the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jul 2022
Nitrate isotope investigations reveal future impacts of climate change on nitrogen inputs and cycling in Arctic fjords: Kongsfjorden and Rijpfjorden (Svalbard)
Abstract. Ongoing climate change in the Arctic has caused tidewater glacier retreat and increased discharge of freshwater and terrestrial material into fjords. These land inputs bring nutrients into the fjords and their cycling within the fjord system can vary with the influence of tidewater glaciers and the presence of sub-glacial meltwater plumes. In this study, we assess the influence of tidewater glaciers on nitrogen inputs and cycling in two fjords in Svalbard during the summer using stable isotopic analyses of dissolved nitrate (δ15N and δ18O) in combination with nutrients and hydrographic data. Kongsfjorden receives inputs from tidewater glaciers, whereas Rijpfjorden mainly receives surface inputs from land-terminating glaciers. Results showed that both fjords are enriched in nutrients from terrestrial inputs, where the inputs exceed Redfield ratios with excess Si and P relative to N. In both fjords, terrestrial nitrate from snowpack and glacier melting are identified as the dominant sources based on high δ18O-NO3- and low δ15N-NO3- of dissolved nitrate. In Kongsfjorden, mixed-layer nitrate is completely consumed within the fjord system which we attribute to vigorous circulation at the glacial front influenced by the subglacial plume and longer residence time in the fjord. This is in contrast with Rijpfjorden where nutrients are only partially consumed perhaps due to surface river discharge and light limitation. In Kongsfjorden, we estimate terrestrial and marine N contributions to the nitrate pool from nitrogen isotopic values (δ15N-NO3-) and this suggests that nearly half the nitrate in the subglacial plume (50 ± 3 %) and the water column (44 ± 3 %) originates from terrestrial sources. In addition, we show that terrestrial N also contributes significantly to regenerated N pool (63–88 %) within this fjord suggesting its importance in sustaining productivity within Kongsfjorden. Given this importance of terrestrial nutrient sources within the fjords, increase in these inputs due to climate change can enhance the fjord nutrient inventory, productivity and nutrient export offshore. Specifically, increasing Atlantification and warmer Atlantic Water will encourage tidewater glacier retreat and in turn increase surface discharge. In fjords akin to Rijpfjorden this is expected to foster more light limitation and less dynamic circulation, ultimately aiding the export of nutrients offshore contributing to coastal productivity. Climate change scenario postulated for fjords such as Kongsfjorden include more terrestrial N-fuelled productivity and N cycling within the fjord, less vigorous circulation and the expansion of oxygen depleted deep waters inside the sill.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(13211 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2022-584', Andy Hodson, 24 Jul 2022
The paper could consider using published seasonal (and multi-year) values of d15N-NO3 and d18O-NO3 in glacial runoff entering Kongsfjord. Linked to this, the paper could also be potentially improved by considering the role of nitrification (which is currently limited to a brief mention in the context of guano). Nitrification has been shown to become the dominant source of NO3 to glacial runoff in Kongsfjord after mid-July, and the “excess nitrate” it creates seems to be present in a worldwide selection of glaciers. While I am unsure of how this will affect the authors’ important assertions about the future nitrogen balance of the two fjords being studied, I think it is really important to demonstrate a full appreciation of the role played by microorganisms in supplementing the nitrate content of runoff whilst glaciers retreat onto land. Two published studies of direct relevance to Kongsjorden are:
Wynn, P.M., Hodson, A.J., Heaton, T.H. and Chenery, S.R., 2007. Nitrate production beneath a High Arctic glacier, Svalbard. Chemical geology, 244(1-2), pp.88-102.
Ansari, A.H., Hodson, A.J., Kaiser, J. and Marca-Bell, A., 2013. Stable isotopic evidence for nitrification and denitrification in a High Arctic glacial ecosystem. Biogeochemistry, 113(1), pp.341-357.
