Application of floating treatment wetlands in a highly eutrophic lake, Indonesia: a new tool for lake restoration and provision of microhabitat

Two sets of floating treatment wetlands (FTWs) have been evaluated as a tool for lake restoration and microhabitat provision for aquatic biota in a highly eutrophic and polymictic Lake Maninjau, Indonesia. Each FTW system consisted of 14 units square floating frame built using PVC pipes and palm sugar fibres as material mat transplanted onto net. All units of FTW were planted with different vegetation composition arrangement then placed inside the square metal frame attached on buoyant plastic drums to support the FTWs against the wave. The plants used in this study were Canna variegata, Vetiveria zizanioides, Cyperus papyrus, Echinodorus polaefolius and Limnocharis flava. FTWs were evaluated for 7 months for nutrient removal efficiency and other water quality indicators. Despite unstable lake conditions accompanied by strong waves and lake mixing, the cage and the PVC frame of FTWs applied remained intact and providing the buoyancy to support the plant growth. The plants in FTWs grew and adapted well in the turbulence water and even in the hypoxic conditions. Improvement of lake water quality in the FTWs area may be small but the accumulated nutrients content in the plant shoots suggests high removal of nutrients. In total by the end of observation, one FTWs system could remove > 100 g N and > 7 g P from the lake water. During field observation, important local fish and diverse wildlife activities were recorded including inhabiting, foraging, breeding, nursing and resting in the FTWs. Our study suggests that FTWs can be a promising tool for lake restoration to improve lake water quality and for microhabitat provision.