NEW Publication: How low oxygen impacts coral reef ecosystems

March 3, 2025

Dr. Jenny Mallon, PhD

How low oxygen impacts coral reef ecosystems

Deoxygenation is emerging as a major threat to coral reefs that has been somewhat overlooked compared to ocean warming and acidification. Coastal deoxygenation, or reduced seawater dissolved oxygen concentrations, can be caused by pollution and nutrients and is also exacerbated by warming temperatures. Warmer water holds less oxygen, and on tropical reefs, sudden drops in oxygen levels can severely stress coral. In some extreme cases, entire reef communities have died over rapid time scales in response to oxygen depletion. This is because corals, like all animals, need oxygen to survive. There is now a growing body of research looking at the fatal effects of deoxygenation on corals, but the precise thresholds and effects of deoxygenation-induced stress, or hypoxia, are not well understood.

In our study at the Smithsonian Marine Station, we simulated moderately low-oxygen conditions, above what we think are the likely thresholds for coral mortality to occur, so that we could measure the nonlethal effects of low oxygen on different coral species. Over 2 weeks, we measured changes to coral health through visual assessment and direct measurements of coral photosynthesis and respiration. Throughout the 2 weeks of exposure to deoxygenation, the corals did not show any signs of stress, and there was no bleaching or mortality in the 3 species we tested. However, we found that the corals shifted their rates of metabolism – that is, photosynthesis and respiration – and this is likely how they managed to survive the low-oxygen conditions. Through switching their energetic budgets, the corals were able to withstand 2 weeks of low oxygen conditions. 

So, what does this mean at the reef scale? Well, potentially, coral stress caused by exposure to low oxygen in the field might be undetectable during traditional reef monitoring because they do not show any signs of stress that can be visually observed. Importantly, the change in the corals’ metabolism that we measured might affect their long-term growth potential, since energy derived from photosynthesis and respiration is used to build coral skeletons. This has important ramifications for overall reef structural growth and integrity, since reefs are made up of coral skeletal material.

Discover more about deoxygenation in an article co-authored by Dr. Mallon in The Conversation—link below!