This is bizarre! It looks as if someone threw blue paint into the lake...
It smells worse ... like some out-of-date old and wet paint, but many orders of magnitude worse.
The algae, or technically speaking, cyanobacteria, had of course been there all the time, but the tiny organisms were evenly distributed in the water column and thus not directly visible. Days of extreme heat (to us Nordic people, at least), in conjunction with persistent strong winds and waves brought on a mass bloom and concentrated the cyanobacteria along the shoreline. I've seen algal blooms many times in my days as an aquatic scientist, but never amassed like this. It was spectacular. The "painted" zone stretched up to 50 m wide.