Seasonal and Spatial Variability of Silicon Cycle in Godavari Tidal River Zone
Abstract
Dam regulation of tropical monsoonal rivers enhances seasonal contrasts in the tidal river zone (TRZ) hydrology, altering the delivery of dissolved silicon (DSi) to the coastal ocean. Here, we identify the drivers of Si cycling in the Godavari TRZ and quantify their implications for Si delivery to the Bay of Bengal (BoB). We analyzed elemental concentrations and isotopic tracers (δ³⁰Si, δ¹⁸O, δ²H, ⁸⁷Sr/⁸⁶Sr) of the water column and adjacent groundwater during dry (pre‐monsoon) and wet (monsoon) seasons to delineate mixing and biogeochemical processes in the Godavari TRZ. During the dry season, deviations from the river water‐seawater binary mixing line for ⁸⁷Sr/⁸⁶Sr and δ¹⁸O indicate groundwater input, ranging from 7% to 43% across TRZ stations. We observe non‐conservative mixing of DSi and δ³⁰Si values during the dry season, with a significant DSi removal (20%–85%) and enrichment in δ³⁰Si values. The dominance of diatoms in suspended matter and elevated concentrations of the diatom marker pigment fucoxanthin confirm the role of biological uptake in DSi removal. Closure of the Dowleiswaram dam during the dry season reduces river discharge, increases water residence time in the TRZ, and promotes diatom growth. In contrast, the opening of dam gates during the monsoon season, when catchment precipitation is at its peak, leads to high river water influx into the TRZ and elevated suspended matter concentrations (175 ± 50 mg/L), limiting diatom growth during the wet season. Annually, the Godavari River supplies 22.1 ± 5.1 Gmol of Si with a flux‐weighted δ³⁰Si composition of 1.2‰ ± 0.1‰ to the BoB, of which approximately 19% is converted into biogenic silica by diatoms during low discharge periods. The estimated isotopic enrichment associated with this diatom uptake is negligible. The annual groundwater‐derived Si flux to the Godavari TRZ is 0.8 ± 0.5 Gmol Si, representing ∼3.5% of the annual riverine Si flux; however, during the dry season, groundwater constitutes the dominant local Si source within the TRZ, accounting for 47%–79% of the total Si flux, depending on the station.
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BibTeX
@article{kavil2026seasonal,
title = {{Seasonal and Spatial Variability of Silicon Cycle in Godavari Tidal River Zone}},
author = {Sarath Pullyottum Kavil and Jean Riotte and Ramananda Chakrabarti and V. V. S. S. Sarma and Arnaud Dapoigny and Gilles Reverdin and Damien Cardinal},
journal = {Journal of Geophysical Research: Biogeosciences},
year = {2026},
month = oct,
doi = {10.1029/2025jg009437},
url = {https://hal.science/hal-05777495v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Seasonal and Spatial Variability of Silicon Cycle in Godavari Tidal River Zone
AU - Sarath Pullyottum Kavil
AU - Jean Riotte
AU - Ramananda Chakrabarti
AU - V. V. S. S. Sarma
AU - Arnaud Dapoigny
AU - Gilles Reverdin
AU - Damien Cardinal
PY - 2026
DA - 2026/10/01
JO - Journal of Geophysical Research: Biogeosciences
DO - 10.1029/2025jg009437
UR - https://hal.science/hal-05777495v1
AB - Dam regulation of tropical monsoonal rivers enhances seasonal contrasts in the tidal river zone (TRZ) hydrology, altering the delivery of dissolved silicon (DSi) to the coastal ocean. Here, we identify the drivers of Si cycling in the Godavari TRZ and quantify their implications for Si delivery to the Bay of Bengal (BoB). We analyzed elemental concentrations and isotopic tracers (δ³⁰Si, δ¹⁸O, δ²H, ⁸⁷Sr/⁸⁶Sr) of the water column and adjacent groundwater during dry (pre‐monsoon) and wet (monsoon) seasons to delineate mixing and biogeochemical processes in the Godavari TRZ. During the dry season, deviations from the river water‐seawater binary mixing line for ⁸⁷Sr/⁸⁶Sr and δ¹⁸O indicate groundwater input, ranging from 7% to 43% across TRZ stations. We observe non‐conservative mixing of DSi and δ³⁰Si values during the dry season, with a significant DSi removal (20%–85%) and enrichment in δ³⁰Si values. The dominance of diatoms in suspended matter and elevated concentrations of the diatom marker pigment fucoxanthin confirm the role of biological uptake in DSi removal. Closure of the Dowleiswaram dam during the dry season reduces river discharge, increases water residence time in the TRZ, and promotes diatom growth. In contrast, the opening of dam gates during the monsoon season, when catchment precipitation is at its peak, leads to high river water influx into the TRZ and elevated suspended matter concentrations (175 ± 50 mg/L), limiting diatom growth during the wet season. Annually, the Godavari River supplies 22.1 ± 5.1 Gmol of Si with a flux‐weighted δ³⁰Si composition of 1.2‰ ± 0.1‰ to the BoB, of which approximately 19% is converted into biogenic silica by diatoms during low discharge periods. The estimated isotopic enrichment associated with this diatom uptake is negligible. The annual groundwater‐derived Si flux to the Godavari TRZ is 0.8 ± 0.5 Gmol Si, representing ∼3.5% of the annual riverine Si flux; however, during the dry season, groundwater constitutes the dominant local Si source within the TRZ, accounting for 47%–79% of the total Si flux, depending on the station.
SN - 2169-8953
ER -