ABSTRACT Background The link between species richness and ecosystem services remains a central question in ecology. Aims To evaluate the effect of composition and plant richness on water quality improvement. Methods Thirty-nine mesocosms (65 L) were divided into four quadrants and were either planted in monocultures, or in 2- or 4-species combinations. Mesocosms were fed with synthetic wastewater during one growing season and outflow samples were collected weekly for physico-chemical analyses. Results Pollutant removal efficiency varied among plant species and species combinations. Eichhornia crassipes outperformed the other plant species and was the only one whose presence in a plant combination had a positive effect on pollutant removal. Species richness had a small but highly significant effect on nitrogen removal, with 2-species and 4-species systems outperforming by 4% and 5%, respectively, the average removal of the monocultures. The removal efficiency of a combination of two species was occasionally better than the average of these species in monocultures. However, higher plant species richness never showed greater treatment performance over the most efficient monoculture of its constituent species. Conclusions Our study showed some weak but significant biodiversity effects of free-floating plant species on water quality improvement. Nevertheless, total plant biomass remained a better predictor of water purification capacity than species richness.
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