Prymnesium parvum is a microscopically small alga. Despite its small size, it can kill large organisms such as adult sturgeons. © Katrin Preuß/IGB
The algal bloom of Prymnesium parvum in the summer of 2022, favoured by increased salt input, had devastating consequences for the ecosystem of the Oder River: up to 1,000 tons of fish alone died – including more than 20,000 juvenile Baltic sturgeon, which are to be reintroduced into the Oder as part of a reintroduction programme. A large number of other gill-breathing organisms such as certain mussels, crabs and snails also did not survive the disaster. The impact of the catastrophe on other organisms, especially smaller ones such as zooplankton, is still unclear.
The scientists in subproject 7 "Prymnesium toxicity" are investigating the dose-dependent effects of Prymnesium parvum and its toxins on early life stages of fish (eggs and larvae), mussels and zooplankton. To date, little is known about the effects of Prymnesium parvum toxin on groups of organisms such as river mussels and smaller zooplankton, which are part of the diet of fish. However, it has been proven that Prymnesium parvum can impair the reproduction of zooplankton even at low densities.
With their study, the scientists in sub-project 7 want to investigate whether a permanent colonisation of the Oder with Prymnesium poses a threat to the restoration of the ecosystem. To this end, they are conducting experimental studies with various organisms (fish, mussels and zooplankton such as rotifers), some in different life stages (embryos and larvae of fish, juvenile fish). Among other things, they want to clarify the following questions:
- How sensitive are different types of organisms and their life stages to the alga and its toxin?
- Is the alga, which in addition to photosynthesis can also feed on organic material, itself responsible for the mortality of the other organisms or is its poison responsible?
- Can mussels serve as an "early warning system" for Prymnesium blooms in natural river systems?
- How and when do zooplankton react to gradual increases in Prymnesium density and salinity in the water?
It is to be expected that threshold values for the impairment of species communities by toxic algal blooms can be derived from the investigations, which are highly relevant for water management.
