Seasonal and spatial variability of silicon cycle in Godavari tidal river zone

The present work identifies the drivers of Si cycling in the Godavari tidal river zone (TRZ) and the implications for Si delivery to the Bay of Bengal (BoB). We analysed the elemental composition and isotopic tracers (δ30Si, δ18O, δ2H,87Sr/86Sr) of water column and adjacent groundwater during dry (pre-monsoon) and wet (monsoon) seasons to delineate mixing and biogeochemical processes in Godavari TRZ. During dry season, the deviation from the river water-seawater mixing for 87Sr/86Sr and δ18O indicates the input of groundwater, with an estimated proportional groundwater discharge ranging from 7 to 43%. We observe non-conservative mixing of DSi and δ30Si values during dry season, with a significant DSi removal (20-85%) and enrichment in δ30Si values. We also observed a dominance of diatoms in supended matter, and abundance of diatom marker pigment fucoxanthin, confirming the role of diatoms in DSi uptake during dry season. The Godavari river flow retention by dams upstream limits the river discharge during the dry season, increases water residence time in TRZ, favouring diatom growth in the water column. In contrast, monsoonal precipitation opening of the dam lead to high river water influx and suspended matter (175±50 mg/l), limiting diatom growth during the wet season. Annually, the river supplies 22.1±0.3 GmolSi with a weighted δ30Si composition of 1.2±0.2‰ to the Bay of Bengal, and almost 19% is converted into biogenic silica by diatoms during low discharge periods. The annual groundwater discharge input into the Godavari TRZ accounts for 0.8±0.5 GmolSi, which is ~3.5% of the riverine Si flux. Plain language summary The dissolved nutients transported by the rivers are modulated by biogeochemical processes occurring within the mixing zones, before entering the coastal ocean. The dissolved silicon is an essential nutrient for the growth of diatoms, which are one the major drivers of coastal primary productivity. We studied the seasonal and spatial variability of dissolved silicon dynamics in the largest monsoonal river estuary (Godavari) in India. Our study estimated that annually approximately 20% of the dissolved silicon supplied by the Godavari river is taken up by ditaoms. The damming of the river upstream reduces the riverine discharge and favours the growth of diatoms in the mixing zone during dry periods. The opening of the dams can lead to large influx of turbid river water during monsoon, limiting the productivity.