Several floating plants have been used in wastewater treatment systems. These floating plants are not typically a design component in constructed wetlands. The species most likely to occur incidentally in FWS wetlands is Lemna (duckweed). The presence of duckweed on the water surface of a wetland can be both beneficial and detrimental. The benefit occurs because the growth of algae is suppressed; the detrimental effect is the reduction in transfer of atmospheric oxygen at the water surface because of the duckweed mat. The growth rate of this plant is very rapid, and the annual yield can be 18 (dw) ton/ac (20 mt/hat) or more. The tissue composition (dw basis) is approximately 6% N, 2% P; solids 5%. Salinity tolerance is less than 0.5 ppt. These species serve as a food source for ducks and other water birds, muskrat, and beaver. The presence of duckweed on FWS wetlands cannot be prevented because the plant also tolerates partial shade. Open-water zones in FWS wetlands should be large enough so wind action can periodically break up and move any duckweed mat to permit desirable reaeration. The decomposition of the unplanned duckweed may also impose an unexpected seasonal nitrogen load on the system.
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