The mass mortality of mussels in the Rusałka reservoir

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.

Keywords: mussels, Rusałka reservoir, Unionidae

Zarys treści: Zjawisko masowej śmierci małży w poznańskim zbiorniku Rusałka miało miejsce w lipcu 2018 r. W wyniku tego zdarzenia populacja małży Unionidae została uszczuplona. W ramach badań terenowych wykonano liczenie małży, które znajdowały się przy brzegach oraz pływały, unosząc się w toni wodnej. Stwierdzono łącznie około 5500 śniętych sztuk. Ze względu na skalę zjawiska Wojewódzki Inspektorat Ochrony Środowiska w Poznaniu wykonał kompleksowe analizy jakości wody. Pobrano cztery próbki wody z różnych głębokości zbiornika w celu przeprowadzenia analiz fizyczno-chemicznych i biologicznych. Wykonano również dwa profile – tlenowy i temperaturowy w najgłębszym miejscu zbiornika. Wykazano m.in obecność siarkowodoru na 5 m głębokości w zbiorniku. Ponadto stężenie tlenu ulegało obniżeniu w miarę wzrostu głębokości do poziomu bliskiego całkowitemu odtlenieniu (< 0,50 mg·l-1O2) od 5 m do dna. Dodatkowo średnie temperatury powietrza w Poznaniu w maju 2018 r. były wyższe o ponad 3°C w porównaniu do lat 2016 i 2017. W czerwcu i lipcu 2018 r. średnie temperatury powietrza wzrosły średnio o 1 i 2°C. Wykazano, że masowa śmierć małży była prawdopodobnie wynikiem wzajemnego oddziaływania wysokiej temperatury, braku tlenu oraz obecności siarkowodoru. Nie można jednoznacznie wskazać wyłącznie jednego czynnika, który doprowadził do tego zjawiska.

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.