1) Nannoplankton 
nanoplankton, nanoplanktonic 1. Minute (very tiny) planktonic organisms that formerly passed through a 0.03-0.04 mm mesh silk bolting cloth; very small forms of plankton. 2. Plankton of minute size, especially plankton composed of orgnisms measuring from two to twenty micrometers.
Planktonic organisms that are 2-20 micometers in size. 
nannoplankton.