Giving waterbodies the treatment they need: A critical review of the application of constructed floating wetlands.

Water quality is declining worldwide and an increasing number of waterbodies lose their ecological function due to human population growth and climate change. Constructed floating wetlands (CFWs) are a promising ecological engineering tool for restoring waterbodies. The functionality of CFWs has been studied in-situ, in mesocosms and in the laboratory, but a systematic review of the success of in situ applications to improve ecosystem health is missing to date. This review summarises the pollutant dynamics in the presence of CFWs and quantifies removal efficiencies for major pollutants with a focus on in situ applications, including studies that have only been published in the Chinese scientific literature. We find that well designed CFWs successfully decrease pollutant concentrations and improve the health of the ecosystem, shown by lower algae biomass and more diverse fish, algae and invertebrate communities. However, simply extrapolating pollutant removal efficiencies from small-scale experiments will lead to overestimating the removal capacity of nitrogen, phosphorus and organic matter of in situ applications. We show that predicted climate change and eutrophication scenarios will likely increase the efficiency rate of CFWs, mainly due to increased growth and pollutant uptake rates at higher temperatures. However, an increase in rainfall intensity could lead to a lower efficiency of CFWs due to shorter hydraulic retention times and more pollutants being present in the particulate, not the dissolved form. Finally, we develop a framework that will assist water resource managers to design CFWs for specific management purposes. Our review clearly highlights the need of more detailed in situ studies, particularly in terms of understanding the short- and long-term ecosystem response to CFWs under different climate change scenarios.