead to an improvement in ecological status, the researchers emphasize. While weeding creates open space for recreational use in the short term or even increases the flow of ditches and natural streams, the process is also very expensive. In addition, after weeding, plant fragments can drift into previously unaffected areas and become established, in which case macrophyte populations tend to expand. For some species, weedings even result in increased growth rates. Because weedings are not very selective, they also quickly reduce rare plant species, destroy diverse habitats, and kill many animals living in aquatic plant stands. These actions can also lead to the stirring up of settled particles and increased oxygen depletion. "Weedings thus pose risks to biodiversity and can sometimes even lead to a switch to a turbid, phytoplankton-dominated state that supports significantly fewer ecosystem functions and is less attractive for many forms of use," explains Sabine Hilt.
According to the scientists, the ecological value of aquatic plants is often overlooked in the public debate. From the research point of view, economic, ecological and social aspects should be equally included in the consideration and planning of measures in the future. If a reduction of aquatic plant populations is necessary, sustainable measures such as the further reduction of nutrient inputs and, if necessary, the planting of riparian trees should be preferred.
"Small-scale weedings for spatially limited uses such as swimming at bathing sites are usually also justifiable from an ecological perspective if no larger stands of legally protected macrophyte species grow there," explains Jan Köhler. However, large-scale weedings, in which almost all aquatic plants of the water body would sometimes be removed for the interest of only a few users, would have disproportionately high costs and disadvantages.
"Water management by authorities and associations is not an end in itself – within the scope of their tasks and areas of competence, therefore, greater consideration should be given to which traditional routines, such as weeding measures, are really necessary and purposeful in the context of water management. The respective conflicting goals between ecology and use should be specifically named for the respective cases in future water management and, where necessary, viable compromises between protection and use should be worked out," Sabine Hilt summarises in the analysis.
The IGB Fact Sheet (in German) can be downloaded and distributed free of charge from the IGB website.