This is really very interesting, especially since we have ponds that I maintain. While "blooms" were always unsighly for the ponds I never realized they had these potential health concerns.
I googled the term BarryG referred to, cyanobacteria, and there are some interesting articles about this. It appears to be a more immediate reaction (vomitting, skin irritation etc.) concerns, but there could be concerns for long term exposure.
Here is link to a CDC article about it;
http://www.cdc.gov/hab/cyanobacteria/facts.htm
And here is a link to a Health Canada article;
http://www.hc-sc.gc.ca/ewh-semt/pubs...bacter-eng.php
The links may not show up till after Al or other moderators can check them.