Who knew that Cesare Emiliani was a practitioner of linguistic prescription? Emiliani had a journal article titled Nomenclature & Grammar [Journal of the Washington Academy of Science, 1952] where he called for the "proper usage of Greek & Latin in taxonomic pursuits". In 1991, he published another paper entitled Planktic/Planktonic, Nektic/Nektonic, Benthic/Benthonic [Journal of Paleontology, 1991]. To put things in context:

Planktic/Planktonic: 'to drift'; used as a prefix for organisms (plankton) that float around (in the upper column of the ocean) and drift due to oceanic currents.
Nektic/Nektonic: 'to swim'; used to describe creatures that can propel themselves so as to swim voluntarily.
Benthic/Benthonic: 'deep'; an adjective for organisms that live in the deep ocean.

In the paper, he details the Greek origin of the words and states that the proper derivatives of the words always end with ic as opposed to onic (ie planktic is correct usage & planktonic is not). Personally, I have seen more papers that adopt the term planktonic rather than planktic (eg. planktonic foraminifera). Interestingly enough, through a customary spell-check, the word planktic is displayed as an error while planktonic is not.

In any case, both are currently acceptable in the scientific community. However, I try to adhere to the recommended terms - not necessarily "in deference to the Greeks, who did so much to get the Western civilization going", as Emiliani puts it.

It is also interesting to note that Emiliani's contemporary, Nicholas Shackleton (another paleoceanography great) is the only one I could find who has used the less popular benthonic as opposed to benthic (with respect to foraminifera). Hmmm.... a friendly rivalry in literature perhaps? Those were different times...