the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 May 2025
Diel Variability Affects the Inorganic Marine Carbon System in the Sea-Surface Microlayer of a Mediterranean coastal area (Šibenik, Croatia)
Abstract. The ocean plays a crucial role in the global carbon cycle by absorbing and storing substantial amounts of atmospheric carbon dioxide (CO2). It is estimated that the ocean has sequestered approximately 26 % of CO2 emissions over the last decade, resulting in significant changes in the marine carbon system and impacting the marine environment. The sea-surface microlayer (SML) plays a crucial role in these processes, facilitating the transfer of materials and energy between the ocean and the atmosphere. However, most studies on the carbon cycle in the SML have primarily addressed daily variability and overlooked nocturnal processes, which may lead to inaccurate global carbon estimates. We analysed temperature, salinity, pHT25, and pCO2 using data collected over three complete diel cycles during an oceanographic campaign along the Croatian coast near Šibenik in the Middle Adriatic. Our analysis revealed statistically significant differences (p < 0.05) between daytime and nighttime measurements of temperature, salinity, and pHT25. These differences may be related to the occurrence of buoyancy fluxes, which are typically more pronounced during the day and could enhance CO2 fluxes, as observed with values of 1.98 ± 2.52 mmol cm−2 h−1 during the day, while at night, they dropped to 0.01 ± 0.02 mmol cm−2 h−1. These findings emphasise the importance of considering complete diurnal cycles to accurately capture the variability in thermohaline features and carbon exchange processes, thereby improving our understanding of the role of the ocean in climate change.
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RC1: 'Comment on egusphere-2025-2090', Anonymous Referee #1, 08 Jul 2025
The paper presents the results of a field experiment investigating diel variability in the marine carbonate system within the sea-surface microlayer (SML) in the Middle Adriatic, along the Croatian coast. An instrumented catamaran collected continuous measurements of temperature, salinity, and pH over three complete 24-hour cycles and took discrete seawater samples to analyze DIC, TA, and nutrient concentrations, both in the SML and at 1 m depth (ULW).
These data were then used to calculate CO₂ partial pressure (pCO₂) and pH on the total scale at 25 °C (pHT25). The authors used the calculated pHT25, together with temperature, to correct pH values obtained from the pH electrode. The derived pCO₂ values were subsequently used to estimate CO₂ fluxes over the daily cycle.
The results showed some statistically significant differences (p < 0.05) between daytime and nighttime values of temperature, salinity, and pHT25 and/or SML and ULW. These variations are then discussed, taking into account several influencing factors. Differences in CO₂ fluxes between day and night are also highlighted, underscoring the importance of considering the full diel cycle in air–sea CO₂ flux estimations.
The paper addresses an important aspect for improving air–sea CO₂ flux calculations, but it suffers from several weaknesses, both in the methodological description and in the discussion of the results.
The introduction focuses primarily on carbonate chemistry and the relevance of pCO₂ measurements, without adequately introducing the unique characteristics of the SML and its relevance for air–sea CO₂ fluxes (e.g., see the review by Cunliffe et al., 2013: https://doi.org/10.1016/j.pocean.2012.08.004). A more comprehensive and updated bibliographic foundation is needed. For example, in line 40, Cantoni et al. (2012) does not present trends in acidification but rather biogeochemical drivers of pH variability. Doney’s work is a milestone, but more recent references are necessary. Similarly, in line 52, Cantoni et al. (2016) does not mention the SML.
The methods section is unclear. It lacks essential information needed to understand what was done, where, and on how many samples the study is based. The system used for data acquisition is not described, and there is no introduction to the study site or description of where the data were collected. See detailed comments in the manuscript.
Regarding carbonate system parameters, pH was measured continuously, but no details are provided about the electrode, calibration procedures, or accuracy. DIC and TA were measured using state-of-the-art methods on a limited number of discrete samples, as reported in Table 1: 3–4 samples during the day and 1–2 at night. These data were then used to calculate pCO₂ and pH at 25 °C. An estimate of the uncertainty associated with these calculations is necessary before discussing the results.
Using atmospheric pCO₂ values from the Mauna Loa Observatory is not appropriate for estimating air–sea CO₂ fluxes in the Mediterranean. Data from the ICOS station at Lampedusa or other regional/interpolated products should be used instead.