The above papers show that the inferred subglacial d15N-NO3 and d18O-NO3 end member signature in the discussion paper is quite different to those observed in glacial rivers. For example, during the main runoff season, subglacial d15N-NO3 was in the range -2 to -7 o/oo (Wynn et al). I am not entirely sure what this means for the discussion paper, but it would be good to see the authors’ views on this and I hope it can help the discussion in Section 4.5, where I found sources mentioned that were difficult to understand (moulins?). Putting the strong seasonality of glacial outflow nitrate aside, I wonder whether the authors’ inferred subglacial end-member requires more denitrification than is apparent from the published values of subglacial outflow. This might be because the dominant subglacial inputs to Kongsfjord come from far larger glaciers than those studied by Wynn et al and Ansari et al. I find this entirely plausible and also useful, because less denitrification after glacier retreat onto land is also a realistic proposition. It would also be good to question the representativeness of values from the smaller glaciers since they dominate the literature but not the inputs to fjords.
Lastly, a minor point is that N2 fixation is indeed poorly understood, but it was studied on glaciers in the Kongsfjord region by the publication below. For sure, though, N2 fixation is not so important
Telling, J., Anesio, A.M., Tranter, M., Irvine‐Fynn, T., Hodson, A., Butler, C. and Wadham, J., 2011. Nitrogen fixation on Arctic glaciers, Svalbard. Journal of Geophysical Research: Biogeosciences, 116(G3).
Citation: https://doi.org/10.5194/egusphere-2022-584-CC1 - AC1: 'Reply on CC1', Marta Santos-Garcia, 19 Sep 2022
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RC1: 'Comment on egusphere-2022-584', Anonymous Referee #1, 09 Aug 2022
By presenting hydrographic data, macronutrients, and isotopic compositions of nitrate, Santos-Garcia et al. examined the influence of tidewater glaciers on nitrogen inputs and cycling in two fjords in Svalbard during the summertime. Overall, this study is a good demonstration of the application of nitrogen isotopes and nutrients in Arctic fjords to reveal the sources, cycling, and possible response of these nutrients to future changes in two well-characterized fjords. The work could be valuable to improve our understanding of the influence of climate change on polar region nutrient cycles. However, the structure of this manuscript needs to be further condensed and reorganized. The comments are as follows:
Major concerns:
Major concerns:
The Introduction section generally gives a good background to the study, but is structurally too long and does not flow smoothly from one section to the next (a subsection in the introduction is also awkward). It is recommended that the introductory section (e.g. Section 1.1 is too redundant) be streamlined and that the final section of the introduction briefly summarizes the shortcomings of current nutrient or nitrogen isotope studies in the Arctic fjords and surrounding waters to better introduce the purpose of the study.
Section 3.2 titled Nutrient concentrations and isotopic ratios actually contains descriptions of parameters such as temperature and salinity, while other parameters in Figures 4 and 5 (e.g. dissolved oxygen and chlorophyll) are not described. It is suggested that this could be split into two sections on environmental settings and nutrient/nitrogen isotope results respectively.
Throughout the discussion section, the language lacks conciseness, and the same idea or description appears repeatedly. For example, in lines 505-510, statements like “nitrate uptake is complete with near-zero nitrate values” appear several times, and similar phenomena occur repeatedly throughout the text. The narrative and language of the article needed major improvement, and the manuscript needed significant refinement of language and reduction in length. Another suggestion is to consider merging some of the sub-section and renaming the subheadings. For example, the analysis in 4.1 and 4.2 is mainly both derived from the nutrient stoichiometry relationship, these two sections can be combined and the description of 4.2 needs to be simplified.
Section 4.3, where it is mentioned that the slope deviation from 1 in Figure 13 may be due to the regeneration of nitrate, i.e., the occurrence of nitrification, is it possible to assess the magnitude of this process? Does the fact that nitrification may introduce 15N signatures from particulate nitrogen mineralization have an impact on the assessment of the source of nitrate? These need to be carefully evaluated.
In general, the article deals with two well-characterized fjords, involving relatively trivial descriptions of features. Consider using a table or conceptual figure to summarize the commonalities, differences, and responses of the two fjords in this paper in terms of hydrology and nutrient cycling under climate change.
Specific comments:
- Figures 1-3 are suggested to be merged into one figure, and Figure 1 should be labeled with latitude and longitude.
- For the paragraphs, some indent the first line and some paragraphs don't, please keep the format consistent.
