the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2026
Biogeochemical controls on nutrient fluxes in small tropical mountainous river basins: Mechanisms for P-limitation and Si-rich conditions
Abstract. Small scale mountainous rivers with their quick flowing mechanisms provide a clear understanding on the nutrient flux dynamics in assessing their role in biogeochemical cycles and coastal nutrient budgets. The present study examines the spatio-temporal variability of dissolved inorganic nutrients in the Karamana River Basin (KRB) and Vamanapuram River Basin (VRB), flowing through Western Ghats, which emphasis the hydro geochemistry, segment-wise nutrient fluxes, and their biogeochemical cycle implications. The results reveal marked spatio-temporal variability in nutrient fluxes similar to the hydrochemistry, with higher fluxes generally recorded during the MON due to enhanced runoff and weathering. The segment-wise average fluxes of DIN, DIP, and DSi in the KRB were estimated at 6.84, 0.05, and 127.25 kg ha-1 yr-1, respectively. In comparison, the corresponding values for the VRB were 9.44, 0.07, and 81.66 kg ha⁻¹ yr⁻¹, respectively. A clear indication of low concentration conditions in the upstream followed by slight enrichment in the mid and downstream regions further highlight pristine environment and the role of land use and anthropogenic influence, respectively. The stagnant conditions after reaching the downstream regions with favorable tropical climate conditions promoting the consumption of nutrients through in-situ production. The DSi flux of VRB (127.25 kg ha-1 yr-1) is comparable to that of large global rivers such as the Amazon (108.6 kg ha-1 yr-1) and Mississippi (118.21 kg ha-1 yr-1), further supports the claim of intense chemical weathering derived silica-rich conditions. Overall, the study highlights the critical role of climate and topography in regulating nutrient fluxes and confirms that nutrient inputs from these small-scale mountainous rivers have a relatively limited influence on coastal eutrophication in the receiving zones.
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RC1: 'Comment on egusphere-2026-995', Anonymous Referee #1, 01 Apr 2026
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AC1: 'Reply on RC1', Badimela Upendra, 16 Jul 2026
Referee#1
- What are the criteria for dividing the study area into upper, middle, and lower reaches? How might uneven sampling density in the upper, middle, and lower reaches affect the results?
Author Response: Thank you for this important comment.
The study area was divided into upper, middle, and lower reaches based on equal-area segmentation of the river basin to ensure a consistent spatial framework for analysis. Sampling locations within each reach were selected primarily based on river accessibility while striving to achieve representative spatial coverage. Although the number of sampling sites differs across the three reaches due to logistical and accessibility constraints, the selected sites adequately represent the hydro-geochemical characteristics of each reach. To improve clarity, we have explicitly described the basis for reach delineation in the Methods section and highlighted the sampling density and spatial distribution of the sampling sites in the Results section, enabling readers to interpret the findings appropriately in the context of the sampling coverage.
- Although the two rivers are classified as "mountainous rivers," they both flow through agricultural and urban areas. The fact that NH₄⁺-N concentrations are below the detection limit is puzzling. The authors need to provide detailed detection limits (in mg/L) for nutrients for reader comparison, rather than using percentages, and clarify the reasons.
Author Response: We sincerely thank the reviewer for this valuable comment.
The manuscript has been revised to include the analytical detection limits of dissolved inorganic nutrients in Section 2.3. The detection limits were 0.001 mg L⁻¹ for NO₃⁻-N + NO₂⁻-N, NH₄⁺-N, and PO₄³⁻-P, and 0.1 mg L⁻¹ for DSi (Line No. 132-134). We have also clarified that NH₄⁺-N remained below the detection limit because the well-oxygenated, fast-flowing river conditions promote rapid nitrification and biological uptake, resulting in NO₃⁻-N as the dominant inorganic nitrogen species (Line No. 422-437).
- Supporting data for flux calculations, such as water discharge values, were recorded only at the basin outlet. This significantly impacts the results, so the limitations and shortcomings of the study need to be discussed.
Author Response: Thank you for this valuable comment.
We would like to clarify that the flux calculations were performed separately for the upper, middle, and lower reaches using the corresponding segment-specific water discharge rather than using only the discharge measured at the basin outlet. We recognize that the original manuscript did not clearly describe this methodology, which may have led to this misunderstanding. To address this, we have revised the Methods section to explicitly explain the segment-wise flux calculation approach. In addition, the Results and Discussion sections have been updated accordingly to present and interpret the segment-wise flux estimates based on the corresponding water discharge values. These revisions provide a more accurate representation of spatial variations in fluxes across the basin.
- The authors state, "The supporting data to calculate the flux, such as water discharge values, were recorded exclusively at the basin outlet." How was Qs obtained?
Author Response: We thank the reviewer for pointing out this methodological concern.
We agree that the methodology for estimating segment-wise discharge (Qs) requires further clarification. Direct discharge measurements were available only at the outlet of each river basin. Therefore, segment-wise discharge was estimated using the relative discharge approach proposed by David et al. (2016) and subsequently applied to tropical river basins of Kerala by Upendra et al. (2025). This method assumes that the long-term average discharge is proportional to the contributing drainage area under comparable climatic conditions.
First, the specific (relative) discharge was calculated by dividing the measured outlet discharge (Qm) by the total basin area (Am):
Q = Qm / Am
where Q is the relative discharge (km³ km⁻² yr⁻¹), Qm is the measured annual discharge at the basin outlet (km³ yr⁻¹), and Am is the total basin area (km²).
The discharge for each river segment (Qs) was then estimated by multiplying the relative discharge by the drainage area of the corresponding segment (As):
Qs = As × Q
where As represents the drainage area of the upstream, midstream, or downstream segment. The estimated segment-wise discharge values were subsequently used to calculate nutrient loads and yields.
To improve clarity, we have revised Section 2.3 of the manuscript to explicitly describe this procedure and emphasize that segment-wise discharge values were estimated using an area-weighted relative discharge approach rather than direct field measurements.
- How was HCO3- measured?
Author Response: We thank the reviewer for highlighting this.
Bicarbonate concentrations were determined using an alkalinity test kit (Metrohm). The same is now provided in the revised MS (Line No. 138-139).
- In Figure 2, the captions should clearly indicate the meanings of "pre," "mon," and "pom." These should be specified in all captions so readers do not have to search elsewhere. Note the correct notation for SO42-, etc.
Author Response: Thank you for the correction.
The figure is now modified according to the reviewer's suggestion.
- Figures 3 and 4 are blurry, with font sizes too small and incorrect formatting of legends, making them unreadable.
Author Response: Thank you for the correction.
Both figures have been modified according to the reviewer's suggestion and uploaded as separate files.
- Figure 5 has inconsistent formatting, axis font sizes too small, lacks significance analysis, and the labels "abcd" are placed inside the figure, reducing readability.
Author Response: Thank you for the correction.
Figure 5 has now been removed.
- Figure 7 looks visually appealing, but much of it remains speculative and requires further evidence.
Author Response: Thank you for the correction.
Figure 7 has been modified according to the reviewer's suggestion and uploaded as a separate file.
- In Figure S3, KPB-PRE and VRB-MON are not distinguished.
Author Response: Thank you for the correction.
Figure S3 has been modified according to the reviewer's suggestion.
- The abstract contains many abbreviations, some of which (e.g., MON) are not commonly used and their meaning is unclear.
