Causes of macrophyte mass development and management recommendations.

Aquatic plants (macrophytes) are important for ecosystem structure and function. Macrophyte mass developments are, however, often perceived as a nuisance and are commonly managed by mechanical removal. This is costly and often ineffective due to macrophyte regrowth. There is insufficient understanding about what causes macrophyte mass development, what people who use water bodies consider to be a nuisance, or the potential negative effects of macrophyte removal on the structure and function of ecosystems. To address these gaps, we performed a standardized set of in situ experiments and questionnaires at six sites (lakes, reservoirs, and rivers) on three continents where macrophyte mass developments occur. We then derived monetary values of ecosystem services for different scenarios of macrophyte management ("do nothing", "current practice", "maximum removal"), and developed a decision support system for the management of water courses experiencing macrophyte mass developments. We found that (a) macrophyte mass developments often occur in ecosystems which (unintentionally) became perfect habitats for aquatic plants, that (b) reduced ecosystem disturbance can cause macrophyte mass developments even if nutrient concentrations are low, that (c) macrophyte mass developments are indeed perceived negatively, but visitors tend to regard them as less of a nuisance than residents do, that (d) macrophyte removal lowers the water level of streams and adjacent groundwater, but this may have positive or negative overall societal effects, and that (e) the effects of macrophyte removal on water quality, greenhouse gas emissions, and biodiversity vary, and likely depend on ecosystem characteristics and macrophyte life form. Overall, we found that aquatic plant management often does not greatly affect the overall societal value of the ecosystem, and we suggest that the "do nothing" option should not be easily discarded in the management of perceived nuisance mass developments of aquatic plants.

Community stability and seasonal biotic homogenisation emphasize the effect of the invasive tropical tanner grass on macrophytes from a highly dynamic neotropical tidal river.

We described the spatial and temporal dynamics of aquatic macrophytes in a Neotropical coastal estuarine river, and identified the negative effects associated to the presence and dominance of the invasive tanner grass. We compared macrophyte beds along the Guaraguaçu River (South Brazil) over four years, using taxonomic and functional dimensions. Biodiversity descriptors were higher in the driest periods compared to the rainiest, although this difference seems to be decreasing over the studied years. Moreover, the spatial organization of biodiversity and community structure slightly changed over time. Such spatial community stability was highlighted by the dominance of the highly invasive tanner grass. In beds dominated by the invasive species, the biodiversity was reduced. As aquatic macrophytes represent an important group in water bodies, the long-term loss of seasonal differences in community structuring is of concern, mainly regarding its potential to impact other groups and ecosystem functioning. By analysing data from standardized monitoring, we were able to identify a poorly discussed facet of biotic homogenisation-the seasonal homogenisation. We also discussed the impact of massive development of invasive species and its consequences for biodiversity in a Neotropical river of outmost importance for biological conservation. Supplementary Information: The online version contains supplementary material available at 10.1007/s00027-022-00858-3.