Biogeochemical and absorption properties of different size fractions of suspended matter in the western Spitsbergen fjords (Svalbard Archipelago) in the summer

Meler, Justyna; Stoń-Egiert, Joanna; Zabłocka, Monika

doi:10.5194/egusphere-2025-2800

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https://doi.org/10.5194/egusphere-2025-2800

Preprints

28 Aug 2025

| 28 Aug 2025

Biogeochemical and absorption properties of different size fractions of suspended matter in the western Spitsbergen fjords (Svalbard Archipelago) in the summer

Justyna Meler, Joanna Stoń-Egiert, and Monika Zabłocka

Abstract. In the summers of 2022 and 2023, the variability of biogeochemical properties and light absorption of suspended particulate matter (SPM) was investigated in three western Spitsbergen fjords: Hornsund, Kongsfjorden, and Isfjorden. Analyses included SPM and its organic (POM) and inorganic (PIM) fractions, chlorophyll a (Tchla), and light absorption coefficients of total particles (a_p(λ)), phytoplankton (a_ph(λ)), and detritus (a_d(λ)). Measurements were conducted on unfractionated seawater and on size-fractionated samples obtained by cascade filtration, allowing the assessment of four particle classes: pico- (0.2–2 µm), ultra- (2–5 µm), nano- (5–20 µm), and micro-particles (20–200 µm).

The obtained results revealed clear differences in particle size contributions in the fjord waters. Ultra-particles dominated the total concentrations, contributing 39 %–56 % of SPM, POM, PIM, and Tchla. Pico-particles also had large contribution: 16 %–29 %, and micro- and nano-particles accounted for 12 %–17 %. The average POM/SPM ratio was 33 %, indicating the dominance of inorganic matter. However, organic contributions varied with size of particles: micro- and pico-particles contained more organic material (45 % and 43 %), while nano- and ultra-particles had lower proportions (38 % and 26 %).

Ultra-particles also played a leading role in light absorption at 443 nm, contributing 44 %–62 % to a_p(443), a_d(443), and a_ph(443). Pico-, nano-, and micro-particles contributed 24 %–28 %, 10 %–19 %, and 4 %–9 %, respectively. Considerable variability in absorption properties was observed across size classes. Since variability ranges often overlap for different size classes, this may prevent unambiguous identification of particle size class based on light absorption spectra.

Received: 12 Jun 2025 – Discussion started: 28 Aug 2025

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Justyna Meler, Joanna Stoń-Egiert, and Monika Zabłocka

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-2800', Anonymous Referee #1, 30 Aug 2025
The authors investigate the biogeochemical composition and light absorption characteristics of SPM across four size fractions in three western Svalbard fjords during the summers of 2022 and 2023. The use of cascade filtration provides some insight into the distribution of SPM in fjordic systems, and the dataset may have value for the polar biogeochemical community as a seasonal snapshot sensitive to climate-driven change.
However, in its current form, the manuscript functions more as a data report than as a research article. It presents percentages and descriptive summaries without rigorous analysis, mechanistic interpretation, or clear hypotheses. As such, the study does not yet meet the standards for a publishable research contribution. Major revisions are required, including the addition of statistical and comparative analyses, stronger ecological and optical interpretation, and a clear positioning within the existing literature, to elevate this work from a dataset report to a substantive scientific article.
General comments are as follows:
First and foremost, the overall novelty of the manuscript is weak. The authors must articulate more clearly how this dataset provides new insights into Arctic size-fractionated optics and biogeochemistry, rather than simply reporting percentage contributions. Without methodological strengthening, statistical rigor, and broader contextualization, the study risks being perceived as a limited dataset report rather than a substantive scientific contribution.

The Introduction is currently too weak to guide readers toward the purpose of the study. It provides minimal background, insufficient literature context, and lacks a clear articulation of the knowledge gap. As written, readers cannot easily understand what scientific question this paper seeks to address or why it matters. The stated aims are descriptive (“to determine the contribution of…”) rather than hypothesis-driven, and there is no strong link between the particle size–resolved measurements and the broader ecological or optical processes in fjord systems. Overall, the Introduction requires substantial revision to provide context, define the gap, and present clear, hypothesis-driven objectives.

A major limitation lies in the underspecified sampling design. Critical details such as sampling depth, instrumentation, replication strategy, tidal or glacial influences, and spatial gradients within fjords are not reported. Without this information, it is impossible to assess the representativeness of the dataset or the robustness of the derived patterns. Similarly, no statistical tests are presented to determine whether contributions of different size classes are significantly distinct, or whether differences exist among fjords or between years. This omission reduces the study to a purely descriptive account rather than a quantitative analysis.

