Abstract. This research is motivated because of increasing eutrophication problems in water bodies. This happens as a result of excess nitrogen and phosphorus. Too many of these nutrients can cause algae to grow, leading to blooms. Harmful algal blooms consume dissolved oxygen in water. As a result, the oxygen level decreases, affecting the aquatic ecosystem. The stenobiont plant, which belongs to the group of the most sensitive bioassays, is the small duckweed Lemna minor L. This plant is sensitive to increasing levels of nutrients and blooms of algae. This research aimed to determine the level of water toxicity using the bioassay of duckweed (Lemna minor L.) to establish dangerous concentrations of nitrogen compounds (NO2 - , NO3 - , NH4 + ) (CA) for higher plants of aquatic ecosystems with subsequent risk forecast for Dnipro basin. The research was carried out in accordance with DSTU 32426-2013 “Testing of chemicals of environmental hazard. Lemna sp. Growth Inhibition. Test”. The number of effects associated with nitrogen compounds (NO2 - , NO3 - , NH4 + ) on plant growth and development during the testing period was detected. The number of green layers is the main variable that was investigated in the experiment. Changes in the morphological features of duckweed were also studied, in accordance with the standard, the toxicity of water was assessed by changes in leaf colour, manifestations of chlorosis, the transformation of whole plants into individuals, the appearance of young leaves. To estimate the number of effects associated with the nitrogen compounds toxicity, the half-maximal effect (EC50) was studied. It should be noted that studies of water toxicity on NO2 - content using the Lemna minor L. test showed that even at the lowest concentration of 0.1 mgNO2 - /L aquatic biota will be significantly affected by the toxicity with possible subsequent death. Summarizing the results of the research of the Lemna minor L reaction to the toxicity of water with different concentrations of NO3 - came to the conclusion that even the lowest concentration (0.1 mgNO3 - /L) will have a negative impact on their growth and development of the plant. Generally, 40.0 mgNO3 - /L is considered “safe” for fish farming. However, this is not a case for aquatic vegetation, as shown by this result – such concentration will be toxic and cause death. If we take into account NH4 + salts, the reaction was even more progressive, the deterioration of the leaf layers of duckweed began to occur immediately from the first days of the research, and the number of damaged duckweed was 30 % more on day 24 than in salts NO3 - , NO2 - . For algae of aquatic ecosystems the highest level of toxicity is shown by nitrogen compounds in the form of NO3 - , the median concentration of EC50 (96 hours) is 7.7 mg/L. Therefore, the regulation of pollution of aquatic ecosystems by nitrogen compounds should be based primarily on the content of NO3 - . To avoid the negative effects of compounds such as NH4 + and NO2 - it is necessary to take into account their toxicity level: EC50 (96 hours) NH4 + – 250 mg/L, EC50 (96 hours) NO2 - – 720 mg/L. From the above, the free-floating hydrophyte Lemna minor L. is a promising object of testing, as it easily absorbs all the elements and quickly shows the result. It is able to quickly accumulate harmful substances, because in the process of life, its leaves absorb absolutely all the elements that fall into the water.
aquatic biota, level of toxicity, pollution of aquatic ecosystems