Integrated effects of light intensity, spectral composition and photoperiod fragmentation on growth and nutrient removal of three duckweed species

The performance of duckweed-based systems used for wastewater treatment and biomass production depends strongly on light conditions, yet the combined effects of light intensity, spectral composition, and photoperiod structure on plant growth and nutrient removal remain insufficiently understood. This study evaluated how these light parameters influence growth and nutrient removal in three widely distributed and commonly studied duckweed species (Lemna minor, Landoltia punctata and Spirodela polyrhiza) cultivated under controlled conditions. A range of sub-saturating to saturating and potentially supra-saturating light intensities (50-1220 µmol m⁻² s⁻¹), spectral composition (blue, green, red, white and mixed spectra), and fragmented photoperiod regimes (12 L:12D to 1 L:1D) were examined. Spectral and photoperiod treatments were conducted at a constant daily light integral (~ 12.96 mol m⁻² d⁻¹), allowing comparison of light-quality and temporal effects under the same total photon input. Relative growth rate (RGR), dry biomass as well as nitrogen (N-NO₃) and phosphorus (P-PO₄) removal were assessed. Growth responses showed saturation with increasing light intensity, with RGR reaching saturation at lower levels (160-296 µmol m⁻² s⁻¹) than dry biomass (~ 343-440 µmol m⁻² s⁻¹). Red-enriched white light generally enhanced growth and nutrient removal across species, whereas fragmented photoperiods reduced growth despite identical daily light integral.