Coastal waters around the world are increasingly losing oxygen, with dramatic consequences for both ecosystems and the people who depend on them. The Baltic Sea is a well-known example: with the consequences of spreading hypoxic or anoxic zones evident in fish kills, the decline of spawning grounds and toxic blue-green algae blooms. So why not introduce oxygen into the sea where it is most urgently needed?
'Various technical approaches have already been tested, some of which have had a positive effect on lakes,' says Prof Dr Andreas Oschlies, Professor of Marine Biogeochemical Modelling at the GEOMAR Helmholtz Centre for Ocean Research Kiel. 'However, artificial oxygenation cannot work miracles — it only temporarily alleviates the symptoms and does not address the underlying causes.'
Together with Prof. Dr Caroline P. Slomp, Professor of Geomicrobiology and Biogeochemistry at Radboud University in the Netherlands, Andreas Oschlies heads the Global Ocean Oxygen Network (GONE). GO2NE is an international expert committee of the United Nations Intergovernmental Oceanographic Commission (IOC UNESCO) researching the causes and consequences of declining oxygen levels in the ocean. GO2NE held its first international workshop on artificial oxygenation in autumn 2024. The results of this workshop were published last week in the scientific journal EOS.
Main causes of oxygen loss in coastal seas
Coastal seas naturally obtain oxygen through exchange with the atmosphere and through photosynthesis by phytoplankton on the surface. Deeper water layers can only obtain oxygen through exchange with surface water. Seawater loses oxygen through bacteria consuming it when decomposing organic material. These bacteria can thrive particularly well when the nutrient supply is high, which is why excessive nutrient inputs (especially nitrogen and phosphorus) from wastewater and agriculture are among the main causes of falling oxygen levels. In addition, water bodies are warming, meaning less oxygen can be dissolved in warmer water. Warm layers of water overlying cooler ones also inhibit the mixing of the water layers.
Oschlies: "There are now huge zones in the Baltic Sea where there is no oxygen at all. We call these zones anoxic, i.e. oxygen-free. They are colloquially referred to as 'dead zones'. They are not completely devoid of life, as there are bacteria that can still survive in this environment. However, these areas are absolutely hostile to all other organisms."
Limits and risks of artificial oxygen input
Oschlies and Slomp investigated two technical approaches for supplying oxygen to bodies of water: air or pure oxygen injection (bubble diffusion), and pumping oxygen-rich surface water into deeper layers (artificial downwelling). Both methods have already been tested locally, producing partially positive results. However, as soon as the measures are discontinued, the anoxia usually returns very quickly. Slomp: "This artificial introduction of oxygen can be used successfully in lakes, shallow estuaries or small bays. However, the effect only lasts as long as the operation is maintained." The Chesapeake Bay near Baltimore in the USA is one example of this. After decades of aerating a shallow tributary, the systems were switched off and the oxygen levels fell back to their original levels within a day.
The artificial supply of oxygen also poses ecological risks. For instance, the injection of oxygen can intensify the upward movement of gases such as methane, which is a potent greenhouse gas. Changes in temperature and salinity distributions, as well as underwater noise, could affect marine habitats and, in extreme cases, lead to a further decrease in oxygen levels. "These processes should only be used after thorough testing and accompanied by environmental monitoring," emphasises Oschlies.
No substitute for climate protection and reducing nutrient inputs
The expansion of plants for the production of green hydrogen is currently a topic of debate. Green hydrogen is produced by electrolysis, which splits water into hydrogen and oxygen. If the electrolysers are located near the sea, the oxygen produced as a by-product could be used for oxygen enrichment measures in coastal marine regions. However, the researchers urge caution, stating that while technical interventions could be beneficial where suitable conditions prevail, they would need to be part of comprehensive water protection strategies.
Slomp's conclusion: "The technical possibilities for supplying oxygen do not replace the need for consistent climate protection and the reduction of nutrient inputs from agriculture and wastewater. However, under certain conditions, they can help mitigate the worst consequences of oxygen deficiency, at least temporarily."
