the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2025
Seasonal Interplay of Water Mass Mixing and Nutrient Dynamics in an Arctic Fjord: A Case Study of Kongsfjorden, Svalbard
Abstract. This study examined seasonal variations in water mass structure and nutrient dynamics in Kongsfjorden, a high Arctic fjord where water mass composition varies seasonally due to mixing among Atlantic Water, Polar Surface Water, and glacial meltwater. In spring, the dominance of Modified Atlantic Water (MAW) facilitated active vertical mixing, leading to relatively high, uniform nutrient concentrations throughout the water column. In summer, the enhanced influence of glacial meltwater and warmer Polar Surface Waters (PSWw) resulted in strong surface stratification and significant nutrient depletion in the upper layer. To disentangle the effects of physical mixing from biological consumption, theoretical nutrient concentrations were calculated based on a four-component water mass mixing model. The positive differences between theoretical and observed concentrations (ΔNutrient) were indicative of significant biological uptake, which accounted for substantial nutrient reductions in observed surface concentrations from spring to summer: approximately 69 ± 18 % for nitrate, 74 ± 15 % for phosphate, and 47 ± 18 % for silicate. Crucially, ΔNutrient values served as a 'biogeochemical memory,' reflecting the cumulative net biological consumption since the spring bloom rather than just instantaneous phytoplankton biomass. These biological processes also altered nutrient stoichiometry, causing the surface nitrogen-to-phosphorus (N/P) ratio to increase from 15.0 in spring to 18.8 in summer, indicating a shift in nutrient limitation patterns. Consequently, summer surface waters transitioned toward potential co-limitation, with concentrations of phosphate (~0.13 ± 0.07 µM) and silicate (~1.66 ± 0.39 µM) approaching their respective limitation thresholds. These findings highlight a clear seasonal transition from a physically controlled, nutrient-replete spring to a biologically regulated, nutrient-limited summer. This understanding is crucial for predicting how Arctic fjord ecosystems, and their primary productivity, will respond to ongoing Atlantification and increased freshwater input under climate change.
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RC1: 'Comment on egusphere-2025-2845', Anonymous Referee #1, 30 Jun 2025
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General overview: This was an interesting article about shifts in nutrient limitations in a fjord. It was focused on water mass characterization and mixing, showing potential impacts of biological processes on nutrient ratios. The conclusions suggested the increasing influences of Atlantic water in the region, especially in the context of future warming. Specific comments: the article suggested a shift to phosphate limitation, and in this case the phosphate in Fig. 5 could be shown on a secondary axis as this would highlight changes in P concentrations, which are obviously lower than N or Si. The authors do admit that glacial silicate sources are excluded, but it would be interesting to know more about this. It would also be helpful to see the full T/S from the CTD sensor data, to make the water mass definitions clearer (in Fig. 2a). Technical comments: In addition to the comments relating to Fig. 2 and Fig 5. there are some minor technical points to consider such as consistency within the references - in most cases the date is the last part of the reference, with no brackets (eg: Wassmann, P., Title, Ref, 2011). However on some occasions the (more usual) method of 'author (date)' formatting has been used eg: Miller, Arthur (1950) - though this particular example is the only time that the forename is written in full. Another inconsistency is in the use of DIN/DIP in Fig.8, which does not match the caption. On the whole this is a very readable article and a valuable contribution to the literature.
Citation: https://doi.org/10.5194/egusphere-2025-2845-RC1 -
AC1: 'Reply on RC1', TAE HOON Kim, 04 Jul 2025
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Dear Referee #1 of Ocean Science,
Thank you for your valuable feedback and the opportunity to revise our manuscript, "Seasonal Interplay of Water Mass Mixing and Nutrient Dynamics in an Arctic Fjord: A Case Study of Kongsfjorden, Svalbard" (OS-2025-2845). We are grateful to the Referee #1 for their insightful and constructive comments, which have helped us to improve the quality and clarity of our paper.
We have carefully addressed all the points raised by the Referee #1. Below is a point-by-point response to the comments, detailing the changes made in the revised manuscript.
Referee #1's General Comment:
General overview: This was an interesting article about shifts in nutrient limitations in a fjord... On the whole this is a very readable article and a valuable contribution to the literature.Our Response:
Thank you for your encouraging words and positive assessment of our work. We are pleased that you found our study to be a valuable contribution.Specific Comments:
- Comment on Figure 5 (Phosphate visualization):
"...the article suggested a shift to phosphate limitation, and in this case the phosphate in Fig. 5 could be shown on a secondary axis as this would highlight changes in P concentrations, which are obviously lower than N or Si."
