Front. Mar. Sci. Frontiers in Marine Science Front. Mar. Sci. 2296-7745 Frontiers Media S.A. 10.3389/fmars.2026.1748933 Original Research El Niño enhances coral larval dispersal potential of Dongsha Island—a biodiversity hotspot in the South China Sea GuoPu 1 2 Software Writing – original draft Writing – review & editing Data curation ZhaoMeixia 3 4 * Writing – review & editing Methodology Supervision LianZhan 5 6 * Writing – review & editing Project administration Visualization Marine Environmental Engineering Center, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Nansha Islands Coral Reef Ecosystem National Observation and Research Station, Guangzhou, China The State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Sanya Marine Ecological Environment Engineering Research Institute, Sanya, China Institute of Marine Science, Guangdong Provincial Key Laboratory of Marine Disaster Prediction and Prevention, Shantou University, Shantou, China Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China Correspondence: Meixia Zhao, zhaomeix@scsio.ac.cn; Zhan Lian, zhanlian@stu.edu.cn 25 02 2026 2026 13 1748933 18 11 2025 02 02 2026 19 01 2026 Copyright © 2026 Guo, Zhao and Lian. 2026 Guo, Zhao and Lian https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Introduction

Larval dispersal and connectivity play a critical role in sustaining the diversity and resilience of coral ecosystems. Dongsha Island, which exhibits extraordinarily high coral biodiversity and lies along a key pathway from the Coral Triangle to the South China Sea (SCS), has unclear coral larval dispersal and demographic linkages with other coral reefs.

 Methods

Using biophysical simulations, this study depicts the spatiotemporal pattern of coral larval dispersal potential of Dongsha Island.

 Results

Following a strong El Niño event, coral larval dispersal potential of Dongsha Island is expected to enhance toward the southwest, driven by intensified westward ocean currents.

 Discussion

This finding highlights the influence of climate change on coral larval dispersal in the SCS from an air-sea dynamic perspective and underscores the pivotal role of Dongsha Island in the West Pacific marine ecosystem. Dongsha Island may act as a key modulatory node for connectivity of coral communities in the Coral Triangle and the SCS, particularly under changing climate conditions, as the frequency of super El Niño events is projected to increase.

 biophysical simulation coral reefs Dongsha Island El Niño larval dispersal National Natural Science Foundation of China 10.13039/501100001809 U25A20794, 42376165, 42176001, 42276166 National Key Research and Development Program of China 10.13039/501100012166 2021YFC3100600 Basic and Applied Basic Research Foundation of Guangdong Province 10.13039/501100021171 2022A1515240024 The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the National Natural Science Foundation of China (U25A20794; 42376165; 42176001; 42276166), National Key R&D Program of China (2021YFC3100600); Guangdong Basic and Applied Basic Research Foundation (2022A1515240024); Hainan Province Science and Technology Special Fund (ZDYF2022SHFZ072). section-at-acceptance Coral Reef Research Introduction

Coral reefs, which host the most diverse marine ecosystems on Earth, sustain their coral connectivity through the dispersal of coral larvae (Cowen and Sponaugle, 2009). Connectivity plays a key role in augmenting diversity and enhancing the resilience of coral ecosystems (Ayre and Hughes, 2004). Therefore, high dispersal potential and connectivity enables coral ecosystems to survive natural disasters and anthropogenic stresses, including typhoons (Hongo et al., 2012), coral bleaching (Douglas, 2003), outbreaks of coral predator or macroalgae (Reimer et al., 2019), and illegal fishing and overfishing (Morton, 2002).

The dynamics of the air-sea system have significant impacts on the larval dispersal potential of coral reefs. The released larvae are neutrally buoyant, and thus float within the upper ocean (Cowen et al., 2006). They can be transported by ocean currents over distances exceeding 1,000 km (Caley et al., 1996). During their viable period, if the larvae reach suitable sites, they will settle and metamorphose (Grasso et al., 2011). Since their swimming ability is very weak, their dispersal paths and extents are near passively controlled by oceanic dynamics.

