Changing occurrence of fall phytoplankton bloom and its impact on phytoplankton size structure in the Pacific Arctic

Phytoplankton bloom in the Pacific Arctic has been characterized as a huge single bloom in spring. However, several studies have reported recent increases in occurrence of a relatively small but evident fall bloom. Here we explored spatiotemporal variations in occurrences of fall boom based on satellite remote sensing for 2003–2017. Seasonal time series variation in satellite-derived chlorophyll-a for individual years was modeled using a Gaussian function to distinguish whether evident fall chlorophyll-a peak was present, and we recognized an occurrence of fall bloom if the model detected the presence of fall chlorophyll-a peak. In addition, the phytoplankton size structure was also estimated from satellite to investigate influence of fall bloom on the seasonal variations in the phytoplankton size structure. Our findings suggested that fall bloom occurred in wide area of the Pacific Arctic, and the size structure of phytoplankton was apparently larger during fall bloom than before/after and absence of fall blooms. Examining interannual variations in occurrence of fall bloom, there were clear increasing and decreasing trends in the southern Chukchi Sea and St. Lawrence Island polynya region, respectively. Since the phytoplankton cell size largely determines the sinking rate of phytoplankton, temporal trends in occurrence of fall bloom modulating seasonal variations in phytoplankton size structure could significantly influence on marine ecosystems. These results suggest that the spatiotemporal monitoring of phytoplankton community not only in spring but also after the period could improve our understanding about marine ecosystem variations in the Pacific Arctic.