the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Jun 2026
Thermal regulation of benthic fluxes in temperate estuaries
Abstract. The effects of short-term heatwave extremes on biogeochemical cycling and fluxes in a temperate estuary of a semi-dry climate were studied using an experimental setup of temperature-controlled benthic incubations. The results demonstrated a strong thermal effect, notably under extreme warming events, for shifts in exchanges across the sediment-water interface. Extreme heatwave conditions (+5 °C of the seasonal mean) boosted acidification, hypoxia, and ammonification, due to accelerated remineralization rates, resulting in strong effluxes of NH4, Si(OH)4, and PO4 to the overlying water. These excessive nutrient loads may increase eutrophication risk via runoff or tidal action, specifically in adjacent oligotrophic coastal waters. CO2 production rates reached ~4000 µatm under extreme hypoxia and acidification, 2.3-fold higher than the ambient rate, with a maximal flux of ~27.0 mmol m-2 d-1. Hence, our experiments show that marine heatwaves amplify CO2 emissions while reducing the CO2 buffering capacity of temperate estuaries. It emphasizes temperate estuaries as highly sensitive ecosystems to climate change.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3235', Anonymous Referee #1, 17 Jul 2026
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RC2: 'Comment on egusphere-2026-3235', Sebastiaan van de Velde, 23 Jul 2026
This manuscript presents an experimental investigation of the impact of temperature changes related to heatwaves on sedimentary metabolism. While the overall conclusion is not very surprising – higher temperatures increase microbial metabolism and thus increase oxygen consumption, nutrient release, and DIC production – the study could be a valuable addition, as it shows experimentally what rates can be expected for an estuary that is exposed to higher temperatures.
In its current form, however, this manuscript is not ready for publication. The experimental design and methods are vague and do not make it clear why the specific approach was chosen. As I read it, this seems to be a combination of two separate approaches: classical flux incubations – which are typically done with cores and mixing of the overlying water, and slurry incubations – which are typically done with continuous mixing of the slurry to get rates, and really only allow comparison between treatments. The authors now seem to set up slurry incubations, but they discuss the results using flux units (mmol m-2 d-1), which they then compare to other fluxes in the literature. The problem with this becomes quite clear when they discuss oxygen fluxes, which are way lower than what you would expect from these types of sediments.
To make this manuscript publication-worthy, the authors would need to
(i) rewrite the methods so the experiment can be reproduced (which should always be the case for any publication), and also clarify the reason why a certain design was chosen (e.g. you use annual mean and mean winter, and then call the treatments ambient winter and ambient summer -> why not take the annual summer temperature?)
(ii) present all results in rates (e.g., umol cm-3 d-1) – not fluxes. Your experimental setup does not allow you to derive fluxes that are comparable to in-situ situations
(iii) adjust the discussion accordingly, the experimental design makes it almost impossible to compare to other studies, not does it allow you to make large inferences about any system-scale processes (even of your own site), the mixing you did before starting the experiment almost certainly introduced artefacts, which implies that most of the oxygen consumption is likely related to reoxidation processes, which might not have the same temperature dependence as carbon oxidation.
Instead, you could put more effort into extracting the coupled processes from your incubations – with the different production rates, you could try to make a mass budget or mechanistic model of your experiment to derive the processes (e,g, denitrification, aerobic respiration) and see if any processes respond non-linearly (although that is hard to derive from 3 different treatment points).I have listed some small remarks that appeared as I was reading the manuscript below, some of which are repetitions of what I have written here.
Kind regards
Sebastiaan van de Velde
L51: why annual average and not mean summer temperature?
L52: that statement should not be in methods
L57: introducing mixing artefacts – are you sure you are measuring temperature effects?
L71: did you collect several timepoints or just before/after?
L102: these fluxes are very low for intertidal sediments
L117: not oxygen consumption but DIC production related to the metabolism is the reason for acidification
Figure2: your oxygen profile suggests non-linear trends (I assume that red dot is overlying the other coloured dots?), so you would underestimate the actual oxygen consumption at timepoint 0 if you do not use a quadratic fit here. The same goes for other solutes (e.g. NH4). The fact that the production is non-linear suggests that there are quite some transient effects occurring in the experiment – which might be related to the potential mixing artefact.
L161: I would not be too sure that those NOx fluxes are significant, so I would not put too much faith in those results
Figure5: why would you do cross-plots of the concentrations? It would make more sense to compare the fluxes
Citation: https://doi.org/10.5194/egusphere-2026-3235-RC2
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- 1
Review on “Thermal regulation of benthic fluxes in temperate estuaries”
Summary of the study:
The study aims to quantify the impact of heat waves on estuarine sedimentary fluxes, and particularly the subsequent CO2 flux from the overlying water into the atmosphere. For this purpose, two experiments were carried out, one in winter, one in summer. For each experiment, three temperatures were chosen that reflect “ambient low”, “ambient high”, and “extreme” conditions. The key findings of the study are that higher temperatures speed up chemical reactions and turnover rates. These in turn increase CO2 release first from sediments and subsequently from the water to the atmosphere.
