Long-term nitrogen burial exceeds denitrification in global fjords

Domain: biology
Published: 2026-04-01
Editorial outcome: minor_revision

Abstract
Nitrogen (N) availability regulates primary productivity and hence directly affects global oceanic carbon sequestration. Global fjords account for up to 11% of marine carbon burial. However, N loss via sediment burial remains largely unquantified. Here, we show that global fjords are hotspots of N burial, accounting for up to 18% of oceanic N burial despite only covering 0.1% of the ocean area. Burial is the dominant N loss mechanism, exceeding microbial N loss via denitrification and anammox, which are generally considered the major N loss mechanisms. Microbial N loss dominates in anoxic fjords and appears to be a function of temperature and nutrient availability. Overall, fjords efficiently sequester excess N in sediments over long timescales. Accelerated warming will promote both N burial from increased primary production and microbial N loss from warmer temperatures, affecting N budgets in fjords and in the ocean in general.

Introduction
Nitrogen (N) availability is intimately linked to carbon held within sequestered in marine sediments. Excessive N in fjords can be removed marine organic matter1,2. In addition to typical blue carbon habitats via microbial N production as well as burial of particulate organic N such as mangroves, seagrass meadows, and salt marshes, fjords are and mineral-bound inorganic N. Denitrification, the microbial reduc- also key coastal marine ecosystems, burying ~11% of the global marine tion of nitrate to N , is often considered the main N removal 2 organic carbon3,4. Since N availability is a key driver of marine primary mechanism in the ocean13,14. As a consequence, earlier fjord studies productivity, N uptake and loss are intimately linked to the efficiency have mainly focused on N loss via denitrification15,16. The contribution of carbon sequestration5. Growing anthropogenic N inputs to coastal of global fjords to marine N burial17 remains largely unknown. In oceans have promoted eutrophication and subsequent oxygen (O ) addition, the overall importance of anammox–the second most 2 depletion6. Additionally, warming-induced stratification reduces O important microbial N production pathway18–remains unclear in fjord 2 supply to fjord deep waters, thereby favouring deoxygenation. Coastal systems.

Fjord N loss can be directly and indirectly related to anthro- N enrichments not only modify carbon sequestration efficiency7,8, but pogenic stressors. Weak mixing limits O replenishment in fjord deep 2 also enhance greenhouse gas emissions, which offset the climate- waters, which may lead to hypoxic or even anoxic conditions19. mitigating capacity of coastal systems9,10. Quantification of N loss in Deoxygenation has been spreading in fjords worldwide, stimulating mid- to high-latitude blue carbon ecosystems is hence critical to microbial N loss20–23. As a consequence of increased anthropogenic N understanding their role in mitigating N enrichment from both direct loads to the ocean, N inventories in global fjords have increased over (agricultural runoff, aquaculture, and wastewater) and indirect (glacial the past decades24. Anthropogenic activities also indirectly impact N meltwater and soil erosion) sources.

Fjords are mid- to high-latitude coastal ecosystems with steep cycles in remote, polar fjords25. Arctic atlantification, i.e., the extension catchments and restricted deep water movement. Such features result of warm, nutrient-rich Atlantic waters into the Arctic26, amplifies in high sedimentation and organic matter burial11. Despite numerous warming in the Arctic Ocean27, favouring deoxygenation28 and thereby studies on carbon cycling in global fjords3,12, less is known about N altering microbial N transformation29. Accelerating glacier melting cycling processes, including burial rates of blue nitrogen–N influences fjord biogeochemistry via the introduction of N-rich meltwater30,31. It is thus essential to resolve key N loss mechanisms to understand N cycling in global fjords under increasing anthropogenic pressures.

Results
Large nitrogen burial rates in global fjords
Nitrogen burial, represented as nitrogen accumulation rates (N ), AR ranged from 0.10 to 16 g N m−2 yr−1 in global fjords (Fig. 1a). Our spatial weighted bootstrap analysis yielded a global N median (interquartile AR range, IQR) of 3.9 (3.7–4.1) g N m−2 yr−1 (Fig. 2a). This areal rate exceeds most marine systems and even global lakes (2.5 g N m−2 yr−1)33, implying that fjords are hotspots for N burial. Arctic, North Atlantic, and North Pacific fjords exhibited N burial rates 14 to 19% higher than the global median, whereas those in the South Pacific (Chile and New Zealand) and Antarctica have rates 45 to 85% lower than the median (Fig. 2a). Fjord N burial rates significantly increased with latitude (Fig. S1). Large N burial rates were mostly observed in high-latitude fjords (≥60°) in Greenland, Canadian Arctic, and Svalbard (Fig. S2). Scaling to the updated global fjord area (259,899 km2; ref. 34), we estimated a median (IQR) global N burial of 1.0 (1.0–1.1) Tg N yr-1 (Table S1), representing 2–10% of N burial in global ocean sediments (10–58 Tg N yr−1; ref. 17. and references therein). Using the larger, commonly referred fjord area (455,535 km2; ref. 35) increases estimate to median of 1.8 (1.7–1.8) Tg N yr-1, or 3–18% of global marine N burial17. Despite only representing 0.1% of the global ocean area34,36, such disproportionately high N burial rates reveal that fjords are hotspots for N burial in the ocean.

Since N burial rates are derived from mass accumulation rates (M ), the high N burial capacity is necessarily linked to M (Fig. 3a). AR AR Similar to C burial, N burial rates are predominantly associated with M 3,37,38. N and C burial were highly correlated across global fjords AR (Fig. S1). High M was often observed in polar and subpolar regions AR (Fig. 2b). The global pattern of M generally followed that of N burial AR across ocean basins, except for the two glacier-influenced Antarctic fjords, where M was comparable to the global median yet with lower AR N (Fig. 2a, b). Higher N were observed at the head of polar fjords AR AR (e.g., Hornsund and Kongsfjorden) (Fig. S2) with marine-terminating glaciers acting as large allochthonous sediment sources5,37,39. In polar fjords, large sediment inputs from glacier meltwater and erosion11 can act as key sources of fjord inorganic N40, which could consequently sustain high N burial rates found in Arctic fjords and Greenland fjords in the North Atlantic (>60° N).

Sediment stoichiometry revealed latitudinal variations in the quality of buried organic matter, as indicated by the C:N ratio, and N burial (Fig. S1). The negative correlation between this ratio and N burial implies higher N burial rates in fjords with high labile organic matter. A clear shift from preponderantly terrestrial (refractory) to marine (labile) organic matter in sediments occurred from temperate towards polar fjords (Fig. S1). Labile organic matter from marine primary production has a lower C:N ratio (<10) than terrestrial organic matter (>15)41,42. This aligns with the low sediment C:N ratios in polar fjords, where organic matter is predominantly derived from marine primary production5. Notably, high productivity can be sustained by nutrient inputs from nutrient-rich deep water upwelling, subglacial discharge, and/or N-rich meltwater from retreating glaciers43–45. High sedimentation rates shorten O exposure for organic matter degradation, 2 allowing efficient preservation of labile organic matter46 and associated N burial in Arctic fjords47. Combined with high sedimentation rates from retreating calving glaciers48, an even higher N burial is currently occurring in high-latitude fjords. With increasing sedimentation and decreasing organic matter C:N ratio (Fig. 3a), N burial in fjords is also expected to increase. In contrast, more refractory organic matter in temperate, terrestrial-dominated fjords (Fig. S1) reflects the underlying oligotrophic conditions with low primary production2, and associated lower M and N burial, particularly in the Southern AR Hemisphere (Fig. 2a, b).

