Explanation:

- Phytoplankton are microscopic organisms found in aquatic environments.

- Primary producers - Phytoplankton belong to this group as they convert sunlight into energy through photosynthesis, forming the base of the food web.

- Primary consumers - These are organisms that eat primary producers; typically herbivores.

- Secondary consumers - These eat primary consumers; they are usually carnivores.

- Decomposers - These break down dead and decaying organisms for energy, such as bacteria and fungi.