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Decoding environmental regimes and spring phytoplankton bloom occurrence in the central Yellow Sea

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Abstract

Phytoplankton blooms, defined as a periods of high biomass, are key indicators of climate-driven ocean responses. Shifts in their timing and magnitude can substantially alter the marine ecosystem, yet the environmental regimes governing bloom development remain poorly constrained. We analyzed long-term environmental data (2003–2023) from the Central Yellow Sea (CYS) to decode the drivers of the spring phytoplankton bloom (SPB), which is defined into four developmental stages based on changes in chlorophyll-a (Chl-a). A machine-learning decision tree (DT) was employed to identify specific quantitative critical thresholds associated with each phase. Results show that the SPB initial stage represented low-light intensity in early-April. The peak stage was determined by strong-light intensity; thus, the Chl-a increased rapidly in mid-April. The decline stage corresponded to a high sea surface temperature (SST(:>)14.40 °C) in May, while the termination stage indicated no SPB occurrence after late-May due to very-high SST ((:>)17.27 °C). We classified four SPB types from phenology and discussed the unique environmental characteristics of each type. SPB peak timing is set by the coupled physical oceanic structure (SST-mixing-light), whereas atmospheric inputs modulate bloom magnitude. The study provides a consistent baseline and a physically interpretable phenology-threshold approach for integrated interpretation of timing and conditions.

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Data availability

Chlorophyll-a (Chl-a), sea surface temperature (SST), and photosynthetically available radiation (PAR) products (version R2022) were obtained from the NASA Ocean Biology Processing Group ( [https://oceandata.sci.gsfc.nasa.gov](https:/oceandata.sci.gsfc.nasa.gov) ). Aerosol optical depth (AOD; Collection C061) was obtained from the NASA LAADS Distributed Active Archive Center (LAADS DAAC) ( [https://ladsweb.modaps.eosdis.nasa.gov/](https:/ladsweb.modaps.eosdis.nasa.gov) ). Mixed layer depth (MLD) was calculated from temperature and salinity (GOFS 3.1) provided by HYCOM ( [https://www.hycom.org/dataserver](https:/www.hycom.org/dataserver) ). Wind divergence (WD) was computed from the 10 m wind components (u and v) provided by ERA5, and total precipitation (TP) was obtained from the same dataset ( [https://cds.climate.copernicus.eu](https:/cds.climate.copernicus.eu) ). The processed dataset generated in this study is available at [Baek, J.Y. (2025).](https:/doi.or.kr/10.22808/DATA-2024-5).

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Acknowledgements

We thank the NASA Ocean Biology Processing Group (OBPG) and LAADS DAAC for satellite products, HYCOM for GOFS 3.1 fields, and the Copernicus Climate Change Service for ERA5 reanalysis. We also appreciate helpful discussions and comments from colleagues at Pusan National University and the Korea Institute of Ocean Science and Technology, as well as the collegial support of the College of Marine Science, University of South Florida.

Funding

This research was supported by the Basis Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (RS-2023-00274699). This study was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2023-00280650). This research was supported by Korea Institute of Marine Science & Technology Promotion(KIMST) funded by the Ministry of Oceans and Fisheries(RS-2022-KS221660).

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Authors and Affiliations

  1. Korea Ocean Satellite Center, Korea Institute of Ocean Science and Technology, Busan, 49111, Republic of Korea

    Ji-Yeon Baek

  2. Department of Oceanography, Pusan National University, Busan, 46241, Republic of Korea

    Ji-Yeon Baek, Hyun-Jin Yang & Young-Heon Jo

  3. Marine Research Institute, Pusan National University, Busan, 46241, Republic of Korea

    Jisun Shin & Young-Heon Jo

  4. BK21 School of Earth Environmental Systems, Pusan National University, Busan, 46241, Republic of Korea

    Hyun-Jin Yang

  5. College of Marine Science, University of South Florida, St. Petersburg, FL, 33701, USA

    Yingjun Zhang & Chuanmin Hu

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  1. Ji-Yeon Baek
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Contributions

J-YB: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. JS: Formal Analysis, Writing – review & editing. H-JY: Data curation, Formal Analysis, Methodology. YZ: Methodology, Writing – review & editing. CH: Writing – review & editing. Y-HJ: Conceptualization, Funding acquisition, Project administration, Supervision, Writing – review & editing. All authors have read and agreed to the published version of the manuscript.

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Young-Heon Jo.

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Baek, JY., Shin, J., Yang, HJ. et al. Decoding environmental regimes and spring phytoplankton bloom occurrence in the central Yellow Sea.
Sci Rep (2026). https://doi.org/10.1038/s41598-026-37301-8

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