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.太平洋年代际振荡（Pacific Decadal Oscillation, PDO）是北太平洋海表温度（sea surface temperature, SST）异常的主导模态。PDO位相不仅是海洋物理属性的重要影响因子，同时也与渔业及水资源密切相关。然而，目前仅有少量研究探讨了PDO对东日本海（East/Japan Sea）海洋生态系统的影响。为此，本研究利用1998年至2018年的遥感海洋色度数据集，探究了PDO与初级生产（primary production, PP）之间的关联，及其对东日本海海洋生态系统的后续影响。研究期内，东日本海的PDO指数与海表温度呈显著负线性相关（R²=0.6350，p<0.01）；而PDO指数与混合层深度（mixed layer depth, MLD）则呈显著正线性相关（R²=0.6443，p<0.01）。年度PDO指数与年度初级生产总量呈极强正线性相关（R²=0.8251，p<0.01）；而小型浮游植物对年度总初级生产的贡献率（small phytoplankton contribution）与PDO指数则呈显著负线性相关（R²=0.5818，p<0.01）。PDO负位相期间较浅的混合层深度表明，相较于PDO正位相阶段，此时海表温度更高导致层结更强，从而可能削弱垂向混合作用。据此，我们提出假设：在PDO与混合层深度呈负相关的位相阶段，垂向混合作用减弱会减少向真光层输送的营养盐，从而形成利于小型浮游植物生长的环境条件。值得注意的是，PDO驱动的混合层深度变浅现象主要出现在冬季，这可能会影响次年的年度初级生产总量。本研究表明，东日本海的年度初级生产总量可在PDO位相的特定阶段通过多种环境条件的相互作用受到显著影响，进而作用于该海域潜在的渔业资源。