- Line 42: “Nitrate is the predominant form of fixed N…” should be “Nitrate (NO3-) is the predominant form of fixed nitrogen (N)…”
- Line 70: in Eqn.2 “14 k/ 15k” should be “14k/15k”; line 229 “1.8 ±0.2 (USGS 34)” should be “1.8±0.2 (USGS 34)”
- Line 455: “The degree of stratification, and its associated nutrient dilution effect, can be ruled out as salinity alone could not explain the isotope effect in Figure 12.” How was this conclusion reached? For example: if the isotope effect is due to the above factors, how should salinity be reflected in this graph?
- Figure 13a Why is it important to represent depth by color, this message doesn't seem to be utilized much, at the moment this chart is a bit confusing, could different colors be used to represent groups?
- AC2: 'Reply on RC1', Marta Santos-Garcia, 19 Sep 2022
-
RC2: 'Comment on egusphere-2022-584', Anonymous Referee #2, 22 Aug 2022
Santos-Garcia et al. through their manuscript have presented sources of nutrients and their cycling at two Svalbard fjords. Their analysis is detailed and employs a multi-fold approach. They did their best to present a (first) quantitative account of terrestrial contribution to the nitrate pool. The study also relates these processes to climate change and provides a convincing account of its future trends under a warming scenario. High Arctic fjords and Kongsfjorden in particular have become centers of multi-disciplinary research on the impact of warming and climate change in the Arctic. This study will surely benefit other aspects of the research being conducted at these sites. While I support its publication, there are a few concerns, comments, and suggestion that needs to be addressed.
- While I feel all the visualizations (both in-text and supplementary plots) to be appealing, uncluttered, and informative, the opposite is true for the text presented in the manuscript. Most of the sentences and sections are unnecessarily cluttered and difficult to read. English usage and grammar need a major overhaul throughout the manuscript in my opinion.
- Get rid of the equations in section 1.1. You may include them as a supplementary note. Reduce the number of sentences and also words within a sentence to better convey these ideas.
- A1 is very good as it contains all the information/ideas (of section 1.1) one needs to follow the rest of the manuscript. The authors may move this inside the main text and move the equations to the supplementary file. This will enhance the reading experience.
- Why a particular water-mass classification scheme was adopted? Why not widely used Cottier et al., 2005?
- The x-axis of Fig 8 and several others (distance) is not clear. Please mark this distance as a line in Fig 2.
- Lines (260-265): Water mass structures are dynamic. You may like to add the sampling period in parenthesis. This approach may be followed throughout the text.
- Lines (321-323): Why incomplete nutrient utilization? Wouldn’t stratification enhance relative nutrient utilization?
- Lines (349-353): What about low nutrient uptake close to glacier front and inner fjord in general. Instead of solely attributing the cause to plume discharge, I would also see the prevailing axial productivity gradient (see Kumar et al., 2016) as a potential reason. Perhaps it’s a combination of both these factors.
- Line 474: You may check a modified form of this equation, which uses more representative sampling (Tiwari et al., 2018, doi: 10.1016/j.gsf.2017.12.007).
Citation: https://doi.org/10.5194/egusphere-2022-584-RC2 - AC3: 'Reply on RC2', Marta Santos-Garcia, 19 Sep 2022
Interactive discussion
Status: closed
-
CC1: 'Comment on egusphere-2022-584', Andy Hodson, 24 Jul 2022
The paper could consider using published seasonal (and multi-year) values of d15N-NO3 and d18O-NO3 in glacial runoff entering Kongsfjord. Linked to this, the paper could also be potentially improved by considering the role of nitrification (which is currently limited to a brief mention in the context of guano). Nitrification has been shown to become the dominant source of NO3 to glacial runoff in Kongsfjord after mid-July, and the “excess nitrate” it creates seems to be present in a worldwide selection of glaciers. While I am unsure of how this will affect the authors’ important assertions about the future nitrogen balance of the two fjords being studied, I think it is really important to demonstrate a full appreciation of the role played by microorganisms in supplementing the nitrate content of runoff whilst glaciers retreat onto land. Two published studies of direct relevance to Kongsjorden are:
Wynn, P.M., Hodson, A.J., Heaton, T.H. and Chenery, S.R., 2007. Nitrate production beneath a High Arctic glacier, Svalbard. Chemical geology, 244(1-2), pp.88-102.
