[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-10] Estimating primary production processes in the southern Sea of Okhotsk using modeling, satellite remote sensing, and in-situ observation data.

★Invited Papers

*Koji Suzuki¹, Yuki Tanoue¹, Pat Wongpan², Peter Strutton², Yuri Fukai¹, Takenobu Toyota¹, Jun Nishioka¹ (1.Hokkaido University, 2.University of Tasmania)

Keywords:Marine phytoplankton, Primary productivity, Sea ice, Spring bloom

The Sea of Okhotsk is a marginal sea of the subarctic North Pacific and is the southernmost sea in the Northern Hemisphere with a sizeable seasonal sea ice cover. In spring, ice melting and enhanced river run-off, together with rapid warming of the upper, less saline waters by increased irradiance, create a shallow pycnocline and stimulate phytoplankton photosynthesis and growth. Phytoplankton bloom in the Sea of Okhotsk plays a vital role in its ecosystems and biogeochemical processes. However, understanding of their spatiotemporal variations and controlling environmental factors remains limited. Previous research has hinted at the influence of nitrate availability on the abundance and composition of spring bloom-forming diatoms toward the end of the bloom, yet the evidence is still lacking. This study aimed to integrate field observations, satellite observations, and modeling using ACCESS-OM2 coupled with biogeochemical components to quantitatively analyze the dynamics of primary production processes in the southern Sea of Okhotsk. The recently developed global ocean-ice coupled numerical model ACCESS-OM2, the Australian Community Climate and Earth System Simulator - Ocean Model version 2 specializes in a high-resolution simulation of complex interactions between the ocean and sea ice, particularly in the detailed analysis of the ocean component. ACCESS-OM2 has been implemented locally, such as in the Kuroshio region south of Japan. This study found significant correlations (Pearson’s r > 0.8, p < 0.01) between ACCESS-OM2 outputs and satellite data for sea-ice areas in the southern regions of the Sea of Okhotsk during February 1979–2022. On the other hand, significant discrepancies were observed in phytoplankton abundance or net primary production between ACCESS-OM2 and ocean color remote sensing, suggesting the requirement for model improvement. Satellite-based chlorophyll-a images using SeaWiFS and MODIS/Aqua suggested recent enhancement of spring phytoplankton blooms in the southern Sea of Okhotsk. However, temporal changes in the magnitude of autumn phytoplankton blooms were little observed. To investigate the effects of nutrient enrichment on changes in phytoplankton community composition that can hardly be assessed by modeling and remote sensing, we conducted on-deck bottle incubation experiments in May 2022. As a result, we found that nitrate availability would be crucial to the proliferation of spring diatoms in the southern Sea of Okhotsk. On the other hand, nutrient (nitrate, silicate, and both) enrichment did not significantly affect the diatom community composition during incubation. We conclude that multidirectional approaches using modeling, remote sensing, and in-situ observation data would be indispensable to better understanding the dynamics of primary production processes in the southern Sea of Okhotsk under global environmental changes.