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1In certain countries, poorly managed freshwaters need to be seriously controlled to not lead to ecosystems 2 degradation. This control can be achieved by a biomonitoring using living organisms as ecological alert 3 indicators. In this context, biotests were undertaken on Lemna gibba (L. gibba) to assess the toxic effect of 4 textile pollutants reaching a natural ecosystem. L. gibba were exposed to different concentrations (5-100 mg/L) 5 of two dyes, namely Direct Red 89 (DR-89) and Vat Blue 20 (VB-20), under laboratory conditions. Our findings 6 showed that at concentrations > 50 mg/L, visible damage of toxicity (chlorosis and dislocation of fronds) were 7 appeared from the third day of toxicity tests leading to serious necrosis. However, at dye concentration ≤ 50 8 mg/L, duckweed showed no visible signs of toxicity within an exposure time of 4 days. However, these 9 concentrations exhibited a significant inhibition in L. gibba growth rate with a clear abatement in the 10 photosynthetic pigments contents. The dye concentration that reduces by 50% the growth rate of the plants 11 (IC50) was 36.3 and 26.9 mg/L for DR-89 and VB-20, respectively. The concentration-dependant reduction in 12 growth and photosynthetic pigments demonstrated their sensitivity in detection of dyes in aquatic systems. The 13 current findings proving physiological alterations of L. gibba following dye exposure, suggested that the species 14 are strongly suitable for the testing of textile effluents contaminating water bodies. 15

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