Diatoms are unique forms of algae that grow a silica shell that is preserved in underwater sediments after they die. The diatom shell, called a frustule, is different for each species, so you can identify them through a microscope. By looking at the fossil frustules in the layers of sediment, you can see which kind of diatoms lived when that layer was deposited. If you figure out the date corresponding to each layer of sediment, and you know what water conditions each diatom species prefers, you will know what the water quality was like through history. There are over a hundred thousand species identified.
Diatoms live anywhere there is water, including lakes, streams, estuaries, oceans, and puddles.
That brown scum you see on the rocks in the stream is probably diatoms. Diatomaceous earth is sometimes used in gardening as a pest control. For more diatom info, check out the Diatom Home Page .