Fjord nitrogen burial surpasses microbial nitrogen loss by denitrification and anammox
Denitrification (nitrate→N ) and anammox (ammonium+nitrite→N ) 2 2 are often considered the dominant N loss pathways in marine ecosystems49. Nitrogen loss by these two N production processes 2 ranged from 0.10 to 21 g N m−2 yr−1 across 20 fjords globally, including our observations from five fjords (Fig. 1b). Spatially weighted bootstrap analysis yielded a global median (IQR) of 2.4 (2.0–2.8) g N m−2 yr−1. Fjord N production decreased at high latitudes and increased in 2 temperate and subpolar fjords (<60°) in the North Atlantic (Fig. 1a). The lower regional median (~0.9 g N m−2 yr−1) in Greenland and Svalbard likely reflect slower temperatures and organic content availability for N production processes compared to temperate fjords, given that 2 both temperature and organic carbon were positively correlated with sediment N production (Fig. S1). Overall, water temperature, nitrate + 2 nitrite concentrations, and oxygen concentrations are significant factors driving fjord N production (Fig. 3b). While based on measure- 2 ments from only two anoxic fjords, N production was notably higher 2 under anoxic conditions (Fig. 4a). Median N production rates in 2 anoxic (<1 µM O ) fjords is 9 times higher than in oxic fjords (Fig. 4). 2 Hence, anoxic fjords, in particular, are hotspots of N production at 2 rates up to 6 times higher than the global fjord median.

Denitrification and anammox primarily occur in anoxic sediment layers, where fixed N derives mostly from nitrification or diffusion from the overlying water. Benthic denitrification increases with decreasing O , particularly below the typical threshold of hypoxia 2 (<63 µM; ref. 50.). For instance, greater sediment denitrification and anammox rates were measured when bottom water O concentrations 2 decreased from 260 to 61 µM in Gullmar Fjord, Sweden (Fig. S3). Similarly, greater N production was observed in the hypoxic Loch 2 Etive51 compared to oxic fjords (Fig. S4). When anoxia develops in fjord basin waters, benthic N production will cease due to restricted nitri- 2 fication and limited nitrate concentration in deep water52. The active nitrate reduction zone then rises from the sediment to the water column53. Typically, this layer’s thickness increases orders of magnitude from millimetres-centimetres in sediments54 to decimetres- meters scale in the water column oxic-anoxic interface21,22. Such sub- stantial increases in the volume of the nitrate reduction zone enhance overall N production in anoxic fjords. 2

Including our measurements from five fjords, paired quantifications of N burial and N production become available for a total of 16 2 fjord worldwide (Fig. 1c). N burial exceeded microbial N production 2 as the major N loss mechanism in 75% of these fjords. The median N burial contribution to total N loss (N burial + N production) was 65% 2 across fjords (Fig. S4). High-latitude fjords with large sedimentation rates and low N production rates in Greenland and Svalbard exhibit 2 remarkably high N burial contributions reaching 90% (e.g., Kobbefjord). In contrast, microbial N production remarkably exceeded burial 2 in three anoxic/hypoxic fjords (Saanich Inlet, By Fjord, and Loch Etive) (Fig. S4). However, even including exceptionally high N production 2 hotspots in anoxic/hypoxic fjords, these global estimates show that N burial exceeds N production (Fig. 4c), challenging general paradigms 2 for N removal in the marine environment55–57. Our analysis clearly indicates the importance of N burial as a long-term N storage mechanism in fjords globally.

Fate of nitrogen in deoxygenated, eutrophic and warmer fjords
Up to nine-fold greater N production in anoxic than in oxic fjords 2 (Fig. 4a) implies that complete deoxygenation shifts the dominant N removal pathway from sedimentary burial toward microbially mediated N loss. Fjord N production increased gradually from oxic to 2 hypoxic conditions with a first threshold at 99 µM O , followed by a 2 sharp increase as O declined toward anoxia down to low micromolar 2 range (<20 µM) (Fig. S5; Table S2). Remarkably, N production rates in 2 anoxic fjords were comparable to or even higher than those in the anoxic, highly eutrophic Baltic Sea (0.30 to 11 g N m−2 yr−1)58. The sub- stantial increase in average N production in anoxic fjords is primarily 2 due to the regime shift and expansion of the nitrate reduction zone, i.e., the anoxic but nitrate containing zone58 (Fig. 5). Indeed, anoxia can inhibit nitrification and subsequentially limit N production, resulting 2 in a transient phase of elevated N production between oxic and anoxic 2 conditions. Since no significant differences in N burial were found between oxic and anoxic fjords (Fig. 4b) we propose that N produc- 2 tion will surpass N burial and dominate N loss with developing anoxia. Combining the two mechanisms, median (IQR) total N loss was 5.3 (3.5–6.1) g N m−2 yr−1 in oxic fjords and increased to 6.2–14.0 and to 3.6 – 25.0 g N m−2 yr−1 in hypoxic and anoxic fjords, respectively. The extent of future changes in fjord N loss pathways are intrinsically sensitive to perturbations of O levels, temperature and nutrient 2 availability (Fig. 3b), which are all intimately linked to a warming climate59. In particular, intensified warming in the Arctic, or Arctic amplification, will stimulate deoxygenation and will likely create hypoxic zones in narrow and inner coastal basins28. Hence, we expect a stronger contribution of N production to N loss in fjords as currently 2 observed in hypoxic and anoxic fjords worldwide (Fig. 4c). Ocean warming also promotes the northward expansion of warmer, nutrient- rich waters from the Atlantic into the Arctic60–62. Warmer fjord waters favour sediment N production (Fig. S1)63. Higher inorganic N 2 availability (for example from increased Atlantic inflow64) can also stimulate sediment N production (Fig. 3b), particularly via anammox65. In 2 addition to the direct stimulatory effects of warming and nutrient enrichment onto N production (Fig. 3b), the increased availability of 2 labile organic matter due to atlantification-driven primary production may increase sediment N production in Arctic fjords45,63,66. Climate 2 change stimulates primary production in high-latitude fjords and subsequent deposition of labile organic matter on sediments45, favouring both N burial and benthic N production (Fig. S1). Combined 2 with potentially higher N burial due to warming-mediated glacial retreat and increased sedimentation rates48, our results suggest an increased N loss in global fjords along with a warmer and less oxygenated ocean.

Our global analysis demonstrates that sediment burial is the most effective N sink mechanism in fjord systems under current conditions. Despite covering <0.1% of the ocean, fjords contribute up to 18% of global N burial. This makes them hotspots of blue nitrogen, i.e., nitrogen sequestered in marine sediments. On average across fjords globally, sediment burial accounts for 60.6±5.0% of total N loss, exceeding denitrification (34.8±4.5%) and anammox (4.6±2.0%). Thus, burial plays a surprisingly significant role in the long-term N removal of fjords globally, making it a crucial mechanism for mitigating coastal N enrichment at medium and high latitudes. Additionally, N burial is an effective removal pathway with minimum climatic impact, compared to microbial nitrification and denitrification, which produce the greenhouse gas nitrous oxide (N O; equivalent to ~1% of N pro- 2 2 duction in an anoxic fjord; ref. 21,67.). Burial in fjords is driven by sedimentation rates and the higher lability of deposited organic particles (Fig. 3a), which can be favoured by increasing rates of primary production, particle deposition, and glacial retreat68,69. However, natural hazards such as landslides and floods can alter sediment delivery and hence N burial efficiency70,71.

The control of environmental factors on fjord microbial N loss via denitrification and anammox is intricate72. At the regional scale, warming and increasing N concentrations will enhance microbial N loss following expanding deoxygenated areas24,28. Given the hydrogeomorphic characteristics of fjords and the warming climate, O 2 deficiency is expected to develop more frequently, including high- latitude fjords impacted by Arctic amplification and atlantification26,59. Indeed, we show that microbial N loss dominates over geological burial loss due to the shift in N production regimes following the develop- 2 ment of anoxia (Fig. 5). Rates of microbial N loss are governed by redox conditions, substrate availability, and the extent of the active nitrate reduction zone. Despite large rate variability, our observations establish a robust link between fjord N loss and deoxygenation. Additional work should focus on the climatic effect of fjord N loss via N O pro- 2 duction, and on refining the role of fjords in the global N cycle. Whether N burial can counter coastal nutrient overenrichment (i.e., eutrophication) and curb greenhouse gas emissions largely depends on coastal nutrient management. The future partitioning of N loss mechanisms in fjords is predominantly linked to their deoxygenation level, eutrophic status, and hydrological response to global warming.

