Investigation of Nutrient Removal Capacity and Growth Rate of Duckweed (Lemna minor) Under Different Harvesting Protocols in Aquaponics

In aquaculture systems, a high proportion of nutrients end up in the water as a by-product of metabolic processes. These must be neutralized through filtration, but to increase efficiency, the integration of some aquatic plants is advisable. Through the nutrient uptake capacity of these plants, the environmental impact of aquaculture systems can be decreased, so they become more sustainable. In this experiment, common duckweed (Lemna minor) was used under different harvesting protocols (control, and 25% and 50% of surface area harvested) to examine the nutrient uptake capacity of the plant and the effects on fish (common carp—Cyprinus carpio) production parameters. It can be concluded that the treatments used did not have a significant effect on fish production parameters. However regular duckweed harvesting had a positive effect on the plant’s biomass production and daily growth rate. By the end of the experimental period, the harvested groups had accumulated more biomass than the control group, though there was no difference between the 25% and 50% harvest rates. In our experiment, the control group achieved a yield of 17.9 t/ha/year, while the regularly harvested (25% and 50%) treatments achieved yields of 23.4–24 t/ha/year (based on extrapolated data). Regular harvesting of duckweed resulted in lower ammonia levels, as the free water surface available to the plants after harvesting allowed for more intensive growth, enabling them to absorb more organic matter. The dynamics of nitrite, nitrate and orthophosphate concentrations are primarily determined by the internal biochemical processes of the system and temporal development, while treatments such as duckweed harvesting had no direct effect on these parameters.