Upper ocean biogeochemistry of the oligotrophic North Pacific Subtropical Gyre : from nutrient sources to carbon export

Dai, Minhan and Luo, Ya‐Wei and Achterberg, Eric P. and Browning, Thomas J. and Cai, Yihua and Cao, Zhimian and Chai, Fei and Chen, Bingzhang and Church, Matthew J. and Ci, Dongjian and Du, Chuanjun and Gao, Kunshan and Guo, Xianghui and Hu, Zhendong and Kao, Shuh‐Ji and Laws, Edward A. and Lee, Zhongping and Lin, Hongyang and Liu, Qian and Liu, Xin and Luo, Weicheng and Meng, Feifei and Shang, Shaoling and Shi, Dalin and Saito, Hiroaki and Song, Luping and Wan, Xianhui Sean and Wang, Yuntao and Wang, Wei‐Lei and Wen, Zuozhu and Xiu, Peng and Zhang, Jing and Zhang, Ruifeng and Zhou, Kuanbo (2023) Upper ocean biogeochemistry of the oligotrophic North Pacific Subtropical Gyre : from nutrient sources to carbon export. Reviews of Geophysics, 61 (3). e2022RG000800. ISSN 1944-9208 (https://doi.org/10.1029/2022rg000800)

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Subtropical gyres cover 26%–29% of the world's surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time-series and the Bermuda Atlantic Time-series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N 2 fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone (EZ) in subtropical gyres as a two-layered vertically structured system: a nutrient-depleted layer above the top of the nutricline in the well-lit upper ocean and a nutrient-replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long-term variations in key biogeochemical parameters in both of these EZ layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies.