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