the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Nov 2025
Lagoon temperature and hydrodynamics of Reao Atoll (Tuamotu Archipelago, French Polynesia) before and during the 2024 El-Niño Marine Heatwave
Abstract. In French Polynesia, semi-closed atolls of the eastern Tuamotu Archipelago are home to fisheries and mariculture activities based on giant clam resources. These activities are increasingly vulnerable to global warming, as evidenced by the mass bleaching event observed in Reao atoll in March 2024, triggered by the most intense and prolonged Marine Heatwave (MHW) ever recorded in this lagoon. The GAIA project (manaGement strAtegy evaluatIon for small-scale fisheries in Atoll lagoons) was launched to address this issue and aimed to explore the sustainability of giant clam fisheries under climate change, with a particular focus on processes driving thermal exposure during MHWs. To achieve this, spatial and temporal temperature fluctuations, lagoon circulation, and water exchange dynamics between ocean and lagoon across the atoll rim were monitored using moored autonomous oceanographic sensors. A total of 6 monitoring periods took place between December 2016 and May 2025, with the most extensive instrumentation covering the entire February–March 2024 MHW event. The strength of these observations lies not only in their coverage of a unique and poorly sampled ecosystem, but also in their comprehensive documentation of an exceptional MHW event, from its onset and peak to its dissipation, providing an unprecedented record of the full life cycle of a major thermal anomaly in a tropical semi-closed atoll lagoon. This dataset also supports future modelling efforts to simulate temperature dynamics and identify thermal refugia in atoll lagoons under changing climate scenarios. All data were post-processed, quality-controlled, and formatted into interoperable NetCDF files. The full dataset is openly accessible through the SEANOE marine data platform: https://doi.org/10.17882/105885 (Le Gendre et al. 2025a).
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Status: open (until 19 Feb 2026)
- RC1: 'Comment on essd-2025-692', Alejandra Sanchez-Rios, 15 Jan 2026 reply
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RC2: 'Comment on essd-2025-692', Camille Grimaldi, 03 Feb 2026
reply
General comments
Thank you for the opportunity to review this interesting manuscript. The study investigates lagoon circulation and temperature dynamics at Reao Atoll before and during the austral summer 2023–2024 marine heatwave. The authors present a unique and comprehensive dataset of both ocean currents and temperature, the collection of which requires substantial effort and is rarely available in the literature. Indeed, there are few in situ records documenting reef-scale hydrodynamic and thermal processes during an MHW, making this dataset very valuable. However, the analysis frequently focuses on short time periods, and several conclusions appear to be drawn without sufficient supporting analysis. In addition, the manuscript would benefit from clearer presentation and stronger framing to guide the reader through the study objectives, methods, results, and implications. Notably, of the 19 pages of the manuscript, 7 pages are focused on the Methods section. While the dataset itself is rich in both spatial and temporal coverage, it is not yet fully leveraged to place the observed MHW into the broader hydro- and thermodynamic context of the atoll. I have reviewed the manuscript in detail and provide specific comments and suggestions below.
Abstract
The abstract would benefit from being reframed around clear research gap or objectives. As currently written, it emphasizes the comprehensive documentation of an MHW event in a semi-enclosed lagoon, but this aspect is only briefly addressed in the manuscript (L316–325). The abstract should more clearly highlight the key results and their significance; at present, it is difficult to identify the main findings of the study.
Introduction
The introduction states that the manuscript aims to “characterize the spatial and temporal variability in local temperature regimes and understand their drivers” (L57–58). As it stands, it is unclear whether this objective has been fully achieved. The introduction would benefit from a clearer description of the specific MHW being studied, including its intensity, duration, and how it compares to previous heatwave events affecting Reao Atoll. If this was the most prolonged or intense event on record, additional context is needed regarding earlier events, their drivers, and their impacts. This background is essential for framing the novelty and relevance of the 2023–2024 MHW and for justifying the focus on potential drivers.
Methods
The Methods section is very long, yet several key pieces of information are missing. Much of the detailed instrument description (currently spanning several pages) could be condensed into a summary table, with standard deployment details moved to the Supplementary Material. I recommend substantially condensing this section, potentially to a single page of essential information, while adding clarity where needed. Additional missing information include:
- Consider adding/merging Figure A1 and Figure 3, summarizing instrument deployments and timing, including the CTD casts.
- Describe the bathymetry dataset used in Figure 1.
- Specify the sources of wind, wave, SST and tidal (FES) datasets, including where they were extracted from.
- Explain how inflow and outflow are calculated. Were current velocities rotated to align with reef pass orientations?
- Explain the MHW analysis presented in Figure 6 was done?
Results
- Important background context is missing. The hydrodynamics of the atoll are described primarily using a 5-day subset of data, despite the availability of a much longer and more comprehensive dataset. The rationale for this choice needs to be more clearly justified.
- Subsection titles could be improved to better guide the reader. For example, Section 6.1 appears to characterize either non-MHW conditions or maximum flushing conditions, but this is not made explicit.
- Given the title of the manuscript, a more thorough description of temperature dynamics before and during the MHW is expected. Currently, the analysis relies largely on a single average temperature difference between SST and one lagoon site.
- The conclusions drawn at the end of Section 6.3 appear to be over-interpreted given the data presented (see detailed comments below).
Overall, the dataset provides an excellent opportunity to place the MHW in the context of background hydrodynamic and thermal variability at the atoll scale, but this potential is not yet fully realized.
Results and Discussion
The results and discussion sections are combined, but the discussion does not sufficiently place the findings in a broader scientific context. Instead, it relies heavily on self-citation, while several relevant references appear to be missing. The discussion would benefit from a more balanced engagement with the wider literature. Additional references on atoll thermodynamics could include:
Rogers, J. S., Monismith, S. G., Koweek, D. A., Torres, W. I., & Dunbar, R. B. (2016). Thermodynamics and hydrodynamics in an atoll reef system and their influence on coral cover. Limnology and oceanography, 61(6), 2191-2206.
Green, R. H., Lowe, R. J., Buckley, M. L., Foster, T., & Gilmour, J. P. (2019). Physical mechanisms influencing localized patterns of temperature variability and coral bleaching within a system of reef atolls. Coral Reefs, 38(4), 759-771.
Grimaldi, C. M., Lowe, R. J., Benthuysen, J. A., Cuttler, M. V. W., Green, R. H., & Gilmour, J. P. (2023). Hydrodynamic and atmospheric drivers create distinct thermal environments within a coral reef atoll. Coral Reefs, 42(3), 693-706.
Reid, E. C., Lentz, S. J., DeCarlo, T. M., Cohen, A. L., & Davis, K. A. (2020). Physical processes determine spatial structure in water temperature and residence time on a wide reef flat. Journal of Geophysical Research: Oceans, 125(12), e2020JC016543.
In addition, the manuscript would be strengthened by addressing potential ecological implications of the observed MHW, if any are known or can be inferred. Finally, please check carefully for typographical and formatting errors throughout the manuscript (e.g., L316, L340).
Minor comments
- L33: Please define what is meant by “shallow.”
- L39: Do you mean “regional” instead of “external”?
- L78: Consider indicating “permanent” for readers unfamiliar with such systems. Please also indicate average island elevation above sea level. Could large swell events combined with spring tides overtop the reef? This is important context.
- L89: Ensure correct formatting of m s⁻¹ (or use m/s), add °N, and correct subscripts for Hs and Tp.
- Figure 1: Please clarify what is meant by “most/least open” hoa. Is this related to depth, width, or another metric?
- L95: This is important context, does the lack of northern reef passes imply higher permeability elsewhere? Please also describe the tidal regime and ranges.
- L114–122: This information is fairly standard and could be moved to the Supplementary Material.
- Figure 2: Panel (A) does not appear to show an SBE56. Panel (B) is a Nortek Aquadopp, and panels (D) and (E) appear to be RDI ADCP Sentinels.
- L173: Define or avoid acronyms.
- Sections 3–5: These sections are very long and could be substantially condensed. Consider moving to supplementary.
- Figure 1: Add “(A)” to the main panel, change “Ouest” to “West” on y-axis labels, and make colorscale discrete.
- L246: Were currents rotated to indicate inflow/outflow?
- L261: Why focus on a 5-day window? Please define what constitutes an “energetic swell event,” including duration, mean Hs, Tp, and direction. How typical are those events?
- L265: The offshore tidal regime has not yet been described. Did this event occur during spring or neap tides? Please indicate tidal ranges.
- L271–273: Would different behaviour be expected under these conditions?
- L273–274: Faster and increased compared to what?
- L275: Currents at A2 appear consistent with increased flushing during large Hs, please clarify how they differ from other sites.
