Just curious. Did you send in a sample to a lab to find this out?
Again, i am so happy to see some information on this topic. I know this has been a very common issue for people around my area for the last few years.
This was 3 or 4 years ago. It was sent to a lab. At that time I was living in Western MA and there was an infestation of Zebra Mussels that made their way into the Quabbin. For some reason, Zebra Mussels bring Cyanobacteria with them. Something to do with the Nitrogen to Phosphorus levels. Whatever strain of Cyanobacteria that I had was resistant to Chlorine.