What are these toxins in our water?
Here's how the EPA describes them:
Microcystins are toxins produced by cyanobacteria. Cyanobacteria are also known as blue-green algae and are ubiquitous in surface water when conditions are favorable for growth and formation of algal blooms.
Cyanobacteria release toxins upon cell death or lysis. When released, toxins may persist for weeks to months.  Toxins of most concern are microcystins. Microcystins take their name from the genera Microcystis. Most microcystins are hepatotoxins (liver toxins).  Hepatotoxins are produced by species of the genera Microcystis, Anabena, Nodularia, Oscillatoria among others.  Most microcystins are associated with Microcystis aeruginosa. While the liver is the primary target of microcystins, it is also a skin, eye and throat irritant.