Nutrient retention behind a tropical mega‑dam: a case study of the Sardar Sarovar Dam, India

摘要

In this study, we demonstrate the impact of the construction of a mega-dam on the nutrient export regime of a large tropicalriver into the Arabian Sea. Long-term (11 years) fortnight nutrient parameters, upstream and downstream to SardarSarovar (SS) Dam, were examined to determine the periodical change in nutrient fluxes from the Narmada River, India.During this 11-year period, the average discharge of the Narmada River upstream to Rajghat (35.3 km~3 year~(−1)) was higherthan that of downstream at Garudeshwar (33.9 km~3 year~(−1)). However, during the same period, the suspended sedimentload was reduced by 21 million tons (MT) from 37.9 MT at Rajghat to 16.7 MT at Garudeshwar. Similarly, mean concentrationsof dissolved silica (Dsi) reduced from 470 (upstream) to 214 μM (downstream), dissolved inorganic phosphate (DIP)from 0.84 to 0.38 μM, and dissolved inorganic nitrogen (DIN) from 43 to 1.5 μM. It means that about 54%, 55%, and 96%flux of Dsi, DIP, and DIN retained behind the dam, respectively. The estimated denitrification rate (80,000 kg N km~(−2) year~(−1))for the reservoir is significantly higher than N removal by lentic systems, globally. We hypothesize that processes such asbiological uptake and denitrification under anoxic conditions could be a key reason for the significant loss of nutrients,particularly of DIN. Finally, we anticipated that a decline in DIN fluxes (by 1.13 × 109 mol year~(−1)) from the Narmada Riverto the Arabian Sea might reduce the atmospheric CO2fixation by 7.46 × 109 mol year~(−1).