Our Response:
This is an excellent suggestion for improving the data visualization. We agree that the changes in ∆Phosphate were not clearly visible due to the scale difference with ∆NOx and ∆Silicate. As suggested, we have revised Figure 5 to include a secondary y-axis on the right for ∆Phosphate. This revision now effectively highlights the seasonal and spatial variations in phosphate anomaly, which strongly supports our discussion on the shift toward phosphorus limitation.- Comment on Glacial Silicate Sources:
"The authors do admit that glacial silicate sources are excluded, but it would be interesting to know more about this."
Our Response:
Thank you for this insightful comment. We agree that a more detailed discussion on the treatment of glacial silicate input is crucial for interpreting our results. We have revised the manuscript to provide a clearer rationale, supported by relevant literature.Our decision not to include glacial meltwater (GMW) as a discrete fifth end-member was based on two primary challenges:
- High and Unpredictable Variability of the Source: Defining a stable and representative silicate concentration for a GMW end-member is scientifically challenging. Studies focusing on Svalbard's tidewater glaciers, including those that discharge into Kongsfjorden, report a wide and highly variable range of silicate concentrations in summer runoff. For instance, values typically range from 2 µM to 6 µM (e.g., Nowak & Hodson, 2014; Hatton et al., 2019; Hopwood et al., 2016). This variability is driven by complex factors like subglacial water residence time, watershed lithology, and melt dynamics. Choosing any single value within this wide range would introduce a significant, unquantifiable bias into our mixing model.
- Model Parsimony and Robustness: Adding a fifth, highly variable end-member would not only increase the model's complexity but also its associated uncertainty, potentially reducing the robustness of the calculated contributions from the other, better-constrained water masses (AW, MAW, PSW).
Therefore, we adopted a more conservative and scientifically defensible approach by subsuming the freshwater influence into our Polar Surface Water warm (PSWw) end-member. We explicitly acknowledge that this methodological choice means our calculated ∆Silicate values inherently underestimate the true biological consumption.
Crucially, this limitation strengthens our overall conclusion. Our own data show a strong inverse correlation between observed silicate and salinity in summer (r² = 0.94), empirically confirming a significant, non-conservative freshwater source of silicate. The fact that we still calculate a substantial biological silicate drawdown—even with a model that systematically underestimates it—provides powerful and compelling evidence that biological uptake is the dominant process regulating silicate dynamics in Kongsfjorden during the summer, far outweighing the effects of physical mixing alone.
We have revised Section 3.3 (now lines 326-338 on page 16) to include this detailed rationale and a discussion of the literature. We believe this clarification significantly strengthens the manuscript and addresses your concern directly.
- Comment on Figure 2a (T-S Diagram):
"It would also be helpful to see the full T/S from the CTD sensor data, to make the water mass definitions clearer (in Fig. 2a)."
Our Response:
Thank you for your thoughtful suggestion regarding the T-S diagram. We appreciate your perspective on providing a broader hydrographic context to strengthen the manuscript.We would like to clarify the specific scope and objective of our study. Our primary goal is not to provide a comprehensive physical oceanography of Kongsfjorden, but rather to quantify the relative impacts of physical mixing versus biological processes on the nutrient dynamics within the discrete water samples we collected.
To this end, the T-S diagram in Figure 2a intentionally and exclusively displays the data points that are directly paired with our nutrient measurements. We firmly believe that this approach is the most transparent and scientifically rigorous way to present the foundation for our biogeochemical model, as it shows the exact hydrographic properties of the water for which all subsequent calculations were made.
While adding the full continuous CTD data would certainly illustrate the broader physical setting, we are concerned that it would shift the focus away from the central hypothesis of our paper. Including a vast amount of data not directly tied to our nutrient analysis could dilute the clarity of our core argument, which is about disentangling processes at our specific sampling locations.
Therefore, we maintain that the current version of Figure 2a is the most appropriate representation for the stated goals of this study. It accurately reflects the precise dataset used for our model and analysis. To avoid any potential misunderstanding for the reader, we will, however, revise the figure caption to state more explicitly that the points represent the complete set of discrete samples analyzed for nutrients in this study. We hope this explanation clarifies our rationale for maintaining the current figure, and we trust that our approach is understood within the specific context of our research questions.
Technical Comments:
- Comment on Reference Consistency:
"...there are some minor technical points to consider such as consistency within the references..."
Our Response:
Thank you for pointing out these inconsistencies. We have thoroughly reviewed and reformatted the entire reference list to ensure a consistent style, following the guidelines for the journal Ocean Science. The specific issues you noted, such as the formatting of the Miller (1950) reference, have been corrected.- Comment on Terminology in Figure 8 (DIN/DIP):
"Another inconsistency is in the use of DIN/DIP in Fig.8, which does not match the caption."
