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

In the MadMacs project, we studied six sites in rivers and lakes with mass development of macrophytes. The sites were in Norway, Germany, France, South Africa, and Brazil, and the macrophytes were perceived as problematic by water managers and residents. At each site, we mechanically removed the macrophytes from areas ranging from 550 m2 to 70,000 m2, reflecting current management practices. We quantified the short-term consequences of macrophyte removal on biogeochemistry and biodiversity, comparing each macrophyte removal site with a nearby site in which the macrophytes were left standing. We distributed questionnaires, in which we asked residents and tourists how they perceive the aquatic vegetation. We also quantified ecosystem services and compared the current situation with management regimes where the macrophytes were fully removed, as well as with a “do-nothing” scenario, i.e. where the macrophytes were left standing. Overall, we learned the following lessons: • Mass developments of macrophytes often occur in ecosystems which (unintentionally) were turned into a «perfect habitat» for aquatic plants • Reduced ecosystem disturbance can cause macrophyte mass developments even if nutrient concentrations are low • Macrophyte removal treats the symptom rather than the cause • Removal of non-native macrophytes may lead to nuisance growth of other macrophytes • The effect of macrophyte removal on ecosystem carbon emissions is site-specific • The consequences of partial macrophyte removal on the biodiversity of other aquatic organism groups are variable but generally small • Dense stands of macrophytes raise the water level of streams and adjacent groundwater • Nobody likes macrophyte mass developments, but visitors tend to regard them as less of a nuisance than residents do • Aquatic plant management often does not affect overall societal value of the ecosystem much • Different forms of recreation often contribute most to the total estimated societal value derived from all ecosystem services, an important message for management is therefore to consider the importance of forms of recreation before engaging in costly removal.

MadMacs combined basic science with applied science and cross disciplinary science and communicated the results in an understandable way to relevant stakeholders, including the general public, water managers and hydropower companies. MadMacs was working towards a change in attitude of these stakeholders, with the goal to improve the management of water courses with dense aquatic vegetation. Dense macrophyte stands were generally perceived as a nuisance and managers as well as the general public almost automatically thought that dense macrophyte stands are a sign that something is “wrong” and therefore must be removed. MadMacs had the following impacts: • Improved knowledge among water managers, hydropower companies and the general public about the underlying causes for macrophyte mass developments. This was achieved through info-meetings and is expected to reach a wider audience in the future due to the publication of the “cookbook” and the MadMacs key messages; • Improved knowledge among stakeholders enabled a less emotional and instead more informed discussion on the management of macrophyte mass developments, what can and cannot be expected from macrophyte removal, and who must pay for the management; • Stakeholders have started to re-think the sustainability of macrophyte removal. This is an on-going process. Our main knowledge outputs are 1. MadMacs key messages (https://www.niva.no/en/projectweb/madmacs/madmacs-key-messages). The key messages are easily understandable information on the complicated and interacting causes of macrophyte mass development, as well as the consequences of macrophyte removal for ecosystem and society. They are targeted at water managers and the general public. Lack of this knowledge previously was a main reason for unsustainable macrophyte management and unrealistic expectations from to macrophyte removal. The key messages form a basis for future, more informed management of water courses with dense aquatic vegetation. 2. Guidelines for managing mass developments of aquatic plants: A Cookbook Tool (https://www.niva.no/en/projectweb/madmacs/madmacs-cookbook). The Cookbook Tool is a set of easy-to-follow guidelines designed to aid managers of waterbodies in assessing and balancing the benefits and dis-benefits of macrophyte removal. The purpose of the guidelines is to assist managers of invaded waterbodies to make decisions on macrophyte removal, given the needs of the system. The aims of removal would be to rehabilitate the ecosystem, thereby increasing biodiversity and improving ecosystem structure and functioning of these systems. The guidelines outline appropriate methods to manage mass developments of macrophytes, while maximising ecosystem services. 3. A Bayesian tool (https://www.niva.no/en/projectweb/madmacs/bayesian-modelling-tool) to illustrate the complicated and interacting causes of macrophyte development, and the consequences of macrophyte removal.

Mass development of aquatic macrophytes in rivers and lakes is a worldwide problem, and substantial resources are spent on macrophyte removal. However, macrophyte stands either quickly grow back, or the removal causes other problems to surface. Macrophyte mass developments have known negative effects, but well-developed macrophyte stands also provide many ecosystem services. These are often poorly known to the public or to water managers. 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). Also, there is a lack of standardized before-after-control-impact studies on the direct and indirect costs 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? 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. Macrophytes will be removed from an area of at least 1000 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.