I am deeply honored to have been awarded the 1992 Okada Prize of the Oceanographic Society of Japan. The present paper reviews my previous works regarding the biological processes in the lower trophic levels in the subarctic waters of the North Pacific Ocean. The outline can be summarized as follows: 1) Regional and seasonal variations in the size composition of phytoplankton communities were studied in the western North Pacific Ocean and the neritic water of Funka Bay, Hokkaido, respectively. Phytoplankton community in the stratified water column was characterized by the < 2μm size fraction of which dominant taxon was coccoid cyanobacterium. Spring phytoplankton bloom was evidently occurred in the subarctic water. During the bloom, phytoplankton community was predominated by diatoms of larger than 10μm in size. More than 50% of annual primary production occurred during the spring bloom period. 2) Food requirements of large copepods, small copepods, and microzooplankton were estimated following the Ikeda-Motoda's method. During summer period, the food requirement of the latter two groups was almost same as the daily primary production in Funka Bay. Sinking loss of intact phytoplankton was less than 1% of the primary production during summer. These results suggested that most of the primary production may be grazed by zooplankton. On the other hand, the total food requirement and sinking phytoplankton carbon were estimated to be 3% and 2% of primary production during spring bloom period, respectively. If zooplankton grazes on phytoplankton only to meet their food requirement, large portion of the fate of primary production connot be identified. 3) Alternative estimation of grazing impact based on Huntley-Boyd's method well explained the phytoplankton carbon budget during spring bloom. Most of the total grazing impact on the spring bloom was due to microheterotrophs (microzooplankton and phagotrophic dinoflagellates). The organic carbon as the microheterotrophs' body tissue will be efficiently transported into large zooplankton. It is thought that impact of zooplankton grazing may control phytoplankton biomass through out a year. Moreover preliminary results suggested that grazing impact of zooplankton community altered size composition of phytoplankton community.