Author Response: We thank the reviewer for this valuable suggestion. The manuscript has been carefully reviewed, and abbreviations have been revised throughout.
- In Figure 2, it is recommended to remove the orange line and adjust the font to improve readability.
Author Response: Thank you for the correction.
Figure 2 has been modified according to the reviewer's suggestion.
- Notations such as Ca-Mg-HCO3 need to be verified.
Author Response: Thank you for the suggestion. The notation is now verified and corrected.
- "Whereas KRB’s steeper slopes and urbanized surface promote hydrological flashiness and rapid nutrient export, particularly limiting DIP accumulation despite high anthropogenic loading." What is the principle behind this? Shouldn’t high anthropogenic loading increase DIP?
Author Response: We thank the reviewer for this insightful comment.
We agree that anthropogenic activities generally increase phosphorus inputs to river systems. However, in steep tropical mountainous rivers such as the KRB and VRB, the rapid runoff and short water residence times promote the swift transport of nutrients and limit the accumulation of dissolved phosphorus in the water column. Furthermore, phosphate is readily removed from the dissolved phase by adsorption onto Fe- and Al-rich soils and suspended sediments, as well as by rapid biological uptake by aquatic microorganisms and primary producers. Consequently, with the relatively low anthropogenic phosphorus inputs, dissolved DIP concentrations remain comparatively low. We have revised the corresponding text in the Discussion to clarify these mechanisms and avoid implying that anthropogenic loading alone controls DIP concentrations (Line No. 402-432).
- There is no significant difference in land use between the middle and lower reaches of the VRB, as both are predominantly cropland.
Author Response: Thank you for this insightful comment.
We would like to clarify that the upper, middle, and lower reaches of the VRB were delineated using an equal-area segmentation approach to provide a consistent spatial framework for basin-scale analysis, rather than based on land-use differences. Consequently, the middle and lower reaches are both dominated by cropland, reflecting the basin's natural land-use distribution rather than the segmentation methodology. We have revised the manuscript to clarify the basis for reach delineation and have acknowledged that the similarity in land use between the middle and lower reaches may reduce the contrast in land-use-related influences when interpreting the results.
- How did the authors determine the contribution of groundwater to runoff?
Author Response: We thank the reviewer for this important comment.
We would like to clarify that the groundwater contribution to river runoff was not quantified directly in the present study using techniques such as baseflow separation, isotopic tracers, or end-member mixing analysis. Our interpretation of groundwater influence is based on the observed hydrogeochemical characteristics of the river water, including the dominance of Ca–Mg–HCO₃ facies, elevated dissolved silica concentrations indicative of silicate weathering, and the perennial nature of the river systems. These observations are further supported by previous hydrogeological studies conducted in the study area, which have documented substantial groundwater discharge sustaining streamflow, particularly during non-monsoon periods. To avoid any misunderstanding, we have revised the manuscript to clarify that the groundwater contribution is inferred from hydrogeochemical evidence and existing literature rather than being directly quantified in this study.
- The authors frequently mention the spatial patterns of nutrients such as DIN in different regions of the watershed, but no figures or tables support this.
Author Response: Thank you for the correction.
A supplementary figure has been added in the revised version.
- Line 335: "Increasing DIN and DSi concentrations toward the outlet in both basins arise through contrasting mechanisms: rapid urban flushing in KRB versus sediment-mediated retention and release in VRB, while tributaries modulate these patterns by reflecting localized land-use pressures, from industrial hotspots (KRB) to agricultural sub-basins (VRB)." How is the role of sediments demonstrated?
Author Response: We thank the reviewer for this insightful comment.
We agree that the present study did not directly investigate sediment nutrient dynamics through sediment sampling or adsorption–desorption experiments. Therefore, the phrase "sediment-mediated retention and release" was an overinterpretation of the observed nutrient patterns. We have revised the manuscript to avoid implying a direct assessment of sediment processes. The revised text attributes the contrasting nutrient dynamics in VRB to differences in hydrological conditions, land-use characteristics, and longer water residence time, which are supported by our hydro-geochemical observations. The discussion has also been modified to clarify that any potential influence of sediment processes is inferred from previous studies rather than demonstrated in the present investigation.
- In Discussion 4.4, the authors provide valuable insights, but no evidence of phytoplankton is presented.
Author Response: We sincerely thank the reviewer for this valuable suggestion.
To strengthen the ecological interpretation presented in Section 4.4, we incorporated an independent assessment of phytoplankton biomass using Sentinel-2-derived chlorophyll-a (Chl-a) distribution. Chl-a was estimated from Sentinel-2 imagery using the Normalized Difference Chlorophyll Index (NDCI) and an established empirical calibration equation. The resulting spatial distribution maps have been added as Figure 6 & 7, and the methodology has been described in Section 2.5. The satellite-derived Chl-a maps indicate negligible phytoplankton biomass in the upstream and most midstream reaches of both river basins, with only localized enrichment in the downstream reaches. These observations are consistent with the nutrient stoichiometry, persistent phosphorus limitation, and negative ICEP values reported in this study, thereby providing independent evidence supporting the conclusion that both river systems currently exhibit a low potential for widespread eutrophication (Line No. 402-432).
Citation: https://doi.org/10.5194/egusphere-2026-995-AC1
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AC1: 'Reply on RC1', Badimela Upendra, 16 Jul 2026
- CC1: 'Comment on egusphere-2026-995', Qingqing Sun, 23 Apr 2026
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RC2: 'Comment on egusphere-2026-995', Anonymous Referee #2, 19 May 2026
Overview and general recommendation:
This manuscript investigates the spatio-temporal evolution of dissolved inorganic nutrients (DIN, DIP, and DSi) in two small tropical mountainous river basins, the KRB and VRB, located in the Western Ghats, India. By applying a segment-wise flux calculation and the ICEP indicator, the authors suggest that these basins exhibit persistent P limitation and Si richness driven by intense chemical weathering and anthropogenic inputs. Specifically, the work lacks rigorous data consistency, methodological transparency, and depth in mechanistic discussion. The pervasive numerical errors suggest a lack of careful preparation, and the simplified hydrological assumptions undermine the validity of the nutrient flux estimations. Therefore, I recommend rejection of this manuscript.
Detailed Comments:
- The data in the manuscript seems inconsistent throughout, which undermines the reliability of the conclusions. For example, the abstract states that the DSi flux of the VRB is 127.25 kg ha-1 yr-1, yet Table 1 lists the average DSi yield for the VRB as 81.66 kg ha-1 yr-1. Conversely, the abstract reports a DSi flux of 127.25 kg ha-1 yr-1 for the KRB, while Table 1 shows an average of 127.25 kg ha-1 yr-1 for the KRB, suggesting a possible swap or mislabeling of data between the two basins. Furthermore, Text S1 in the supplement reports a DSi yield of 143.37 kg ha-1 yr-1 for the KRB downstream segment, while Table 1 lists it as 161.94 kg ha-1 yr-1. The authors must conduct a rigorous audit of all numerical values in the text, tables, and figures to ensure absolute consistency.
- The study’s reliance on a “relative discharge approach” to estimate nutrient loads is scientifically problematic for the Western Ghats. This method assumes a linear relationship between drainage area and discharge, which fails to account for the extreme spatial heterogeneity in rainfall and runoff characteristic of tropical mountainous terrains. Furthermore, the annual nutrient fluxes are derived from only three sampling events. In small, flashy mountainous systems, this sampling frequency is insufficient to capture storm-driven nutrient pulses, likely leading to a significant underestimation of annual loads and rendering the comparison with large global rivers (Amazon/Mississippi) statistically tenuous.
