An aquatic food chain begins with phytoplankton like algae which are producers, these phytoplankton are eaten by zooplankton like protozoan (primary consumer). The primary consumers are eaten by secondary consumers which are small fishes in an aquatic food chain. These secondary consumers (fishes) are eaten by large fishes which are tertiary consumers. The tertiary consumers are further eaten by quaternary consumers, usually whale in an aquatic food chain. (Secondary, tertiary, and quaternary consumers are also known as carnivores).