Ecological Systems and Devices Annotation << Back TIME OF DAY AS A BIOLOGICALLY IMPORTANT FACTOR IN BIOASSAY A.M. Lazareva, V.I. Ipatova The effect of the time of day at the moment of adding potassium dichromate K2Cr2O7 on the integral biological indicators of cell abundance and their survival in the culture of the green alga Scenedesmus quadricauda was studied. The effect of the time of day at the moment of adding the toxicant on the results of biotesting was evaluated at concentrations of 0.1, 1, 5, and 10 mg/L K2Cr2O7. Two experiments were carried out - acute and chronic, in which the toxicity of potassium dichromate was assessed depending on the time it was added to the culture medium - in the evening at the time coinciding with the end of the light period, and in the morning at the time coinciding with the beginning of the light period. The growth and development of the test object in the norm and under toxic exposure on the Uspensky No. 1 medium was studied. The sensitivity of the culture to the toxicant was determined and its toxicity was assessed when added at different times of the day. It was found that the time of the toxicant addition affects the results of the bioassay. In terms of changes in the number and ratio of living and dead cells in the culture, a greater inhibition was noted when K2Cr2O7 was added in the morning compared to the evening addition. The calculated LC50 values for different periods of observation of the chronic experiment confi rm the fact that the toxicity of potassium dichromate increases when the experiment is carried out in the morning compared to those for the evening experiment. Keywords: potassium dichromate, Scenedesmus quadricauda, toxicity, time of the toxicant addition, bioassay, circadian rhythms. DOI: 10.25791/esip.4.2022.1292 Pp. 28-37.