Placing giant tubes in the oceans has been theorized as a possible way to increase upward flow of nutrient rich colder water from deep layers of the ocean into warmer sunlit zones where algae can grow. Algae are plants that use carbon dioxide and create oxygen. The movement of water through the giant tubes would be powered by waves and theoretically reduce the risk of hurricane level storms by redirecting wave energy. (1)
Potential negatives would be that part of the reason oxygen free zones have occurred is due to excess algae blooms, which then die off in mass once the extra supply of nutrients are used. The dead algae are then decomposed by bacteria that use oxygen in the process - leaving an oxygen free zone. So a possible solution might be to combine mussel aquaculture within the area where the giant tubes are placed. The tubes would be anchored to bottom of the ocean and stabilized at the top with floating buoys. (1) Other floating buoys could be holding dangling strings of mussel shellfish.
The series: 1. Oceans need oxygen, 2. Oxygenating the ocean's dead zone, 3. Non-traditional windmills can increase efficiency, 4. Upwelling tubes, hurricanes, and mussels. 5. Baking Soda to the rescue! - Yes really. 6. Supplementing the Ocean, 7. Mussels - Delicious and Sustainable. 8. The big picture of climate change is bigger than businesses may consider. 9. Inventions Occur in Stages. 10. Stacked Horizontal Turbines. The series in one document, minimal images, on Google-docs: (Oxygenating the Ocean).
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References:
Jennifer Horton, Why do some scientists want to scatter tubes throughout the open ocean?, HowStuffWorks, https://science.howstuffworks.com/environmental/green-science/ocean-pipes.htm