Ocean acidification in the Northern Indian Ocean: a review

Highlights

  • Multiple oceanographic processes functioning together and increased anthropogenic CO2 make Northern Indian Ocean more susceptible to ocean acidification than other major oceans of the world.
  • ACD and CCD have been defined chemically and biologically, in the NIO.
  • Palaeo-records suggest shoaling of ACD and CCD during warmer (interglacial) periods leading to dissolution of pteropod and foraminiferal shells.
  • The ACD lies within the OMZ and thus the strength of the latter directly affects the ACD.
  • While OMZ reduces the pH causing shoaling of ACD, denitrification in the Arabian Sea leads to increase in pH. Bay of Bengal is not conducive to denitrification.
  • Impacts of ocean acidification on ecosystems are completely lacking.
  • Need to collate abundant data collected since International Indian Ocean Expedition-1, laboratory and in-situ culture experiments for biological responses, and water column studies.

Abstract

Characterised by one of the largest fresh water influxes in the world, the Northern Indian Ocean (NIO) is also bathymetrically, hydrodynamically and climatologically fragmented into smaller basins, making it a complex ocean basin. The monsoon system is unique to the NIO, bringing in several processes like ocean warming, dissolved oxygen depletion, evaporation, freshwater runoff induced by precipitation as well as glacial melting, nutrients derived from land runoff as well as coastal upwelling, biological productivity, oxic-degradation of organic matter and formation of oxygen minimum zones, all functional together at any given time. The seasonality and magnitude of all these processes control the pH in the NIO, as against other global oceans wherein only one or a couple of these processes are in play. The thermohaline circulation is another process that continuously affects the water chemistry in the NIO and has varied in strength over glacial-interglacial cycles. Its interaction with the NIO has the potential of affecting water masses in other ocean basins as well. While there exists limited work in the NIO directly addressing the issue of ocean acidification, substantial research has been done to understand the different hydrological and climatological processes, which influence the acidity in the basin. The present review summarises such studies which offer significant clues to the processes leading to- and impacts of- ocean acidification in the NIO. However, ocean acidification studies are in their nascent stage in the NIO. Understanding the influence of thermohaline ventilation in the Bay of Bengal, interbasinal hydrological teleconnections, impacts of ocean acidification on different trophic levels of the food chain, estimation of anthropogenic CO2 flux, estimation of pre-industrial pH and dissolution horizons by collating past chemical metrics collected, study of submarine volcanic regions as mimics of climate change and qualitative / quantitative ecosystem responses / adaptations are all promising prospects for further study in the Northern Indian Ocean.

Panchang R. & Ambokar M., 2021. Ocean acidification in the Northern Indian Ocean : a review. Journal of Asian Earth Sciences: 104904. doi: 10.1016/j.jseaes.2021.104904. Article (subscription required).


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