- Figure 4: Add units to the left axis. Replace “FES” with “offshore water level” (or equivalent). Modify the legend to “depth-averaged currents.” Add a grid. Clarify what “projected Hs” means. Why is water level at T11 so flat?
- L288: Do you mean current speed?
- L292–295: Define wind slope and sea surface slope; both should be described in the Methods.
- L298: Please clarify what is meant by “overturning circulation.” Given the year-long dataset, can the frequency of such events be quantified?
- Figure 5: The “main axis” is referred to as “cross axis” in Figure 3. What does “slope” in panel (b) represent? Consider using contours for clarity.
- L311: What does “cover a range of values” mean? The ERA dataset should be described in the Methods.
- L317: Which site is used for this average? It would be informative to compare across sites to assess spatial variability and controls (e.g., depth, proximity to hoa).
- L322: High levels of what?
- L323–325: The term “marginal” is unclear. Without even a simple heat budget, these conclusions are difficult to assess. Figure 6 suggests higher Hs during the MHW than during the earlier event, yet reduced inflow is inferred. Temperature variability is driven by both advection and air–sea heat fluxes, neither of which are sufficiently quantified. Consider estimating flushing rates and/or air–sea heat fluxes.
- Figure 6:
- (b) Consider adding SST to the zoomed panel. Why does temperature appear flat during parts of the diurnal cycle?
- (c) Is “depth” water depth or offshore tidal water level?
- (d) Add “A1” to indicate the measurement location.
- The legend states that the MHW was calculated from a statistical hindcast model—this requires further explanation in the Methods.
- L334: This section would benefit from a clearer introduction. Are you investigating whether the MHW affected the full water column or only surface waters? Is the CTD snapshot representative of the full 40-day MHW? Were additional profiles collected?
Citation: https://doi.org/10.5194/essd-2025-692-RC2
Data sets
Lagoon temperature and hydrodynamics of Reao atoll (Tuamotu Archipelago) Romain Le Gendre et al. https://doi.org/10.17882/105885
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General Assessment
This manuscript presents a valuable dataset collected at Reao Atoll. The scope and quality of the observations are impressive, and the dataset will likely serve as the foundation for multiple future studies aimed at understanding environmental variability and changes in both wave and atmospheric forcing in reef systems. Overall, the paper is strong, and most suggested changes are editorial in nature and intended to improve clarity, readability, and accessibility. In a few places, tighter definitions and more careful framing of data interpretation would further strengthen the manuscript and increase its impact.
Specific Comments
Section 3: Data and Instrumentation (Lines 107–113)
The description of the dataset and data access is clear and well done. However, the readability and orientation of the manuscript would benefit from additional contextual information at the start of Section 3. In particular, consider briefly including in the first paragraphs:
Lines 106–165
The inclusion of direct links to the instrumentation and datasets is appreciated and improves transparency and accessibility. One suggestion is to replace the long-form URL with a shortened or embedded link for readability. The repeated “last accessed: 7 November 2025” note was slightly confusing, as it was unclear whether this referred to the data itself or the webpage.
Section 3.3: CTD Profiles Line 160-163
A brief description of how the CTD profiles were collected would be helpful. Specifically, clarifying whether the CTD was hand-lowered, the approximate lowering speed, and whether any stabilization or stopping protocol was used would provide useful context for data quality and interpretation.
Lines 254–258
It would be helpful to briefly state whether salinity spiking was observed in this dataset and, if so, how it was identified and handled during processing.
Figure 5 and Associated Text (Lines 293–302)
There appears to be a possible mismatch between the color scheme described in the text and the currents shown in Figure 5. The text suggests that surface currents are directed northwestward (red) and bottom currents southeastward, consistent with expected pressure gradients. However, this pattern is not immediately clear in the top or middle panel, and the current intensification during wind reversal appears to be oriented northeastward rather than in the opposite direction described as a “current reversal.” ( might be reading this wrong). This may be a misunderstanding of the reference frame or color scheme, but adding directional arrows or a small schematic indicating flow orientation along the primary axis would greatly reduce ambiguity and help readers interpret the figure correctly.
Lines 314–315
A brief definition of what constitutes Category I and Category II events is needed here. Even a short descriptive phrase would help readers unfamiliar with the classification fully appreciate the results.
Lines 338–340
The observed difference in stratification is intriguing. If there is any speculation of what created this difference. A short comment would add valuable context and highlight the significance of this result.