The abundance of phytoplankton and its size groups is analyzed in three regions of the Pacific Ocean: Subarctic, Northern Subtropical, and Eastern Equatorial. Both our own and published data were used. Picoplankton dominates all three regions. The role of nanoplankton is especially high in biomass; however, sometimes the contribution of microplankton to phytoplankton biomass is significant. Both
... [Show full abstract] the Northern Subtropical Region and the Eastern Equatorial Region are more productive than was previously realized. The system of the Northern Subtropical Region is the most stable; there, summer is long and winter is short, and the factors of the environment vary insignificantly. In the Subarctic Region the abundance and species composition of phytoplankton vary with season; however, these seasonal variations are evident near the shores and less pronounced in the open part of the ocean. The system of the Eastern Equatorial Region is the most variable. The reasons for phytoplankton variability in this region are not yet clear.