Methods
Global fjord database collection and upscaling
Our global fjord database combining sediment mass accumulation rates and N content, consists of 158 observations across 79 fjords. Major fjord regions are covered11, including North-western Europe (Scotland, Sweden, Norway, Iceland, and Faroe Islands), Greenland, Svalbard, Western Canada, Eastern Canada, the Canadian Arctic archipelagos, Alaska, Patagonia, New Zealand, and Antarctica. We excluded shallow fjards (which are not true fjords but rather drowned estuaries) with average water depth <10 meters, such as those in Denmark, due to the hydrodynamical and physicochemical differences. We assumed that sediment particle size was relatively homogenous across sites, given a mean porosity of 0.78±0.06. Global rates were determined by upscaling to the global fjord area34,35 (Table S1).

Nitrogen burial rates, expressed as N , were further compared to AR microbial N loss rates (i.e., N production) that were measured in the 2 corresponding fjords. Our global compilation of fjord N production 2 measurements consists of 47 observations across 20 fjords. Reported areal rates were measured by either intact core incubations, water column incubations, or in situ chamber incubations using isotope pairing techniques73,74. Areal N production rates obtained in the water 2 column were obtained in the case of anoxic fjords. Measurements of both N production and N burial are available for a total of 16 fjords 2 located mainly in NW Europe, Svalbard, Greenland, and Eastern Canada.

Original observations in Swedish and Icelandic fjords
Data on both N burial and N production were collected in five 2 temperate (57–62˚N) and subpolar (65°N) fjords along the west coast of Sweden and the east coast of Iceland, respectively (Fig. S6). These fjords have distinct watershed characteristics, anthropogenic influences, and redox gradients, including oxic, seasonally hypoxic, and long-term anoxic conditions (Table S3). Samples were collected at three sampling stations at the head, centre, and mouth of each fjord aboard the R/V Skagerrak. For the Swedish fjords, samples for mass accumulation rates and sediment stoichiometric analysis were collected in September-October 202175 and N production rates in 2 sediments and water columns were determined by 15N-labelling incubation experiments in 2023 and 2024. For Icelandic fjords, samples for all analyses and incubations were collected in June 2023. Mass accumulation rates and sediment stoichiometry from Icelandic fjords were reported in a previous publication76, which were determined based on particle-reactive radiotracer-derived age models and sediment stoichiometry. Detailed descriptions of sampling procedures of each campaign are given in the Supplementary Materials.

Determination of nitrogen accumulation rates and N production
2
Total N burial rate, defined as N accumulation rate (N ) in the sedi- AR ment, was calculated from mass accumulation rates, porosity, bulk density, and sediment N content in collected sediment cores and in the literature (Supplementary Materials and references therein). Mass accumulation rates (M ) were determined based on particle-reactive AR radiotracer-derived age models in the sediment cores and were obtained from published data75. Age models were created for each core using 210Pb, 137Cs and/or 241Am profiles. Dry bulk density and porosity were determined for each sediment slice by measured wet and dry weights. Radionuclides 210Pb, 137Cs, and 241Am were analysed using an ORTEC HPGe GWL-series well-type coaxial low-background germanium detector. Linear sedimentation rates were estimated by the constant flux–constant sedimentation (CF-CS) model, ensuring a logarithmic regression fit with R²>0.75, and were further constrained by 137Cs and 241Am peaks. Mass sediment accumulation rates were calculated using the constant rate of supply model and validated against 137Cs peaks. N accumulation rates were derived from multiplying M by the weight fraction of total nitrogen (TN%). When only AR organic carbon accumulation rates and sediment C:N ratio were reported in the literature, N were calculated considering the sedi- AR ment stoichiometric ratio.

Sediment N production rates were determined through 2 15N-labelling incubation experiments by the revised-isotope pairing technique74. Collected intact cores were spiked with 15NO – (11, 24, 56, 3 and 120 µM mean final concentration) to determine rates of N pro- 2 duction (denitrification and anammox) and N recycling (dissimilatory nitrate reduction to ammonium rates, DNRA). At the centre station of By Fjord, total N production rates were determined in the water col- 2 umn as N production is restricted in the sediment due to depletion of 2 O , NO – and any other N oxide species23. 15N-labelling incubation 2 x experiments were conducted with samples collected 21 m below the surface, which covered the NO – containing oxic-anoxic water layer. x Additional incubation procedures and rate calculations are given in the Supplementary Materials.

Spatial weighted bootstrap analysis
We performed spatial weighted bootstrap analysis to minimize the impact of geographical sampling bias on our estimates of the global mean of N loss processes. Such an analysis helps to minimise uncertainties derived from small datasets with geographical sampling biases. We assigned sample weights based on their spatial proximity to other samples using the inverse weighting algorithm32,77 with a spatial scale of ~55 km (0.5 degree). Using the resulting weights (Fig. S7), we performed a bootstrap analysis to generate distributions of 1000 weighted bootstrapped means for both N and N pro- AR 2 duction rates. The observed raw data distribution and bootstrapped data used in the analyses were illustrated in histograms (Fig. S8). Both spatial weighing and bootstrapping analysis were performed in R version 4.3.2.