- The discussion of P limitation remains largely speculative and lacks local empirical validation. While the authors suggest that low DIP concentrations result from P-sorption in acidic soils and biological uptake, they provide no site-specific data on soil mineralogy, pH, or adsorption capacity to support these claims. Additionally, the manuscript fails to provide biological evidence, such as Chl a data or phytoplankton community analysis, to prove that the identified chemical nutrient ratios actually manifest as biological limitations in these specific ecosystems. Without this link, the conclusion that these rivers are “P-limited systems” remains an unverified hypothesis rather than a demonstrated fact.
- The comparison between these small tropical basins and global rivers like the Amazon or Mississippi is intriguing but requires more nuance. The authors should discuss the unique “high-efficiency” transport characteristics of small basins, where shorter residence times may limit the in-stream transformation of nutrients compared to large-scale systems. On a technical note, the assertion that NH4+ was consistently below the detection limit needs to be supported by the specific detection limit values of the Continuous Flow Analyzer used. The authors should assess the potential error introduced into the DIN and ICEP calculations by excluding the ammonium fraction. Finally, the figure captions and labels, particularly in Fig. 1 and Fig. 4, should be reviewed for clarity to ensure that segment boundaries and sampling locations are easily distinguishable for the reader.
Citation: https://doi.org/10.5194/egusphere-2026-995-RC2 -
AC2: 'Reply on RC2', Badimela Upendra, 17 Jul 2026
- The data in the manuscript seems inconsistent throughout, which undermines the reliability of the conclusions. For example, the abstract states that the DSi flux of the VRB is 127.25 kg ha-1 yr-1, yet Table 1 lists the average DSi yield for the VRB as 81.66 kg ha-1 yr-1. Conversely, the abstract reports a DSi flux of 127.25 kg ha-1 yr-1 for the KRB, while Table 1 shows an average of 127.25 kg ha-1 yr-1 for the KRB, suggesting a possible swap or mislabeling of data between the two basins. Furthermore, Text S1 in the supplement reports a DSi yield of 143.37 kg ha-1 yr-1 for the KRB downstream segment, while Table 1 lists it as 161.94 kg ha-1 yr-1. The authors must conduct a rigorous audit of all numerical values in the text, tables, and figures to ensure absolute consistency.
Author response: We thank the reviewer for this valuable comment.
The entire manuscript has been carefully reviewed, and the supplementary information has been removed. All relevant values, calculations, tables, figures, and associated text have been verified for accuracy and consistency, and necessary corrections have been incorporated throughout the revised manuscript and the supplementary information.
- The study’s reliance on a “relative discharge approach” to estimate nutrient loads is scientifically problematic for the Western Ghats. This method assumes a linear relationship between drainage area and discharge, which fails to account for the extreme spatial heterogeneity in rainfall and runoff characteristic of tropical mountainous terrains. Furthermore, the annual nutrient fluxes are derived from only three sampling events. In small, flashy mountainous systems, this sampling frequency is insufficient to capture storm-driven nutrient pulses, likely leading to a significant underestimation of annual loads and rendering the comparison with large global rivers (Amazon/Mississippi) statistically tenuous.
Author Response: We sincerely thank the reviewer for this thoughtful and constructive comment.
We acknowledge that estimating annual nutrient fluxes in small tropical mountainous catchments is inherently challenging due to pronounced spatial rainfall variability, rapid hydrological responses, and episodic storm-driven nutrient transport. The relative discharge approach adopted in this study was selected because continuous discharge observations were available only at the basin outlets. This method has previously been applied for estimating nutrient fluxes in tropical river basins where spatially distributed discharge measurements are unavailable (David et al., 2016; Upendra et al., 2025). We agree that this approach assumes proportionality between drainage area and long-term average discharge and therefore does not explicitly account for localized rainfall variability or short-duration hydrological events. Similarly, we recognize that the three seasonal sampling campaigns (pre-monsoon, monsoon, and post-monsoon) represent the major hydrological periods but may not fully capture high-frequency storm-event nutrient pulses that can contribute substantially to annual nutrient export in flashy mountainous rivers. Consequently, the estimated nutrient fluxes should be regarded as first-order annual estimates representative of seasonal conditions rather than precise annual budgets. To address this concern, we have revised the manuscript to explicitly acknowledge these methodological limitations in the Discussion and have clarified that future studies incorporating continuous discharge monitoring and higher-frequency water-quality sampling during storm events would improve annual flux estimation. Furthermore, the comparison with larger global rivers (e.g., Amazon and Mississippi) was intended to provide a broad biogeochemical context rather than a direct quantitative comparison. We have revised the text to emphasize that differences in basin size, hydrology, climatic regime, and sampling strategy should be considered when interpreting these comparisons.
- The discussion of P limitation remains largely speculative and lacks local empirical validation. While the authors suggest that low DIP concentrations result from P-sorption in acidic soils and biological uptake, they provide no site-specific data on soil mineralogy, pH, or adsorption capacity to support these claims. Additionally, the manuscript fails to provide biological evidence, such as Chl a data or phytoplankton community analysis, to prove that the identified chemical nutrient ratios actually manifest as biological limitations in these specific ecosystems. Without this link, the conclusion that these rivers are “P-limited systems” remains an unverified hypothesis rather than a demonstrated fact.
Author Response: We sincerely thank the reviewer for this constructive comment.
The mineralogical data from the soils support the claim that the adsorbed DIP is not available in the current study. However, the Chl-a data are presented to substantiate the evidence that DIP is consumed due to in-situ production (Line No. 402-432).
- The comparison between these small tropical basins and global rivers like the Amazon or Mississippi is intriguing but requires more nuance. The authors should discuss the unique “high-efficiency” transport characteristics of small basins, where shorter residence times may limit the in-stream transformation of nutrients compared to large-scale systems. On a technical note, the assertion that NH4+ was consistently below the detection limit needs to be supported by the specific detection limit values of the Continuous Flow Analyzer used. The authors should assess the potential error introduced into the DIN and ICEP calculations by excluding the ammonium fraction. Finally, the figure captions and labels, particularly in Fig. 1 and Fig. 4, should be reviewed for clarity to ensure that segment boundaries and sampling locations are easily distinguishable for the reader.
Author Response: We sincerely thank the reviewer for these constructive comments.
The methodological details, including the detection limits, are presented in the revised MS (Line No. 130-133). Fig. 1 and Fig. 4 have been revised.
Citation: https://doi.org/10.5194/egusphere-2026-995-AC2
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CC3: 'Comment on egusphere-2026-995', Qingqing Sun, 20 May 2026
Publisher’s note: this comment is a copy of RC3 and its content was therefore removed on 26 May 2026.
