Is phytoplankton a herbivore?

Plankton is not a herbivore, nor any type of consumer. Plankton is a photosynthetic autotroph; an autotroph being an organism that synthesizes its required organic compounds either through photosynthesis or inorganic chemosynthesis. Other autotrophs include plants, algae and some bacteria.

Conversely, consumers, such as herbivores, carnivores, omnivores and detrivores, are known collectively as heterotrophs, meaning that they must derive their required compounds from the consumption of other preexisting organic compounds.