In the marine ecosystem, one of the primary consumers of phytoplankton are tiny organisms known as copepods. These small crustaceans graze on phytoplankton, playing a crucial role in the food web.
As for decomposers, certain types of bacteria and fungi can also decompose organic material produced by phytoplankton. When phytoplankton die or when they are consumed, their organic matter is broken down by these microorganisms, returning essential nutrients to the environment.
So, to summarize: copepods primarily eat phytoplankton, while bacteria and fungi serve as decomposers that break down the organic matter in the ecosystem.