By Reich, A Blackmore, C; Hopkins, R; Lazensky, R; Geib, K; Ngo- Seidel, E A "red tide" is a harmful algal bloom that occurs when toxic, microscopic algae in seawater proliferate to a higher-thannormal concentration (i.e., bloom), often discoloring the water red, brown, green, or yellow.

You can find them in almost any lake or river, scientists say. Just don't expect to see them - not until the tiny cells of the algae split a thousand, 10 thousand, a million times - growing into a bright-green blanket that covers the water like a coat of slick paint.