Ansari, A.H., Hodson, A.J., Kaiser, J. and Marca-Bell, A., 2013. Stable isotopic evidence for nitrification and denitrification in a High Arctic glacial ecosystem. Biogeochemistry, 113(1), pp.341-357.
The above papers show that the inferred subglacial d15N-NO3 and d18O-NO3 end member signature in the discussion paper is quite different to those observed in glacial rivers. For example, during the main runoff season, subglacial d15N-NO3 was in the range -2 to -7 o/oo (Wynn et al). I am not entirely sure what this means for the discussion paper, but it would be good to see the authors’ views on this and I hope it can help the discussion in Section 4.5, where I found sources mentioned that were difficult to understand (moulins?). Putting the strong seasonality of glacial outflow nitrate aside, I wonder whether the authors’ inferred subglacial end-member requires more denitrification than is apparent from the published values of subglacial outflow. This might be because the dominant subglacial inputs to Kongsfjord come from far larger glaciers than those studied by Wynn et al and Ansari et al. I find this entirely plausible and also useful, because less denitrification after glacier retreat onto land is also a realistic proposition. It would also be good to question the representativeness of values from the smaller glaciers since they dominate the literature but not the inputs to fjords.
Lastly, a minor point is that N2 fixation is indeed poorly understood, but it was studied on glaciers in the Kongsfjord region by the publication below. For sure, though, N2 fixation is not so important
Telling, J., Anesio, A.M., Tranter, M., Irvine‐Fynn, T., Hodson, A., Butler, C. and Wadham, J., 2011. Nitrogen fixation on Arctic glaciers, Svalbard. Journal of Geophysical Research: Biogeosciences, 116(G3).
Citation: https://doi.org/10.5194/egusphere-2022-584-CC1 - AC1: 'Reply on CC1', Marta Santos-Garcia, 19 Sep 2022
-
RC1: 'Comment on egusphere-2022-584', Anonymous Referee #1, 09 Aug 2022
By presenting hydrographic data, macronutrients, and isotopic compositions of nitrate, Santos-Garcia et al. examined the influence of tidewater glaciers on nitrogen inputs and cycling in two fjords in Svalbard during the summertime. Overall, this study is a good demonstration of the application of nitrogen isotopes and nutrients in Arctic fjords to reveal the sources, cycling, and possible response of these nutrients to future changes in two well-characterized fjords. The work could be valuable to improve our understanding of the influence of climate change on polar region nutrient cycles. However, the structure of this manuscript needs to be further condensed and reorganized. The comments are as follows:
Major concerns:
Major concerns:
The Introduction section generally gives a good background to the study, but is structurally too long and does not flow smoothly from one section to the next (a subsection in the introduction is also awkward). It is recommended that the introductory section (e.g. Section 1.1 is too redundant) be streamlined and that the final section of the introduction briefly summarizes the shortcomings of current nutrient or nitrogen isotope studies in the Arctic fjords and surrounding waters to better introduce the purpose of the study.
Section 3.2 titled Nutrient concentrations and isotopic ratios actually contains descriptions of parameters such as temperature and salinity, while other parameters in Figures 4 and 5 (e.g. dissolved oxygen and chlorophyll) are not described. It is suggested that this could be split into two sections on environmental settings and nutrient/nitrogen isotope results respectively.
Throughout the discussion section, the language lacks conciseness, and the same idea or description appears repeatedly. For example, in lines 505-510, statements like “nitrate uptake is complete with near-zero nitrate values” appear several times, and similar phenomena occur repeatedly throughout the text. The narrative and language of the article needed major improvement, and the manuscript needed significant refinement of language and reduction in length. Another suggestion is to consider merging some of the sub-section and renaming the subheadings. For example, the analysis in 4.1 and 4.2 is mainly both derived from the nutrient stoichiometry relationship, these two sections can be combined and the description of 4.2 needs to be simplified.
