Abstract <p>The causes of the disturbance of the radioactive equilibrium between long-lived parent ⁹⁰Sr and short-lived daughter ⁹⁰Y in water and aquatic microbiota of V-2 natural reservoir (Lake Kysyl-Tash), serving as a nuclear facility and a liquid radioactive waste repository, are discussed, and the extent of this disturbance is estimated. A sample of green foam taken from the reservoir surface, containing various phytoplankton species, was used as microbiota. Radiometric measurements with variation of the exposure time and processing of the decay/accumulation curves were performed to determine the ⁹⁰Sr and ⁹⁰Y activities in water and in the microbiota filtered off using the blue band paper filter. By the moment of sampling, the ⁹⁰Y/⁹⁰Sr specific activity ratio was ~0.5 in water and ~50 in microbiota (the contribution of the ⁹⁰Y to the total activity of β-emitting nuclides was ~98%), with the ⁹⁰Sr and ¹³⁷Cs activities making up ~1.5 and ~0.5%, respectively. The specific activity of the most finely dispersed fraction of the microbiota, obtained after filtration, reaches 31 kBq/g, exceeding by a factor of ~20000 the specific activity of ⁹⁰Sr in V-2 water. The results obtained should be taken into account when estimating the dose to microbiota.</p>