Investigating the ecological dispersal of the coral reefs at Dongsha Island in the South China Sea (SCS), as well as the impacts of climate change on the dispersal potential, have profound implications. First, the coral ecosystem at Dongsha Island exhibits extraordinarily high biodiversity in the SCS. With 297 species, 64 genera, and 14 families of hermatypic corals, Dongsha Island ranks third among all coral reefs in the SCS (Huang et al., 2021). Given the small area of Dongsha Island, about two orders of magnitude smaller than the top two coral systems (the Nansha Islands and Taiwan Island) in the SCS, the diversity at Dongsha Island particularly highlights its status as a biodiversity hotspot in the region.

Secondly, Dongsha Island lies in a pivotal position. As seen from Figure 1, the Kuroshio Current flows from the Coral Triangle, intruding into the SCS as the Branch of the Pacific-to-Indian Ocean Throughflow (BPIOT) (Zheng et al., 2006). The BPIOT passes by Dongsha Island, therefore, the island has the potential to be a bio-hub connecting the Coral Triangle and the coral reefs within the SCS (Liu et al., 2021). Lastly, although threatened by global warming, the corals at Dongsha Island are likely to be protected by the cooling effects caused by the world’s strongest marine internal waves (Alford et al., 2015). Therefore, investigating the coral larval dispersal from Dongsha Island, a potential thermal refuge (Tkachenko and Soong, 2017), plays a key role in forecasting the evolution of the entire coral reef systems within the SCS.

Geographical distributions of coral reefs in the regions around the South China Sea. The coral reefs are outlined in red (Dongsha Island) and purple (other reefs). The black arrows indicate the main circulations within the study region. The abbreviations stand for the following: KC, Kuroshio Current; BPIOT, Branch of the Pacific-to-Indian Ocean Throughflow; SCSWC, South China Sea Warm Current; LG, Luzon Gyre; TWC, Taiwan Warm Current.

Colored map of the South China Sea with labeled currents, islands, and regions such as DongSha Island and the Coral Triangle. Depth is indicated by a blue gradient bar from zero to five thousand meters.

So far, the spatiotemporal characteristics of the dispersal distances of the larval released from Dongsha Island remain unclear. Genetic structure analyses are capable of revealing the connections among various coral communities (Hedgecock et al., 2007). Previous studies have depicted genetic structure of the coral reefs in the SCS [D Huang et al., 2015, 2018; Tkachenko et al., 2020). The existing population genetic data confirm that Dongsha Island maintains stepping-stone gene flow across the SCS for the majority of coral-reef species (Liu et al., 2021). However, gene analyses provide an integrated overview of bio-connections but lack detailed temporal variations in dispersal and connectivity.

The particle tracking approach treats coral larvae as Lagrangian particles in ocean currents. Tracking the particle trajectories delineates the larval dispersal and connectivity of the larva-releasing coral reef to adjacent coral communities (Feng et al., 2016; Raitsos et al., 2017; Wang et al., 2019). More importantly, this approach can elucidate the long-term temporal variations of larval dispersal of a target coral reef (Figueiredo et al., 2022; Grimaldi et al., 2022). These temporal variations are likely induced by climate events, such as the El Niño-Southern Oscillation (ENSO) and global warming (Gurdek-Bas et al., 2022; Wood et al., 2016).

The circulation system in the northern SCS undergoes significant variations forced by climate anomaly events, thus, the coral larval dispersal from Dongsha Island might be characterized by similar features. Among various factors, such as typhoons (Shi et al., 2020), monsoon (Wang et al., 2009), and intra-seasonal atmospheric circulation and moist convection anomalies (Wang et al., 2013), previous studies have elucidated that El Niño modulates the SCS circulations during both boreal summer and winter (Bi et al., 2024; Wang et al., 2020). However, the corals at Dongsha Island spawn in boreal spring (generally at the first full moon midnight in April)(Baird et al., 2009; Lin and Nozawa, 2017; Sun et al., 2024). In April, the circulations (Figure 1) around Dongsha Island consist of the upwind South China Sea Warm Current (SCSWC), the BPIOT, and the cyclonic Luzon Gyre (LG). It is necessary to investigate the relation between coral larval dispersal from Dongsha Island and the complex circulations in the northern SCS.

