Rio’s Olympic rowing lake filled with 65 tonnes of dead fish

This is a main story that was recently made on Yahoo.
This seems to be happening a hell of a lot recently, I don't quite buy the deoxygenated water although I don't write it off either.
Anyone else have any theories?

A huge clean-up has been launched in the Rodrigo de Freitas Lagoon after 65 tonnes of dead fish filled the waters which are set to play host to the 2016 Olympic rowing events. The lagoon, authorities say, became deoxygenated after heavy storms, which washed ‘a large amount of organic matter’ into the lake. The clean-up is still underway, and is not the first time that the lagoon has suffered such misfortune. In 2009 an estimated 100 tonnes of dead fish, predominantly shad, had to be removed from the waters after a similar incident. As you can see from the picture above, taken during the Pan-Am Games in 2007, the lagoon hosted rowing successfully.

Link

This reminds me of the dead pigs found in china and the gov gave the OK to continue drinking the water, or so I saw on a thread yesterday.
Since it's a rowing lake I suppose no one depends on this for drinking water but it's still really sad to so many animals dying in these mass amounts.

Strange times are among us ..

Actually de-oxygenated water can cause this very quickly. If organic material is allowed to produce oxygen consuming algae in ponds and lakes it will cause deaths of any and all life within the lake. My father-in-law works on treatments for this exact issue on fisheries and farms by taking out the phosphates and nitrates with pellets and allowing the water to become naturally oxygenated again. Organic material itself, in large quantities, uses enough oxygen to do this as well. If enough was dumped then it takes oxygen to break down.