Statistical analysis
The relationship between environmental variability and N removal rates was estimated from the correlation of N burial vs. total N production rates with individual environmental factors. 2 Spearman’s rank correlation coefficients were calculated to minimize impacts of collinearity. A one-way permutation test was used to reveal the difference of N and N production rates AR 2 across oxic and deoxygenated fjords, as well as the difference in mean N production processes between oxic and hypoxic sedi- 2 ment in Gullmar Fjord. To examine potential changepoints in the relationship between N production and O concentration across 2 2 redox regimes, we fitted a Bayesian segmented regression model with three linear segments using the mcp package (Table S2). Correlations and differences across conditions were considered statistically significant with p value<0.05. All statistical analyses were performed in R version 4.3.2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a well-written, interesting and important paper, which I believe may be accepted for publication after minor edits. I\nmade some comments and proposed some changes directly in the manuscript and supplementary materials word files. The\npaper is easy to read and conveys the main messages in a very clear manner.\nWhat are the noteworthy results? Revealing the importance of nitrogen burial in fjord systems worldwide and how its relative\nimportance relates with the presence of anoxic environments, where denitrification and anammox gain importance.\nWill the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nI think so, and it looks to be original.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nI believe that the conclusions are well grounded. Additional evidence is always welcome but a considerable amount of data\nwas used to support the claims made by the authors, covering fjords in both hemispheres and at various latitudes.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nI did not detect any fails that should prevent publication or require revision. I found some statements that I believe should be\nrevised but that do not compromise the claims made by the authors. I added comments to the manuscript about these small\nthings that should be corrected.\nIs the methodology sound? Does the work meet the expected standards in your field?\nI believe so.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes. Moreover, the authors provide many methodological details in the Supplementary material.", "reviewer_2": "# General assessment\nThis review concerns the manuscript “Long term nitrogen burial exceeds denitrification in global fjords” by Cheung, Levin,\nSmeaton, Politi, Thamdrup, Santos, and Bonaglia. I would like to recognize the considerable effort invested in this study,\nparticularly the integration of multiple techniques, extensive literature review, and conceptual development. The work\naddresses an important gap in nitrogen cycling in fjords and provides valuable insights. The comments below reflect my\npersonal opinion and are intended to improve the clarity, robustness, and relevance of the manuscript.\nThis manuscript addresses an important gap in global nitrogen cycling by quantifying burial versus microbial N loss in fjords.\nThe topic is novel and relevant, and the integration of new measurements with a global synthesis is a major strength. The\nmethods (radiotracer dating, isotope pairing, spatial bootstrap) are technically sound, and the conceptual model (Fig. 5) is a\nuseful framework. However, several aspects require clarification or improvement to strengthen the manuscript. In its present\nform, the discussion is clear which makes the key results easy to understand. However, some of the interpretations remain\n“too obvious” and I think the manuscript will benefit a more thorough discussion of the results to provide more critical\nfindings. I also have concerns with the statistical analyses.\nBelow are more detailed comments:\n- Abstract\nL17–18: The statement on long-term sequestration is too categorical. Geological stability can be disrupted by natural\nhazards (e.g., landslides), which should be acknowledged.\nL18–20: The link between accelerated polar warming and increased burial/microbial N loss needs clearer mechanistic\nexplanation.\n# Main\nL29–31: Expand on anthropogenic pressures in mid/high-latitude fjords (e.g., aquaculture, salmon farming, wastewater\ninputs).\nThe phrasing of “anthropogenic enrichment” is misleading. Climate-driven processes (glacier melt, soil erosion) are not\ndirect anthropogenic inputs. Consider explicitly distinguishing direct human activities from climate-mediated changes.\nAdd a note on stratification and its role in isolating deep waters, as this is critical for deoxygenation.\n# Large nitrogen burial rates in global fjords\nL75 : This needs a reference (percentage of global marine N burial)\nL78–79: Since N burial rates are derived from MAR, the observed correlation seems circular. Perhaps clarify this in the text\nto avoid misinterpretation as an independent relationship.\n# Fjords N burial surpasses microbial N loss by denitrification and anammox\n(Sometimes you use « nitrogen » and other « N » in title. Please be consistent)\nL111-112 : Please clarify the statement on lower OM in arctic fjords\nL120 : Please clarify « hypoxic sediment ». Do you mean hypoxic bottom water or sediment layer ?\nL122 : Comparing to the St. Lawrence Estuary is problematic, it is not a fjord. Crowe et al. studied the estuary, not Saguenay\nFjord in the St Lawrence system. Same for Thibodeau et al. I am not aware of published data on N cycling in the Saguenay\nFjord.\n# Fate of nitrogen in deoxygenated, eutrophic and warmer fjords\nL141-148: This paragraph is confusing. It begins by stating that N2 production is higher in anoxic than in oxic fjords, but then\nclaims that N2 production is similar to or higher in oxygen-depleted fjords (interpreted here as hypoxic but not fully anoxic)\nthan in the anoxic Baltic Sea. However, the subsequent discussion focuses solely on anoxic fjords.\nCould anoxia reduce N2 production because nitrification cannot occur? This might explain why N2 production is similar to or\nhigher in oxygen-depleted (hypoxic) than in fully anoxic fjords. In that case, assuming climate change will lead to anoxia\n(L148), the increase in N2 production would represent a transient state between oxic and anoxic conditions.\nL142 : Please clarify « geological vs biotic dominated N-loss »\nL144: Could N2 production occur in anoxic microenvironments within suspended organic matter? Worth mentioning but from\nyour data it doesn’t seem so.\nClarify “displacement” vs. “expansion” of the nitrate reduction zone.\nL162-163 : This needs a reference.\nOverall, in the present form, the climate prjoections seem speculative and would need more support to appear robust. Also,\nthe statistical analyses used in this study, while correct, are not the most efficient ones to make robust predictions.\n# Summary and implication\nL168-169 : You mean « currently » ?\nL170 : Could you define « blue nitrogen » ?\nL170-171 : The percentages do not add up to 100\nL171-172 : You mean « N loss » other wise it is somewhat obvious that burial play a key role in long-term N storage.\nL174-175 : Nitrification is often the main process producing N2O\nL175-178 : It is not easy for me to follow the rationals here. What comes from your results and from the littérature ?\nL185 : You show a clear shift following anoxia, not deoxygenation\nL189 : I do not think you defined « eutrophication » inthe manuscript\nL189-190 : Do you think that enhanced N burial in the sediment could also stimulate CO2 and CH4 production which, if\nreleased, might offset the reduction in N2O production?\n# Methods\n## Global fjord database collection and upscalling\nFor your upscaling, mention the assumption that fjord seafloors always consist of soft sediments. This holds for deep basins\nbut not always for sills, steep fjord « walls », or areas with boulders from glacier/sea-ice melt.\nDo you have details on sample locations within the fjords for the data you retrieve from litterature? High spatial heterogeneity\noften occurs between the head and mouth.\n## Statistical analyses\nThe Spearman rank correlation test assesses monotonic relationships (not necessarily linear), so why is a linear model\nshown in Figure 3? Moreover, no predictions can be drawn from the Spearman correlation test. Since you show linear\nmodels, could you consider using GLMs (or GAMs if not linear) on these data instead, and potentially incorporate multiple\npredictors simultaneously? This could strengthen your points and broaden the scope of your data.\nGiven the differences in n between groups (see Figures 2 and 4), could you also report effect sizes?\nI think the Wilcoxon rank-sum test is not the best option here because you have three groups. It is like doing pairwise tests\nwithout applying a p-value correction, which increases the risk of false positives.\nAnother point: one group has n = 2, which is too small for reliable statistical testing. It is nice that the data are actually shown\nin Figures 2 and 4, but from my perspective the anoxic group should be treated as exploratory observations. You could try\npermutation tests, although this would not fully resolve the issue of such a small group. As mentioned above, effect sizes\nmay be more meanigful in this case. To be fully transparent about the likely large uncertainty in this group, you could also\nreport bootstrap confidence intervals which sould be large.