phytoplankton vs plant life

phytoplankton and plant life both are nouns.

Word	Noun	Adjective	Verb	Adverb
phytoplankton	Yes	No	No	No
plant life	Yes	No	No	No

As nouns, plant life is a hypernym of phytoplankton; that is, plant life is a word with a broader meaning than phytoplankton:

phytoplankton: photosynthetic or plant constituent of plankton; mainly unicellular algae
plant life: (botany) a living organism lacking the power of locomotion

Other hypernyms of phytoplankton include flora, plant.

phytoplankton (noun)	plant life (noun)
photosynthetic or plant constituent of plankton; mainly unicellular algae	(botany) a living organism lacking the power of locomotion