During the environmental disaster in the Oder in the first three weeks of August 2022, an estimated 1000 tonnes of fish died along a stretch of more than 300 km of the Oder, as well as a significant proportion of mussels, water snails and presumably other invertebrates. Some of the fish species affected, such as the Atlantic sturgeon and the Baltic golden stone loach, only occur in the Oder, while Baltic houting, zope and burbot have their main distribution area here in Germany. According to the findings of the German expert group, the massive mortality of aquatic organisms was caused by a combination of different factors. Massive discharges of common salt, together with the very low water flow of the Oder, its high nutrient load and a prolonged period of sunshine, enabled a mass proliferation of the toxin-releasing brackish water alga Prymnesium parvum in one or more upper parts of the Oder system, which then moved downstream, presumably continuing to grow (https://de.wikipedia.org/wiki/Umweltkatastrophe_in_der_Oder_2022)

Such increased salt concentrations have already been measured in the Oder in the past, so that a repetition cannot be ruled out if high salt loads continue to be discharged into the Oder or are discharged again at low water. Furthermore, the occurrence of these algae in other saline waters cannot be ruled out. For the survival of some of the fish populations in such toxin waves, side waters of the floodplains are of considerable importance, provided they are still connected to the main river course. In addition, the dynamic types of floodplain waters are characterised by a particularly high level of biodiversity. However, the lateral connectivity required for the refugial function, habitat function and other ecosystem services of floodplains is being greatly reduced in the Oder as groyne construction on the Polish bank progresses.