\nFrom my perspective, the unfortunate lack of data from anoxic fjords, combined with the global variability in the dataset,\nundermines the robustness of the findings linking nitrogen cycling to deoxygenation.\n## Figures\nMake sure to use a color-blind-friendly palette.\nFigure 3: I think the figure would benefit from a clearer rationale for how the panels are arranged. At the moment, the\nsequence does not follow an obvious logic, at least to me. For example, NAR is shown as a function of latitude (a) and of the\nC:N ratio (d), and then the C:N ratio is shown as a function of latitude (e). What are the objectives of presenting each of these\nrelationships? Are these panels simply a subset of all significant relationships among the possible variable combinations or\nrely on hypotheses?\n# Supplementary material\n## Methods\nThe methodology is technically sound; a few suggestions:\nL91–108: For reproducibility, could you report the 15N-NO3 concentration used for this?\n## Figures\nFigure S3 is not referenced in the main text. I understand the conceptual idea behind the exponential model, but would a\nsegmented model or GAM be more informative? With n=2 in anoxia, you may not be able to detect a significant tipping point,\nbut using the segmented or the mcp R package within a Bayesian framework might still allow you to explore potential « real »\nchange points.\nDoes the reported O2 concentration correspond to bottom-water values, and did you test for a relationship between N burial\nand O2 ? Finally, from this figure it is not obvious that N burial exceeds N2 production, especially under well-oxygenated\nbottom waters, where most organic matter may already have been remineralized through oxic processes before reaching the\nseafloor. This point seems worth discussing.\nI look forward seeing this work refined."}, "author_response": "We thank Reviewer #1 for the supportive feedback and for the suggested changes, which positively\nshaped our revisions and further improved the quality of the manuscript.\nComments on the manuscript\nL. 10: “Global fjord systems account for up to 11% of total marine C sequestration”.\nThis does not seem correct. Please see my comment below about these numbers. The value of 11% is\nbased on reference number 3, which suggests that fjord system bury 11% of the total buried by marine\ncarbon burial. However, burial is not the only form of sequestration. A large part of sequestration is\ndue to cooling and under saturation of ocean water at high latitudes.\nWe have edited this passage to be precise and avoid confusion:\n“Global fjord systems account for up to 11% of total marine C burial.” (Page 1, Line 10).\nL. 24: “fjords are also key coastal marine ecosystems that bury up to 11% of marine carbon globally”.\nPlease see my comment in the Abstract. I suggest specifying here that is 11% of the total carbon buried\nin marine systems to avoid confounding with total sequestration.\nWe have reformulated this sentence to avoid confusion:\n“… , fjords are also key coastal marine ecosystems burying ~11% of the global marine organic\ncarbon 3, 4..” (Page 2, Lines 24–25).\nL. 47: “Arctic amplification favours deoxygenation and enables atlantification”.\nI do not think that this sentence is accurate. How does Arctic amplification favour Atlantification? I\ndon’t recall any study supporting this statement. I would put it the other way around: ”Atlantification\nenhances Arctic amplification”. Amplification likely results from the climate-albedo feedback but areas\nmore exposed to Atlantification are those warming faster in the Arctic.\nWe agree and have revised the statement with new references to avoid confusion:\n“Arctic atlantification, i.e., the extension of warm, nutrient-rich Atlantic waters into the Arctic\n26, amplifies warming in the Arctic Ocean 27, favouring deoxygenation 28 and thereby altering\nmicrobial N transformation 29.” (Page 3, Lines 49–51).\nL. 96: “primary production sustained by glacier meltwater nutrients”.\nI don’t think one may generalize that primary production is sustained by glacier meltwater nutrients.\nIn several fjords, nutrient inputs from the sea are the major source and in many cases the subglacial\nfjord discharges promote upwelling of nutrient-rich fjord deep water, without providing by themselves\na significant input of nutrients. See, for example, the following references: 10.1038/S41467-018-\n05488-8, 10.1038/S41598-025-06953-3\nWe reformulated the statement as follows:\n“This aligns with the low sediment C:N ratios in polar fjords, where organic matter is\npredominantly derived from marine primary production 5. Notably, high productivity can be\nsustained by nutrient inputs from nutrient-rich deep water upwelling, subglacial discharge,\nand/or N-rich meltwater from retreating glaciers 43, 44, 45.” (Page 5, Lines 97–100).\nL. 100: “This burial is expected to increase, giving fjords a central role in global N cycle.”.\nWhy is it expected to increase?\nWe have incorporated a predictive model for N burial, which includes sediment C:N ratio (i.e., lability\nof deposited organic matter) and sedimentation rates as key predictors (Fig. 3a). Considering their\nstimulatory effects, we reformulated this section to better support this statement:\n“High sedimentation rates shorten O exposure for organic matter degradation, allowing\n2\nefficient preservation of labile organic matter 46 and associated N burial in Arctic fjords 47.\nCombined with high sedimentation rates from retreating calving glaciers 48, an even higher N\nburial is currently occurring in high-latitude fjords. With increasing sedimentation and\ndecreasing organic matter C:N ratio (Fig. 3a), N burial in fjords is also expected to increase.”\n(Page 5, Lines 100–104).\nFigure 3 Predictive models for sediment nitrogen burial and N production across fjords globally. (a) Observed versus\n2\npredicted fjord nitrogen accumulation rates (N ) derived from a generalized linear model including sediment mass\nAR\naccumulation rates (M ) and sediment C:N ratio (C:N). (b) Observed versus predicted fjord N production rates derived from\nAR 2\na generalized linear model including water temperature (temp), nitrate and nitrite concentrations (NO), and restricted\nx\nmaximum likelihood fitted oxygen concentrations (O ). Data points are colour-coded on the basis of fjord regions and the\n2\ndashed line indicates the 1:1 relationship. Model coefficients, goodness-of-fit (R2) and effective degrees of freedom for the O\n2\nfit (edf\nfO2\n) are shown.\nL. 156–158: “Ocean warming also stimulates atlantification – the northward inflow of warmer,\nnutrient-rich waters from the Atlantic into the Arctic”.\nAs I mentioned above, I never saw any clear demonstration that Atlantification is stimulated by ocean\nwarming. May the authors clarify the statement? I understand that the effects of Atlantification will\nbe stronger under ocean warming since Atlantic water will be warmer, but I am not sure that the\ninflow of Atlantic water changes because of ocean warming. I never saw any evidence for that.\nWe thank the reviewer for pointing out this issue. To clarify, we have reformulated the statement and\nadded references as follows:\n“Ocean warming also promotes the northward expansion of warmer, nutrient-rich waters\nfrom the Atlantic into the Arctic 61, 62, 63.” (Page 8, Lines 168–170).\nL. 160–162: “However, these microbial feedbacks may occur more slowly due to the resistance of\nsediments to temperature shifts, and sediment N2 production might not always be limited by inorganic\nN availability”.\nI may have misunderstood the sentence, but it looks like the first part challenges the increase in\nmicrobiological N2 removal due to sediment temperature shifts, whereas the second seems to argue\nin the opposite direction, claiming that N2 production might not be limited ny inorganic N availability.\nMay you please clarify?\nWe have reformulated this statement to improve clarity:\n“In addition to the direct stimulatory effects of warming and nutrient enrichment onto N\n2\nproduction (Fig. 3b), the increased availability of labile organic matter due to atlantification-\ndriven primary production may increase sediment N production in Arctic fjords 45, 64, 67.” (Page\n2\n8, Lines 172–175).\nL. 194: Section – Global fjord database collection and upscaling. I suggest removing details about the\ncalculation of N burial and N2 production from this paragraph, since such details are provided below\nin a dedicated paragraph. Here you may just list the geographic detaIls of the data and upscaling.\nWe have minimised details about N burial as suggested by the reviewer. Instead of describing the\nprocedure to calculate N production rates provided in another paragraph, this section now only\n2\ndescribes the methodological criteria for data collection in the literature to avoid methodological\nartefacts (Eyre et al., 2002).\nComments on Supplementary Materials\nL. 33: “Small river inputs from the heads of both fjords regarding the drainage basins <160 km2”. Please\nrephrase for clarity\nWe have reformulated the sentence to improve clarity:\n“The relatively small drainage basins (<160 km2) resulted in limited river inputs to both fjords.”