Indian sector of Southern Ocean sees rising oxygen levels contrary to global trend Study

Panaji, Nov 9 (PTI) Dissolved oxygen levels in the Indian sector of the Southern Ocean (ISSO) have significantly risen over the past three decades, even as oceans elsewhere are showing the trend of declining oxygen levels, a study by scientists at a Goa-based institute has found.

The scientists attributed the oxygenation primarily to enhanced ocean ventilation - the process through which surface waters are mixed and transported into the ocean interior, replenishing oxygen at greater depths.

Oxygenation in the ISSO intensified between 1993 and 2025, particularly in the Polar and Antarctic zones, as per the study conducted by researchers from the National Centre for Polar and Ocean Research (NCPOR), Goa.

The findings stand in contrast to the broader trend of declining oxygen levels in oceans worldwide, which scientists have linked to global warming, increased ocean stratification and weakening circulation.

"The Southern Ocean acts as the primary 'lung' of the global ocean and plays a defining role in regulating global heat, carbon, nutrients and oxygen. Changes in its circulation and ventilation can therefore have implications for the global ocean oxygen inventory," NCPOR's Director Dr Thamban Meloth told PTI.

The paper, titled 'Trends and drivers of recent oxygenation in the Indian sector of the Southern Ocean', was published on October 30 in the journal Environmental Research Letters.

The study combined ship-based observations from 2011 and 2025 with satellite and ocean reanalysis datasets spanning 1993 to 2025.

As per the research paper, the researchers found that the strongest increase in dissolved oxygen occurred in intermediate waters, at depths of 300 to 2,000 metres, with the Antarctic Zone recording a rise of 2.37 micromoles per kilogram per year and the Polar Zone 1.41 micromoles per kilogram per year.

Deep waters, between 2,000 and 5,000 metres, also showed a significant increase in oxygen levels in the Antarctic Zone, at around 1.95 micromoles per kilogram per year, it said.

Observations along 57.5 degrees East longitude showed higher dissolved oxygen levels in 2025 compared with 2011. Oxygen concentration in intermediate layers rose to about 5.5 ml per litre in 2025 from 4.5 ml per litre in 2011, while the oxygen-minimum layer showed partial shoaling.

The study also recorded a southward retreat of the winter water layer by around five degrees of latitude between 2011 and 2025, indicating changes in the region's ocean structure.

According to the study, warming and increasing salinity have weakened ocean stratification, allowing greater vertical exchange of water. The shoaling of circumpolar deep water and changes in ocean circulation have also contributed to the process.

The decline in apparent oxygen utilisation supported enhanced ventilation as the dominant explanation for the increase, NCPOR's scientist Dr Sabu P said.

The Antarctic Zone also showed rising chlorophyll and silicate levels, suggesting that enhanced biological production may have contributed to oxygen enrichment in surface waters.

It was further observed that the oxygen levels in the region's intermediate waters are currently at the higher end of the natural range recorded over approximately the last 80,000 years, based on sedimentary records.

The two ship-based observational sections, collected 14 years apart, represent snapshots, NCPOR project scientist and the paper's lead author, Dr Vishnu Narayanan Nampoothiri, said. However, they are consistent with the longer-term trends identified through reanalysis datasets, he added.

"Sustained observations, including expanded deployment of Argo floats and repeat hydrographic surveys, would be necessary to better understand the pathways of oxygen change and th.eir implications for marine ecosystems and climate," NCPOR Group Director Dr Rahul Mohan said.

(This story has not been edited by THE WEEK and is auto-generated from PTI)