Masowa śmiertelność małży w zbiorniku Rusałka
The mass mortality of mussels in the Rusałka reservoir

Summary/Abstract: The phenomenon of mass death of mussels in the Rusałka reservoir in Poznań took place in July 2018. As a result of this incident, the Unionidae population has been depleted. The number of dead mussels was counted, which floated around the banks and in the water. A total of approximately 5,500 deaths were found. Due to the scale of the phenomenon, the analysis of water quality was carried out by the Voivodeship Inspectorate for Environmental Protection in Poznań. Four samples of water were taken from different depths of the reservoir to perform physico-chemical and biological analyses. Two profiles, oxygen and temperature, were made in the deepest place of the Rusałka reservoir. There was a presence of hydrogen sulphide at the 5-meter depth of the reservoir. Additionally, oxygen concentration decreased strongly with depth to total deoxidation (< 0.50 mg•l-1O2 from 5 meters depth to the bottom). Furthermore, the average air temperatures in May 2018, in Poznań, were more than 3°C higher than in 2017 and 2016. In June and July, temperatures increased on average by 1 and 2°C compared to the previous. It was shown that the mass death of mussels was probably the result of the interaction of high temperature, a lack of oxygen, and the presence of hydrogen sulphide. It is not possible to clearly indicate only one factor that led to this phenomenon.

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