Citation: https://doi.org/10.5194/egusphere-2026-995-CC3 -
RC3: 'Comment on egusphere-2026-995', Qingqing Sun, 22 May 2026
More compelling evidence is needed to support the mechanistic explanations and conclusions regarding nutrient limitations and biogeochemical controls.1. This study mainly reports observational patterns rather than mechanistic evidence. Some related geochemical processes, such as phosphorus adsorption, lack quantitative basis.2. The text repeatedly mentions intense silicate weathering, in-situ production, sediment-water interaction, but there is no direct evidence. At present, it is only a speculation.3. The calculation of nutrient fluxes adopted the relative flow proportion method, as only the outflow flow data was available. This will bring about great uncertainty to the accounting process.4. The different slopes, land use and rainfall responses in mountainous river basins cannot be scaled linearly by area.5. The phytoplankton and chlorophyll data are missing. You may need find more relevant evidence in the literature, such as carbon and nitrogen, to support your stoichiometric view and the phosphorus limitation theory.6. The discussion in the "Ecological Consequences" section is highly speculative as there is a lack of further observational evidence. You can refer to relevant literature to support your viewpoint.7. The conclusion and the observational logic are somewhat contradictory. For instance, when you mentioned urban erosion and output enhancement; Downstream stagnation is beneficial for biological production. It is recommended to add more explanations in the conceptual model.8. The language can be further optimized. For example, "both the rivers exhibiting P-limit systems" can be changed to "both rivers exhibit P-limited conditions". Overuse of “signify”, “support the claim”, “further validates”.9. Verify whether the values in the context are consistent, such as 81.66 kg ha−1 yr−1 for DSi and 127.25 kg ha−1 yr−1.10. There are some issues in the draft that have not been corrected. For instance, the chemical formulas in the supporting documents are not written in a standard format.Citation: https://doi.org/
10.5194/egusphere-2026-995-RC3 -
AC3: 'Reply on RC3', Badimela Upendra, 21 Jul 2026
More compelling evidence is needed to support the mechanistic explanations and conclusions regarding nutrient limitations and biogeochemical controls.
- This study mainly reports observational patterns rather than mechanistic evidence. Some related geochemical processes, such as phosphorus adsorption, lack quantitative basis.
Author Response: We thank the reviewer for this constructive comment.
We agree that the present study is primarily based on field observations rather than direct process-based experiments. Accordingly, the manuscript has been revised to moderate mechanistic interpretations and clearly distinguish observed patterns from inferred processes. Statements regarding phosphorus adsorption have been supported by relevant literature on phosphate retention in Fe- and Al-rich tropical soils, while emphasizing that adsorption was not directly quantified in this study.
- The text repeatedly mentions intense silicate weathering, in-situ production, sediment-water interaction, but there is no direct evidence. At present, it is only a speculation.
Author Response: We appreciate this observation.
We acknowledge that silicate weathering intensity, in-situ production, and sediment–water interactions were inferred from hydrochemical signatures and published studies rather than directly measured. The revised manuscript replaces definitive statements with cautious wording (e.g., 'suggest', 'likely', 'may indicate') and explicitly states that these processes represent plausible explanations that require future validation through mineralogical, isotopic, and sedimentary analyses.
- The calculation of nutrient fluxes adopted the relative flow proportion method, as only the outflow flow data was available. This will bring about great uncertainty to the accounting process.
Author Response: We agree that the relative discharge approach introduces uncertainty because continuous discharge measurements were only available at the basin outlets. The revised manuscript explicitly discusses this limitation and clarifies that the estimated nutrient fluxes represent first-order annual approximations rather than precise segment-wise budgets. The need for continuous discharge monitoring and event-based sampling has been highlighted as future work.
- The different slopes, land use and rainfall responses in mountainous river basins cannot be scaled linearly by area.
Author Response: We thank the reviewer for this important comment.
We acknowledge that rainfall, slope, land use, and hydrological responses vary considerably across mountainous basins and therefore discharge does not necessarily scale linearly with drainage area. This limitation has now been clearly acknowledged, and the interpretation of spatial nutrient fluxes has been appropriately moderated.
- The phytoplankton and chlorophyll data are missing. You may need find more relevant evidence in the literature, such as carbon and nitrogen, to support your stoichiometric view and the phosphorus limitation theory.
Author Response: We appreciate this valuable suggestion.
To strengthen the stoichiometric interpretation, Sentinel-2-derived chlorophyll a observations have been incorporated into the revised manuscript as an independent line of evidence. In addition, the discussion has been expanded using published studies relating nutrient stoichiometry, carbon cycling, and phosphorus limitation. We acknowledge that direct measurements of the phytoplankton community and primary productivity were beyond the scope of the present study and are recommended for future investigations.
- The discussion in the "Ecological Consequences" section is highly speculative as there is a lack of further observational evidence. You can refer to relevant literature to support your viewpoint.
Author Response: We agree that parts of the previous discussion were overly speculative.
The Ecological Consequences section has been substantially revised by removing unsupported statements, incorporating additional peer-reviewed literature, and restricting interpretations to evidence supported by hydrochemical observations, nutrient stoichiometry, ICEP results, and satellite-derived chlorophyll a.
- The conclusion and the observational logic are somewhat contradictory. For instance, when you mentioned urban erosion and output enhancement; Downstream stagnation is beneficial for biological production. It is recommended to add more explanations in the conceptual model.
Author Response: We thank the reviewer for this insightful comment.
The conceptual model has been revised to better explain the sequence of hydrological and biogeochemical processes. The revised framework distinguishes enhanced urban runoff and nutrient transport from downstream conditions that may locally favour biological activity, while emphasizing that persistent phosphorus limitation constrains large-scale phytoplankton growth. This improves consistency between the observations, discussion, and conclusions.
- The language can be further optimized. For example, "both the rivers exhibiting P-limit systems" can be changed to "both rivers exhibit P-limited conditions". Overuse of “signify”, “support the claim”, “further validates”.
Author Response: We appreciate these corrections.
The manuscript has undergone comprehensive language editing to improve clarity and scientific precision. Repetitive expressions such as 'signify', 'support the claim', and 'further validates' have been replaced with more appropriate wording, and phrases such as 'both the rivers exhibiting P-limit systems' have been revised to 'both rivers exhibit P-limited conditions'.
- Verify whether the values in the context are consistent, such as 81.66 kg ha−1yr−1 for DSi and 127.25 kg ha−1 yr−1.
Author Response: We thank the reviewer for identifying this issue.
All numerical values presented in the manuscript, tables, figures, and supplementary information have been carefully cross-checked and corrected where necessary to ensure complete consistency, including the DSi yield values (81.66 and 127.25 kg ha⁻¹ yr⁻¹).
- There are some issues in the draft that have not been corrected. For instance, the chemical formulas in the supporting documents are not written in a standard format.
Author Response: We appreciate this observation.
The manuscript and Supporting Information have been thoroughly reviewed, and all chemical formulae have been reformatted using standard scientific notation (e.g., HCO₃⁻, SO₄²⁻, PO₄³⁻, NH₄⁺, NO₃⁻) throughout the revised version.
Citation: https://doi.org/10.5194/egusphere-2026-995-AC3
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AC3: 'Reply on RC3', Badimela Upendra, 21 Jul 2026
Status: closed
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RC1: 'Comment on egusphere-2026-995', Anonymous Referee #1, 01 Apr 2026
The author studied the inorganic nutrients in mountainous rivers and emphasized the crucial role of climate and topography in regulating nutrient fluxes. It was also confirmed that the nutrient input from these small-scale mountainous rivers had a relatively limited impact on coastal eutrophication in the receiving areas. However, there are still some problems.
- What are the criteria for dividing the study area into upper, middle, and lower reaches? How might uneven sampling density in the upper, middle, and lower reaches affect the results?