Section 4.3, where it is mentioned that the slope deviation from 1 in Figure 13 may be due to the regeneration of nitrate, i.e., the occurrence of nitrification, is it possible to assess the magnitude of this process? Does the fact that nitrification may introduce 15N signatures from particulate nitrogen mineralization have an impact on the assessment of the source of nitrate? These need to be carefully evaluated.
In general, the article deals with two well-characterized fjords, involving relatively trivial descriptions of features. Consider using a table or conceptual figure to summarize the commonalities, differences, and responses of the two fjords in this paper in terms of hydrology and nutrient cycling under climate change.
Specific comments:
- Figures 1-3 are suggested to be merged into one figure, and Figure 1 should be labeled with latitude and longitude.
- For the paragraphs, some indent the first line and some paragraphs don't, please keep the format consistent.
- Line 42: “Nitrate is the predominant form of fixed N…” should be “Nitrate (NO3-) is the predominant form of fixed nitrogen (N)…”
- Line 70: in Eqn.2 “14 k/ 15k” should be “14k/15k”; line 229 “1.8 ±0.2 (USGS 34)” should be “1.8±0.2 (USGS 34)”
- Line 455: “The degree of stratification, and its associated nutrient dilution effect, can be ruled out as salinity alone could not explain the isotope effect in Figure 12.” How was this conclusion reached? For example: if the isotope effect is due to the above factors, how should salinity be reflected in this graph?
- Figure 13a Why is it important to represent depth by color, this message doesn't seem to be utilized much, at the moment this chart is a bit confusing, could different colors be used to represent groups?
- AC2: 'Reply on RC1', Marta Santos-Garcia, 19 Sep 2022
-
RC2: 'Comment on egusphere-2022-584', Anonymous Referee #2, 22 Aug 2022
Santos-Garcia et al. through their manuscript have presented sources of nutrients and their cycling at two Svalbard fjords. Their analysis is detailed and employs a multi-fold approach. They did their best to present a (first) quantitative account of terrestrial contribution to the nitrate pool. The study also relates these processes to climate change and provides a convincing account of its future trends under a warming scenario. High Arctic fjords and Kongsfjorden in particular have become centers of multi-disciplinary research on the impact of warming and climate change in the Arctic. This study will surely benefit other aspects of the research being conducted at these sites. While I support its publication, there are a few concerns, comments, and suggestion that needs to be addressed.
- While I feel all the visualizations (both in-text and supplementary plots) to be appealing, uncluttered, and informative, the opposite is true for the text presented in the manuscript. Most of the sentences and sections are unnecessarily cluttered and difficult to read. English usage and grammar need a major overhaul throughout the manuscript in my opinion.
- Get rid of the equations in section 1.1. You may include them as a supplementary note. Reduce the number of sentences and also words within a sentence to better convey these ideas.
- A1 is very good as it contains all the information/ideas (of section 1.1) one needs to follow the rest of the manuscript. The authors may move this inside the main text and move the equations to the supplementary file. This will enhance the reading experience.
- Why a particular water-mass classification scheme was adopted? Why not widely used Cottier et al., 2005?
- The x-axis of Fig 8 and several others (distance) is not clear. Please mark this distance as a line in Fig 2.
- Lines (260-265): Water mass structures are dynamic. You may like to add the sampling period in parenthesis. This approach may be followed throughout the text.
- Lines (321-323): Why incomplete nutrient utilization? Wouldn’t stratification enhance relative nutrient utilization?
- Lines (349-353): What about low nutrient uptake close to glacier front and inner fjord in general. Instead of solely attributing the cause to plume discharge, I would also see the prevailing axial productivity gradient (see Kumar et al., 2016) as a potential reason. Perhaps it’s a combination of both these factors.
- Line 474: You may check a modified form of this equation, which uses more representative sampling (Tiwari et al., 2018, doi: 10.1016/j.gsf.2017.12.007).
Citation: https://doi.org/10.5194/egusphere-2022-584-RC2 - AC3: 'Reply on RC2', Marta Santos-Garcia, 19 Sep 2022
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Marta Santos-Garcia
Raja Singaravelu Ganeshram
Robyn Elizabeth Tuerena
Margot Christine Frédérique Debyser
Katrine Husum
Philipp Assmy
Haakon Hop
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(13211 KB) - Metadata XML