Blue-green alga are able to fix Nitrogen gas. Because of this they often
flourish in situations where N is limiting. Blue-green algae can also
produce nerve toxins (short chain proteins) which can be deadly to animals
and small children. This is, however, uncommon though here on the
Canadian prairies every once in a while a slough becomes toxic during the
short season of liquid water.
Chemistry Textbook: The authoritative tome is by Stumm and Morgan, 1981,
Aquatic Chemistry: An Introduction (ha!) emphasizing chemical equilibria
in natural waters. Wiley and Sons, 780 pp.