- Although the two rivers are classified as "mountainous rivers," they both flow through agricultural and urban areas. The fact that NH₄⁺-N concentrations are below the detection limit is puzzling. The authors need to provide detailed detection limits (in mg/L) for nutrients for reader comparison, rather than using percentages, and clarify the reasons.
- Supporting data for flux calculations, such as water discharge values, were recorded only at the basin outlet. This significantly impacts the results, so the limitations and shortcomings of the study need to be discussed.
- The authors state, "The supporting data to calculate the flux, such as water discharge values, were recorded exclusively at the basin outlet." How was Qs obtained?
- How was HCO3- measured?
- In Figure 2, the captions should clearly indicate the meanings of "pre," "mon," and "pom." These should be specified in all captions so readers do not have to search elsewhere. Note the correct notation for SO42-, etc.
- Figures 3 and 4 are blurry, with font sizes too small and incorrect formatting of legends, making them unreadable.
- Figure 5 has inconsistent formatting, axis font sizes too small, lacks significance analysis, and the labels "abcd" are placed inside the figure, reducing readability.
- Figure 7 looks visually appealing, but much of it remains speculative and requires further evidence.
- In Figure S3, KPB-PRE and VRB-MON are not distinguished.
- The abstract contains many abbreviations, some of which (e.g., MON) are not commonly used and their meaning is unclear.
- In Figure 2, it is recommended to remove the orange line and adjust the font to improve readability.
- Notations such as Ca-Mg-HCO3 need to be verified.
- "Whereas KRB’s steeper slopes and urbanized surface promote hydrological flashiness and rapid nutrient export, particularly limiting DIP accumulation despite high anthropogenic loading." What is the principle behind this? Shouldn’t high anthropogenic loading increase DIP?
- There is no significant difference in land use between the middle and lower reaches of the VRB, as both are predominantly cropland.
- How did the authors determine the contribution of groundwater to runoff?
- The authors frequently mention the spatial patterns of nutrients such as DIN in different regions of the watershed, but no figures or tables support this.
- Line 335: "Increasing DIN and DSi concentrations toward the outlet in both basins arise through contrasting mechanisms: rapid urban flushing in KRB versus sediment-mediated retention and release in VRB, while tributaries modulate these patterns by reflecting localized land-use pressures, from industrial hotspots (KRB) to agricultural sub-basins (VRB)." How is the role of sediments demonstrated?
- In Discussion 4.4, the authors provide valuable insights, but no evidence of phytoplankton is presented.
Citation: https://doi.org/10.5194/egusphere-2026-995-RC1 -
AC1: 'Reply on RC1', Badimela Upendra, 16 Jul 2026
Referee#1
- What are the criteria for dividing the study area into upper, middle, and lower reaches? How might uneven sampling density in the upper, middle, and lower reaches affect the results?
Author Response: Thank you for this important comment.
The study area was divided into upper, middle, and lower reaches based on equal-area segmentation of the river basin to ensure a consistent spatial framework for analysis. Sampling locations within each reach were selected primarily based on river accessibility while striving to achieve representative spatial coverage. Although the number of sampling sites differs across the three reaches due to logistical and accessibility constraints, the selected sites adequately represent the hydro-geochemical characteristics of each reach. To improve clarity, we have explicitly described the basis for reach delineation in the Methods section and highlighted the sampling density and spatial distribution of the sampling sites in the Results section, enabling readers to interpret the findings appropriately in the context of the sampling coverage.
- Although the two rivers are classified as "mountainous rivers," they both flow through agricultural and urban areas. The fact that NH₄⁺-N concentrations are below the detection limit is puzzling. The authors need to provide detailed detection limits (in mg/L) for nutrients for reader comparison, rather than using percentages, and clarify the reasons.
Author Response: We sincerely thank the reviewer for this valuable comment.
The manuscript has been revised to include the analytical detection limits of dissolved inorganic nutrients in Section 2.3. The detection limits were 0.001 mg L⁻¹ for NO₃⁻-N + NO₂⁻-N, NH₄⁺-N, and PO₄³⁻-P, and 0.1 mg L⁻¹ for DSi (Line No. 132-134). We have also clarified that NH₄⁺-N remained below the detection limit because the well-oxygenated, fast-flowing river conditions promote rapid nitrification and biological uptake, resulting in NO₃⁻-N as the dominant inorganic nitrogen species (Line No. 422-437).
- Supporting data for flux calculations, such as water discharge values, were recorded only at the basin outlet. This significantly impacts the results, so the limitations and shortcomings of the study need to be discussed.
Author Response: Thank you for this valuable comment.
We would like to clarify that the flux calculations were performed separately for the upper, middle, and lower reaches using the corresponding segment-specific water discharge rather than using only the discharge measured at the basin outlet. We recognize that the original manuscript did not clearly describe this methodology, which may have led to this misunderstanding. To address this, we have revised the Methods section to explicitly explain the segment-wise flux calculation approach. In addition, the Results and Discussion sections have been updated accordingly to present and interpret the segment-wise flux estimates based on the corresponding water discharge values. These revisions provide a more accurate representation of spatial variations in fluxes across the basin.
- The authors state, "The supporting data to calculate the flux, such as water discharge values, were recorded exclusively at the basin outlet." How was Qs obtained?
Author Response: We thank the reviewer for pointing out this methodological concern.
We agree that the methodology for estimating segment-wise discharge (Qs) requires further clarification. Direct discharge measurements were available only at the outlet of each river basin. Therefore, segment-wise discharge was estimated using the relative discharge approach proposed by David et al. (2016) and subsequently applied to tropical river basins of Kerala by Upendra et al. (2025). This method assumes that the long-term average discharge is proportional to the contributing drainage area under comparable climatic conditions.
First, the specific (relative) discharge was calculated by dividing the measured outlet discharge (Qm) by the total basin area (Am):
Q = Qm / Am
where Q is the relative discharge (km³ km⁻² yr⁻¹), Qm is the measured annual discharge at the basin outlet (km³ yr⁻¹), and Am is the total basin area (km²).
The discharge for each river segment (Qs) was then estimated by multiplying the relative discharge by the drainage area of the corresponding segment (As):
Qs = As × Q
where As represents the drainage area of the upstream, midstream, or downstream segment. The estimated segment-wise discharge values were subsequently used to calculate nutrient loads and yields.
To improve clarity, we have revised Section 2.3 of the manuscript to explicitly describe this procedure and emphasize that segment-wise discharge values were estimated using an area-weighted relative discharge approach rather than direct field measurements.
- How was HCO3- measured?
Author Response: We thank the reviewer for highlighting this.
Bicarbonate concentrations were determined using an alkalinity test kit (Metrohm). The same is now provided in the revised MS (Line No. 138-139).
- In Figure 2, the captions should clearly indicate the meanings of "pre," "mon," and "pom." These should be specified in all captions so readers do not have to search elsewhere. Note the correct notation for SO42-, etc.
Author Response: Thank you for the correction.
The figure is now modified according to the reviewer's suggestion.
- Figures 3 and 4 are blurry, with font sizes too small and incorrect formatting of legends, making them unreadable.
Author Response: Thank you for the correction.
Both figures have been modified according to the reviewer's suggestion and uploaded as separate files.
- Figure 5 has inconsistent formatting, axis font sizes too small, lacks significance analysis, and the labels "abcd" are placed inside the figure, reducing readability.
Author Response: Thank you for the correction.
Figure 5 has now been removed.
- Figure 7 looks visually appealing, but much of it remains speculative and requires further evidence.
