by Than Kyaw — When lim­it­ing nu­tri­ents like ni­trates and phos­phates are in­tro­duced into a body of wa­ter, mi­cro­scopic al­gae mul­ti­ply so much that the wa­ter changes color. For ex­am­ple, in re­sponse to fer­til­izer runoff, a 2011 cyanobac­te­ria bloom changed the clear wa­ters of Lake Erie into a dark-green soup and forced the nearby city of Toledo into a state of wa­ter cri­sis.