Mysterious mass fish kill in Oder River: Climate change or poison?
dw.com
dw.com
Daily cycles on this graph [1] and oxygen surplus can be explained by hyperactivity of organisms that do photosynthesis - cyanobacteries and algae. Something boosted their activity. It might be natural, caused by high temperature very low level of water. Might be because someone contaminated the river with something that worked as fertilizer.
[1] https://nitter.it/pic/orig/enc/bWVkaWEvRmFDNl9EeVgwQUFRNmpRL...
Either you have mercury or you haven't. Blue algae are ephemeral and don't store mercury, If I'm not wrong (I could be wrong)
Wildfires can release mercury, stirring the bottom could (in theory) release mercury and environmental terrorists (from Poland, Germany or other countries) can trow poison in the water, of course. We can see a sudden global pattern of environmental crimes all around Europe since the last months (and don't tell me that is climate change, most wildfires were deliberate and systematic).
A start would be identifying the organisms killed, the level of poisons found in the last years and the type of poison(s) found in their corpses.
You seem to be missing one potential candidate state already responsible for some huge environmental disasters this year.
The presence of salts is particularly interesting in its three possible ways to end there. Either:
1) Salt was dumped directly in the river by criminals or ...
2) Salt was a by-product of the toxic interacting with the river (Who has access to a lot o mercury salts? We have chemistry experts in HN, maybe some of them could enlight us about some possible chemical paths that release mercury and salt?) or ...
3) Salt (Iodine??) was a second dump aiming to fix the disaster and neutralize the poison after the fishes were killed. Maybe by the government, maybe by the perpetrators.
Many fishes can stand salt for hours or days using their liver to detox, time enough to quit the area. We used salt and freshwater baths routinely to treat our ill freshwater and marine fishes. If is progressive they survive. Carps can stand low quality waters.
The parasites could provide an answer. I would suggest to take a look to them.
In the past, the Soviet Union controlled this area, and they are well known for the massive ecological disasters they created.
It wouldn't surprise me at all if this turned out to be the case here.