the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Glacier loss and vegetation expansion alter organic and inorganic carbon dynamics in alpine streams
Andrew Lean Robison
Nicola Deluigi
Camille Rolland
Nicolas Manetti
Tom Battin
Abstract. High-mountain ecosystems are experiencing acute effects of climate change, most visibly through glacier recession and the greening of the terrestrial environment. The streams draining these landscapes are affected by these shifts, integrating hydrologic, geologic, and biological signals across the catchment. We examined the organic and inorganic carbon dynamics of streams in four Alpine catchments in Switzerland to assess how glacier loss and vegetation expansion are affecting the carbon cycle of these high mountain ecosystems. We find that organic carbon concentration and fluorescence properties associated with humic-like compounds increase with vegetation cover within a catchment, demonstrating the increasing importance of allochthonous carbon sources following glacier retreat. Meanwhile, streams transitioned from carbon dioxide sinks to sources with decreasing glacier coverage and increased vegetation coverage, with chemical weathering and soil respiration likely determining the balance. Periods of sink behavior were also observed in non-glaciated streams, indicating geochemical consumption of carbon dioxide may be more common in high-mountain, minimally vegetated catchments than previously described. Together, these results demonstrate the dramatic shifts in carbon dynamics of alpine streams following glacier recession, with significant changes to both the organic and inorganic carbon cycles. The clear link between the terrestrial and aquatic zones further emphasizes the coupled dynamics with which all hydrologic and biogeochemical changes in these ecosystems should be considered, including the role of mountain streams in the global carbon cycle.
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Andrew Lean Robison et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-12', Anonymous Referee #1, 07 Mar 2023
This manuscript presents data from a space for time approach to characterize DOC and DIC fluxes from streams draining high mountain and glacierized watersheds to understand how glacier melt will impact carbon source/sink behavior. Overall, this is a well written paper and presents a clear picture and conceptual model hypothesizing how weathering, soil and stream respiration, and transport impact stream CO2 fluxes. It will make a nice contribution to our collective understanding of the role of glaciers and mountain systems in local and global biogeochemical cycles. I have a few questions and comments regarding data and details/analyses that were omitted.
This is somewhat minor, but in multiple places, the authors refer directly or indirectly to soil development allowing for expansion of vegetation when these really are concomitant processes. Vegetation can drive further development of soils, see Klaar et al. 2015 (among many others). I would suggest that you acknowledge this and adjust the way you discuss landscape evolution. On a related note, can you provide more information on the type of vegetation in these watersheds? Vegetation type varies with elevation, are there any potential differences in DOC export as a function of vegetation type? Or are you assuming this is entirely related to soil development and overall accumulation of biomass?
You state that samples were collected monthly during the snow free season. Was there any temporal variability in sampling within and across seasons? How might this seasonal variability impact your results and estimates of each flux? (See Bergstrom et al., 2021) I would have liked to see a plot or at least a description of seasonal and interannual variability. You get out ahead of the fact that your flux and sink/source estimates have high uncertainty for a number of reasons and therefore are designed to be more of a comparison, but I expect that there may be some interesting information to be gained from examining this across the snow free season or from a wetter to a drier year.
You mention in the site description that two watersheds are dominated by carbonate and sedimentary lithology while the other two are metamorphic but there is no further discussion of this until the end where you simply state that more direct geologic perspectives are needed. It seems like you at least have the ability to include the fact that two watersheds are carbonate dominated in your analysis and interpretation. Can you include the percent carbonate in your PLS analysis? Or interpret your major ion data and potential impact on your DIC concentrations in the context of what is likely being weathered? Possibly the resolution of your geologic data is not good enough to directly include but I believe that there should at least be more direct discussion of the impact of the differing lithologies on your results.
Minor Comments:
Line 63 – this sentence could use a citation
Line 68 – consider changing to underneath? Below somewhat implies downstream
Lines 99-102 – This sentence is hard to parse as written
Line 127 – Is this a mean elevation? It’s not clarified here or in the table, a mean and an elevation range would probably helpful to include.
Line 279 – catchment is misspelled
Line 422 – Events is misspelled
Line 422 – How might snowmelt impact the mobilization of DOC? You only discuss rain events, this is related to the major comment above.
Line 502 – This paragraph would benefit from a summarizing sentence
Line 538-541- This is a repeat of information recently stated- I would suggest deleting or shortening.
Klaar, M. J., Kidd, C., Malone, E., Bartlett, R., Pinay, G., Chapin, F. S., and Milner, A. (2015), Vegetation succession in deglaciated landscapes: implications for sediment and landscape stability. Earth Surf. Process. Landforms, 40, 1088– 1100. doi: 10.1002/esp.3691.
Bergstrom, A., Koch, J. C., O'Neel, S., & Baker, E. (2021). Seasonality of solute flux and water source chemistry in a coastal glacierized watershed undergoing rapid change: Wolverine Glacier watershed, Alaska. Water Resources Research, 57, e2020WR028725. https://doi.org/10.1029/2020WR028725
Citation: https://doi.org/10.5194/bg-2023-12-RC1 -
RC2: 'Comment on bg-2023-12', Anonymous Referee #2, 16 Mar 2023
The authors describe impacts of glacial retreat on carbon dynamics by comparing organic carbon concentration and quality and PCO2 variations across a range of Alpine catchments with variations in glacial coverage. The manuscript is well-written and concise. Methods are appropriate and conclusions are supported by results. One aspect of the study that may warrant further discussion is the role of dominant lithology type on carbon dynamics. The authors describe variability in the abundance of carbonate versus silicate minerals between catchments, which should have a major impact on weathering dynamics and CO2 concentrations. Some analysis of catchment lithology impacts on weathering and CO2/d13C-CO2 would be interesting to address this possibility. This could be through major ion ratios or geochemical modeling, both of which are commonly employed methods in glacial mineral weathering studies.
Citation: https://doi.org/10.5194/bg-2023-12-RC2 -
RC3: 'Comment on bg-2023-12', Anonymous Referee #3, 19 Mar 2023
The paper investigates how the glacier retreat, the soil development and related vegetation expansion alter organic and inorganic carbon dynamics in alpine streams. The paper is well-written and organized, providing a clear presentation of the results, well discussed and synthesized in a conceptual model.
Some minor points are listed below:
Line 128: Please add information about the main soil types which characterize the study area as recommeded also for lines 413-414.
Lines 321-324: Why not using as a measure unit mgL-1? Please change the DOC concentrations into mgL-1
Lines 366-368: Maybe it’s better to move these sentences to the Discussion paragraph
Lines 413-414: May we assume that an increase in vegetation cover corresponds to an increase in soil development? Line 425 decribes the concept, which I think is a key concept. Do you have an idea of the soil Corg content in your study area? See also line 496. Or at least it would be useful to report the main soil types in the study area according to the standard classification systems (e.g. WRB and/or Soil Taxonomy)
Line 418: Do you exclude that in some catchments the contribution of glacier melt to DOC concentration in stream water is not negligible? See lines 63-65.
Table 1: Please include also the information about the geology of the area
Table 2: As reported for Line 324, I suggest to transform the data of DOC concentation into mgL-1 as you did in Figure 2
Citation: https://doi.org/10.5194/bg-2023-12-RC3
Andrew Lean Robison et al.
Andrew Lean Robison et al.
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