Combining high-resolution ocean current simulations and a parameterization scheme of larva mortality, this study establishes a biophysical model to depict the spatiotemporal characteristics of the coral larval dispersal potential of Dongsha Island. The results illustrate the main pathways through which the corals at Dongsha Island connect with the surrounding coral communities and demonstrate that the dispersal potential is significantly controlled by ENSO events. The data and methods are described in Section 2. In Section 3, we analyze and discuss the results and identify the associated mechanism. Finally, the conclusions are presented in Section 4.

 Methodology and data Coral data

The coral distribution data in this study are mainly based on Global Distribution of Coral Reefs V4.1 (https://data.unep-wcmc.org/datasets/1). The coral reefs along the China mainland coast and in the Zhongsha Islands are digitalized and included based on field investigations (Hu et al., 2021; Huang et al., 2012; Tong et al., 2015; Xiao et al., 2020) and satellite observations and detections (Lyons et al., 2024).

 Selection of representative coral species

This study targets Acropora millepora to simulate the movements of larvae. Reefs in the SCS are dominated by species of the staghorn coral genus Acropora (Mao et al., 2018; Qin et al., 2019). Previous studies show that larvae of Acropora millepora have representative spawning time and mortality rates, thus are suitable to be chosen as a representative species in the study of coral connectivity (Connolly and Baird, 2010; Figueiredo et al., 2013).

 Biophysical simulation

The biophysical simulation in this study includes the larvae release process, dispersal process, and larvae dying process.

According to the spawning habit of the coral genus Acropora in the northern SCS, the larval release date is set at midnight on the first full moon in April each year (Baird et al., 2009; Sun et al., 2024). A total of 12,810 larvae are evenly released at the atoll on the spawn date during each breeding season.

The dispersal process is jointly governed by tidal currents and ocean circulations, which were both interpolated to a uniform spatial resolution of 1/12° and a temporal resolution of 1 hour. The tidal currents are calculated from the TPXO Global Tidal Models, including 8 primary tides M2, S2, N2, K2, K1, O1, P1, Q1, 2 long period tides Mf, Mm, and 3 non-linear tides M4, MS4, MN4. (Egbert and Erofeeva, 2002). The ocean circulations in the SCS are obtained from Global Ocean Forecasting System (GOFS, https://www.hycom.org/dataserver/) with spatial and temporal resolutions of 1/12° and 3 hours, respectively. GOFS 3.0 Reanalysis data for 1993, GOFS 3.1 Reanalysis data for 1994-2015, and GOFS 3.1 Analysis data for 2016–2022 are used. The main circulations in the northern SCS have been compellingly depicted by the GOFS (Figure 2). The TPXO tidal currents are predicted at grids of the circulation product of GOFS, and the two datasets are uniformly calculated at hourly intervals ( Δt=1 h). The floating larvae are transported by the vertically integrated (< 10m) velocities ( v) during 1993 to 2022.

Long-term (1993-2022) and vertical (< 10 m) mean circulations in the study region simulated by the GOFS.

Vector field map shows ocean surface current velocities in the South China Sea with vectors indicating speed and direction. Speed legend ranges from five to eighty centimeters per second. Coastlines are highlighted in yellow and purple. A red dot marks a circular current feature near 115 degrees East longitude and 20 degrees North latitude. Longitude and latitude lines border the map for reference.

The floating movements of coral larvae can be expressed as (Thomas et al., 2014):

xn+1=xn+vnΔt+Rnr2KΔt

where xn and xn+1 are the position of the target particle at time n and n+1, respectively; K denotes the horizontal diffusivity coefficient calculated using the formulation in de Brye et al. (2010); and R is a random number generated to simulate the stochastic motions of particles, with a mean value of 0 and variance r.

In the larvae dying process, the survival rate of larvae at time t, denoted as S(t), can be expressed as:

S(t)=e−∫τ=0tμ(τ)dτ

where μ is the larva mortality rate, which is a function of time defined by a generalized Weibull model:

μ(t)=λν(λt)ν−11−σ(λt)ν

In this study, based on the investigation by Figueiredo et al. [2022] we set the temperature-dependent parameters λ, ν, and σ to 1.38×10-4, 0.2069, and 2.1545, respectively. The survival rate S(t) declines over time (Supplementary Figure S1). We stopped tracking the larvae on the 45th day after the release date.

