Why Are 96,000,000 Black Balls on This Reservoir?

I've seen a video by another youtuber about it. That is one odd place.

 

If I recall, it's to cut down on evaporation.

 

One of the reasons, that reduced such effect by 80 - 90 %, and half of the balls price ($12M), pays for it self, due that, yet that wasn't the main reason.

The main reason was, to reduce some chemical reaction, which becomes active once the sun heats the water.