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Journal of Tropical Oceanography

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Carbon‑Oxygen Isotope Signatures of Benthic Foraminifera and Their Implications for Vertical Connectivity Between Shallow‑Water and Upper‑Mesophotic Zones of Xisha Coral Reefs

CHEN Tianran1, ZHOU Wen1, LIN Xianzhi1, LI Shu2, WU Jingqi3, WANG Ziyang4, LIU Sheng1    

  1. 1.  State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;

    2.  Laboratory of Marginal Sea and Ocean Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;

    3.  Guangzhou No. 3 Middle School, Guangzhou 510120, China;

    4.  Guangzhou Experimental School Affiliated to Beijing Normal University, Guangzhou 510700, China



  • Received:2026-08-28 Revised:2026-09-07 Accepted:2026-10-15
  • Supported by:

    : Science & Technology Fundamental Resources Investigation Program (2022FY100600); Project of State Key Laboratory of Tropical Oceanography (E6YQ2A0203)

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Abstract: Mesophotic coral ecosystems (MCEs) are regarded as potential “deep‑reef refugia” for shallow‑water corals under climate warming. However, their functional integrity largely relies on water‑mass exchange and vertical material connectivity between shallow‑water and upper‑mesophotic zones. Previous studies on vertical connectivity have primarily focused on population genetics, whereas direct geochemical evidence for material connectivity across reef slopes remains limited. In this study, we employed monospecific benthic foraminifera (Amphisorus sp.) extracted from surface sediments on the fore‑reef slopes of Lingyang Reef and Zhaoshu Island, Xisha Archipelago, as geochemical archives. A total of 48 paired δ¹³C and δ¹⁸O isotopic datasets were obtained, comprising 24 samples from the shallow‑water zone (3‑20 m) and 24 from the upper‑mesophotic zone (35‑48 m). The results show that δ¹³C exhibits no statistically significant difference between the shallow‑water and upper‑mesophotic groups, with substantial overlap of datasets, indicating no significant spatial differentiation of DIC isotopic signals. In contrast, δ¹⁸O increases significantly with water depth. After controlling for depth effects via analysis of covariance (ANCOVA), the between‑group effect on δ¹⁸O approaches but does not reach the pre‑set significance level of α = 0.05, suggesting that the δ¹⁸O discrepancy mainly arises from the continuous depth‑dependent temperature gradient. Isotopic evidence supports the hypothesis of vertical material connectivity and implies likely intense cross‑depth water‑mass exchange on Xisha reef slopes. This study provides in‑situ geochemical evidence for carbon‑pool connectivity based on benthic foraminiferal isotopes, offering a material‑based perspective complementary to conventional genetic‑connectivity research. Furthermore, it supplies modern‑process calibration references for paleoenvironmental reconstruction using fossil foraminifera from Xisha coral reefs, and provides scientific support for the conservation and management of mesophotic coral ecosystems in the Xisha Archipelago. Key words: Xisha Islands; mesophotic coral reefs; benthic foraminifera; carbon and oxygen stable isotopes; vertical connectivity

Key words: Xisha Islands, mesophotic coral reefs, benthic foraminifera, carbon and oxygen stable isotopes, vertical connectivity