New study reveals how the Arabian Sea 'breathes'
A combination of strong winds and small eddies is crucial for the Arabian Sea to have sufficient oxygen levels for marine life. However, these processes are sensitive to climate change, and even very small changes can have a huge impact. This is shown in a new doctoral thesis at the University of Gothenburg, which provides a clearer picture of the Arabian Sea's ‘breathing rhythm’.
“The ocean must be able to ‘breathe’. It’s important to understand how oxygen enters and moves through these oxygen-depleted zones in order to make predictions for both marine life and the global climate. Even the slightest change in winds and currents has an impact on oxygenation,” says Estel Font Felez, doctoral student at the Department of Marine Sciences, University of Gothenburg.
Collaboration with local fishermen
Estel Font Felez has visited Muscat in Oman to conduct her research in the Arabian Sea. She has collaborated with local researchers at Sultan Qaboos University and local fishermen and launched autonomous marine robots, known as gliders, from their small fishing boats.
Using data from the gliders, combined with satellite data and numerical models, Estel Font Felez has been able to track oxygen changes through the different layers of water in the ocean.
“We went straight out from the coast for 30 minutes and then deployed the robots into the sea. The robots then dived up and down through the ocean's water layers for a month while measuring oxygen, temperature, salinity, and currents at different depths. We conducted the study over the course of a year, so once a month we had to retrieve the robots and then deploy them again,” says Estel Font Felez.
Three important pathways for oxygen supply
The study revealed three main pathways for oxygen supply that allow the Arabian Sea to breathe.
- The mixing of oxygen-rich waters at the sea surface with the interior occurs during when strong winds cool and stir up the sea surface, and drive oxygen deep into the sea with the help of small eddies.
- When the winds then subside, the mixed sea surface and its oxygen-rich waters are capped for weeks to months below a new surface layer, temporarily oxygenating the upper OMZ.
- Currents from the Persian Gulf, in turn, deliver oxygen to deeper areas that cannot be reached by the processes occurring at the surface.
“These small-scale processes may not be big, but they are critical. Together, these mechanisms create a dynamic ‘breathing rhythm’ that causes oxygen levels to fluctuate on weekly to monthly timescales, and also seasonally,” says Estel Font Felez.
Improving the environment in the Baltic Sea
The results are significant for both the climate and the environment.
If OMZs are allowed to expand, it could shrink fish habitats, threaten coastal livelihoods, and increase emissions of nitrous oxide, a potent greenhouse gas.
“By determining how oxygen is supplied, we can improve climate and ecosystem models, which contributes to sustainable fishing. Some of the processes occurring in the Arabian Sea can also be applied to coastal oxygen-depleted areas in, for example, the Baltic Sea, says Estel Font Felez.
Link to Estel Font Felez's doctoral thesis in oceanography: Ventilation of the Arabian Sea Oxygen Minimum Zone.
Text: Annika Wall
Marine ecosystems around the world are currently threatened by ocean deoxygenation driven by climate-induced warming, increased stratification, and circulation changes. Oxygen-depleted zones – known as Oxygen Minimum Zones – are formed naturally in several places in the world's oceans. These region affect the Earth's climate because they produce nitrous oxide, a gas 300 times more potent compared to carbon dioxide, and they are also very sensitive to climate disturbances.
Naturally occurring OMZs exist all over the world, including in the Arabian Sea and off the coast of Chile. The OMZ in the Arabian Sea is as large as Western Europe. Oxygen depleted zones or low oxygen waters also exist in coastal areas, such as the Baltic Sea, but here they are often a combination of natural conditions and human activity, such as eutrophication.
OMZs have grown in both size and intensity in recent decades as a result of climate change. As sea temperatures rise, the ocean's ability to hold oxygen decreases, leading to a drop in oxygen concentration in the water. Circulation and biological productivity changes also have had an impact.
Read more about research on oxygen-depleted areas in the ocean at the Department of Marine Sciences.