This looks Euglypha acanthophora (Ehrenberg, 1841) Perty (Rhizopoda, Testacea). But should see the mouth, idiosomes etc., need preparation. Spines can be from 3 till 9.
for representatives from subsystems like plankton better to nominate Protists that belongs to like protistoplankton-benthos, -periphyton... Zoo belongs to animals. This. E. may be raised from benthic or per. community as had been underlined by Willem.
As well I'm attaching a key for Rhisopoda, Testacea for alll who are interested. This PDF I remake from bad JPG copy.