Major comments:
The abstract is somewhat fuzzy. There is no introductory sentence and some of the statements are factually wrong (e.g. µatm as a unit for a rate).
The methodology is scarcely described which makes it difficult to comprehend the results. The experimental setup is ambiguous: Why do you expose sediments collected in winter to heatwave conditions and vice versa sediments collected in summer to “ambient low” temperatures? In addition, and more profoundly: The sediments were possibly mixed. That alters the entire chemistry in sediments as well as the microbial community. In that case the entire study might be just a record of artefacts.
The language of the manuscript is very often extremely generic. The authors talk about “other related modifications” and similar things instead of naming them. That reduces the clarity of the study.
Overall, the study fails to point out what new questions it wants to answer. Eventually the discussion fails to answer any real question anyway. The result and discussion part is very much a result part. The discussion occasionally drifts into a review but does not manage to point out what the new findings of this study (are they new?) imply. There is some explanation of the observed data, but no attempt to put the data into context. For example: What is the total magnitude of flux change we look at? How many estuarine systems is the chosen estuary representative for?
The fact that the conclusion is extremely short underscores that the study does not provide many new findings in its current form.
Precise criticism can be found in the line-by-line comments.
Summarizing it is actually very difficult to evaluate this study. The results might be worth being published (if the method section is revised), but then the entire structure of the study needs revision: I suggest a classic format with methods, results and discussion, and a completely revised discussion that clearly tackles the novelty of the results and their implications for the field.
Line 6: The first sentence of the abstract is missing. Please insert a short sentence that guides the reader towards the research question and then start with what you did.
Line 11 this sentence is somewhat weird – runoff ok, but tidal action? Is the idea to say that the enhanced nutrient fluxes lead to eutrophication in the ocean adjacent to the river? If so, just say that. The transport mechanism is somewhat self-explanatory.
Line 13: First: µatm is not a unit for a rate. Second: Where’s the difference between extreme hypoxia and anoxia? Third: Isn’t acidification the consequence of high CO2 production?
Line 14: Since alkalinity was not mentioned to this point the reduced buffering capacity appears very abruptly.
Line 14/15 This last sentence is quite detached from the rest of the abstract
Line 18: Please explain briefly in what way they buffer carbon and nutrients.
Line 20: Comma after Yet.
Line 20: Is that really their depth? Or rather the fact that they are mainly influenced by a riverine system that depending on length and runoff is reflecting air temperatures and incoming heat radiation?
Line 23: What do you mean by “from these estuaries”? If you really mean from the estuaries to the adjacent sea then the rest of the sentence is weird as it is completely obvious that the adjacent sea might then be influenced by the fluxes. Or do you mean sedimentary fluxes in the estuary that can then influence the adjacent sea? Please clarify this sentence.
Line 27: What effect does temperature have? Please be specific.
Lines 20 – 30: Same Problem: What effect?
Line 31: Please reduce the amount of “yet”s in the manuscript.
Line 31: Likely should stand before microbial and ideally be replaced with “possible”.
Line 32 What do you mean by “other related modifications to the fundamentals of these systems”? That formulation can mean anything. Please be precise in your language.
Line 33: Higher temperatures compared to what?
Lines 35 – 38: So far, the authors have not clarified which ecosystems might be affected in what way to justify that SEMS eco-systems can serve as a role model. Hence: Please rewrite the introduction here in a way that clarifies what ecosystems are likely to experience what kind of changes and in what way a SEMS ecosystem is showcasing these features already.
Line 40: The hypothesis that higher temperatures increase metabolic rates has not really been established above. Please make sure that you explain this hypothesis “above”.
Line 42: This would be a very nice spot in the manuscript to introduce DIC as an abbreviation for dissolved inorganic carbon.
Figure1: Please explain in the main text why you assume a lower degree of bioturbation and add to the caption that the bright brown features aim to reflect worm burrows and qualitatively depict the amount of expected bioturbation.
Lines 48 – 54: This section is completely detached from the rest of the methods and the information it contains is (at this point) not needed. Delete it and put the information to sections where it is needed.
Line 55: Summer and winter are not very precise. Please specify at least the month, ideally provide dates.
Line 56: What kind of device was used for sampling?
Line 55 – 60: Was the sediment undisturbed? Or was it mixed? If it was mixed: How do the authors justify that the observed fluxes are not simply an artefact of the mixing?
