the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Geographical controls and anthropogenic impacts on dissolved organic carbon from mountainous rivers: Insights from optical properties and carbon isotopes
Shuai Chen
Jun Zhong
Yuanbi Yi
Wanfa Wang
Zelong Yan
Siliang Li
Khan M. G. Mostofa
Abstract. Mountainous rivers (MRs) are one of the critical systems in transporting dissolved organic carbon (DOC) from terrestrial environments to downstream ecosystems. However, how geographical factors and anthropogenic impacts control the composition and export of DOC in mountainous rivers remains largely unclear. Here, we explore DOC dynamics in three subtropical mountainous catchments (i.e., the Yinjiang, Shiqian, and Yuqing catchments) in southwest China which are highly influenced by anthropogenic activities. Water chemistry, stable and radioactive carbon isotopes of DOC (δ13CDOC and Δ14CDOC) and optical properties (UV absorbance and fluorescence spectra) for river water were employed to assess the biogeochemical processes and controlling factors of DOC. The radiocarbon ages of the DOC in the Yinjiang River varied widely, ranging from 928 years before present to modern. Both allochthonous and autochthonous sources had an important effect on riverine DOC export. Results from carbon isotopes suggested that in-stream processing of POC is also an important source of DOC. DOC in catchments with higher slope gradients and lower annual air temperature was characterized by lower concentration and more aromatic, which was distinct from those with gentle slopes and higher temperature. Variabilities in DOC concentrations and δ13CDOC were also explained by land use, showing that higher DOC concentrations with 13C-depleted characters were observed in urban and agricultural land use areas. Moreover, DOM was less aromatic, less recently produced and had a higher degree of humification in catchments with a higher proportion of urban and agricultural land use area. This research highlights the significance of incorporating geographical controls and anthropogenic impacts into the MRs to better understand their DOC dynamics and quality of dissolved organic matter (DOM).
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Shuai Chen et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-217', Anonymous Referee #1, 21 Jan 2023
This paper evaluated how land use practices and catchment slope impact concentrations and sources of DOC in three mountainous rivers. They find that agricultural activities at lower elevations increase DOC concentrations and lead to more terrestrial (13C-depleted) and old (14C-depleted) organic carbon in rivers. Findings from this study address a current gap on the factors controlling DOC source (as measured by 13C, 14C, absorbance, and fluorescence) in these ecosystems because most of the literature to date has focused on POC cycling. I have three suggestions to improve the manuscript:
- Many of the water chemistries measurements made in the three rivers in Figure 2 were reported in a recent paper (Chen et al. 2021). The study adds to this dataset by comparing water chemistries between the rivers and a spring water source, but there are two shortcomings. First, there is further discussion of how DOC concentrations differed between the sources, but not the other variables measured. Second, no statistical analyses were performed to determine whether the water chemistries of spring water were different from the river waters. Section 3.1 of the results should be revised to clarify when there were statistically significant differences or not between the sites sampled.
- The authors discuss in Section 4.2 how river reaches with shallower slopes had higher DOC concentrations and more terrestrial (13C), less aromatic (SUVA254), and older (14C) DOC sources, and in Section 4.3 how the agricultural activities consistently take place at lower elevations. Because the former result could be due to agricultural activities as well as increased erosion from those activities, and not as much from shallower slopes, I suggest that the authors revise the discussion to make this point more clearly.
- I also wonder if the authors could discuss further how they arrived at the conclusion that 14C-DOC came from POC, and the implication of that finding. For example, if the authors show that DOC is more 14C-depleted in river reaches with more agricultural activity, then couldn’t the DOC be coming from agricultural sources? Or would you expect wastewater and agricultural runoff have different 13C-DOC signatures compared to the POC? Chen et al. (2021) recently showed that aquatic photosynthesis was the main source of POC in these rivers. If DOC is coming from POC, then the authors should discuss how and why more 13C- and 14C-depleted POC from photosynthesis would be present at lower elevations with agricultural activity. Lastly, the discussion would benefit from a comparison of findings on 14C-DOC signatures to other papers that have measured this in mountainous rivers, including Masiello and Druffel 2001, Longworth et al. 2007, Moyer et al. 2012, Schwab et al. 2022.
Other minor questions:
- In the results, what error is being reported? Is it standard error, standard deviation, etc. and what are the sample numbers?
- Do the statistical results shown in Figure 5 hold if Y12 is removed as an outlier?
- How do the patterns (and statistically significances) shown between rivers and spring waters in Figures 2 and 3 change if Y12 is excluded? Do the results from Y12 skew the rest of the findings in any way?
Citation: https://doi.org/10.5194/bg-2022-217-RC1 -
AC1: 'Reply on RC1', Lishan Ran, 05 Mar 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-217/bg-2022-217-AC1-supplement.pdf
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RC2: 'Comment on bg-2022-217', Anonymous Referee #2, 22 Jan 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-217/bg-2022-217-RC2-supplement.pdf
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AC2: 'Reply on RC2', Lishan Ran, 05 Mar 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-217/bg-2022-217-AC2-supplement.pdf
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AC2: 'Reply on RC2', Lishan Ran, 05 Mar 2023
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RC3: 'Comment on bg-2022-217', Anonymous Referee #3, 01 Feb 2023
General Comments:
This study characterizes the geomorphological controls and anthropogenic effects on DOM dynamics in small mountainous rivers. There are some interesting results and discussion that would make this manuscript a good contribution to the literature. However, there are important methodological details that need to be included or rectified before publication. Further, the discussion is lacking a clear structure, and I think the manuscript would be much improved by the removal of tangential and speculative discussion.
