Population dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific

► An index for diatom bloom development has been established. ► Bloom-forming diatoms were mainly Thalassiosira and Chaetoceros species. ► Fucoxanthin can serve as a strong indicator of the diatom carbon biomass.► Abundance of heterotrophic bacteria changed little during the spring diatom bloom.► Viral abundance increased toward the end of the spring diatom bloom.