| 05 Dec 2025
Gliding through marine heatwaves: Subsurface biogeochemical characteristics on the Australian continental shelf
Abstract. Marine heatwaves (MHWs) disrupt ecosystems across multiple trophic levels by altering oxygen and biological productivity through the water column. Yet, most studies focus on the surface, overlooking subsurface processes that shape ecosystem responses, particularly under compound events involving multiple co-occurring extreme environmental conditions. To address this gap, we analysed 16 years of routine and event-based glider observations on the continental shelf around Australia to present the first comprehensive assessment of the subsurface biogeochemical response during surface MHWs across four contrasting coastal regions. Summer surface MHWs were characterised by a shallower mixed layer depth than normal conditions and enhanced stratification, confining warming to the upper ocean, while other seasons allow deeper penetration under weakly stratified conditions. Stratification favoured deeper and intensified deep chlorophyll maxima, aligned with the depth of stratification maxima, and emerged as a useful proxy for the vertical extent of MHWs. Across all regions and seasons, for non-MHW conditions, dissolved oxygen had a bimodal distribution above and below the mixed layer. However, this distribution changed with event severity and included greater concentrations of low dissolved oxygen and reduced concentrations of high dissolved oxygen during strong events. Below the mixed layer, the bimodal distribution was less apparent and oxygen concentrations during strong events were more concentrated towards middle values. During moderate and strong MHWs, chlorophyll concentrations declined in the mixed layer, albeit this trend was not apparent below it. Regional responses were related to the environmental setting, including the continental shelf structure and boundary current influences, underscoring the importance of region-specific monitoring to understand how MHWs influence biogeochemistry, and furthermore, their ecological consequences on coastal waters. The interaction between physical processes, such as seasonal circulation and stratification, and biological feedback, including the presence of deep chlorophyll maxima and potential oxygen production, highlights the complex biogeochemical responses to MHWs.
Review of Mawren et al. “Gliding through marine heatwaves: Subsurface biogeochemical characteristics on the Australian continental shelf”
Marine heatwaves (MHWs) are known to influence oxygen and biological productivity in the water column, but the majority of studies investigating them focus solely on the surface. This study addresses this important gap by analysing 16 years of glider observations on the Australian continental shelf across four contrasting coastal regions.
The authors show that summer surface marine heatwaves had a shallower mixed layer depth and enhanced stratification, confining warming to the upper ocean, whereas in other seasons there is deeper penetration under weakly stratified conditions. The study discusses the impact this has on biogeochemical variables (chlorophyll fluorescence and dissolved oxygen) across contrasting regions and also seasonally. By incorporating rigorous analysis and investigation of the vertical profiles, this study achieves the aim of showing the impact of MHWs goes beyond the surface. The authors have done a good job of summarising a large, multi-regional dataset and presenting a coherent analysis across regions and variables.
Overall, this study fits well within the scope of Ocean Science and is well aligned with the glider special issue theme and I recommend it for publication after the authors address specific comments below:
General comments:
In situ does not need to be italicised. (See English Guidelines in Submission on the Ocean Science webpage).
There is some inconsistency in spelling conventions, with a mix of Oxford and British spelling (e.g., categorized vs characterised). Either convention is acceptable, but consistency throughout the manuscript is required. (See English Guidelines in Submission on the Ocean Science webpage).
Please ensure consistency in the use of abbreviations: define them on the first use only, and use the abbreviated form thereafter. Whilst I have highlighted a few in my specific comments, it may be worth checking through them all.
Within the Results and Discussion, the manuscript refers to different areas within the defined regions such as the ‘continental shelf’ and ‘inner shelf’. For clarity, it would be helpful to explicitly define these terms (e.g. by approximate isobaths) or annotate them in Fig. 1, particularly for readers unfamiliar with the regional shelf structure.
I have a query regarding the calculated of the MHW severity. It is done using the surface based satellite data, but as the authors have pointed out, MHWs can show stronger signals in the subsurface compared to the surface. Would it be possible to justify the use of the SST data only (whether this is quality or sample size) and discuss the limitations.