The stated aim “to capture the greatest possible optical diversity of fjord waters” in Line 161 is not supported by the sampling design. The dataset consists of limited snapshot measurements from two summer campaigns, which cannot adequately represent the strong seasonal and interannual variability known in Svalbard fjords. Optical and biogeochemical properties in these systems are highly dynamic, influenced by spring blooms, glacial meltwater pulses, and autumn mixing, none of which are captured here. As a result, the claim of covering “the greatest possible optical diversity” is overstated and should be substantially tempered. The authors should acknowledge this limitation clearly and frame their dataset as a restricted summer snapshot rather than a comprehensive representation of fjord optical diversity.

The manuscript repeatedly refers to a separate paper for detailed explanations of sample analyses. While cross-referencing related work is acceptable, the current paper must remain methodologically self-contained. Readers should not have to consult another publication to understand how the measurements were obtained, processed, or validated. At minimum, the authors need to provide concise but sufficient descriptions of key analytical methods (e.g., cascade filtration procedure, absorption measurements, organic/inorganic determinations), including potential uncertainties and limitations. Without this, the study is not reproducible or assessable on its own merits. A brief summary of the essential methodological details should therefore be included directly in the present manuscript, even if a companion paper contains additional depth.

The interpretation of results is also weak. The dominance of ultra-particles is repeatedly emphasized, but no mechanistic explanation is provided. The discussion mainly restates percentages rather than linking particle distributions to processes such as fjord circulation, glacial meltwater input, seasonality, or primary production dynamics.

The manuscript is further weakened by its limited engagement with existing literature. Several studies have examined size-fractionated SPM and absorption in other polar and temperate environments, yet the authors do not convincingly explain how their results advance understanding beyond what is already known. The main conclusion—that mid-sized particles dominate bulk properties—is not surprising and risks being seen as incremental without stronger comparative analysis or conceptual framing. Similarly, the ecological implications remain underdeveloped. If absorption is dominated by particles smaller than 5 µm, what does this mean for fjord light climate, primary productivity, or biogeochemical cycling? These connections must be drawn explicitly.
Citation: https://doi.org/10.5194/egusphere-2025-2800-RC1
- AC1: 'Reply on RC1', Justyna Meler, 23 Feb 2026
  
