@futurebird
some plankton are tiny arthropods. But there are many other kinds of plankton not closely related to arthropods (or to each other), diatoms, radiolarians, algae, cyanobacteria, so many more I don't know anything about.
I don't know if anyone has done any equivalent to the "deathfog a dozen different species of trees, see how many new insects fall out, use that to estimate total insect diversity" experiments for plankton.