Significant increase in oxygen levels in the Indian sector of the Southern Ocean

A significant increase in the levels of dissolved oxygen has been observed in the Indian sector of the Southern Ocean (ISSO) over the last three decades. This has come to light in a study conducted by scientists of a Goa-based institution. Interestingly, while a declining trend in oxygen levels is being observed in other oceans of the world, this contrary trend has been seen in the Indian sector.

According to the study conducted by researchers of the National Centre for Polar and Ocean Research (NCPOR), Goa, the process of oxygenation in the Indian sector of the Southern Ocean has accelerated between 1993 and 2025. This increase has been more prominently observed particularly in the Polar and Antarctic zones. Scientists primarily attribute this increase to the process of 'ocean ventilation', during which surface water mixes and is transported to the deeper parts of the ocean, thereby increasing the oxygen content there.

According to a report by PTI, Dr. Thamban Meloth, Director of NCPOR, stated that the Southern Ocean acts as the main 'lungs' of the global ocean and plays a crucial role in regulating global heat, carbon, nutrients, and oxygen. Therefore, changes in its circulation and ventilation can affect the global ocean's oxygen stock.

The research paper, published in the journal 'Environmental Research Letters' on 30 October, stated that researchers used ship-based observations from 2011 and 2025, as well as satellite and ocean reanalysis datasets from 1993 to 2025. The study found that the largest increase in oxygen levels occurred in the intermediate waters at depths of 300 to 2,000 metres. An annual increase of 2.37 micromoles per kilogram was recorded in the Antarctic zone and 1.41 micromoles per kilogram in the Polar zone. An annual increase of approximately 1.95 micromoles per kilogram was also observed in the deep waters at depths of 2,000 to 5,000 metres in the Antarctic zone.

Observations at 57.5 degrees East longitude show that the level of dissolved oxygen in 2025 was higher compared to 2011. The oxygen content in intermediate layers was 4.5 ml per litre in 2011, which increased to approximately 5.5 ml per litre in 2025. Additionally, the study noted that between 2011 and 2025, the winter water layer has retreated southwards by about five latitudes, indicating a change in the ocean structure of this region.

According to the study, ocean stratification has weakened due to increasing heat and salinity, which has increased vertical exchange of water. NCPOR scientist Dr. Sabu P.A. stated that the decrease in oxygen consumption proves that increased ventilation is the main reason for the oxygen increase. An increase in chlorophyll and silicate levels was also observed in the Antarctic zone, suggesting that increased biological productivity may have also helped in increasing oxygen in surface waters. Based on sedimentary records, the oxygen levels in the intermediate waters of this region are currently at high levels within the natural range of the last approximately 80,000 years.

Dr. Vishnu Narayanan Nambuthiri, Project Scientist at NCPOR and lead author of this paper, stated that these two observations taken at a 14-year interval are similar to snapshots, but are consistent with the long-term trends identified through reanalysis datasets. NCPOR Group Director Dr. Rahul Mohane added that continuous observations, such as the use of Argo floats and frequent hydrographic surveys, are necessary to better understand the pathways of oxygen changes and their impacts on the marine ecosystem and climate.