a floating water treatment device for biological treatment of water in a body of water , the device comprising : a submerged biomass - carrying system having at least one biomass - carrying element ; a water aeration device adapted to aerate the water in the body of water in the vicinity of the biomass - carrying system ; and at least one flotation system , having at least one floating element , the at least one flotation system adapted to float on the surface of the body of water and to support the submerged biomass - carrying system and water aeration device from totally submerging into the body of water .