the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2026
Nitrogen fixation and atmospheric deposition influences on Mediterranean surface and intermediate water nutrient dynamics
Abstract. The Mediterranean Sea is an oligotrophic basin characterised by anti-estuarine circulation and low productivity. In comparison to the global average, nutrients in the upper water column have high nitrate: phosphate ratios (or N*, nitrate minus phosphate x 16), suggested to arise from high external nutrient inputs. Modified Atlantic Water (MAW) enters with a low inorganic/organic nitrogen ratio, becomes saltier and denser moving eastwards, sinks to form Levantine Intermediate Water (LIW) ultimately feeding the Mediterranean Overflow Water (MOW), supplying nutrients to the Atlantic. This study employed stoichiometric and isotopic analyses of nitrate, dissolved organic nitrogen (DON), and particulate nitrogen to assess the roles of atmospheric deposition and nitrogen fixation on nutrient dynamics within MAW and LIW formation regions.
In the western basin, atmospheric deposition, primarily from anthropogenic sources, dominates nitrogen inputs, maintaining a high N:P ratio (N*) and low δ15N (~3.5 ‰) in MAW. Moving eastward, anthropogenic nitrogen inputs decline, lowering the atmospheric N:P contribution, allowing dust-derived phosphorus to be increasingly important and potentially stimulating nitrogen fixation. This further decreases δ15N (< 3 ‰) while altering MAW nutrient stoichiometry (lower N* and nutrient concentrations, higher organic contributions), passing these into initial LIW properties. During LIW formation, dissolved organic phosphorus (DOP) concentration decreased, suggesting DOP hydrolysis via high alkaline phosphatase activity could support nitrogen fixation along with a dust-derived nutrient source. This could contribute 54.94 ± 29.03 Gg N of new nitrogen, alongside direct atmospheric nitrogen inputs (7.4–37.2 Gg N). These new nitrogen additions are transferred to depth, forming the source composition of the LIW and influencing the properties of subsequent water masses formed further along the LIW pathway. At the basin scale, the LIW nitrogen content is largely determined by Atlantic DON, modified by changes in nitrogen inputs from atmospheric nitrogen deposition and nitrogen fixation to the surface water, controlled by phosphorus availability.
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- RC1: 'Comment on egusphere-2026-4372', Anonymous Referee #1, 17 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-4372', Anonymous Referee #2, 29 Oct 2026
reply
The manuscript addresses an interesting question in Mediterranean biogeochemistry by combining nutrient distributions, nitrogen isotopes, organic nutrient measurements, and atmospheric deposition data. However, the quantitative attribution of nitrogen fixation to newly formed Levantine Intermediate Water (LIW) remains uncertain. I have the following comments.
Major comments
- Nitrogen fixation estimate: The isotope mass balance compares δ¹⁵N-DON in incoming water with δ¹⁵N-NO₃ in LIW, assuming a nitrogen-fixation endmember of −1‰. These pools are not directly equivalent, and atmospheric nitrogen deposition may have a similar isotope signature. The authors should test the sensitivity of the estimate to endmember selection and acknowledge that the sources cannot be uniquely distinguished.
- Water-mass transformation and nitrogen budget: The comparison between pre-formation and post-formation waters relies on samples collected at different locations and times. Mixing, remineralisation, and seasonal variability could contribute to the observed differences. The authors should clarify these limitations and provide a more transparent mass-balance framework, including the transport timescale and uncertainty propagation.
- Nitrogen fixation and isotope evidence: The direct nitrogen-fixation measurements show decreasing rates from west to east, whereas the isotope-based interpretation needs to be reconciled with this pattern. As nitrogen-isotope endmembers overlap, δ¹⁵N-POM alone cannot uniquely establish the contribution of nitrogen fixation.
- Phosphorus limitation: High N:P ratios alone do not establish P limitation. Absolute nutrient concentrations, bioavailability, and the biological process being considered are also relevant. The authors should clarify what is supposedly P-limited and provide stronger support for this interpretation.
- DOP removal and nitrogen fixation: The observed decrease in dissolved organic phosphorus (DOP) is consistent with phosphorus regeneration but does not demonstrate that this process stimulated nitrogen fixation. This mechanism should be presented as a hypothesis unless supported by direct evidence.
- Seasonal extrapolation: The study uses summer observations to infer nitrogen cycling during winter LIW formation. The authors should justify this extrapolation and qualify the resulting nitrogen budget accordingly.
Specific comments
- Lines 31–33: “This could contribute 54.94 ± 29.03 Gg N of new nitrogen, alongside direct atmospheric nitrogen inputs (7.4–37.2 Gg N).” There is no clear timescale. I would advise expressing these quantities in flux terms, with the relevant integration period specified.
- Line 40: “Significant external nutrient inputs.” The word significant is vague here. Rather, provide the magnitude of the inputs.
- Lines 40–41: “High nitrogen (N) to phosphorus (P) (N:P) ratios.” Please mention the median value or a range.
- Line 60: “2.5–3.3 times higher primary production.” Are these numbers precise to the first decimal place? If not, I would suggest “2–3 times higher.”
- Lines 60–64: How do high N:P ratios alone indicate P limitation? Shouldn't the absolute P concentration also matter? Many oligotrophic regions have low surface-water P concentrations but remain N-limited. Please clarify what process is supposedly P-limited and support this interpretation.
- Line 80: “δ¹⁵N signals could arise from isotopically depleted DON entering via the Strait of Gibraltar.” What is the nature of this DON? How labile is it, and how could its isotopic signature be transferred to phytoplankton and measured in POM?
- Line 81: Change “tracers” to “tracer.”
- Lines 72–86: I do not think these are necessarily contrasting results. Direct measurements are not a substitute for proxy results, but both must be considered. The direct measurements show decreasing nitrogen-fixation rates from west to east. Given the overlap in nitrogen-isotope endmembers, I would be cautious about relying on δ¹⁵N-POM alone to infer nitrogen fixation.
- Line 138: Please define DOCN at first use and clarify how it differs from DON and total dissolved nitrogen.
- Line 386: “The increased anthropogenic sources of nitrogen in the west.” Do you mean increased anthropogenic nitrogen inputs? If so, what evidence supports this interpretation?
- Lines 387–388: “Increased mixing in the western basin.” Where is the analysis of mixing presented that supports this conclusion?
- Line 391: “Where nutrient requirements are not exceeded.” How were these requirements calculated, and what evidence supports this statement?
Citation: https://doi.org/10.5194/egusphere-2026-4372-RC2
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