the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 May 2026
Nitrous oxide and methane concentrations and air-sea fluxes in undersampled areas of the Mediterranean basin
Abstract. Nitrous oxide (N2O) and methane (CH4) are potent greenhouse gases for which oceanic contributions remain uncertain, particularly in undersampled regions like the Southwest and Southeast margins of the Mediterranean Sea, where there is a major observational gap. This data paper presents a comprehensive dataset of monthly N2O and CH4 concentrations and air-sea fluxes collected over a full seasonal cycle (April 2023 – June 2024 at most sites, with one station extended to September 2024) from eight coastal stations across three distinct Mediterranean ecoregions (Alboran, Balearic, and Levantine Seas) as part of the ROADSTER collaborative project. Sampling, preservation, and analytical procedures were standardized across sites, and dissolved-gas analyses were performed in a single laboratory to ensure comparability. We detail standardized sampling and analytical methodologies, including ancillary variables (temperature, salinity, dissolved oxygen, chlorophyll a and inorganic nutrients). The complied dataset reveals distinct seasonal and spatial variability: N2O concentrations exhibit a strong negative correlation with temperature, with all stations acting as moderate N₂O sources. Conversely, CH₄ concentrations show greater variability and a positive correlation with temperature, with the Levantine sub-basin stations displaying episodic high-flux events (up to 35.20 μmol m⁻² d⁻¹) indicative of localized seafloor sources. This dataset bridges significant data gaps in the Mediterranean, providing a crucial baseline for regional climate modeling, understanding biogeochemical processes, and future climate change impact assessments.
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Status: final response (author comments only)
- RC1: 'Comment on essd-2026-259', Anonymous Referee #1, 17 Jun 2026
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RC2: 'Comment on essd-2026-259', Anonymous Referee #2, 18 Jun 2026
This paper presents a valuable and comprehensive dataset of surface N2O and CH4 concentrations and air-sea fluxes collected in the coastal regions of the Mediterranean Sea. I would say this study was designed well and presented clearly. I think the data can enhance our understanding of variation in non-CO2 greenhouse gases concentrations and their influencing factors in this region. The topic fits within ESSD’s scope. Thus, I recommend publication only after minor revisions. The following comments are for the author's reference.
- Line 325: ‘samples were discarded with a CV higher than 4.5% (n=3) for CH4 and1% for N2O (n=1)’. The removal procedure of outlier datapoints for N2O needs justification. I am wondering if it may remove real “hotspot” observations rather than erroneous measurements.
- Line 492: ‘there was a positive correlation between N2OSat and temperature’, please give the fit equation, n, r and p value.
- Figures 2 and 4: It’s better to use different symbols together with colors to distinguish the data from different study sites more clearly.
- Figure 3: The figure captions should give more detailed information, for example, what does x represent? Which represents mean and median values?
- Tables: I recommend using a three-line table.
- Table 4: please provide the CH4 mean value for Strait of Gibraltar
Citation: https://doi.org/10.5194/essd-2026-259-RC2
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- 1
L 44 : GHG abbreviation not defined
L 47: “CO equivalent” ?
L 55: you could consider also adding the synthesis by Rosentreter et al. (2021)
L 56: “Similarly, N2O accounts for about one-third of the natural sources to the atmosphere” there seems that something missing from this sentence. “natural sources” of what ?
L58: also DIN
L 61: “extensive coastline” but very narrow continental shelf, so that most of the Mediterranean is deep ocean. Major truly coastal regions in the Mediterranean (Adriatic Sea and Aegean Sea) were not sampled.
L67: is warming rate equivalent in the Eastern and Western Mediterranean regions ?
L76-85: there are some extra measurements of CH4 in seagrass meadows (Champenois et al. 2021).
L 87-89: I do not think it is “vital” to expand data collection for CH4 in the ocean. The ocean has been repeatedly shown to be a very minor natural source of CH4 to the atmosphere, and mainly confined to shallow nearshore regions. It does not mean it is not interesting to measure CH4 in the ocean, but the choice of words is important, and “vital” seem inadequate here.
L109: As far as I’m aware reporting GHG marine emissions is not an obligation of the Marine Strategy Framework Directive. But I can be mistaken, please provide more details (reference or link).
L321: Scripps’ CRMs are used to calibrate total alkalinity, pH, and DIC measurements. However, CH4 and N2O are methodologically closer to pCO2 measurements which are not calibrated with Scripps’ CRMs. Instead, certified calibration gas cylinders are used to calibrate pCO2 measurements, for instance from NOAA. I think that there are equivalent certified gas cylinders for CH4 and N2O measurements that can be used to calibrate the GCs.
L 325: please provide detection and quantification limits of CH4 and N2O measurements.
L 329: section 2.3: there is a need to explain how the Ceq was derived (from which atmospheric pCH4 and pN2O values).
Table 2: “Fosfate” ?
Figure 2 = there are 2 CH4 values for LEV-B1 (?) that seem to be = 0. Is this real of data that were not filtered correctly ?
Figure 2 = the figure is difficult to read because round filled symbols are used for all the stations. It would be advisable to use a combination of different symbols (circles, squares, triangles, diamonds) filled/open to improve readability.
L488 : section 3.3.: it is strange that CH4 was not tested against station depth, as there is ample evidence that CH4 strongly correlates with depth in the ocean (Weber et al.).
It is notable that there is no correlation between CH4 and PO43- (p=0.01). Indeed, there has been emphatic claims about aerobic CH4 production in PO43- depleted areas of the ocean (Wang et al. 2026), but this does not seem to apply in the Mediterranean Sea despite the fact that this ocean region is an oligotrophic zone.
Refs
Champenois W & AV Borges (2021) Net community metabolism of a Posidonia oceanica meadow, Limnology and Oceanography, 66, 2126–2140, https://doi.org/10.1002/lno.11724
Rosentreter JA et al. (2021) Half of global methane emissions come from highly variable aquatic ecosystem sources, Nature Geoscience, 14, 225-230 https://doi.org/10.1038/s41561-021-00715-2
Wang S. et al (2026) Phosphate scarcity governs methane production in the global open ocean, Proc. Natl. Acad. Sci. U.S.A. 123 (12) e2521235123, https://doi.org/10.1073/pnas.2521235123.