 Results Spatial pattern of coral larval dispersal from Dongsha Island

The dispersal of viable larvae is significantly regulated by the ocean circulations in the northern SCS (Supplementary Figure S2). Carried by the BPIOT, the main cohort of floating larvae drift southwestward along the isobaths. The leading edge of larvae reaches the Xisha Islands within 30 days and sequentially the Zhongsha Islands in 35 days. Although the Hainan Island coast lies adjacent to the main path of larval dispersal, it is not a suitable settlement area for larvae released from Dongsha Island. Onshore currents are insufficient to transport the larvae across the isobaths and into the nearshore waters of Hainan Island within the larvae’s viable period.

The SCSWC and Taiwan Warm Current (TWC) also contribute to larval dispersal. A minor portion of larvae, on one hand, reach the coral reefs in the Taiwan Strait within 25 days and arrives at the coast near the Pearl River Estuary and eastern Taiwan Island within 35 days. Conversely, no larvae are capable of dispersing from Dongsha Island to the western coast of Taiwan Strait or the coastline of the Indo-China Peninsula.

To provide an integrated overview of dispersal potential from both the dispersal distance ( L) and survival rate (S) perspectives, we introduce an index LD. This index is calculated as follows: first, the maximum product of L and S is determined for each individual larval particle; then, the mean of these maximum values across all particles is computed as the final LD value. The rose diagram of the maximum LD (Figure 3) indicates that the southwest waters relative to Dongsha Island exhibit the highest dispersal potential. The maximum survival-rate-weighted distance reaches more than 400 km to the west-southwest, the subsequent maxima are to the southwest and west of Dongsha Island. The maximum LD to the northeast corresponds to larvae floating toward the Taiwan Strait, which is fewer and isolated compared with the larvae cluster in the western SCS.

(A) Rose diagram of the maximum LD in each direction with an interval of 22.5°. (B) Annual mean maximum LD (blue bars) to the southwest of Dongsha Island (the target direction is highlighted by the blue sector in the upper panel) and NINO 3.4 index (red line, source from https://psl.noaa.gov/data/correlation/nina34.anom.data) during 1993 to 2022.

Panel A shows a polar bar chart with data concentrated around the west and southwest directions. Panel B displays a time series from 1993 to 2023 with blue bars for LD values and a red line for NINO3.4 index values, both showing periodic variation. Temporal variation of dispersal potential of Dongsha Island

The annual mean maximum LD within 270° to 202.5° of Dongsha Island ( LDSW) reveals obvious temporal variations during 1993 to 2022 (blue bars in Figure 3). The peaks of LDSW, such as in 1993, 1999, and 2017, are subsequent to the typical strong El Niño years (Yeh et al., 2009), suggesting a potential relationship between high dispersal potential of Dongsha Island and strong El Niño events. For an El Niño event, its maximum intensity occurs during December to the following April (DJFMA). Using the Pearson correlation method, we calculate the correlation coefficient between the annual DJFMA mean NINO 3.4 index (sea surface temperature anomalies averaged in the region 5°N–5°S, 90°W–150°W, red line in Figure 3) and the corresponding LDSW in the subsequent year. The correlation coefficient is 0.59 (p < 0.01), confirming that a strong El Niño would lead to the enhancement of dispersal potential from Dongsha Island into its western regions.

The regulation of El Niño on the coral larval dispersal from Dongsha Island can be explained by the influence of El Niño on the ocean circulations in the northern SCS during coral spawning seasons. The coral at Dongsha Island spawns at the first full moon midnight in April, and we target the features of the circulations in 45 days after the larval release date. Therefore, we regress the spawning-season averaged ocean currents in each year onto the NINO 3.4 index averaged over DJFMA of the previous year (Figure 4). The results exhibit that the part of the BPIOT near Dongsha Island significantly enhances following a positive NINO 3.4 index. Therefore, the larvae tend to drift a relatively long distance to the southwest, carried by the enhanced cyclonic current. Conversely, the currents toward the Taiwan Strait do not significantly correlate with the index.

Circulations in the study region regression on the NINO 3.4 index. The index is set to 1.5 to indicate a moderate El Niño. The red vectors indicate that the regress is significant (P <0.05).

