Ecological Systems and Devices Annotation << Back The Effect of Water Hardness on the Toxicity of Potassium Dichromate and Sodium Dodecylsulphate to algae Chlorella vulgaris Vorobieva O.V., Gershkovich D.M., Egorova M.D., Karyakina P.A., Lebedeva A.D., Lupolova A.E., Chukhrova S.S. The toxicity of two standard comparison toxicants, potassium dichromate and sodium dodecyl sulfate, was assessed on the green microalgae Chlorella vulgaris Beijerinck, 1890, depending on water hardness. Water from a natural reservoir with a low anthropogenic load was used for the study. Calcium and magnesium salts were added (at a calcium to magnesium ratio of 4:1), thereby increasing the water hardness by 3°F - 9°F. During acute experiments, changes in cell number, growth rate, and the maximum quantum efficiency of photosystem II photochemistry were assessed in control lines and with the addition of toxicants. Water hardness did not change the cell number and growth rate of Chlorella in the control lines, but it did alter the toxicity of the toxicants: with an increase in water hardness, the toxic effect of both potassium dichromate and sodium dodecyl sulfate on cell number and the efficiency of primary photosynthetic processes decreased. A comparison of the values EC50 at different hardness levels showed that a 4-fold increase in water hardness from 3°G to 12°G led to an increase in the EC50 of potassium dichromate by 3,6 times, and a decrease in the same indicator by 2,7 times for sodium dodecyl sulfate. Keywords: biotesting, water hardness, sodium dodecyl sulfate, potassium dichromate, algae, Chlorella vulgaris. DOI: 10.25791/esip.12.2025.1564 Pp. 47-55.