Sensitivity of indicator photosynthetic and physiological endpoints of Canadian waterweed (Elodea canadensis) to light flux density was investigated to find optimal light regime in the system “water - sediment”, suggested earlier for contact biotesting of natural bulk bottom sediments. Based on of response of shoot and root growth (length and weight), and state of photosynthetic system (concentration and ratio of photosynthetic pigments: chl.a, chl b, and carotenoids) of Elodea to the change of light flux density, it was shown that in the range of light flux density from 50 to 140 µmol quanta × m^-2×s^-1 no limitation or inhibition of growth and photosynthesis of Elodea was revealed. Hence, the level of irradiance from this range can be recommended for experimental biotesting of bulk bottom sediments using E.canadensis as indicator species.