Mass development of aquatic macrophytes – causes and consequences of macrophyte removal for ecosystem structure, function, and services – MadMacs

Mass development of aquatic macrophytes (water plants) in rivers and lakes is a worldwide problem, and substantial resources are spent annually on removal of macrophytes. This approach, however, does not address the causes of the mass development and is not sustainable. Macrophyte stands either quickly grow back, or the removal causes other problems to surface (e.g. the mass development of algae or cyanobacteria). Macrophyte mass developments have known negative effects, but well-developed macrophyte stands also provide many ecosystem services, including nutrient and carbon retention (= purification of water), as well as providing shelter and nursery habitat for many organisms (= affecting biodiversity). The ecosystem services provided by macrophytes are often poorly known to the public or to water managers. Consequently, management decisions, despite being costly, are generally based on a prevailing intuitive negative perception rather than a rational knowledge-based decision.
The specific regional reasons for macrophyte mass development are still poorly understood, likely because there is typically a combination of factors which together cause nuisance growth (multiple pressures). This makes analysis of causes of nuisance growth at a particular site challenging. Also, there is a lack of standardized before-after-control-impact (BACI) studies on the direct and indirect costs of macrophyte removal (= loss of ecosystem services provided by macrophytes) across multiple sites. This greatly hampers the possibility to generalize results, and makes giving general management advice difficult. In our project, 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? 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 BACI design across six case studies in five countries (Norway, Germany (2), France, South Africa, Brazil). Macrophytes will be removed from an area = 1000 m2 at each site, and the following parameters will be quantified before and after the removal at control and impact sites, respectively: phytoplankton, zooplankton, benthic algae, macrophytes, macroinvertebrates, fish, nutrient and carbon retention and removal, impoundment of flowing waters, shoreline erosion, as well as all relevant ecosystem services related to recreation and water use, including agriculture and industry. We will develop a general risk assessment tool of macrophyte mass development and associated ecological impacts under multiple pressures, as well as of the effects of macrophyte removal, using causal pathway analyses and a probabilistic approach, and the tool will then be tested and improved based on the case study results. We will seek to detect consistent effects of macrophyte presence versus removal, and forecast consequences of macrophyte removal in aquatic ecosystems. This will enable us to directly compare benefits and dis-benefits of macrophyte removal, and generalize the findings. Based on these, we will formulate guidelines for the management of water courses with dense aquatic vegetation (“cookbook” tool to assess and balance benefits and dis-benefits of aquatic macrophyte removal). This can potentially save a substantial amount of money, by preventing management measures which cost more than they gain. MadMacs will help move the management of water courses with dense aquatic vegetation from “perception” to rational knowledge-based decisions.