  Dear Reviewer
  Thank you very much for insightful critical comments on our work. Below we present our responses and the description of actions taken in regards to your comments. We believe that we have provided satisfactory explanations to your criticisms, and have made appropriate revisions in our manuscript.
  Reviewer's comment:
  The authors investigate the biogeochemical composition and light absorption characteristics of SPM across four size fractions in three western Svalbard fjords during the summers of 2022 and 2023. The use of cascade filtration provides some insight into the distribution of SPM in fjordic systems, and the dataset may have value for the polar biogeochemical community as a seasonal snapshot sensitive to climate-driven change.
  However, in its current form, the manuscript functions more as a data report than as a research article. It presents percentages and descriptive summaries without rigorous analysis, mechanistic interpretation, or clear hypotheses. As such, the study does not yet meet the standards for a publishable research contribution. Major revisions are required, including the addition of statistical and comparative analyses, stronger ecological and optical interpretation, and a clear positioning within the existing literature, to elevate this work from a dataset report to a substantive scientific article.
  Author's response:
  We thank the Reviewer for constructive comments. In accordance with the Reviewer's suggestions, we have modified and supplemented the text of the manuscript.
  Reviewer's comment:
  First and foremost, the overall novelty of the manuscript is weak. The authors must articulate more clearly how this dataset provides new insights into Arctic size-fractionated optics and biogeochemistry, rather than simply reporting percentage contributions. Without methodological strengthening, statistical rigor, and broader contextualization, the study risks being perceived as a limited dataset report rather than a substantive scientific contribution.
  Author's response:
  We have supplemented the methodological part, presented the results of Kruskal-Wallis statistical analyses and added a broader context of the research and analyses conducted.
  Reviewer's comment:
  The Introduction is currently too weak to guide readers toward the purpose of the study. It provides minimal background, insufficient literature context, and lacks a clear articulation of the knowledge gap. As written, readers cannot easily understand what scientific question this paper seeks to address or why it matters. The stated aims are descriptive (“to determine the contribution of…”) rather than hypothesis-driven, and there is no strong link between the particle size–resolved measurements and the broader ecological or optical processes in fjord systems. Overall, the Introduction requires substantial revision to provide context, define the gap, and present clear, hypothesis-driven objectives.
  Author's response:
  The introduction, including the literature context, the statement of the study's aims and hypotheses, and the gaps in knowledge, has been updated according to the Reviewer's suggestions. We have linked size fractions to ecological and optical processes occurring in fjord ecosystems.
  Reviewer's comment:
  A major limitation lies in the underspecified sampling design. Critical details such as sampling depth, instrumentation, replication strategy, tidal or glacial influences, and spatial gradients within fjords are not reported. Without this information, it is impossible to assess the representativeness of the dataset or the robustness of the derived patterns. Similarly, no statistical tests are presented to determine whether contributions of different size classes are significantly distinct, or whether differences exist among fjords or between years. This omission reduces the study to a purely descriptive account rather than a quantitative analysis.
  Author's response:
  We have supplemented the methodology and description of the empirical material. Seawater samples were collected from the surface water layer (0-1 m). Between 40 and 100 L of seawater were collected and then subjected to cascade filtration to obtain suspended solids size fractions, followed by refiltration to determine biogeochemical and optical parameters. A description of the cascade filtration setup was added. Replication per se was not included due to the time required for complete cascade filtration and filtration to obtain the appropriate parameters (4-6 hours for each station). Given the tight schedule of the AREX expedition (which has been held annually for 40 years), this was not possible, despite 24-hour operation. The SPM collection method, where triplicate sampling was always performed, can be considered as a form of replication. We conducted additional statistical analysis of the obtained results (Kruskal-Wallis) to determine whether the contributions of individual size fractions were significant. In most cases, these differences are not significant (p>0.05). Significant differences (p<0.05) were observed only for Tchla and the picoparticle fraction. We have added appropriate descriptions in the manuscript.
  Reviewer's comment:
  The stated aim “to capture the greatest possible optical diversity of fjord waters” in Line 161 is not supported by the sampling design. The dataset consists of limited snapshot measurements from two summer campaigns, which cannot adequately represent the strong seasonal and interannual variability known in Svalbard fjords. Optical and biogeochemical properties in these systems are highly dynamic, influenced by spring blooms, glacial meltwater pulses, and autumn mixing, none of which are captured here. As a result, the claim of covering “the greatest possible optical diversity” is overstated and should be substantially tempered. The authors should acknowledge this limitation clearly and frame their dataset as a restricted summer snapshot rather than a comprehensive representation of fjord optical diversity.
  Author's response:
  Our phrase, "capturing the greatest possible optical diversity of fjord waters," does not encompass the full seasonal variation in fjords. Our goal was to obtain the greatest possible optical diversity, determined by the Secchi disk (different water opacities), solely during the summer season we studied.
  Reviewer's comment:
  The manuscript repeatedly refers to a separate paper for detailed explanations of sample analyses. While cross-referencing related work is acceptable, the current paper must remain methodologically self-contained. Readers should not have to consult another publication to understand how the measurements were obtained, processed, or validated. At minimum, the authors need to provide concise but sufficient descriptions of key analytical methods (e.g., cascade filtration procedure, absorption measurements, organic/inorganic determinations), including potential uncertainties and limitations. Without this, the study is not reproducible or assessable on its own merits. A brief summary of the essential methodological details should therefore be included directly in the present manuscript, even if a companion paper contains additional depth.
  Author's response:
  The Methodology section has been modified and supplemented.
  Reviewer's comment:
  The interpretation of results is also weak. The dominance of ultra-particles is repeatedly emphasized, but no mechanistic explanation is provided. The discussion mainly restates percentages rather than linking particle distributions to processes such as fjord circulation, glacial meltwater input, seasonality, or primary production dynamics.
  Author's response:
  The interpretation of the results is included in the Discussion and Conclusions.
  Reviewer's comment:
  The manuscript is further weakened by its limited engagement with existing literature. Several studies have examined size-fractionated SPM and absorption in other polar and temperate environments, yet the authors do not convincingly explain how their results advance understanding beyond what is already known. The main conclusion—that mid-sized particles dominate bulk properties—is not surprising and risks being seen as incremental without stronger comparative analysis or conceptual framing. Similarly, the ecological implications remain underdeveloped. If absorption is dominated by particles smaller than 5 µm, what does this mean for fjord light climate, primary productivity, or biogeochemical cycling? These connections must be drawn explicitly.
  Author's response:
  Supplemented in the Introduction, Discussion and Conclusions.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2800-AC1
RC2:
'Comment on egusphere-2025-2800', Anonymous Referee #2, 30 Dec 2025