Our Response:
Thank you for catching this oversight. To ensure clarity and consistency with standard oceanographic terminology, we have revised the manuscript to use "DIN/DIP ratio" uniformly throughout the text (Section 3.4), in the caption for Figure 8, and on the figure's axis label. This change ensures that the terminology is consistent across all parts of the manuscript.We believe that these revisions have addressed all your concerns and have substantially strengthened the manuscript. We thank you once again for your constructive feedback.
Sincerely,
Sincerely,
Tae-Hoon Kim
Corresponding Author
Department of Oceanography, Chonnam National University
thkim80@jnu.ac.krCitation: https://doi.org/10.5194/egusphere-2025-2845-AC1 - Comment on Figure 5 (Phosphate visualization):
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AC1: 'Reply on RC1', TAE HOON Kim, 04 Jul 2025
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RC2: 'Comment on egusphere-2025-2845', Louise Delaigue, 08 Jul 2025
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This manuscript presents a carefully executed and scientifically robust investigation into seasonal changes in water mass mixing and nutrient dynamics in Kongsfjorden, Svalbard. The authors successfully apply a four-end-member mixing model to disentangle the relative roles of physical mixing and biological processes in shaping nutrient distributions, a methodological approach that is both appropriate and insightful. A key strength of the study is the introduction and effective use of the ΔNutrient metric, which serves as a novel and compelling proxy for cumulative biological uptake—a concept aptly framed as "biogeochemical memory." This framework not only captures seasonal transitions in nutrient regimes but also offers broader utility for interpreting biological dynamics in polar marine systems. Overall, the study is timely, highly relevant, and makes a valuable contribution to our understanding of Arctic fjord biogeochemistry in the context of accelerating Atlantification and glacial meltwater inputs under climate change.
However, there are several issues that warrant further consideration before publication.
Main comments
1. The authors assume that glacial meltwater (GMW) is adequately represented within the PSWw end-member. This simplification is questionable, particularly given that silicate shows clear glacial signatures (e.g., strong salinity-silicate correlation). It might be worthwhile to treat GMW as a separate fifth end-member in the mixing model or at least conduct sensitivity analyses showing the effect of its exclusion.
2. The study spans two time points (spring and summer) in a single year (2023), which limits its generalizability regarding interannual variability. The authors should explicitly acknowledge this as a limitation and suggest future work with multi-year seasonal coverage.
3. The calculation of theoretical nutrient concentrations assumes conservative behavior of water mass properties, but nutrients are often influenced by remineralization and benthic fluxes, especially in fjords. The authors should discuss the validity of this assumption more critically in the methodology or discussion. I would also encourage them to take a look at the following article in which the authors expanded the OMP analysis to include processes like photosynthesis - thus directly assessing the biological influence on their analysis (which is also in a similar environment):
Dinauer, A., & Mucci, A. (2018). Distinguishing between physical and biological controls on the spatial variability of pCO2: A novel approach using OMP water mass analysis (St. Lawrence, Canada). Marine Chemistry, 204, 107-120.
3. The weak correlation between chlorophyll-a and ΔNutrient is well interpreted as "biogeochemical memory." However, more robust proxies (e.g., primary production rates or phytoplankton community data) would strengthen this conclusion. The authors might want to clarify that chlorophyll-a is only a proxy for standing biomass and does not reflect total productivity or uptake.
4. The model assumes that observed nutrient depletions are due solely to biological uptake, without considering possible remineralization at depth that could bias ΔNutrient calculations. Nuancing this part of the discussion by including this possibility in the discussion as a caveat to the interpretation may be beneficial.
5. While a ±10% variation was tested, this may not fully capture natural variability, especially given different water origins and seasonal effects. The authors might consider either justifying the 10% range more explicitly with literature or testing a broader uncertainty range.
Minor comments
-Clearly define “nitrate” vs. “NOx” earlier in the methods section to avoid confusion.
-Consider adding chlorophyll-a and ΔNutrient time series or depth profiles instead of just scatter plots.
-Some parts (e.g., "biogeochemical memory") are a bit jargon-heavy and could be explained more clearly for general readers.
-As a final comment, I would encourage the authors to briefly place their findings into a broader global context. The observed shifts in nutrient stoichiometry and limitation in Kongsfjorden could be more explicitly linked to global-scale trends in marine biogeochemistry. In this regard, the recent study by Liu et al. (2025), Global-scale shifts in marine ecological stoichiometry over the past 50 years (Nat. Geosci.), may provide a useful reference and framework for situating the local dynamics of Arctic fjords within larger oceanic patterns and trajectories.
Citation: https://doi.org/10.5194/egusphere-2025-2845-RC2
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