It is almost certain that a substantial toxic algal bloom, supported by human activity, caused the ecological destruction
Around 360 tonnes of fish in the Oder River were impacted by toxic algal bloom in 2022. Photo shared by Virginijus Sinkevičius / @EU_Commission Commissioner for Environment, Oceans and Fisheries / Twitter_ Last summer, the banks of the River Oder in Poland was a picture of doomsday — hundreds of dead fish washed up every day, making headlines in the regional media.
From mid-July through mid-September, over 360 tonnes of fish would have died, along with many other aquatic creatures – a clear sign that the ecosystem of the river has been pushed beyond its tipping point. The rot spread over 500 kilometres along the river.
A new report by the European Union (EU) investigating the incident has confirmed initial concerns — the destruction was caused by toxic algal bloom made possible by anthropogenic factors.
The report published February 16, 2023 noted:
It is almost certain that a substantial toxic algal bloom caused their deaths. The causal species: Prymnesium parvum, is adapted to brackish salinities. A key factor enabling the proliferation of this species was the high salinity of the river during this time, probably in part resulting from discharges of saline industrial wastewater e.g. from mining.
Other contributing factors were the drought and resulting low water levels reducing dilution and flow and also hydromorphological modifications to the river. High nutrient concentrations, especially phosphorus and nitrogen, are also key in promoting such blooms.
The river is one of Europe’s 20 large rivers and is valuable to the livelihoods of and recreation of the 16 million people residing in its catchment area and beyond, the authors of the report noted. “It also serves as a focus for nature preservation with numerous Natura 2000 sites located
along its length.”
It originates in the Czech Republic, flows through western Poland, forms the border between Poland and Germany, then drains north to the Szczecin Lagoon near Szczecin.
The West Pomeranian region was among those that recorded the largest fish kills, mostly occurring over a 12 day period in mid-August. Apart from fish, aquatic organisms like freshwater bivalves and other molluscs alongside the mortality of birds, ducks, beavers and other
Wildlife were also impacted, the EU wrote in the latest report.
Affected areas in the Oder river
Source: An EU analysis of the ecological disaster in the Oder River of 2022
The massive algal blooms of the toxic brackish-water algae Prymnesium parvum would not have been possible under natural conditions, the analysts noted.
Laboratory tests of the waters of the Oder River, the Gliwice Canal and reservoirs directly adjacent and connected with the Oder River showed high concentrations of Prymnesium parvum, exceeding the level of 50-100 million cells per litre, at which, according to the literature, fish deaths may already be recorded.
The surroundings of the Gliwice Canal that runs through many industrial areas, carrying effluents, was one of the first sites from where fish death was reported last year. Experts speculated that extremely saline discharge from any of these industries may have caused an atmosphere for the toxic algae to flourish, causing the calamity downstream.
“The disaster also directly and indirectly impacted nature protection areas /protected habitats and their protected species alongside the Oder River e.g. ‘Stettiner Haff’, the Natura 2000 area "Dolna Odra /Unteres Odertal”, and many others,” the authors observed in the report.
Online monitoring of these river stretches should be improved and communication of pollution events across international river basin districts made mandatory to prevent such events in the future, the scientists said.
It may also be necessary to review and implement dynamic control of all licenced discharges and review the role of hydromorphological modifications in slowing the flow allowing time for blooms to develop, they added. “A complete investigation of discharges in the catchment should be carried out to explain the increase in salt load that played a key role in bloom development.”
The presence and spread of this invasive and toxic algal species will most likely continue, according to the analysts. “Therefore, management strategies to prevent its proliferation must now be prioritised in this catchment but also in all other susceptible European river basin districts,” they added.
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