Sandy silt implies very low permeability. Most likely the natural sediments’ porewaters had distinct gradients. Likewise, the microbial community was set up for these gradients. If the sediment is mixed it can take weeks to several months before a sedimentary system is back to a natural equilibrated balance. In short: If the Sediments were mixed this might be an absolute red flag for publication. I want to urge the authors to clarify this in the manuscript and if need be, argue why they believe their results are still valid.
Line 57: What material were the bottles made of?
Lines 60 ff: For clarification: Sediments collected in Winter and Summer were exposed to the same three different temperatures? If so: Why? What was the temperature in the field when the sediment was collected? Why not do one set at “in situ” temperatures and the other two at elevated temperatures that are expected under certain emission scenarios?
Line 64: delete internal. No direct sunlight, but windows? What would indirect sunlight look like?
Line 65: Why the different times?
Line 66/67: This sentence does not make much sense: “What does “these” refer to? What do you mean by “conditions that would be dominated by heterotrophic activity”?
Line 67 – 70: These sentences (reasoning for measured values) belong either to the introduction or the discussion.
Line 71: What do you mean by removed? Permanently? What subset? (number, which bottles, and most importantly: why?)
Line 71: How much water? Was it replaced?
Line 73: What do you mean by “as close to conditions as possible”?
Line 74: Filtered at what size? What filter (method) was used?
Line 75: How was the pH meter calibrated? On what scale are pH values reported?
Line 78: This is confusing. What pH method was used now?
Line 79: Why no alkalinity for the summer experiments?
Line 81: Why at 25°C? Please rewrite the section on pH and alkalinity measurements in a clear and understandable way.
Line 83: So far, I have not heard of undissolved organic nutrients.
Line 95: Replace “through” with “throughout”
Line 99: How were these fluxes calculated? How was oxygen measured?
Line 101: These figures show concentrations, not consumption rates.
Line 99 – 105: Since it is not clear, how oxygen consumption was calculated, this section is difficult to follow. In addition, the described observations are of course very expected.
Lines 106/107: What experiments do you refer to here?
Line 118: What does “acidification rate” refer to? How is it defined/calculated?
Line 123: If temperature is the primary factor, which process is responsible for acidification? Is it only CO2 partial pressure?
Line 136: What do you mean by “setting up phase”?
Lines 138 – 141: “This initial pulse is not captured in detail in this work, as it represents a different mechanism related to the immediate response to the perturbation rather than the intermediate-term response investigated here, for which the post-initial-perturbation mode is the baseline.” Please rephrase this. What mechanism does the initial pulse represent, what do you mean by “the mediate response to the perturbation”, what does intermediate-term response even mean, and what exactly is a “post-initial-perturbation mode”?
Line 167: What model do you refer to here?
Line 173: What does “these” refer to here? The mentioning of microbial community function comes very abrupt here. Is there any evidence for this? Please clarify your train of thought here.
Line 183: What you are saying here is: The fact that recycling of biogenic silica is also important in summer suggests strong recycling of biogenic silica. Need I say more? Please rephrase this.
Line 195: What does “it” refer to here?
Line 198: What do you mean by downstream?
Line 201: “This stands in somewhat contrast to open marine systems, in which net CO2 degassing decreases due to marine heatwaves”: This sentence stands without any reasonable context.
Line 202: Rather use the plural “emissions”.
Line 209: How did you calculate this percentage?
Line 216: What do you mean by footprint?
Line 217: Would you really describe a river mouth as a confined water body?
Line 221/222: This sentence is rather a review of previous literature that would be suitable for the introduction, not the discussion section.
Line 225: Did you mean represents instead of presents?
Line 227: What effects? These generic formulations are used very often in the script making it very hard to read.
Line 230/231: This sentence is a bit weird. Especially the part “but possibly also aerobic ones”. That is super generic and does eventually not provide any information.
Line 231: What do you mean by “this region”?
Line 232/233: “Oxygen level in the course of the experiments here also allows for aerobic remineralization, notably of dissolved organic matter, which is high in local estuaries in summer and promotes high bacterial activity (Bar-Zeev and Rahav, 2015).” This sentence is a nightmare to understand. Delete “also” and “notably”. Define “local”. And bacterial activity is not per se promoted by oxygen. That depends highly on the bacteria you look at.
Line 234/235: The fungi story comes quite out of the blue and lacks explanation.
Line 238: See comment on the abstract.
Line 240: There is a “that” missing after fact.
Line 252: You already mentioned the 4000 µatm.
Line 253: How was this flux correlated to wind speed? Please provide formulas for the respective calculations.
Line 254: How was this estuary flux calculated/measured? What estuary are you referring to?
Lines 262 – 266: The fact that the conclusions are four sentences over four lines speaks for itself.