Contrasting patterns in pH variability in the Arabian Sea and Bay of Bengal

Continuous understanding of the ongoing ocean acidification (OA) is essential for predicting the future impact of OA on marine ecosystems. Here we report the results of open ocean time-series measurements (19 cruises) of seawater pH in total hydrogen ion scale (pH_T) and associated parameters in the Arabian Sea (AS) and the Bay of Bengal (BoB). During southwest monsoon (SWM), the pH_T within the 30 to 100 m water column shows the maximum difference between the two basins with BoB pH_T being lower (up to ~0.39 units) than AS which could be due to freshwater influx from rivers, mixed layer dynamics, and cold-core eddies. However, during Spring inter-monsoon (SIM), the pH_T of BoB follows the trend of AS. A contrasting finding is that the lowest pH_T occurs at 350 to 500 m in the BoB while it is ~1000 m in the AS. The pH_T within the 150 to 1500 m layer of these two basins shows lower values by 0.03 (±0.02) in the BoB as compared to the AS. The possible reasons for the low pH_T within the BoB oxygen minimum zone (OMZ) could be due to intrusion of western Pacific water in the BoB, freshwater influx from rivers, variations in OMZ of the two basins, higher temperature (~2°C) within the OMZ of the AS, and denitrification in the AS. The pH_T in both the basins (500 to 1000 m) is lower than in the North Atlantic and higher than in the North Pacific waters; however, the pH_T in the 200 to 500 m is lower in the BoB than in all these basins. This study highlights the under-saturation of calcium carbonate at very shallow depths (~ 100 m) in the BoB, indicating that the plankton in the BoB are facing a major risk from OA compared to the AS and need further investigation.