Cu(II) stress responses and nutrient removal performance of Spirodela polyrrhiza and Lemna minor in aquaculture effluent

The rapid growth of aquaculture has led to the release of effluents rich in nutrients and heavy metals, posing significant ecological risks. This study evaluated the physiological responses and nutrient removal capacity of Spirodela polyrrhiza and Lemna minor exposed to a Cu(II) gradient (0–5 mg/L) in aquaculture wastewater. S. polyrrhiza exhibited greater Cu(II) tolerance, maintaining significantly higher activities of superoxide dismutase, peroxidase, and ascorbate peroxidase than L. minor . Increasing Cu(II) concentrations reduced chlorophyll and carotenoid contents, inhibited growth, and impaired photosynthetic structures in both species. At low Cu(II) levels (0–1 mg/L), marked reductions in total nitrogen (up to 78.3%), ammonium-nitrogen (up to 80.6%), and total phosphorus (nearly 100%) were observed in systems containing both duckweed species, but these reduction efficiencies declined sharply at ≥ 3 mg/L. Spectroscopic analyses revealed that both species regulated dissolved organic matter composition and fluorescence, with S. polyrrhiza showing stronger modulation of humic-like substances. These findings clarify the mechanisms of duckweed adaptation to Cu(II) stress and highlight the Cu tolerance of S. polyrrhiza and support its application for aquaculture effluent treatment under Cu(II) stress.