Author Response: Thank you for the correction.
Figure 7 has been modified according to the reviewer's suggestion and uploaded as a separate file.
- In Figure S3, KPB-PRE and VRB-MON are not distinguished.
Author Response: Thank you for the correction.
Figure S3 has been modified according to the reviewer's suggestion.
- The abstract contains many abbreviations, some of which (e.g., MON) are not commonly used and their meaning is unclear.
Author Response: We thank the reviewer for this valuable suggestion. The manuscript has been carefully reviewed, and abbreviations have been revised throughout.
- In Figure 2, it is recommended to remove the orange line and adjust the font to improve readability.
Author Response: Thank you for the correction.
Figure 2 has been modified according to the reviewer's suggestion.
- Notations such as Ca-Mg-HCO3 need to be verified.
Author Response: Thank you for the suggestion. The notation is now verified and corrected.
- "Whereas KRB’s steeper slopes and urbanized surface promote hydrological flashiness and rapid nutrient export, particularly limiting DIP accumulation despite high anthropogenic loading." What is the principle behind this? Shouldn’t high anthropogenic loading increase DIP?
Author Response: We thank the reviewer for this insightful comment.
We agree that anthropogenic activities generally increase phosphorus inputs to river systems. However, in steep tropical mountainous rivers such as the KRB and VRB, the rapid runoff and short water residence times promote the swift transport of nutrients and limit the accumulation of dissolved phosphorus in the water column. Furthermore, phosphate is readily removed from the dissolved phase by adsorption onto Fe- and Al-rich soils and suspended sediments, as well as by rapid biological uptake by aquatic microorganisms and primary producers. Consequently, with the relatively low anthropogenic phosphorus inputs, dissolved DIP concentrations remain comparatively low. We have revised the corresponding text in the Discussion to clarify these mechanisms and avoid implying that anthropogenic loading alone controls DIP concentrations (Line No. 402-432).
- There is no significant difference in land use between the middle and lower reaches of the VRB, as both are predominantly cropland.
Author Response: Thank you for this insightful comment.
We would like to clarify that the upper, middle, and lower reaches of the VRB were delineated using an equal-area segmentation approach to provide a consistent spatial framework for basin-scale analysis, rather than based on land-use differences. Consequently, the middle and lower reaches are both dominated by cropland, reflecting the basin's natural land-use distribution rather than the segmentation methodology. We have revised the manuscript to clarify the basis for reach delineation and have acknowledged that the similarity in land use between the middle and lower reaches may reduce the contrast in land-use-related influences when interpreting the results.
- How did the authors determine the contribution of groundwater to runoff?
Author Response: We thank the reviewer for this important comment.
We would like to clarify that the groundwater contribution to river runoff was not quantified directly in the present study using techniques such as baseflow separation, isotopic tracers, or end-member mixing analysis. Our interpretation of groundwater influence is based on the observed hydrogeochemical characteristics of the river water, including the dominance of Ca–Mg–HCO₃ facies, elevated dissolved silica concentrations indicative of silicate weathering, and the perennial nature of the river systems. These observations are further supported by previous hydrogeological studies conducted in the study area, which have documented substantial groundwater discharge sustaining streamflow, particularly during non-monsoon periods. To avoid any misunderstanding, we have revised the manuscript to clarify that the groundwater contribution is inferred from hydrogeochemical evidence and existing literature rather than being directly quantified in this study.
- The authors frequently mention the spatial patterns of nutrients such as DIN in different regions of the watershed, but no figures or tables support this.
Author Response: Thank you for the correction.
A supplementary figure has been added in the revised version.
- Line 335: "Increasing DIN and DSi concentrations toward the outlet in both basins arise through contrasting mechanisms: rapid urban flushing in KRB versus sediment-mediated retention and release in VRB, while tributaries modulate these patterns by reflecting localized land-use pressures, from industrial hotspots (KRB) to agricultural sub-basins (VRB)." How is the role of sediments demonstrated?
Author Response: We thank the reviewer for this insightful comment.
We agree that the present study did not directly investigate sediment nutrient dynamics through sediment sampling or adsorption–desorption experiments. Therefore, the phrase "sediment-mediated retention and release" was an overinterpretation of the observed nutrient patterns. We have revised the manuscript to avoid implying a direct assessment of sediment processes. The revised text attributes the contrasting nutrient dynamics in VRB to differences in hydrological conditions, land-use characteristics, and longer water residence time, which are supported by our hydro-geochemical observations. The discussion has also been modified to clarify that any potential influence of sediment processes is inferred from previous studies rather than demonstrated in the present investigation.
- In Discussion 4.4, the authors provide valuable insights, but no evidence of phytoplankton is presented.
Author Response: We sincerely thank the reviewer for this valuable suggestion.
To strengthen the ecological interpretation presented in Section 4.4, we incorporated an independent assessment of phytoplankton biomass using Sentinel-2-derived chlorophyll-a (Chl-a) distribution. Chl-a was estimated from Sentinel-2 imagery using the Normalized Difference Chlorophyll Index (NDCI) and an established empirical calibration equation. The resulting spatial distribution maps have been added as Figure 6 & 7, and the methodology has been described in Section 2.5. The satellite-derived Chl-a maps indicate negligible phytoplankton biomass in the upstream and most midstream reaches of both river basins, with only localized enrichment in the downstream reaches. These observations are consistent with the nutrient stoichiometry, persistent phosphorus limitation, and negative ICEP values reported in this study, thereby providing independent evidence supporting the conclusion that both river systems currently exhibit a low potential for widespread eutrophication (Line No. 402-432).
Citation: https://doi.org/10.5194/egusphere-2026-995-AC1
- CC1: 'Comment on egusphere-2026-995', Qingqing Sun, 23 Apr 2026
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RC2: 'Comment on egusphere-2026-995', Anonymous Referee #2, 19 May 2026
Overview and general recommendation:
This manuscript investigates the spatio-temporal evolution of dissolved inorganic nutrients (DIN, DIP, and DSi) in two small tropical mountainous river basins, the KRB and VRB, located in the Western Ghats, India. By applying a segment-wise flux calculation and the ICEP indicator, the authors suggest that these basins exhibit persistent P limitation and Si richness driven by intense chemical weathering and anthropogenic inputs. Specifically, the work lacks rigorous data consistency, methodological transparency, and depth in mechanistic discussion. The pervasive numerical errors suggest a lack of careful preparation, and the simplified hydrological assumptions undermine the validity of the nutrient flux estimations. Therefore, I recommend rejection of this manuscript.
Detailed Comments:
- The data in the manuscript seems inconsistent throughout, which undermines the reliability of the conclusions. For example, the abstract states that the DSi flux of the VRB is 127.25 kg ha-1 yr-1, yet Table 1 lists the average DSi yield for the VRB as 81.66 kg ha-1 yr-1. Conversely, the abstract reports a DSi flux of 127.25 kg ha-1 yr-1 for the KRB, while Table 1 shows an average of 127.25 kg ha-1 yr-1 for the KRB, suggesting a possible swap or mislabeling of data between the two basins. Furthermore, Text S1 in the supplement reports a DSi yield of 143.37 kg ha-1 yr-1 for the KRB downstream segment, while Table 1 lists it as 161.94 kg ha-1 yr-1. The authors must conduct a rigorous audit of all numerical values in the text, tables, and figures to ensure absolute consistency.
