Long-term changes in Nutrient Concentration and Fluxes from the Godavari Estuary: Role of River Discharge and Fertilizer Inputs

The discharge from the Indian peninsular rivers is strongly controlled by the intensity of monsoonal rainfall, which is strongly influenced by atmospheric extreme events, such as El Niño, La Niña, and Indian Ocean Dipole (IOD). To examine the effect of river discharge on concentrations of nutrients, N/P ratios in the estuary and its fluxes to the coastal Bay of Bengal, time-series observations were conducted from 2008 to 2019. An increase in river discharge was observed in recent years due to an increase in monsoon rainfall, expansion of hydropower plant, and siltation of the dam reservoir. The increase in fertilizer application per hectare of agricultural land was observed in recent years. In response to an increase in both river discharge and fertilizer usage, an enhanced concentration of nutrients in the estuary was observed. Low N/P ratios were observed during the peak discharge (June to September) period due to less removal of phosphate through geochemical pathways due to low residence time of water respectively. An increase in N/P ratios, associated with phytoplankton blooms, was observed during moderate and low discharge periods, suggesting faster phosphate removal through biological or geochemical pathways. The flux of nitrogen, phosphate, and silicate increased by 675, 440, and 906% between 2008 and 2019. This study suggested that increases in river discharge and fertilizer application have both contributed to increased nutrient fluxes from the Godavari estuary to the coastal Bay of Bengal. The impacts of these increased fluxes on coastal ecosystems warrant further investigation.