Variations in Phytoplankton Primary Production Driven by the Pacific Decadal Oscillation Based on the Ocean Color Data in the East/Japan Sea, South Korea
The Pacific Decadal Oscillation (PDO) is a leading mode of sea surface temperature (SST) anomalies in the north Pacific. The PDO regime is a major factor not only for the physical properties of the ocean but also for fishery and water resources. However, only a few studies have examined the impact of the PDO on the marine ecosystem in the East/Japan Sea. Therefore, in this study, we investigated the relationship between the PDO and primary production (PP) and subsequent effects on the marine ecosystem in the East/Japan Sea using remotely sensed ocean color datasets from 1998 to 2018. The PDO index and SST showed a negative linear relationship in the East/Japan Sea during the study period (R2 = 0.6350, p < 0.01), while the PDO index and mixed layer depth (MLD) showed a positive linear relationship (R2 = 0.6443, p < 0.01). The annual PDO index and annual PP showed a strong positive linear relationship (R2 = 0.8251, p < 0.01), while the contribution of the small phytoplankton to the total annual PP (small phytoplankton contribution) and PDO index showed a negative linear relationship (R2 = 0.5818, p < 0.01). The shallower MLD during the negative PDO phase indicates that vertical mixing may be weakened due to the stronger stratification caused by the higher SST than observed during the positive PDO phase. Consequently, we hypothesized that weakened vertical mixing may reduce nutrient supply to the euphotic layer, providing environmental conditions that favor the growth of small-sized phytoplankton during the phases in which the PDO is negatively related to a shallower MLD. It is noteworthy that PDO-induced shoaling of the MLD was mainly observed in winter, which may influence the annual PP of the following year. This study shows that the annual PP in the East/Japan Sea can be largely affected through interactions among several environmental conditions during certain phases of the PDO regime, which subsequently affects potential fishery resources in the East/Japan Sea.