- The study’s reliance on a “relative discharge approach” to estimate nutrient loads is scientifically problematic for the Western Ghats. This method assumes a linear relationship between drainage area and discharge, which fails to account for the extreme spatial heterogeneity in rainfall and runoff characteristic of tropical mountainous terrains. Furthermore, the annual nutrient fluxes are derived from only three sampling events. In small, flashy mountainous systems, this sampling frequency is insufficient to capture storm-driven nutrient pulses, likely leading to a significant underestimation of annual loads and rendering the comparison with large global rivers (Amazon/Mississippi) statistically tenuous.
- The discussion of P limitation remains largely speculative and lacks local empirical validation. While the authors suggest that low DIP concentrations result from P-sorption in acidic soils and biological uptake, they provide no site-specific data on soil mineralogy, pH, or adsorption capacity to support these claims. Additionally, the manuscript fails to provide biological evidence, such as Chl a data or phytoplankton community analysis, to prove that the identified chemical nutrient ratios actually manifest as biological limitations in these specific ecosystems. Without this link, the conclusion that these rivers are “P-limited systems” remains an unverified hypothesis rather than a demonstrated fact.
- The comparison between these small tropical basins and global rivers like the Amazon or Mississippi is intriguing but requires more nuance. The authors should discuss the unique “high-efficiency” transport characteristics of small basins, where shorter residence times may limit the in-stream transformation of nutrients compared to large-scale systems. On a technical note, the assertion that NH4+ was consistently below the detection limit needs to be supported by the specific detection limit values of the Continuous Flow Analyzer used. The authors should assess the potential error introduced into the DIN and ICEP calculations by excluding the ammonium fraction. Finally, the figure captions and labels, particularly in Fig. 1 and Fig. 4, should be reviewed for clarity to ensure that segment boundaries and sampling locations are easily distinguishable for the reader.
Citation: https://doi.org/10.5194/egusphere-2026-995-RC2 -
AC2: 'Reply on RC2', Badimela Upendra, 17 Jul 2026
- The data in the manuscript seems inconsistent throughout, which undermines the reliability of the conclusions. For example, the abstract states that the DSi flux of the VRB is 127.25 kg ha-1 yr-1, yet Table 1 lists the average DSi yield for the VRB as 81.66 kg ha-1 yr-1. Conversely, the abstract reports a DSi flux of 127.25 kg ha-1 yr-1 for the KRB, while Table 1 shows an average of 127.25 kg ha-1 yr-1 for the KRB, suggesting a possible swap or mislabeling of data between the two basins. Furthermore, Text S1 in the supplement reports a DSi yield of 143.37 kg ha-1 yr-1 for the KRB downstream segment, while Table 1 lists it as 161.94 kg ha-1 yr-1. The authors must conduct a rigorous audit of all numerical values in the text, tables, and figures to ensure absolute consistency.
Author response: We thank the reviewer for this valuable comment.
The entire manuscript has been carefully reviewed, and the supplementary information has been removed. All relevant values, calculations, tables, figures, and associated text have been verified for accuracy and consistency, and necessary corrections have been incorporated throughout the revised manuscript and the supplementary information.
- The study’s reliance on a “relative discharge approach” to estimate nutrient loads is scientifically problematic for the Western Ghats. This method assumes a linear relationship between drainage area and discharge, which fails to account for the extreme spatial heterogeneity in rainfall and runoff characteristic of tropical mountainous terrains. Furthermore, the annual nutrient fluxes are derived from only three sampling events. In small, flashy mountainous systems, this sampling frequency is insufficient to capture storm-driven nutrient pulses, likely leading to a significant underestimation of annual loads and rendering the comparison with large global rivers (Amazon/Mississippi) statistically tenuous.
Author Response: We sincerely thank the reviewer for this thoughtful and constructive comment.
We acknowledge that estimating annual nutrient fluxes in small tropical mountainous catchments is inherently challenging due to pronounced spatial rainfall variability, rapid hydrological responses, and episodic storm-driven nutrient transport. The relative discharge approach adopted in this study was selected because continuous discharge observations were available only at the basin outlets. This method has previously been applied for estimating nutrient fluxes in tropical river basins where spatially distributed discharge measurements are unavailable (David et al., 2016; Upendra et al., 2025). We agree that this approach assumes proportionality between drainage area and long-term average discharge and therefore does not explicitly account for localized rainfall variability or short-duration hydrological events. Similarly, we recognize that the three seasonal sampling campaigns (pre-monsoon, monsoon, and post-monsoon) represent the major hydrological periods but may not fully capture high-frequency storm-event nutrient pulses that can contribute substantially to annual nutrient export in flashy mountainous rivers. Consequently, the estimated nutrient fluxes should be regarded as first-order annual estimates representative of seasonal conditions rather than precise annual budgets. To address this concern, we have revised the manuscript to explicitly acknowledge these methodological limitations in the Discussion and have clarified that future studies incorporating continuous discharge monitoring and higher-frequency water-quality sampling during storm events would improve annual flux estimation. Furthermore, the comparison with larger global rivers (e.g., Amazon and Mississippi) was intended to provide a broad biogeochemical context rather than a direct quantitative comparison. We have revised the text to emphasize that differences in basin size, hydrology, climatic regime, and sampling strategy should be considered when interpreting these comparisons.
- The discussion of P limitation remains largely speculative and lacks local empirical validation. While the authors suggest that low DIP concentrations result from P-sorption in acidic soils and biological uptake, they provide no site-specific data on soil mineralogy, pH, or adsorption capacity to support these claims. Additionally, the manuscript fails to provide biological evidence, such as Chl a data or phytoplankton community analysis, to prove that the identified chemical nutrient ratios actually manifest as biological limitations in these specific ecosystems. Without this link, the conclusion that these rivers are “P-limited systems” remains an unverified hypothesis rather than a demonstrated fact.
Author Response: We sincerely thank the reviewer for this constructive comment.
The mineralogical data from the soils support the claim that the adsorbed DIP is not available in the current study. However, the Chl-a data are presented to substantiate the evidence that DIP is consumed due to in-situ production (Line No. 402-432).
- The comparison between these small tropical basins and global rivers like the Amazon or Mississippi is intriguing but requires more nuance. The authors should discuss the unique “high-efficiency” transport characteristics of small basins, where shorter residence times may limit the in-stream transformation of nutrients compared to large-scale systems. On a technical note, the assertion that NH4+ was consistently below the detection limit needs to be supported by the specific detection limit values of the Continuous Flow Analyzer used. The authors should assess the potential error introduced into the DIN and ICEP calculations by excluding the ammonium fraction. Finally, the figure captions and labels, particularly in Fig. 1 and Fig. 4, should be reviewed for clarity to ensure that segment boundaries and sampling locations are easily distinguishable for the reader.
Author Response: We sincerely thank the reviewer for these constructive comments.
The methodological details, including the detection limits, are presented in the revised MS (Line No. 130-133). Fig. 1 and Fig. 4 have been revised.
Citation: https://doi.org/10.5194/egusphere-2026-995-AC2
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CC3: 'Comment on egusphere-2026-995', Qingqing Sun, 20 May 2026
Publisher’s note: this comment is a copy of RC3 and its content was therefore removed on 26 May 2026.