Specific comments:
Introduction
Lines 63 – 65: Consider explicitly linking MHW characteristics to stratification here. This relationship recurs throughout the manuscript and may benefit from being introduced earlier.
Line 71: The emphasis on coastal areas could be clarified. I appreciate the next paragraph addresses this, but as one of the examples of subsurface MHWs is in the North Pacific, the coastal comment here lacks context and the importance of coastal regions could be emphasised here.
Lines 85-87: This is where it is important to understand the link between MHWs and stratification.
Line 111: Please indicate the temporal limits of glider coverage and comment on how this sampling frequency relates to the duration of extreme events, particularly MHWs.
Lines 113 – 114: Sensors on gliders do not directly measure stratification or phytoplankton, it may be better to list the variables they do measure (e.g. temperature, salinity, oxygen, chlorophyll fluorescence) and then indicate that from these stratification and phytoplankton dynamics can be inferred.
Lines 117 – 120: Were any of the cited studies specifically focused on MHWs? If so, please clarify. Additionally, please define what is meant by “short-term” (e.g. hours, days).
Methods
Line 151: Extra space between “is” and “used”.
Line 163 – 164: IMOS has already been defined on line 123
Line 194 – 197: What was the quenching depth? Is it worth retaining daytime measurements from this depth and below to limit the removal of valuable data? Particularly given several hypotheses rely on chlorophyll interpretation.
Line 202: Consider using “few” instead of “rare”.
Line 217: There appears to be a typesetting issue here and the link doesn’t work.
Line 229 – 230: Is excluding negative seasonal anomalies standard practise when identifying MHWs? Please clarify whether this refers to identifying events or calculating the severity index.
Line 238: Add “defined” before “as”
Line 328: Potentially an extra space between “sampled” and “off”.
Lines 334 – 341: Where possible, figure captions should appear on the same page as the figure.
Results
Lines 369 – 370: How do you define the continental shelf and offshore waters? Also, clarify the area spoken about here is in the QLD region (if I have understood correctly).
Lines 370 – 372: Not sure what is meant here by “In agreement with the higher frequency”, please clarify the link between frequency and duration.
Line 372: GBR has not yet been defined.
Lines 379 – 381: It could be helpful to briefly clarify what types of quality control criteria led to the exclusion of these profiles (e.g. sensor issues, incomplete dives), and whether this could bias the representation of the most extreme conditions.
Lines 389 – 391: Could more frequent summer and autumn events reflect increased glider sampling during these seasons?
Figure 4: Can the axis labels and colourbar labels be larger for readability?
Line 410: DOX has already been defined on line 270.
Line 415: For clarity, refer to the MLD rather than “both layers”.
Lines 434 – 435: Could you elaborate on how the regional and seasonal regimes do affect the multi-modal structure?
Line 476: Missing comma after Holbrook and Bindoff.
Line 501: Should this refer to oxygen saturation rather than DOX?
Lines 504 – 510: Caption placement should ideally be on the same page as the figure.
Line 525: Consider using “DCM” here for consistency.
Line 546: GBR should be defined here rather than later on line 648.
Lines 566 – 569: Could this pattern also reflect autumn stratification breakdown and nutrient entrainment from deeper waters?
Lines 576 – 578: Could nutrient limitation toward the end of the bloom period also contribute?
Line 604: Is oxygen saturation intended rather than DOX?
Discussion
Since results consistency with hypotheses (2) and (4) is discussed, it may be useful to explicitly link findings back to hypotheses (1) and (3) as well, either here or in the conclusions.
Line 668: Do you have a suggestion for why it is different for NSW?
Line 716 – Earlier referred to as the Eastern Australian current on line 473 – be consistent with East or Eastern
Supplementary
Line 6: Add a space after “Panels”.
Fig S6: Also doesn’t have the colour coding Fig. 10 has; consider mentioning this in the caption.