When a big fish eats a small fish, which in turn eats water fleas that are supported by phytoplankton, the water fleas are

At a trophic level, the plants are the producers. They are the first trophic level of a pyramid. Phytoplanktons are microscopic plants that are capable of conducting photosynthesis, and thus are called producers. Water fleas feed upon these producers, and are called primary consumers. These fleas are then eaten by the small and large fish, who form the secondary and tertiary consumers, respectively.