A coral reef ecosystem might include all of the following organisms: phytoplankton, clams, octopus, moray eel and barracuda. The following shows a basic food chain of these organisms:

Basic food chain

In this food chain, which organism would be the primary producer?

Phytoplankton are primary producers and are the foundation of the aquatic food web. They feed everything from microscopic, animal-like zooplankton to multi-ton whales. Small fish and invertebrates also graze on the plant-like organisms, and then those smaller animals are eaten by bigger ones.