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The Pacific Decadal Oscillation (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, the relationship between PDO and primary production (PP), and subsequent effects on the marine ecosystem were investigated in the East/Japan Sea using satellite data sets. PDO index showed a negative relationship with sea surface temperature (SST) and the contribution of the small phytoplankton to the total PP during the study period, whereas the mixed layer depth (MLD) and the PP showed a positive relationship with PDO index. 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 small‐sized phytoplankton favored environmental conditions during the negative PDO. 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 between SST, MLD and subsequent changes in nutrient regime according to the PDO regime, which subsequently affects potential fishery resources in the East/Japan Sea. Plain Language Summary: Pacific Decadal Oscillation (PDO) is very important climatological factor that can affect ocean environmental conditions and its various components. However, only a few studies were conducted to examine the impact of the PDO on the marine ecosystem in the East/Japan Sea. In order to understand how the marine ecosystem will change according to the PDO in the East/Japan Sea, we investigated changes in marine environmental conditions according to the PDO using satellite data sets. Sea surface temperature was warmer, and mixed layer depth was shallower during the negative phase of the PDO. In addition, total primary production of phytoplankton was lower, while the contribution of small‐sized phytoplankton to the total primary production was higher during the negative phase. The higher ocean surface temperature and shallower mixed layer depth suggest that the ocean is more stratified, which can lead to weakening of vertical mixing. Weakened vertical mixing may reduce nutrient supply to the euphotic layer, providing small‐sized phytoplankton favored environmental conditions. Primary production of phytoplankton in the East/Japan Sea can be affected through environmental changes in related to the PDO, which can subsequently affect potential fisheries in the East/Japan Sea. Key Points: Various marine environmental conditions in the East/Japan Sea were closely related to the Pacific Decadal Oscillation (PDO)Primary production and phytoplankton community structure in the East/Japan Sea could be changed according to the PDOA conceptual model for the responses of phytoplankton and potential changes according to the PDO phase in the East/Japan Sea was suggested [ABSTRACT FROM AUTHOR]

Little information on the phytoplankton community in the Yellow Sea (YS)—especially size-fractionated phytoplankton—is currently available, in comparison to the various physicochemical studies in the literature. Using high-performance liquid chromatography (HPLC), size-fractionated phytoplankton communities were seasonally investigated in the YS in 2019. In the study period, diatoms (55.0 ± 10.2%) and cryptophytes (16.9 ± 9.3%) were the dominant groups. Due to the recent alteration in inorganic nutrient conditions reported in the YS, the contribution of diatoms was lower than in previous studies. The large-sized phytoplankton group (>20 µm) was dominated mostly by diatoms (89.0 ± 10.6%), while the small-sized phytoplankton group (<20 µm) was also dominated by diatoms (41.9 ± 9.1%), followed by cryptophytes (19.2 ± 9.8%). The contributions of small-sized diatoms (<20 µm) have been overlooked in the past, as they are difficult to detect, but this study confirms significant amounts of small-sized diatoms, accounting for 62.3% of the total diatoms in the YS. This study provides an important background for assessing the seasonal variations in different-sized diatom groups in the YS. Further detailed studies on their potential ecological roles should be conducted, in order to better understand marine ecosystems under future warming scenarios. [ABSTRACT FROM AUTHOR]

Dramatic environmental changes have been recently reported in the Yellow Sea (YS), the South Sea of Korea (SS), and the East/Japan Sea (EJS), but little information on the regional primary productions is currently available. Using the 13C-15N tracer method, we measured primary productions in the YS, the SS, and the EJS for the first time in 2018 to understand the current status of marine ecosystems in the three distinct seas. The mean daily primary productions during the observation period ranged from 25.8 to 607.5 mg C m−2 d−1 in the YS, 68.5 to 487.3 mg C m−2 d−1 in the SS, and 106.4 to 490.5 mg C m−2 d−1 in the EJS, respectively. In comparison with previous studies, significantly lower (t-test, p < 0.05) spring and summer productions and consequently lower annual primary productions were observed in this study. Based on PCA analysis, we found that small-sized (pico- and nano-) phytoplankton had strongly negative effects on the primary productions. Their ecological roles should be further investigated in the YS, the SS, and the EJS under warming ocean conditions within small phytoplankton-dominated ecosystems. [ABSTRACT FROM AUTHOR]

Understanding size-differential productivity characteristics of phytoplankton is important to evaluating climate-driven environmental changes in marine ecosystems. In this study, we derived the relative contribution of small phytoplankton (P small ; <5 μm) to the total primary production (TPP) and characterized their photosynthetic traits in comparison to large phytoplankton (P large) based on field-measured observations in the Chukchi Sea, 2012. The overall chlorophyll- a (51.8 ± 23.1%) and TPP contributions (52.3 ± 29.8%) of P small observed in this study were closely related to freshwater contents. In regards to nitrogen uptake, P small contributed 39.0% (S.D. = ± 28.4%) and 73.4% (S.D. = ± 50.1%) to the total nitrate uptake rate (TNP) and the total ammonium uptake rate (TAP), respectively, which is consistent with previous findings. Particulate and assimilated carbon/nitrogen (C/N) ratios were significantly (p < 0.01 and 0.05, respectively) lower in P small compared to P large , indicating their different natural capacities during growths. A significantly negative relationship was observed between the P small contribution and the TPP in the Chukchi Sea. These results imply that the more P small contribution in marine ecosystems could lead the less C sequestration from euphotic depths to deep ocean. In comparison to other regions, a more rapid decreasing TPP in the Chukchi Sea with increasing P small contribution is distinctively noticeable, which suggest that the Chukchi Sea ecosystem could be more vulnerable to an expected increase in P small contribution. • Contribution of small phytoplankton is closely related to freshwater content in the Chukchi Sea. • Significantly lower C/N ratios of small phytoplankton in comparison to large phytoplankton. • Strong negative relationship between small phytoplankton contribution and the total primary production. • Chukchi Sea ecosystem could be more vulnerable to increasing small phytoplankton. [ABSTRACT FROM AUTHOR]