Protists that play an important role in aquatic food webs are called

Plant like Protists - also called algae - autotrophs

Fungus like Protists - heterotrophs, decomposers, external digestion

From the above their role in the aquatic food chain is clear.

They perform their role as

1) producers = example = Plant like Protists - also called algae - autotrophs

2) consumers = example = Animal like Protists - also called protozoa (means "first animal") - heterotrophs

and

3) Decomosers = example = = Fungus like Protists - heterotrophs, decomposers, external digestion