Remote Sensing of the Seasonal and Interannual Variability of Surface Chlorophyll-a Concentration in the Northwest Pacific over the Past 23 Years (1997-2020)

Phytoplankton in the northwest Pacific plays an important role in absorbing atmospheric CO2 and promoting the ocean carbon cycle. However, our knowledge on the long-term interannual variabilities of the phytoplankton biomass in this region is quite limited. In this study, based on the Chlorophyll-a concentration (Chl-a) time series observed from ocean color satellites of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) in the period of 1997-2020, we investigated the variabilities of Chl-a on both seasonal and interannual scales, as well as the long-term trends. The phytoplankton Chl-a showed large spatial dynamics with a general decreasing pattern poleward. The seasonal phytoplankton blooms dominated the seasonal characteristics of Chl-a, with spring and fall blooms identified in subpolar waters and single spring blooms in subtropical seas. On interannual scales, we found a Chl-a increasing belt in the subpolar oceans from the marginal sea toward the northeast open ocean waters, with positive trends (similar to 0.02 mg m(-3) yr(-1), on average) in Chl-a at significant levels (p < 0.05). In the subtropical gyre, Chl-a showed slight but significant negative trends (i.e., <-0.0006 mg m(-3) yr(-1), at p < 0.05). The negative Chl-a trends in the subtropical waters tended to be driven by the surface warming, which could inhibit nutrient supplies from the subsurface and thus limit phytoplankton growth. For the subpolar waters, although the surface warming also prevailed over the study period, the in situ surface nitrate reservoir somehow showed significant increases in the targeted spots, indicating potential external nitrate supplies into the surface layer. We did not find significant connections between the Chl-a interannual variabilities and the climate indices in the study area. Environmental data with finer spatial and temporal resolutions will further constrain the findings.

Automated summary

Phytoplankton in the northwest Pacific plays a crucial role in absorbing atmospheric CO2 and driving the ocean carbon cycle. This study investigated the long-term interannual variations of phytoplankton biomass in this region using satellite data from 1997 to 2020. The results showed significant spatial and temporal dynamics of phytoplankton biomass with decreasing patterns poleward. Seasonal blooms were dominant, with spring and fall blooms in subpolar waters and only a spring bloom in subtropical seas. Interannually, an increasing trend of phytoplankton biomass was observed in subpolar oceans, while a slight but significant negative trend was found in the subtropical gyre. The negative trend in subtropical waters was driven by surface warming and limited nutrient supply, while in subpolar waters, there were potential external nutrient supplies. No significant connections were found between phytoplankton biomass variations and climate indices in the study area.