Use of floating wetlands to treat nutrient enriched lake water

The nutrient attenuation performance of floating treatment wetlands (FTWs) planted with emergent wetland macrophytes was investigated in 4 one-week long batch mesocosm studies using artificial eutrophic lake water. The FTWs were made from buoyant polyester mats (0.6 x 0.6 m squares) planted with one of three native wetland species, Cyperus ustulatus, Juncus edgariae or Schoenoplectus tabernaemontanii. Three of each FTW type were each placed in 0.7 m3 mesocosm tanks and dosed with artificial eutrophic lake water with nutrients at concentrations relevant to eutrophic Rotorua lakes. In addition, three Control mesocosms without FTWs but with shading equivalent to the FTWs were also monitored. Mean areal mass attenuation rates of the FTWs ranged from 638 to 762 mg m-2 d-1 for total nitrogen, however the Controls which simply shaded the surface, had removal rates of 379 mg m-2 d-1 . Thus the average net TN removal attributable to the FTWs was 339 mg m- 2 d-1 . This rate is similar to that for equivalently loaded conventional constructed wetlands with sediment-rooted vegetation. Mean areal mass attenuation rates for TP and DRP ranged from 54 to 58 and from 57 to 64 mg m-2 d -1 respectively. In the Controls the rates were 39 and 44 mg m-2 d-1 , resulting in net FTW removal of 30 and 16 mg m -2 d-1 respectively. Attenuation mechanisms appeared to be dominated by plant and algal uptake and subsequent algal settling beneath the FTWs. Denitrification within the FTW matrix was also apparent when nitrogen was supplied as nitrate, but was limited by a lack of organic carbon available for microbial processing. Direct investigation of these mechanisms will be necessary to determine their long-term sustainability, as they include uptake into temporary and/or finite storage pools. Furthermore, the nutrient attenuation rates recorded in the present study during summer are likely to be higher than would be expected in winter. The FTWs have however demonstrated considerable potential to reduce nutrients from eutrophic lake water. They may be a suitable option for targeted application in smaller lakes, in polluted lakes embayments, or at the mouth of streams or point sources that are important inputs of nutrients into lakes.