Short-term effects of macrophyte removal on aquatic biodiversity in rivers and lakes.

Mass development of macrophytes is an increasing problem in many aquatic systems worldwide. Dense mats of macrophytes can negatively affect activities like boating, fishing or hydropower production and one of the management measures often applied is mechanical removal. In this study, we analyzed the effect of mechanical macrophyte removal on phytoplankton, zooplankton, and macroinvertebrate (pelagic and benthic samples) assemblages. Our study covered five sites in four countries in Europe and Africa with highly variable characteristics. In all sites, dense mats of different macrophyte species (Juncus bulbosus in a river in Norway; a mix of native macrophytes in a German river, Elodea nuttallii in a lake in Germany, Ludwigia spp. In a French lake and Pontederia crassipes in a South African lake) are problematic and mechanical removal was applied. In every country, we repeated the same BACI (Before-After-Control-Impact) design, including "before", "one week after", and "six weeks after" sampling in a control and an impact section. Repeating the same experimental design at all sites allowed us to disentangle common effects across all sites from site-specific effects. For each taxonomic group, we analyzed three structural and three functional parameters, which we combined in a scoring system. Overall, the removal of macrophytes negatively affected biodiversity, in particular of zooplankton and macroinvertebrate assemblages. In contrast, plant removal had positive effects on the phytoplankton assemblages. Effects were more pronounced one week after removal than six weeks after. Consequently, we suggest a stronger consideration of the effect of plant removal on biodiversity to arrive at more sustainable management practices in the future.

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.