The authors characterize biogeochemistry and absorption properties of three fjords on the west coast of Svalbard during two summertime sampling campaigns in 2022 and 2023. The dataset presented is valuable, in part, because of the partitioning of the biogeochemical and absorption property observations across four size fractions: pico, ultra, nano, and micro particles. The authors' findings include that the majority of both organic and inorganic particles observed in the fjords corresponded to the ultra size fraction. The authors note that studies considering particle size classes in comparable detail have been rare. The authors also draw on the importance of this work towards development of optical algorithms. For example, the authors note that the absorption spectra observed were not sufficiently dissimilar across size fractions to support optical approaches for size fractionation.
Despite the potential value of the data collected, my perception of the manuscript is that it is not ready in its present form for publication as a research article. I struggled to extract what hypotheses the authors were testing, or how their findings (briefly that the ultra size fraction dominated the particle size distribution and that the characteristics of the three fjords during the summer campaigns were qualitatively dissimilar from other global waters such as the Baltic Sea) constituted a meaningful advance in knowledge of the dynamics of fjord biogeochemistry or optics. Without clearly linking the observations towards addressing existing knowledge gaps or testing relevant hypotheses, the manuscript reads as a dataset report and does not yet realize the potential impact that the authors' data could support. The manuscript is significantly lacking in synthesis and interpretation of the findings. As a result, the manuscript's length is also not justified.
The lack of synthesis of the observations' meaning is compounded because the sampling is not contextualized in terms of the spatial and temporal dynamics of fjord systems. For example, what depths were observations obtained from, and what were the vertical and mixing dynamics of the water mass that could modify the particle size distribution as sampled? What were spatial gradients observed in the sampling along the fjords? What can the authors discern from their observation or from the literature regarding temporal variability that might help contextualize the representativeness of the authors' summertime observations. Adding this context would be helpful, but the key limitation is still that the study does not adequately synthesize its findings or apply its observations towards hypothesis testing. I encourage the authors to extract more information about fjord dynamics from their valuable dataset.

Citation: https://doi.org/10.5194/egusphere-2025-2800-RC2
- AC2: 'Reply on RC2', Justyna Meler, 23 Feb 2026
  
  Dear Reviewer
  Thank you very much for insightful critical comments on our work. Below we present our responses and the description of actions taken in regards to your comments. We believe that we have provided satisfactory explanations to your criticisms, and have made appropriate revisions in our manuscript.
  Reviewer's comment:
  The authors characterize biogeochemistry and absorption properties of three fjords on the west coast of Svalbard during two summertime sampling campaigns in 2022 and 2023. The dataset presented is valuable, in part, because of the partitioning of the biogeochemical and absorption property observations across four size fractions: pico, ultra, nano, and micro particles. The authors' findings include that the majority of both organic and inorganic particles observed in the fjords corresponded to the ultra size fraction. The authors note that studies considering particle size classes in comparable detail have been rare. The authors also draw on the importance of this work towards development of optical algorithms. For example, the authors note that the absorption spectra observed were not sufficiently dissimilar across size fractions to support optical approaches for size fractionation.
  Despite the potential value of the data collected, my perception of the manuscript is that it is not ready in its present form for publication as a research article. I struggled to extract what hypotheses the authors were testing, or how their findings (briefly that the ultra size fraction dominated the particle size distribution and that the characteristics of the three fjords during the summer campaigns were qualitatively dissimilar from other global waters such as the Baltic Sea) constituted a meaningful advance in knowledge of the dynamics of fjord biogeochemistry or optics. Without clearly linking the observations towards addressing existing knowledge gaps or testing relevant hypotheses, the manuscript reads as a dataset report and does not yet realize the potential impact that the authors' data could support. The manuscript is significantly lacking in synthesis and interpretation of the findings. As a result, the manuscript's length is also not justified.
  Author's response:
  We thank the Reviewer for constructive comments. In accordance with the Reviewer's suggestions, we have modified and supplemented the text of the manuscript.
  Reviewer's comment:
  The lack of synthesis of the observations' meaning is compounded because the sampling is not contextualized in terms of the spatial and temporal dynamics of fjord systems. For example, what depths were observations obtained from, and what were the vertical and mixing dynamics of the water mass that could modify the particle size distribution as sampled? What were spatial gradients observed in the sampling along the fjords? What can the authors discern from their observation or from the literature regarding temporal variability that might help contextualize the representativeness of the authors' summertime observations. Adding this context would be helpful, but the key limitation is still that the study does not adequately synthesize its findings or apply its observations towards hypothesis testing. I encourage the authors to extract more information about fjord dynamics from their valuable dataset.
  Author's response:
  In accordance with the Reviewer's suggestions, the manuscript was modified and supplemented in important respects. The Introduction was expanded, including the context of the conducted research and analyses, the study objectives were revised, and the hypotheses were presented. Sampling was performed only in surface waters, but based on the measured CTD profiles, it is known that the fjord water was three-layered (surface water, Atlantic water, and bottom water, winter Arctic water). In the Results and Discussion sections, we provided further interpretation of the obtained results and connections between these results and the processes occurring in fjord ecosystems.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2800-AC2

Justyna Meler, Joanna Stoń-Egiert, and Monika Zabłocka

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Short summary

We present a variability of absorption properties by different size fractions of particles suspended in the Spitsbergen Fjords waters in the summer season. The light absorption coefficient by all suspended particles, detritus and phytoplankton was determined for four size fractions: pico, ultra, nano and micro-particles. We have shown the proportions of particles from the size classes in the total absorptions, and we have found that dominant contribution had ultra-particles.


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