\n(Page 2, Lines 33–34).\nReviewer #2 (Remarks to the Author):\n# General assessment\nThis review concerns the manuscript “Long term nitrogen burial exceeds denitrification in global fjords”\nby Cheung, Levin, Smeaton, Politi, Thamdrup, Santos, and Bonaglia. I would like to recognize the\nconsiderable effort invested in this study, particularly the integration of multiple techniques, extensive\nliterature review, and conceptual development. The work addresses an important gap in nitrogen\ncycling in fjords and provides valuable insights. The comments below reflect my personal opinion and\nare intended to improve the clarity, robustness, and relevance of the manuscript.\nThis manuscript addresses an important gap in global nitrogen cycling by quantifying burial versus\nmicrobial N loss in fjords. The topic is novel and relevant, and the integration of new measurements\nwith a global synthesis is a major strength. The methods (radiotracer dating, isotope pairing, spatial\nbootstrap) are technically sound, and the conceptual model (Fig. 5) is a useful framework. However,\nseveral aspects require clarification or improvement to strengthen the manuscript. In its present form,\nthe discussion is clear which makes the key results easy to understand. However, some of the\ninterpretations remain “too obvious” and I think the manuscript will benefit a more thorough\ndiscussion of the results to provide more critical findings. I also have concerns with the statistical\nanalyses.\nWe thank Reviewer #2 for their constructive feedback and the encouraging tone. We have carefully\naddressed all points raised by the reviewer in our responses below.\nBelow are more detailed comments:\n- Abstract\nL17–18: The statement on long-term sequestration is too categorical. Geological stability can be\ndisrupted by natural hazards (e.g., landslides), which should be acknowledged\nWe have also acknowledged the influence of natural hazards as follow:\n“However, natural hazards such as landslides and floods can alter sediment delivery and hence\nN burial efficiency 71, 72.” (Page 9, Lines 193–194).\nL18–20: The link between accelerated polar warming and increased burial/microbial N loss needs\nclearer mechanistic explanation.\nWe have reformulated the sentence to explain the mechanism while staying within the abstract word\nlimit:\n“Accelerated warming in polar regions will promote both N burial from increased primary\nproduction and microbial N loss from warmer temperatures, affecting the N budgets in fjords\nand in the ocean in general.” (Page 1, Lines 18–20).\n# Main\nL29–31: Expand on anthropogenic pressures in mid/high-latitude fjords (e.g., aquaculture, salmon\nfarming, wastewater inputs). The phrasing of “anthropogenic enrichment” is misleading. Climate-\ndriven processes (glacier melt, soil erosion) are not direct anthropogenic inputs. Consider explicitly\ndistinguishing direct human activities from climate-mediated changes.\nWe have reformulated the sentence as follow:\n“Quantification of N loss in mid- to high-latitude blue carbon ecosystems is hence critical to\nunderstanding their role in mitigating N enrichment from both direct (agricultural runoff,\naquaculture, and wastewater) and indirect (glacial meltwater and soil erosion) sources.” (Page\n2, Lines 31–33).\nAdd a note on stratification and its role in isolating deep waters, as this is critical for deoxygenation.\nWe have added the following statement to acknowledge the role of stratification in fjord\ndeoxygenation:\n“Additionally, warming-induced stratification reduces O supply to fjord deep waters, thereby\n2\nfavouring deoxygenation.” (Page 2, Lines 27–29).\n# Large nitrogen burial rates in global fjords\nL75 : This needs a reference (percentage of global marine N burial)\nWe have added a reference for global marine N burial (ref. 17; Zhang et al. (2020)). (Page 4, Line 77).\nL78–79: Since N burial rates are derived from MAR, the observed correlation seems circular. Perhaps\nclarify this in the text to avoid misinterpretation as an independent relationship.\nWe have reformulated the sentence to avoid misinterpretation:\n“Since N burial rates are derived from mass accumulation rates (M ), the high N burial\nAR\ncapacity is necessarily linked to M (Fig. 3a).” (Page 4, Lines 80–81).\nAR\n# Fjords N burial surpasses microbial N loss by denitrification and anammox\n(Sometimes you use « nitrogen » and other « N » in title. Please be consistent)\nWe have revised this subtitle for consistency.\nL111-112 : Please clarify the statement on lower OM in arctic fjords\nWe have revised the statement as follow:\n“The lower regional median (~0.9 g N m-2 yr-1) in Greenland and Svalbard likely reflects lower\ntemperatures and organic content availability for N production processes compared to\n2\ntemperate fjords, given that both temperature and organic carbon were positively correlated\nwith sediment N production (Fig. S1).” (Page 6, Lines 114–117).\nWe have reformulated the sentence to improve clarity:\n“For instance, greater sediment denitrification and anammox rates were measured when\nbottom water O concentrations decreased from 260 to 61 µM in Gullmar Fjord, Sweden (Fig.\nL122 : Comparing to the St. Lawrence Estuary is problematic, it is not a fjord. Crowe et al. studied the\nestuary, not Saguenay Fjord in the St Lawrence system. Same for Thibodeau et al. I am not aware of\npublished data on N cycling in the Saguenay Fjord.\nMeasurements reported in Crowe et al. and Thibodeau et al. were conducted in the Lower St. Lawrence\nEstuary (LSLE). The LSLE is often defined as a proper fjord system (Bianchi et al., 2020) due\nto its inherent bathymetric and oceanographic features (El-Sabh & Silverberg, 2012). The LSLE is\ncharacterised by a narrow, glaciated submarine valley as a result of glacial erosion (Dyke & Prest, 1987).\nAlike a typical fjord, the water column in the LSLE is permanently stratified due to the presence of a\n“sill” (Cyr et al., 2015; Saucier et al., 2003). Because of these reasons and to remain consistent with\nclassifications used in the previous studies, we decided to keep the data from the LSLE. To avoid\nconfusion, we have clarified that those data were originated from measurements conducted in the\nfjord-like LSLE (Page 6, Line 130) as in the previous review paper (Bianchi et al., 2020):\n“Similarly, greater N production was observed in the hypoxic fjord-like Lower St. Lawrence\n# Fate of nitrogen in deoxygenated, eutrophic and warmer fjords\nL141-148: This paragraph is confusing. It begins by stating that N2 production is higher in anoxic than\nin oxic fjords, but then claims that N2 production is similar to or higher in oxygen-depleted fjords\n(interpreted here as hypoxic but not fully anoxic) than in the anoxic Baltic Sea. However, the\nsubsequent discussion focuses solely on anoxic fjords.\nWe agree that the previous version was confusing. We have now replaced “O -depleted” with “anoxic”\n2\n(Page 7, Line 153). We have also revised the second sentence in the paragraph that now reads:\n“Fjord N production increased gradually from oxic to hypoxic conditions with a first threshold\n2\nat 100 µM O , followed by a sharp increase as O declined toward anoxia down to low\n2 2\nmicromolar range (<20 µM) (Fig. S5).” (Page 7, Line 151–153).\nCould anoxia reduce N2 production because nitrification cannot occur? This might explain why N2\nproduction is similar to or higher in oxygen-depleted (hypoxic) than in fully anoxic fjords. In that case,\nassuming climate change will lead to anoxia (L148), the increase in N2 production would represent a\ntransient state between oxic and anoxic conditions.\nWe have fixed the previous misunderstanding, i.e., in this context we used “oxygen-depleted” =\n“anoxic” ≠ “hypoxic”. Indeed, N production in anoxic conditions (<20 µM O ), which exceeded that\n2 2\nmeasured in hypoxic and oxic conditions (Fig. S5), was mainly due to the regime shift and expansion\nof the nitrate reduction zone, where anoxia persists yet nitrate is still available for N production (Page\n2\n6, Lines 133–136).\nL142 : Please clarify « geological vs biotic dominated N-loss »\nWe have reformulated the sentence to improve clarity:\n“Up to six-fold greater N production in anoxic than oxic fjords (Fig. 4a) implies that complete\n2\ndeoxygenation shifts the dominant N removal pathway from sedimentary burial toward\nmicrobially mediated N loss.” (Page 7, Lines 149–151).\nL144: Could N2 production occur in anoxic microenvironments within suspended organic matter?\nWorth mentioning but from your data it doesn’t seem so.\nThis is an interesting question. To avoid speculation, we have opted not to comment on anoxic\nmicroenvironments because we do not have measurements or literature support to assess the\ncontribution of microenvironments to overall N production in fjords.\nWe have expanded the sentence as follow:\n“The substantial increase in average N production in anoxic fjords is primarily due to the\n2\nregime shift and expansion of the nitrate reduction zone, i.e., the anoxic but nitrate-containing\nzone 59 (Fig. 5).” (Page 7, Lines 155–157).\nL162-163 : This needs a reference.\nWe added ref. 45, Sørensen et al. (2015). (Page 8, Line 175).\nOverall, in the present form, the climate prjoections seem speculative and would need more support\nto appear robust. Also, the statistical analyses used in this study, while correct, are not the most\nefficient ones to make robust predictions.