Last year’s fish die-off in the Dyje River near the village of Bulhary in the Břeclav region was most likely caused by a combination of factors. The main one was a lack of oxygen. However, high water temperatures and toxic foam formed by cyanobacteria in the Nové Mlýny reservoir also contributed to the die-off. Behind these causes, however, lies human activity. The reservoir water contains high levels of phosphorus, which has proven difficult to reduce. This was confirmed by Radovan Kopp from the Institute of Zoology, Fisheries and Hydrobiology at the Faculty of AgriSciences of Mendel University.
Due to climate change and an excess of nutrients, cyanobacteria form massive biomasses in stagnant waters. “The concentration of dissolved oxygen thus primarily depends on the intensity of their photosynthesis. At the Nové Mlýny reservoir, nitrogen is the limiting nutrient, which allows the development of filamentous cyanobacteria capable of binding atmospheric nitrogen. This process requires anoxic conditions, and atmospheric nitrogen is fixed in the form of ammonia. Ammonia is toxic to fish, and its toxicity increases with high water temperatures and low oxygen levels,” explains the scientist, describing the main mechanism by which cyanobacteria contribute to fish die-offs. According to Kopp, especially in summer, critical conditions in water quality affecting fish life are becoming increasingly frequent in the early morning hours.
Around 30 tonnes of fish died last year in the stretch of the Dyje River below the Nové Mlýny reservoir. Such a massive die-off occurred because this section is highly nutrient-rich, allowing fish populations to grow in large numbers. “When water with parameters threatening fish life flows out of the Nové Mlýny reservoir, most fish try to escape downstream. This escape is stopped by a weir in Bulhary, where a mass die-off can then occur,” explains Kopp. According to the scientist, however, such an event would not have happened without human activity. People built the reservoir, discharge large amounts of nutrients into it, and human activity is also responsible for climate change. “If we did not release such amounts of nutrients, such as phosphorus, into reservoirs, there would not be such extensive phytoplankton growth, and conditions for fish would not reach critical levels,” he adds.
According to Kopp, phosphorus pollution is a key factor. “If we do not start addressing this problem, we cannot realistically improve unsuitable water conditions in the long term. Paradoxically, the biggest polluters are wastewater treatment plants and combined sewer overflows—mixtures of wastewater and rainwater that are discharged into receiving waters without treatment. Other sources include runoff, baiting, and fish feeding,” the scientist lists. Improving the situation would require significantly stricter limits on phosphorus discharge and its removal even at smaller wastewater treatment plants. However, such limits have not yet been adjusted.
As short-term measures, solutions such as installing a floating barrier with a curtain in front of the reservoir outlet could help. “The purpose of this barrier is to reduce the amount of cyanobacteria entering the Dyje River. Flushing the river could also help, but this requires sufficient water in the reservoir,” Kopp adds. Despite these short-term measures, the scientist expresses concern. If unfavorable conditions coincide again in the lower Nové Mlýny reservoir, another fish die-off will occur. Without a significant reduction in nutrient input, water quality cannot be improved in the long term.
Contact for further information:
Prof. Ing. Radovan Kopp, Ph.D.
+420 777 098 790
radovan.kopp@mendelu.cz
Institute of Zoology, Fisheries and Hydrobiology (Faculty of AgriSciences)