Citation: https://doi.org/10.5194/egusphere-2026-995-CC3 -
RC3: 'Comment on egusphere-2026-995', Qingqing Sun, 22 May 2026
More compelling evidence is needed to support the mechanistic explanations and conclusions regarding nutrient limitations and biogeochemical controls.1. This study mainly reports observational patterns rather than mechanistic evidence. Some related geochemical processes, such as phosphorus adsorption, lack quantitative basis.2. The text repeatedly mentions intense silicate weathering, in-situ production, sediment-water interaction, but there is no direct evidence. At present, it is only a speculation.3. The calculation of nutrient fluxes adopted the relative flow proportion method, as only the outflow flow data was available. This will bring about great uncertainty to the accounting process.4. The different slopes, land use and rainfall responses in mountainous river basins cannot be scaled linearly by area.5. The phytoplankton and chlorophyll data are missing. You may need find more relevant evidence in the literature, such as carbon and nitrogen, to support your stoichiometric view and the phosphorus limitation theory.6. The discussion in the "Ecological Consequences" section is highly speculative as there is a lack of further observational evidence. You can refer to relevant literature to support your viewpoint.7. The conclusion and the observational logic are somewhat contradictory. For instance, when you mentioned urban erosion and output enhancement; Downstream stagnation is beneficial for biological production. It is recommended to add more explanations in the conceptual model.8. The language can be further optimized. For example, "both the rivers exhibiting P-limit systems" can be changed to "both rivers exhibit P-limited conditions". Overuse of “signify”, “support the claim”, “further validates”.9. Verify whether the values in the context are consistent, such as 81.66 kg ha−1 yr−1 for DSi and 127.25 kg ha−1 yr−1.10. There are some issues in the draft that have not been corrected. For instance, the chemical formulas in the supporting documents are not written in a standard format.Citation: https://doi.org/
10.5194/egusphere-2026-995-RC3 -
AC3: 'Reply on RC3', Badimela Upendra, 21 Jul 2026
More compelling evidence is needed to support the mechanistic explanations and conclusions regarding nutrient limitations and biogeochemical controls.
- This study mainly reports observational patterns rather than mechanistic evidence. Some related geochemical processes, such as phosphorus adsorption, lack quantitative basis.
Author Response: We thank the reviewer for this constructive comment.
We agree that the present study is primarily based on field observations rather than direct process-based experiments. Accordingly, the manuscript has been revised to moderate mechanistic interpretations and clearly distinguish observed patterns from inferred processes. Statements regarding phosphorus adsorption have been supported by relevant literature on phosphate retention in Fe- and Al-rich tropical soils, while emphasizing that adsorption was not directly quantified in this study.
- The text repeatedly mentions intense silicate weathering, in-situ production, sediment-water interaction, but there is no direct evidence. At present, it is only a speculation.
Author Response: We appreciate this observation.
We acknowledge that silicate weathering intensity, in-situ production, and sediment–water interactions were inferred from hydrochemical signatures and published studies rather than directly measured. The revised manuscript replaces definitive statements with cautious wording (e.g., 'suggest', 'likely', 'may indicate') and explicitly states that these processes represent plausible explanations that require future validation through mineralogical, isotopic, and sedimentary analyses.
- The calculation of nutrient fluxes adopted the relative flow proportion method, as only the outflow flow data was available. This will bring about great uncertainty to the accounting process.
Author Response: We agree that the relative discharge approach introduces uncertainty because continuous discharge measurements were only available at the basin outlets. The revised manuscript explicitly discusses this limitation and clarifies that the estimated nutrient fluxes represent first-order annual approximations rather than precise segment-wise budgets. The need for continuous discharge monitoring and event-based sampling has been highlighted as future work.
- The different slopes, land use and rainfall responses in mountainous river basins cannot be scaled linearly by area.
Author Response: We thank the reviewer for this important comment.
We acknowledge that rainfall, slope, land use, and hydrological responses vary considerably across mountainous basins and therefore discharge does not necessarily scale linearly with drainage area. This limitation has now been clearly acknowledged, and the interpretation of spatial nutrient fluxes has been appropriately moderated.
- The phytoplankton and chlorophyll data are missing. You may need find more relevant evidence in the literature, such as carbon and nitrogen, to support your stoichiometric view and the phosphorus limitation theory.
Author Response: We appreciate this valuable suggestion.
To strengthen the stoichiometric interpretation, Sentinel-2-derived chlorophyll a observations have been incorporated into the revised manuscript as an independent line of evidence. In addition, the discussion has been expanded using published studies relating nutrient stoichiometry, carbon cycling, and phosphorus limitation. We acknowledge that direct measurements of the phytoplankton community and primary productivity were beyond the scope of the present study and are recommended for future investigations.
- The discussion in the "Ecological Consequences" section is highly speculative as there is a lack of further observational evidence. You can refer to relevant literature to support your viewpoint.
Author Response: We agree that parts of the previous discussion were overly speculative.
The Ecological Consequences section has been substantially revised by removing unsupported statements, incorporating additional peer-reviewed literature, and restricting interpretations to evidence supported by hydrochemical observations, nutrient stoichiometry, ICEP results, and satellite-derived chlorophyll a.
- The conclusion and the observational logic are somewhat contradictory. For instance, when you mentioned urban erosion and output enhancement; Downstream stagnation is beneficial for biological production. It is recommended to add more explanations in the conceptual model.
Author Response: We thank the reviewer for this insightful comment.
The conceptual model has been revised to better explain the sequence of hydrological and biogeochemical processes. The revised framework distinguishes enhanced urban runoff and nutrient transport from downstream conditions that may locally favour biological activity, while emphasizing that persistent phosphorus limitation constrains large-scale phytoplankton growth. This improves consistency between the observations, discussion, and conclusions.
- The language can be further optimized. For example, "both the rivers exhibiting P-limit systems" can be changed to "both rivers exhibit P-limited conditions". Overuse of “signify”, “support the claim”, “further validates”.
Author Response: We appreciate these corrections.
The manuscript has undergone comprehensive language editing to improve clarity and scientific precision. Repetitive expressions such as 'signify', 'support the claim', and 'further validates' have been replaced with more appropriate wording, and phrases such as 'both the rivers exhibiting P-limit systems' have been revised to 'both rivers exhibit P-limited conditions'.
- Verify whether the values in the context are consistent, such as 81.66 kg ha−1yr−1 for DSi and 127.25 kg ha−1 yr−1.
Author Response: We thank the reviewer for identifying this issue.
All numerical values presented in the manuscript, tables, figures, and supplementary information have been carefully cross-checked and corrected where necessary to ensure complete consistency, including the DSi yield values (81.66 and 127.25 kg ha⁻¹ yr⁻¹).
- There are some issues in the draft that have not been corrected. For instance, the chemical formulas in the supporting documents are not written in a standard format.
Author Response: We appreciate this observation.
The manuscript and Supporting Information have been thoroughly reviewed, and all chemical formulae have been reformatted using standard scientific notation (e.g., HCO₃⁻, SO₄²⁻, PO₄³⁻, NH₄⁺, NO₃⁻) throughout the revised version.
Citation: https://doi.org/10.5194/egusphere-2026-995-AC3
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AC3: 'Reply on RC3', Badimela Upendra, 21 Jul 2026
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- 1
The author studied the inorganic nutrients in mountainous rivers and emphasized the crucial role of climate and topography in regulating nutrient fluxes. It was also confirmed that the nutrient input from these small-scale mountainous rivers had a relatively limited impact on coastal eutrophication in the receiving areas. However, there are still some problems.