\nTo improve the robustness of our analyses, we have now incorporated model predictions that include\nmultiple environmental predictors (new Fig. 3) as suggested by the reviewer. We also added new\ndiscussion about these findings as follows:\n“In addition to the direct stimulatory effects of warming and nutrient enrichment onto N\n2\nproduction (Fig. 3b), the increased availability of labile organic matter due to atlantification-\ndriven primary production may increase sediment N production in Arctic fjords 45, 64, 67.\ndeposition of labile organic matter on sediments 45, favouring both N burial and benthic N2\nproduction (Fig. S1). Combined with potentially higher N burial due to warming-mediated\nglacial retreat and increased sedimentation rates 48, our results suggest an increased N loss in\nglobal fjords along with a warmer and less oxygenated ocean.” (Page 8, Lines 172–179).\n# Summary and implication\nL168-169 : You mean « currently » ?\nYes, we have revised the sentence accordingly:\n“Our global analysis demonstrates that sediment burial is the most effective N sink mechanism\nin fjord systems under current conditions.” (Page 9, Lines 181–182).\nL170 : Could you define « blue nitrogen » ?\nWe have added the definition accordingly:\n“Despite covering <0.1% of the ocean, fjords contribute up to 18% of global N burial. This\nmakes them hotspots of “blue nitrogen”, i.e., nitrogen sequestered in marine sediments.”\n(Page 9, Lines 182–184).\nL170-171 : The percentages do not add up to 100\nYes, the reported percentages do not sum to 100% because they represent mean relative\ncontributions with associated uncertainty, rather than fractions of a single global total. We have\nreformulated the sentence to improve clarity:\n“On average across fjords globally, sediment burial accounts for 59.4±4.9% of total N loss,\nexceeding denitrification (34.2±4.2%) and anammox (6.4±2.6%).” (Page 9, Lines 184–185)\nL171-172 : You mean « N loss » other wise it is somewhat obvious that burial play a key role in long-\nterm N storage.\nYes, we referred to N removal from the ecosystem. We have reformulated the sentence accordingly:\n“Thus, burial plays a surprisingly significant role in the long-term N removal of fjords globally,\nmaking it a crucial mechanism for mitigating coastal N enrichment at medium and high\nlatitudes.” (Page 9, Lines 185–187)\nL174-175 : Nitrification is often the main process producing N O\n“Additionally, N burial is an effective removal pathway with minimum climatic impact,\ncompared to microbial nitrification and denitrification, which produce the greenhouse gas\nnitrous oxide (N O; equivalent to ~1% of N production in an anoxic fjord; ref. 21, 68).” (Page 9,\n2 2\nLines 187–190)\nL175-178 : It is not easy for me to follow the rationals here. What comes from your results and from\nthe littérature ?\nWe agree that the previous text was not easy to follow and we have restructured these sentences that\nnow reads:\n“N burial in fjords is driven by the sedimentation rates and higher lability of deposited organic\nparticles (Fig. 3a), which can be favoured by increasing rates of primary production, particle\ndeposition, and glacial retreat 69, 70. However, natural hazards such as landslides and floods\ncan alter sediment delivery and hence N burial efficiency 71, 72” (Page 9, Lines 190–194)\nL185 : You show a clear shift following anoxia, not deoxygenation\nWe reformulated accordingly:\n“Indeed, we show that microbial N loss will dominate over geological burial loss due to the\nshift in N production regimes following development of anoxia (Fig. 5).” (Page 9, Lines 199–\n2\n201)\nL189 : I do not think you defined « eutrophication » inthe manuscript\nWe added a definition of eutrophication accordingly:\n“Whether N burial can counter coastal nutrient overenrichment (i.e., eutrophication) and curb\ngreenhouse gas emissions largely depends on coastal nutrient management.” (Page 10, Lines\n205–207)\nL189-190 : Do you think that enhanced N burial in the sediment could also stimulate CO2 and CH4\nproduction which, if released, might offset the reduction in N2O production?\nWe agree and feel that enhanced N burial may potentially stimulate anaerobic sediment processes,\npotentially affecting the production of all three key greenhouse gases. However, because parallel\ngreenhouse gas flux measurements are unavailable, we cannot evaluate this relationship and would\nprefer to avoid excessive speculation in the manuscript.\n# Methods\n## Global fjord database collection and upscaling\nFor your upscaling, mention the assumption that fjord seafloors always consist of soft sediments. This\nholds for deep basins but not always for sills, steep fjord « walls », or areas with boulders from\nglacier/sea-ice melt. Do you have details on sample locations within the fjords for the data you retrieve\nfrom litterature? High spatial heterogeneity often occurs between the head and mouth.\nIn the compiled global dataset, the mean (± SD) sediment porosity was 0.78 ± 0.06 (n = 80). We added\nthis information accordingly:\n“We assumed that sediment particle size was relatively homogenous across sites, given a\nmean porosity of 0.78±0.06.” (Page 10, Lines 218–219).\nIn addition, no observable effect on burial and N production was found. We conclude that the effects\n2\nof sediment heterogeneity on N loss in fjords are negligible.\n## Statistical analyses\nThe Spearman rank correlation test assesses monotonic relationships (not necessarily linear), so why\nis a linear model shown in Figure 3?\nThe linear models were used to indicate linearity of the relationship in some cases and, indeed, not all\ntested datasets showed a linear relationship. We have therefore removed all linear models from the\ncorrelation analysis plots (See Fig. S1)\nMoreover, no predictions can be drawn from the Spearman correlation test. Since you show linear\nmodels, could you consider using GLMs (or GAMs if not linear) on these data instead, and potentially\nincorporate multiple predictors simultaneously? This could strengthen your points and broaden the\nscope of your data.\nWe thank the reviewer for the suggestion to generate better predictive models. We have now included\nGLM and GAM for N and N production to reveal key predictors, respectively, in Fig. 3. These models\nAR 2\nbetter highlight that: (1) N burial were driven primarily by sedimentation rate (M ) and lability of\nAR\nburied material (sediment C:N ratio); (2) N production were mainly driven by water temperature,\n2\nnitrate concentration, and oxygen concentration. These models particularly consider the non-linear\neffect of O on N production.\n2 2\nFigure 3 Predictive models for sediment nitrogen burial and N production across fjords globally. (a) Observed versus predicted\n2\nfjord nitrogen accumulation rates (N ) derived from a generalized linear model including sediment mass accumulation rates\nAR\n(M ) and sediment C:N ratio (C:N). (b) Observed versus predicted fjord N production rates derived from a generalized linear\nAR 2\nmodel including water temperature (temp), nitrate and nitrite concentrations (NO), and restricted maximum likelihood fitted\nx\noxygen concentrations (O ). Data points are colour-coded on the basis of fjord regions and the dashed line indicates the 1:1\n2\nrelationship. Model coefficients, goodness-of-fit (R2) and effective degrees of freedom for the O fit (edf ) are shown.\n2 fO2\nGiven the differences in n between groups (see Figures 2 and 4), could you also report effect sizes? I\nthink the Wilcoxon rank-sum test is not the best option here because you have three groups. It is like\ndoing pairwise tests without applying a p-value correction, which increases the risk of false positives.\nAnother point: one group has n = 2, which is too small for reliable statistical testing. It is nice that the\ndata are actually shown in Figures 2 and 4, but from my perspective the anoxic group should be treated\nas exploratory observations. You could try permutation tests, although this would not fully resolve the\nissue of such a small group. As mentioned above, effect sizes may be more meanigful in this case. To\nbe fully transparent about the likely large uncertainty in this group, you could also report bootstrap\nconfidence intervals which sould be large. From my perspective, the unfortunate lack of data from\nanoxic fjords, combined with the global variability in the dataset, undermines the robustness of the\nfindings linking nitrogen cycling to deoxygenation.\nWe have now used a one-way permutation test to assess differences among groups. Effect sizes, as\nthe reviewer suggested, are now also reported for all tests (See revised Fig. 2 and 4 below). For Fig. 4,\nwe removed pairwise comparisons between groups to avoid misinterpretation of the exploratory\nstatistical analysis.\nAbout the lack of data from anoxic fjords: Rates from anoxic fjords were comparable to those in the\nanoxic, brackish Baltic Sea, supporting their reliability. In addition, the low number of observations\nreflects the fact that, under current conditions, anoxic fjords are generally quite rare. Because of these\nreasons, we have decided to keep the anoxic group for the global analysis, yet interpret it cautiously.