MadMacs key messages
We studied six sites in rivers and lakes with mass development of macrophytes (= aquatic plants). The sites were located in Norway, Germany, France, South Africa and Brazil, and the macrophytes were perceived as problematic by water managers and residents.
At each site, we – in collaboration with local water managers - mechanically removed the macrophytes from areas ranging from 550 m2 (Lake Grand Lieu) to 70,000 m2 (River Spree), reflecting current management practices. We quantified the short-term consequences (up to six weeks) of macrophyte removal on biogeochemistry and biodiversity, comparing each site from which the macrophytes were removed 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 scenarios 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
- Aquatic plant management often does not affect overall societal value of the ecosystem much