phytoplankton 5-60 microns long.

Plankton smaller than 10 microns, which pass through an ordinary plankton net but can be removed from the water by centrifuging samples

Generally any plankton, usually plants, smaller than 80 µm; many important nannoplankton are only 0.2-2.0 µm long. From the Greek, nanno ( nanno), "dwarf, very small."