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

Duckweeds are widely used in wastewater treatment and biomass production systems; however, the combined effects of light intensity, spectral composition and photoperiod structure on their performance remain insufficiently understood. This study evaluated how these light parameters influence growth and nutrient removal in three duckweed species ( Lemna minor, Landoltia punctata and Spirodela polyrhiza ) cultivated under controlled conditions. Light intensity (50–1220 µmol m⁻² s⁻¹), spectral composition (blue, green, red, white and mixed spectra), and fragmented photoperiod regimes (12L:12D to 1L:1D) were examined. Spectral and photoperiod treatments were conducted at a constant daily light integral (~ 12.96 mol m⁻² d⁻¹), enabling isolation of light-quality and temporal effects from total photon dose. Relative growth rate (RGR), dry biomass and nitrogen (N–NO₃) and phosphorus (P–PO₄) removal were assessed. Growth responses showed saturation with increasing light intensity, with RGR saturating 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.