The local vigilantes on my beautiful New Hampshire lake have been spotting and reporting cyanobacteria blooms for two years now. Cyanobacteria are naturally occurring bacterial that do at times give off toxins that can cause harm to mammals. Dogs have been known to become sick, although I am unaware of any confirmed cases of human illness. Last summer I got very concerned when some local researchers reported -- in an unpublished paper that nonetheless got much local press-- a statistical correlation between living close to freshwater lakes and onset of ALS, or Lou Gehrig's disease. I don't know where that research now stands, but in my considered opinion it suffered from serious defects. One of these potential defects was the way cases of ALS were reported. I actually got an email from someone on my lake who said that anyone knowing of ALS cases around our lake should report them to the researchers. Hmmmmm....I wonder if a similar email went out to folks who don't live near a lake?
Anyway, the risk from cyanobacteria is incredibly small, especially if one doesn't swim directly in visible blooms (I like to cite a WHO report that said if you are standing in kneedeep water and cannot see your toes, you probably should not go swimming. Hell, even those of us from the UP would figure that one out!)
But finally, the CDC has come out with a report saying that 1 in 8 public swimming pools pose immediate infection risks.
I always told people who asked about cyanobacteria in my beautiful clean Goose Pond: maybe if you are worried you would prefer to go to the pool at Storrs Pond in Hanover and swim in chlorinated water that a bunch of little kid have....well you don't want to know.
Risks are everywhere.