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Mass development of aquatic macrophytes in rivers and lakes is a problem occurring in many different rivers and lakes around the world. The specific regional reasons for macrophyte mass development are, however, still poorly understood, most likely because there is typically a combination of factors which together cause nuisance growth (multiple pressures). Macrophyte mass developments have known negative effects (such as causing fish kills and impairing the use of the water body for swimming and boating), but well-developed macrophyte stands also provide many ecosystem services (such as providing habitat for fish and waterfowl). These are often poorly known to the public or to water managers. Furthermore, despite the amount of funding spent on macrophyte removal, there is a lack of standardized before-after-control-impact studies on the direct and indirect costs and benefits of macrophyte removal.

We aim to address the following questions:

1)  Which combination of natural conditions and pressures leads to undesired mass development of macrophytes?

2)  What are the direct and indirect consequences of macrophyte removal for ecosystem functions and services?

3)  Which consequences of macrophyte removal are site-specific, and which are general?

In collaboration with key stakeholders, we will execute a set of “real-world experiments” in a harmonized design across five countries (Norway, Germany, France, South Africa and Brazil). Macrophytes will be removed from an area of at least 500 m2 at each site, and the following parameters will be quantified before and after the removal at control and impact sites: phyto- and zooplankton, benthic algae, macrophytes, macroinvertebrates, fish, nutrient and carbon retention and removal, impoundment, shoreline erosion, as well as relevant ecosystem services related to recreation and water use. We will develop a risk assessment tool of macrophyte mass development and its ecological impacts, as well as of the effects of macrophyte removal. We will compare benefits and dis-benefits of macrophyte removal and formulate guidelines for the management of water courses with dense aquatic vegetation.