\nWe explicitly acknowledge these limitations in the text:\n“While based on measurements from only two anoxic fjords, N production was notably\n2\nhigher under anoxic conditions (Fig. 4a).” (Page 6, Lines 119–120)\nBootstrap CIs of the group were large, and values were very close to the range of the two data points\nin anoxic fjords. To avoid potential misinterpretation, we hence chose not to report these values.\nFigure 2 Distribution of accumulation rates and stoichiometry in fjord sediments across ocean basins. The range and\nvariability of sediment (a) nitrogen accumulation rates (N ), (b) mass accumulation rates (M ), and (c) sediment carbon to\nAR AR\nnitrogen molar ratio (C:N) across ocean basins (colour-coded). Dashed horizontal line indicates bootstrapped global median\nvalue. Crosses indicate bootstrapped medians of the corresponding region. Differences among ocean basins were assessed\nusing permutation-based one-way tests, with global p-values reported. Effect sizes were quantified using epsilon-squared (ε²)\nderived from the Kruskal–Wallis statistic.\nFigure 4 Rates of N loss in fjords under different redox conditions. (a) N production and (b) nitrogen accumulation rates (N )\nin anoxic, hypoxic, and oxic fjords. (c) contribution of N burial to total fjord N loss (sum of N production and N burial) under\n2\neach redox condition. Global differences among redox conditions were assessed using permutation-based one-way tests, with\np-values reported. Effect sizes were quantified using epsilon-squared (ε²). Pairwise differences relative to oxic conditions were\nevaluated using permutation-based post hoc tests with Holm correction, with adjusted p-values shown.\n## Figures\nMake sure to use a color-blind-friendly palette.\nWe have revised all plots and have used color-blind-friendly palette accordingly.\nFigure 3: I think the figure would benefit from a clearer rationale for how the panels are arranged. At\nthe moment, the sequence does not follow an obvious logic, at least to me. For example, NAR is shown\nas a function of latitude (a) and of the C:N ratio (d), and then the C:N ratio is shown as a function of\nlatitude (e). What are the objectives of presenting each of these relationships? Are these panels simply\na subset of all significant relationships among the possible variable combinations or rely on hypotheses?\nWe have now included only predictive models of N and N production in revised Figure 3. For other\nAR 2\nrelationships presented in the previous version, we have added supplementary Fig. S1 shown below,\nincluding: (1) Latitudinal trends of N burial, sediment C:N ratio and fjord N production; (2)\n2\ncontrols (bottom water temperature, sediment organic carbon content, and bottom water\nnitrate+nitrite concentration) on sediment N production (water column N production excluded).\n2 2\nFigure S1 Relationship between fjord N loss process rate and environmental factors. (a-c) Latitudinal trends in fjord nitrogen\nburial (N ), sediment C:N ratio, and fjord N production across fjord sites, respectively. (d-f) Environmental controls on fjord\nAR 2\nNAR, including organic carbon accumulation rates (OC ), sediment C:N ratio, and bottom water oxygen (O ) concentration,\nAR 2\nrespectively. (g-i) Environmental controls on fjord sediment N production rate (water column production excluded), including\n2\nbottom water temperature, sediment organic carbon content (OC), and bottom water nitrate + nitrite concentration,\nrespectively. Data points are colour-coded on the basis of fjord regions. Spearman correlation coefficient (ρ) and\ns\ncorresponding p-value are shown.\n# Supplementary material\n## Methods\nThe methodology is technically sound; a few suggestions:\nL91–108: For reproducibility, could you report the 15N-NO3 concentration used for this?\nThe 15N-NO – concentrations were reported in the main text Methods:\n3\n“Collected intact cores were spiked with 15NO – (11, 24, 56, and 120 µM mean final\n3\nconcentration) …” (Page 12, Lines 261–262)\n## Figures\nFigure S3 is not referenced in the main text. I understand the conceptual idea behind the exponential\nmodel, but would a segmented model or GAM be more informative? With n=2 in anoxia, you may not\nbe able to detect a significant tipping point, but using the segmented or the mcp R package within a\nBayesian framework might still allow you to explore potential « real » change points.\nWe thank the reviewer for the suggestion and have adopted the MCP model within our analyses. We\nyielded two change points of N production rates at 9.1 and 100.4 µM O . This aligns well with our\n2 2\ndiscussion on hypoxia and anoxia in the fjord, which stimulates N production rates. We added the\n2\nresults of the MCP model in the text as follows:\n“Fjord N production increased gradually from oxic to hypoxic conditions with a first threshold\n2\nat 100 µM O , followed by a sharp increase as O declined toward anoxia down to low\n2 2\nmicromolar range (<20 µM) (Fig. S5).” (Page 7, Line 151–153).\nFigure S5 Potential changes in fjord N production as a function of oxygen concentrations. Sediment and water column total\n2\nliterature and new data from Sweden and Iceland. A Bayesian segmented regression model was fitted to the observed N\n2\nproduction rates. Dashed lines indicate the estimated change points (cp1 and cp2) of shifts in N production rate. An asterisk\n2\ndenotes a higher benthic N production rate reported in Loch Etive under oxygenated conditions, yet this system is known to\n2\nexperience periodic hypoxia 19.\nDoes the reported O2 concentration correspond to bottom-water values, and did you test for a\nrelationship between N burial and O2 ?\nYes, the reported O concentrations were bottom-water values. In addition to no observable\n2\ndifferences in N burial rates among redox conditions (Fig. 4b), we tested N burial as a function of\nbottom-water O concentrations (Fig. S1), with Spearman’s rho = 0.12 and p = 0.52, suggesting a\n2\nnegligible effect of O concentrations on N burial rates.\noxygenated bottom waters, where most organic matter may already have been remineralized through\noxic processes before reaching the seafloor. This point seems worth discussing.\nMost of the burial literature data does not report bottom water O concentration. Only 32 out of 163\n2\nobservations reported bottom water O concentrations during sampling. Therefore, without proper in\n2\nsitu O data, we categorised fjords according to their prevalent redox condition (Fig. 4b). We therefore\n2\nremoved the N burial data from this figure (new Fig. S5) to avoid misinterpretation.\nI look forward seeing this work refined.\nOnce again, we thank the editor and two reviewers for their constructive feedback. We feel that this\nrevised version has substantially improved."}, {"round": 2, "reviewer_comments": {"reviewer_2": "# General assessment\nThis review concerns the revised manuscript \"Long-term nitrogen burial exceeds denitrification in global fjords\" by Cheung,\nLevin, Smeaton, Politi, Thamdrup, Santos, and Bonaglia. I want to sincerely congratulate the authors for their effort in this\nrevision. I appreciate the care with which they addressed most of my comments and those of the other reviewer. This is a\nvaluable contribution.\n# Specific comments\nTwo comments remain on this manuscript:\n1- Change points analysis\nIt would be more transparent to report the full model output, including a summary of posterior distributions (to convey\nconfidence intervals), model diagnostics such as R-hat (convergence diagnostic) and effective sample size, and a clear\nstatement of any priors used.\n2- Lower St Lawrence estuary as a fjord-like system\nThis is the most critical point.\nFrom my perspective, the Lower St. Lawrence Estuary (LSLE) is not a fjord but an estuarine basin. This basin is a rift system\nsubsequently overdeepened and modified by glacial erosion during the last glaciation/deglaciation (Tremblay et al. 2003 :\n10.1016/S0040-1951(03)00227-0; Duchesne et al. 2010 : 10.1111/j.1365-2117.2009.00457.x). It is true that a cross-\nsectional profile of the Laurentian Channel shows a U-shape, with steep walls at some locations (Figure 2 from\nNormandeau et al. 2015; 10.1016/j.geomorph.2015.03.023). However, the whole LSLE lacks defining characteristics of a\nfjord. For instance, the LSLE is not narrow, and its walls are not steep. Moreover, the sill is located at the head of the LSLE,\nnear Tadoussac, not at its mouth. This is visible in Figure 3 from Cyr et al. (2015; 10.1002/2014JC010272) cited by the\nauthors, and figure 3 from Bluteau et al. (2021; 10.5194/os-17-1509-2021). I therefore strongly suggest the authors to be\nextremely cautious when classifying the LSLE as a fjord or fjord-like system, even if this classification has precedent in the\nliterature. The « Fjord-type estuarie » classification is also problematic since the stratification is not due to a sill at the\nentrance of the Laurentian Channel. Maybe a more descriptive terminology « Laurentian Channel estuarine system » and\njustify in the method why they consider it similar as a fjord in its biogeochemical functioning. I strongly feel that this is\nparticularly important for a publication in a high impact venue that is likely to be widely cited. Such classifications should be\ncritically reassessed rather than inherited."}, "author_response": ""}]

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