Photosynthetic and population growth response of the test alga Selenastrum capricornutum Printz to zinc, cadmium and suspended sediment elutriates

Short-term ¹⁴C-fixation (4 h) Selenastrum capricornutum algal toxicity tests were conducted with Cd (n=8), Zn (n=9) and suspended sediment aqueous elutriates (n=28) and the results were compared to those obtained in a 48 h population growth test. In order to provide more realistic experimental conditions, toxicity tests were carried out in prefiltered nutrient-spiked Lake Geneva water. The population growth inhibition test was significantly more sensitive than the ¹⁴C-fixation test for Cd (median EC50-4h and EC50-48h values of 600 and 118 µg L⁻¹, respectively) whereas no significant difference was measured for Zn toxicity (median EC50-4h and EC50-48h values of 97 and 96 µg L⁻¹, respectively). With suspended sediment aqueous elutriates, the relative sensitivity of the two different end points is sample dependent, with ratios of the EC25 for the ¹⁴C-fixation: population growth test ranging from 53.3. Elutriate toxicity shows no apparent relationship between the acute and chronic test, indicating that population growth inhibition cannot be derived directly or predicted from ¹⁴C-fixation. Both tests with their specific advantages and limitations provide valuable complementary information to measure the impact of single toxicants or complex mixtures on aquatic plants