Vector field map of South China Sea circulations regressed on NINO 3.4 index, with arrows indicating velocity at 0.4 and 0.8 centimeters per second. Land areas are highlighted in yellow and red vectors marking significant regression (P<0.05). Discussions and conclusions

The seawater temperature has impacts on the larval mortality and pelagic larval duration. The key parameters in the biophysical simulation ( λ, ν, and σ) of this study are based on experiments conducted at 27 °C (Figueiredo et al., 2022). In parallel, we calculate the maximum LD to the southwest of Dongsha Island using the parameters at 29 °C ( λ=1.25×10−5, ν=0.1386, and σ=2.3833). When lagging behind DJFMA mean NINO 3.4 by one year, the LDSW have the correlation coefficient of 0.59 (p < 0.01), which is very close to the results at 27 °C. It suggests that although the seawater temperature around Dongsha Island is projected to increase, the regulation from El Niño on dispersal potential of Dongsha Island tends to remain significant under global warming.

The release date of coral larvae at Dongsha Island varies each year (Supplementary Table S1). Yet, the partial correlation coefficient among LDSW, the durations from April 1st to the larval release date, and the NINO 3.4 index is only -0.18, suggesting the moon phase has negligible impact on coral larval dispersal from Dongsha Island. Additionally, the lack of correlation implies that ENSO’s impact on the target dispersal potential does not depend on the release date of coral larvae, even if the release date might vary over time and is potentially decoupled from the lunar phase.

The significant relation between dispersal potential of Dongsha Island and ENSO events suggests that the former might be further modulated under the increasingly intensified climate change. Modeling studies have found a stronger and more consecutive ENSO in global warming scenarios (Cai et al., 2023; Geng et al., 2023; Heede and Fedorov, 2023). Super El Niño tends to occur more frequently in the warming future, consequently enabling the intensified BPIOT transporting more viable larvae from Dongsha Island farther to the southwest of the spawning grounds. Given that Dongsha Island might be a thermal refuge sustained by the most active internal waves, the potential enhanced dispersal potential highlights the importance of Dongsha Island to the coral ecosystem in the SCS facing projected super El Niño in the future. Additionally, as it lies in the coral larvae conveyor from the Coral Triangle into the SCS, Dongsha Island might act as a key modulatory node for connectivity of coral communities in the Coral Triangle and the SCS. Therefore, the comprehensive investigation of coral connectivity of this area is warranted in future studies.

Some simplifications were made in this study. For example, we employ an ocean current product with a spatial resolution of 1/12° that is integrated over the mixed layer. This approach does not capture the complex topography within coral atolls or the vertical variations in the flow field. Moreover, the biophysical model omits the parameterization of coral larval settlement processes and active larval behaviors.

These simplifications do not affect the study’s primary conclusions. Their effects are confined to fine-scale phenomena, such as the transport between lagoons and forereefs and the influence of internal solitary waves. In contrast, this study focuses on large-scale (~102 km) and climatological features of larval dispersal, which are not impacted by small-scale events (~100 km). Although coral larval behaviors may influence mortality and indirectly affect connectivity through changes in settlement preferences, they do not alter the overall larval dispersal range.

In summary, this study finds that coral communities at Dongsha Island have relatively high larval dispersal potential to the coral ecosystems in the Xisha Islands, Zhongsha Islands, surrounding Taiwan Island, and the Pearl River Estuary. The larvae from Dongsha Island do not arrive at the coastline of the Indo-China Peninsula, Hainan Island, or mainland China except near the Pearl River Estuary. Following a strong El Niño event, larval dispersal potential of Dongsha Island is expected to enhance toward the southwest. This finding explains the effects of climate events on the coral network in the SCS from an air-sea dynamic perspective and highlights the pivotal role of Dongsha Island in the West Pacific marine ecosystem under global climate change.

 Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

 Author contributions

PG: Software, Writing – original draft, Writing – review & editing, Data curation. MZ: Writing – review & editing, Methodology, Supervision. ZL: Writing – review & editing, Project administration, Visualization.

 Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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 Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2026.1748933/full#supplementary-material

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Edited by: Wei Jiang, Guangxi University, China

Reviewed by: Yi Guan, Guangdong Ocean University, China

Chinenye Ani, Australian Institute of Marine Science, Australia