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 (δ ³⁰ Si, δ ¹⁸ O, δ ² H, ⁸⁷ Sr/ ⁸⁶ Sr) 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 ⁸⁷ Sr/ ⁸⁶ Sr and δ ¹⁸ O indicate groundwater input, ranging from 7% to 43% across TRZ stations. We observe non‐conservative mixing of DSi and δ ³⁰ Si values during the dry season, with a significant DSi removal (20%–85%) and enrichment in δ ³⁰ Si 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 δ ³⁰ Si 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.