Impact of vertical stratification on the 2020 spring bloom in the Yellow Sea.

The Yellow Sea is one of the world's most abundant marine resources, providing food and economic benefits to the Korean and Chinese populations. In spring 2020, a decrease in the intensity of phytoplankton bloom was observed. While one study attributed this decline to a decrease in nutrient associated with the COVID-19 pandemic, our previous research proposed weakened thermal stratification accompanied by a surface cooling anomaly as the cause. However, the relationship between the marine environment and ecosystem has not been fully elucidated. Using observations and marine physical-biogeochemical model data, we identified the weakened stratification as a critical factor for suppressing the 2020 spring bloom. Intense vertical mixing hindered the accumulation of nutrient and chlorophyll-a concentrations within the euphotic zone, resulting in a diminished phytoplankton bloom. In contrast, reduced nitrate and phosphate concentrations in 2020 were insignificant compared to those in 2017-2019, despite the notable decline in PM2.5 in March 2020 due to COVID-19. In April 2020, nutrient levels fell within the range of interannual variability based on long-term observations, reflecting a negligible effect on the spring phytoplankton bloom. Our findings provide insight into the importance of marine physical factors on the phytoplankton biomass in the Yellow Sea.

A regional-scale approach for modeling primary production and biogenic silica export in the Southern Ocean.

Persistent uncertainties in the representations of net primary production (NPP) and silicate in the Southern Ocean have been noted in recent assessments ofthe ocean biogeochemical components of Earth system models (ESMs). Consequently, more mechanistic studies at the regional scale are required. To reduce these uncertainties, we applied a one-dimensional (1D) marine ecosystem model to different bioregions in the Southern Ocean: the Polar Frontal Zone in the Pacific sector, the seasonal sea ice zone in the northwestern Ross Sea, and the inner shelf of Terra Nova Bay. To make the existing ecosystem model applicable to the Southern Ocean, we modified the phytoplankton physiology (stoichiometry depending on species) and the silicate cycle (dissolution rate of biogenic silica (BSi) depending on latitude) in the model. We quantified and compared seasonal variations in several limitation factors of NPP, namely, iron, irradiance, silicate and temperature, in the three regions. The simulation results showed that dissolved iron plays the most significant role in determining the magnitude of NPP and the phytoplankton community structure during summer. Additionally, the modified model successfully reproduced the vertical flux of BSi and particulate organic carbon (POC). The POC export efficiency was high in the inner shelf zone, which had high levels of iron concentration, NPP, and Phaeocystis biomass. In contrast, BSi export occurred most efficiently in the Polar Frontal Zone, where diatoms are dominant, the BSi dissolution rate is low, and NPP is extremely low. Our results from the integrated mechanistic framework at the regional scale demonstrate which specific processes should be urgently included in ESMs for better representation of the biogeochemical dynamics in the Southern Ocean.