Allochthonous and autochthonous are incorrectly defined here. Autochthonous does not mean “microbial,” but rather, produced within the river (e.g., from primary producers within the aquatic system). Allochthonous means organic C produced outside the river, but can also be "microbial" in composition (for example, if the DOM is derived from soils that have undergone significant microbial degradation).
Specific Comments:
Methods: There are many important details missing in the methodology, particularly in the laboratory analysis section. This makes it challenging to assess whether the methodology in this study is sound. See specific comments below.
Please include methodology on how land-use and slope are calculated.
Ln 100: What were samples collected with? Acid-washed or baked equipment?
Ln 106: How long were samples stored before optical measurement? 0.7 µm pore size allows microbes into filtrate, so long storage times (even at 4°C) can be problematic.
Ln 117-123: Did the authors measure blanks or standards for radiocarbon measurement to test and correct for contamination in the radiocarbon processing setup? Please describe whether these checks and corrections were performed, and if so, what the amount of error from contamination was, as contamination can be quite significant when processing samples for radiocarbon.
Ln 124: Were blanks measured between absorption analyses and were the blanks subtracted from the measured samples? How did the authors calculate the absorption coefficient (which is needed to calculate SUVA), or are the values presented here raw absorbance values? The SUVA values presented in Figure 4a are much too high for the typical range of surface waters, so maybe SUVA was miscalculated by using the raw absorbance value rather than the decadic absorption coefficient as in Weishaar et al. 2003 (doi: 10.1021/es030360x)?
Ln 126: Absorbance and therefore SUVA are highly affected by the presence of iron in filtrate (see Poulin et al. 2014, doi: 1021/es502670r). Do you happen to know the iron concentrations or the iron:DOC ratio in this system? Iron interference could explain why the SUVA values are much higher than typical surface water values (typically < 5.0 L mg-1 m-1, see Poulin et al. (2014) and references therein), though the SUVA values presented here seem too high for just an iron interference issue.
Ln 130: How often were blanks measured? Was inner-filter correction performed (see Kothawala et al. 2013, doi: 10.4319/lom.2013.11.616)? Inner-filter correction is essential when working with fluorescence data. Additionally, PARAFAC results should be compared previous work (for example, using OpenFluor) for context.
Discussion: Many results are presented in the discussion that are not first presented in the results. I suggest the authors move Figures 3-7 to the Results and add text describing these findings. Further, I think the number of figures can be greatly reduced and the discussion streamlined.
Figure 3: Unclear what the purpose of this figure or PARAFAC analysis is as these results are never discussed. This figure can be removed or discussion of PARAFAC results (after contextualizing the components based on OpenFluor, for example) should be added.
Figure 4a: SUVA values are uncharacteristically high (see comment above in the methods). Additionally, this figure (once the high SUVA values are addressed or corrected) would be more effective if is discussed in the section below when discussing geomorphology controls on DOM. Why not include it in Figure 6 when trends with slope are shown?
Figure 4b: This figure is confusing as we do not know much about the site numbers. I suggest the authors present this with changing slope or land-use instead. Additionally, I think this figure would be more effective if the three indices were split into three different plots. The small changes in each parameter are minimized by the large scale of the plot, and the indices shouldn’t be compared directly, so there is no need to include them all on the same axis.
Figure 5a: Why is the x-axis presented as 1/DOC? I feel it is confusing.
Ln 235: I don’t think you have enough evidence to say that the more negative δ13C and ∆14C are from autochthonous production – isn’t aged DIC (if autochthonous DOM is incorporating aged weathering productions) more enriched in 13C (see Mayorga et al. 2005, doi: 10.1038/nature03880)? Can the authors provide an explanation for the positive relationship between ∆14C and δ13C? Further, DOM derived from deeper flow paths is more depleted in 14C, enriched in 13C, and looks more “microbial” in composition (see Barnes et al. 2018, doi: 10.1021/acs.est.7b04717 and Butman et al. 2007, doi: 10.1016/j.orggeochem.2007.05.011). Therefore you may not have allochthonous vs autochthonous DOM driving these trends as you suggest, and it may be differences in flow path depth (surface soils and fresh plant material vs. microbially-degraded deep soils).
Figures 6 and 7 are really strong and interesting. On the other hand, I don’t see the utility of including section 4.1. It is more of a summary of past work with a lot of speculation, and it is mostly tangential to the main purpose of the manuscript (i.e., the effects of geomorphology and anthropogenic impacts on DOM in mountainous rivers). I think some of the text in this section can be incorporated into later sections when the authors are discussing the relationships between DOM parameters and slope / anthropogenic impacts, but much of it can be removed.
Ln 318-319: see above comment about the definition of allochthonous vs. autochthonous DOM
Technical Comments:
Ln 20-21: “Both allochthonous and autochthonous sources had an important effect on riverine DOC export” Unclear what is meant here – what effect? Be specific
A table in the methods describing the characteristics of the three rivers with average slopes, land-use properties, drainage areas, etc. would be helpful.
Ln 146: Confusing wording. Decreasing trend in DO in regards to what? Over time? Downstream?
Ln 230: This sentence should be split into two.
Citation: https://doi.org/10.5194/bg-2022-217-RC3 -
AC3: 'Reply on RC3', Lishan Ran, 05 Mar 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-217/bg-2022-217-AC3-supplement.pdf
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AC3: 'Reply on RC3', Lishan Ran, 05 Mar 2023
Shuai Chen et al.
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