nannoplankton
noun
nan·no·plank·ton ˌna-nō-ˈplaŋ(k)-tən 
-ˌtän
: the smallest plankton that consists of those organisms (such as bacteria) passing through nets of very fine mesh silk cloth
Word History
Etymology
New Latin, from Greek nanos, nannos dwarf + New Latin plankton plankton
First Known Use
1912, in the meaning defined above