The low oxygen or oxygen free areas in the ocean have been increasing in size and new ones are being discovered. The rate of increase in the size and number of these oxygen deprived areas of the ocean is unprecedented per specialists. (1)
In the ancient history of the planet's oceans were all low oxygen. Sulfur-loving bacteria do not produce oxygen and they were the most common species 2.52 billion years ago. (2) They have been found to be increasing in the low oxygen zones of the Baltic Sea. While sulfur-loving bacteria were common in the ancient ocean water, photosynthesis was still newly evolved, and there was very little oxygen in the air. (2)
Algae and other microbes or larger seaweeds and sea grass that use photosynthesis to create energy from sunshine, use carbon dioxide in the chemical reaction and release oxygen just like plants on the land do. Planting more trees on land and mangroves and sea grass along coastal regions can help remove carbon dioxide from the air and increase oxygen in the ocean or air supply.
Over the history of the planet there have been five times when many species became extinct. During three of the times of extinction an increase in sulfur using bacteria in marine waters and the resulting low oxygen level in the water was involved in the loss of species. (3)
"Biodiversity loss among marine taxa, for at least 3 of these mass extinction events (Late Devonian, end-Permian and end-Triassic), has been connected with widespread oxygen-depleted and sulfide-bearing marine water." (3)
We are seeing a sixth major extinction event. Large numbers of plants, animals, insects and other species are endangered or already extinct. Loss of oxygen in the oceans is due in part to excess agricultural chemicals reaching the ocean. The excess can cause an over growth of algae which then eventually run out of the seasonal increase in agricultural chemicals and die off and then decomposition bacteria which use oxygen grow in excess numbers.
Over fishing is also involved. Depleting one type of fish species can upset the balance across the food chain and lead to overgrowth of some types which then overuse nutrients leading to species die-off of other species dependent on the nutrients.
The early posts on this site are a series with inventions that already exist which might help ocean health and brainstorming ideas of mine. I am not a marine expert but am trained as a nutritionist and I care about survival of biodiversity and the planet. Human existence is also dependent on oxygen in the air and we, at a planet wide level, get about a third of our protein rich food from marine species. The series of posts are combined in this document: Oxygenating the Ocean, with fewer pictures than are included among the individual blog posts.
References:
Fiona Harvey, Oceans losing oxygen at unprecedented rate, experts warn. The Guardian, Dec. 7, 2019. https://www.theguardian.com/environment/2019/dec/07/oceans-losing-oxygen-at-unprecedented-rate-experts-warn?CMP=share_btn_tw
Stephanie Pappas, 2.5-Billion Year Old Fossils Predate Earth's Oxygen. livescience.com, Dec. 1, 2016, https://www.livescience.com/57051-ancient-life-fossils-predate-earth-oxygen.html
Schobben M, Stebbins A, Ghaderi A, Strauss H, Korn D, Korte C. Eutrophication, microbial-sulfate reduction and mass extinctions. Commun Integr Biol. 2015;9(1):e1115162. Published 2015 Dec 4. doi:10.1080/19420889.2015.1115162 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4802792/