The primary producers in an ocean grazing food web are usually
plants
animals
fungi
phytoplankton

phytoplankton

Explanation:

Phytoplankton can be defined as a set of photosynthesizing microorganisms that live floating on the water surface. It is composed of microscopic algae and cyanobacteria, which can be unicellular, colonial or filamentous. These microorganisms are defined as the primary producers of an ocean grazing food network.

Because phytoplankton live in aquatic environments - both in limic (eg lakes) and marine environments - they have a number of adaptations that guarantee their survival in the water column.  Some of these microorganisms, for example, have flagella that aid locomotion; others, in turn, have gas vacuoles that aid in flotation, while some of them have mucilage, which surrounds the cells and ensures protection, flotation and locomotion.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.