Biogeochemical controls on nutrient fluxes in small tropical mountainous river basins: Mechanisms for P-limitation and Si-rich conditions

Badimela, Upendra; Reddy, Kiran Kumar; Kamaraj, Jesuraja; Manohar, Ciba; Suresh, Vidya; Perumbully, Vinnetha; Krishnan, Anoop

doi:10.5194/egusphere-2026-995

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https://doi.org/10.5194/egusphere-2026-995

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06 Mar 2026

| 06 Mar 2026

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Biogeochemical controls on nutrient fluxes in small tropical mountainous river basins: Mechanisms for P-limitation and Si-rich conditions

Upendra Badimela, Kiran Kumar Reddy, Jesuraja Kamaraj, Ciba Manohar, Vidya Suresh, Vinnetha Perumbully, and Anoop Krishnan

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.

Received: 20 Feb 2026 – Discussion started: 06 Mar 2026

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Upendra Badimela, Kiran Kumar Reddy, Jesuraja Kamaraj, Ciba Manohar, Vidya Suresh, Vinnetha Perumbully, and Anoop Krishnan

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RC1:
'Comment on egusphere-2026-995', Anonymous Referee #1, 01 Apr 2026 reply
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.

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Citation: https://doi.org/10.5194/egusphere-2026-995-RC1
CC1: 'Comment on egusphere-2026-995', Qingqing Sun, 23 Apr 2026 reply

It is a great honor to review this paper. Review comments are attached.

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Citation: https://doi.org/10.5194/egusphere-2026-995-CC1
CC2:
'Comment on egusphere-2026-995', Xin Liu, 10 May 2026 reply
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.

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Citation: https://doi.org/10.5194/egusphere-2026-995-CC2

Upendra Badimela, Kiran Kumar Reddy, Jesuraja Kamaraj, Ciba Manohar, Vidya Suresh, Vinnetha Perumbully, and Anoop Krishnan

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https://doi.org/10.5194/egusphere-2026-995-supplement

Upendra Badimela, Kiran Kumar Reddy, Jesuraja Kamaraj, Ciba Manohar, Vidya Suresh, Vinnetha Perumbully, and Anoop Krishnan

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