The data discussion is weak. Various hypotheses are proposed to explain the observed variability, but these are not supported by further analysis or robust conclusions. For instance, in line 285 and following, the effect of evaporation on SML cooling is introduced, but the conclusion simply states that this affects pHₜ₂₅, without explaining whether it increases or decreases, and by which mechanisms. Other processes are mentioned without clarification. The overall conclusion is that "it is complicated, with an interplay of several factors," but this does not advance the current understanding.
One of the main conclusions is the importance of including nighttime in the study of air–sea CO₂ fluxes. Wind speed and ΔpCO₂ are the primary variables controlling these fluxes, with wind dependence incorporated into the parameterization of “k” (line 131; Wanninkhof, 2014). Wind speeds were lower during the night (line 174), which would be expected to result in lower fluxes. However, the authors do not consider this aspect, and the relevance of diel ΔpCO₂ variability is not adequately discussed.
Further specific comments are provided in the annotated manuscript.
Overall, the paper, in its current form, is not suitable for publication in Ocean Science. However, if the authors are able to significantly improve the methodological section and strengthen the data analysis and discussion, the manuscript could be reconsidered for publication upon resubmission.
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RC2: 'Comment on egusphere-2025-2090', Anonymous Referee #2, 23 Jul 2025
Lopez Puerta et al describe the results of a short field campaign examining physicochemical parameters in the surface microlayer and underlying waters of the Croatian coast near Sibenik in the Middle Adriatic Sea. They detail differences in temperature , salinity, and pH that they attribute to changes in solar radiation and metabolic activity. Changes in CO2 fluxes during day and night are attributed to changes in wind speeds and buoyancy fluxes. The SML and controls on its various physical and biogeochemical parameters are important for a variety of reasons including the air-sea exchanges that are the focus of this work. The subject of this paper is, therefore, of interest to the air-sea biogeochemical community. However, the paper suffers from deficiencies that must be considered fully in a revised manuscript prior to its acceptance. My primary concerns have to do with a lack of methodological details and a failure to consider explanations for patterns observed in the data. Specifically, the authors need to address the potential effects of tidal mixing on their data. I detail several of these deficiencies below:
General Comments:
- what are the uncertainties in the temperature, pH and salinity measurements collected using the S3 sampler? The differences being discussed between night and day, SML and ULW, and over short timescales are quite small. Similarly, what is the propagated uncertainty in the sigma-T, pH-T25, and pCO2 calculations. One must know the uncertainties in the measurements to understand whether the fluctuations displayed in figures 5 and 6 (as just two examples of many) are relevant.
- pCO2 data from the Mauna Loa observatory does not seem relevant for the calculation of fluxes in the Mediterranean.
- More information is needed on the sampling location and environmental conditions at the time of sampling. Did the S3 move? Was it deployed in exactly the same location the entire 6 days? A better description of deployment details is needed. What is the water depth at the sampling location? Was there rainfall preceding or during the sampling event? What is the tidal range in the sampling location? What is the typical salinity range at the site (and how does this vary daily due to tides?)?
- Tides: I was surprised that the potential influence of tides on the diurnal parameter variations was never discussed given that the sampling location is described as being on the Croatian coast and appears to be at the mouth of the Krka River estuary where even microtidal tides could influence the salinity, CO2, temperature, pH parameters the authors are measuring. The authors should provide text and/or analysis demonstrating the degree to which tidal mixing of water masses does or does not contribute to the observed variations. To what extent could variations in tides influence the data in Figures 3 and 4, for example? Overlaying the tidal cycle on the graphs in Figure 3 could be very instructive and useful for providing support for or against diurnal-nocturnal versus tidal controls.
Specific Comments:
- lines 303-307 - the authors note that the Krka River normally has low discharge in summer months, but what were the discharge conditions during the time of sampling? Presumably, there are at least precipitation data for that time period, if not discharge data for the river. Those data will be effective for justifying their argument.
Lines 335-336 - why does the lack of wind which forces CO2 fluxes to near 0 demonstrate the need for gas transfer parameterizations with an intercept? While this may be true, the justification is not clear in the argument being made here.
Lines 340-342 - can the authors determine the degree to which the physical and biogeochemical differences in the SML contribute to the differences in the estimated CO2 fluxes (as opposed simply to the wind speed differences)? It seems this could be calculated using a sensitivity analysis which would demonstrate which parameters control these differences in CO2 fluxes between day and night. That type of analysis would place the importance of the diurnal-nocturnal differences in the physical and biogeochemical parameters into proper context.
- Note that 'night' is spelled incorrectly in tables 1 and S2
Citation: https://doi.org/10.5194/egusphere-2025-2090-RC2
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