Sustainment of microplankton biomass peak after spring phytoplankton bloom in the Kuroshio and adjacent region

Matching spring phytoplankton blooms is the key to recruiting aquatic living organisms, even though they do not always spawn during that season. This study investigated how the energy assimilated during the spring phytoplankton bloom sustains a microplankton biomass peak in an area adjacent to the Kuroshio Current, south of Japan. Microscopic observations of microplankton (≥5 µm) for 17 yr in the area adjacent to the Kuroshio showed that carbon biomass peaked during the post-bloom period. The carbon biomass of autotrophic microplankton peaked during the bloom period in both coastal (mean with 95% CIs: 8.59 with 6.72-11.0 µg C l -1 ) and offshore areas (3.49 with 2.72-4.49 µg C l -1 ) as well as the in situ chl a concentration. In the spring bloom period, centric diatom biomass was ~5 µg C l -1 in the coastal areas but ~1.0 µg C l -1 in the offshore areas. Still, it was lower than the biomass of heterotrophic microplankton. The contribution of dinoflagellates increased from the bloom to the post-bloom period, and the total microplankton biomass during the post-bloom period (coastal: 198 with 124-316 µg C l -1 , offshore: 10.5 with 7.44-14.8 µg C l -1 ) was as high as during the bloom period (coastal: 62.3 with 38.2-101 µg C l -1 , offshore: 6.81 with 4.41-10.5 µg C l -1 ). Thus, we considered that heterotrophic dinoflagellates, whose biomass cannot be evaluated using plant pigments, take over the spring phytoplankton bloom energy and deliver it to post-bloom biological production.