Algae blooms when water is warm, or full of pesticides

I saw my first algae blooms in Mexico when my family drove past the Tropic of Capricorn to Guadalajara in 1961. I was fascinated with the ponds, all the same color, an opaque pale green without an inch of reflective water. People lived in jacales, round huts made of tropical leaves and branches. The warm rain poured down in the rainy season.

The Tropic of Capricorn seems to be creeping northward. Now they have algae blooms in ponds in Oregon.

Apparently, pesticides affect the grazers that naturally keep algae under control.