Prishlyak S. Radionuclide contamination of higher aquatic plants and the role of helophytes in the migration of 137Cs in freshwater bodies. - Manuscript.

Thesis for obtaining the Doctor of Philosophy degree (PhD)in Biological Sciences, specialty 03.00.17 "Hydrobiology".- Institute of Hydrobiology of the National Academy of Sciences of Ukraine, Kyiv, 2019. The thesis is devoted to the determination of formation peculiarities of radionuclide contamination of higher aquatic plants in water bodies outside the exclusion zone and the explanation of helophytes role in the 137Cs migration in water bodies of various trophic status. During 2010-2017, the maximum values of specific activity of 137Cs in the plants of studied water bodies exceeded the pre-accident levels in 8-3000 times and reached 11000 Bq/kg, while 90Sr - in the plants of some water bodies and reached 80 Bq/kg. It has been shown that in water bodies located outside the exclusion zone, 137Cs forms the radioactivity of higher aquatic plants. Its contribution to the total activity of plants of different species ranged from 52 to 99 %. Analysis of the regularities of radionuclide pollution formation of the aboveground and underground organs of helophytes from water bodies of various types and trophic status showed that high specific activity of 137Cs in soil roots causes the predominant accumulation of radionuclide by underground organs. The vital activity of helophytes contributes to the redistribution of gross contents and exchange and non-exchange forms of 137Cs in the bottom sediments. Annually, dying off the helophytes causes biotransformation of 0,3-1,1 % of the total amount of 137Cs in the upper layer of bottom sediments. The magnitude of the 137Cs radiation dose absorbed by helophytes is 78-95% due to the irradiation of underground organs.