Ocean Deoxygenation

Deoxygenation is emerging as a critical threat to tropical coral reefs.  Hypoxic events, when low oxygen levels drop below the critical threshold to sustain life, have been detected on tropical coral reefs in recent years, and in some cases cause mass mortality of corals and other reef organisms. Declining dissolved oxygen concentrations in seawater occur because warming oceans have a reduced capacity to absorb and retain oxygen. This is further exacerbated by high nutrient loading and eutrophication driven by wastewater runoff. Declining oxygen concentrations have been observed in coastal waters for half a century. However deoxygenation has been largely overlooked in the past 20 years of research on coral reef ecosystems, and the impact of deoxygenation on coral reefs has only recently been documented.

At the Smithsonian in Fort Pierce, Florida, research into deoxygenation impact on corals is being conducted in a custom-built mesocosm facility designed to allow researchers to manipulate seawater chemistry to mimic the current and future conditions on coral reefs. 

The system has 24 individual closed system tanks, each one with independent control of temperature, pH, DO and light to facilitate replication and multi-stressor experiments.

While we know that critically low oxygen levels can kill some coral species, some species can survive under the same low-oxygen conditions. However, we do not know about the sublethal effects this may have on the coral’s health, growth, and reproductive capacity. 

Most studies measuring the effect of low oxygen on corals have focused on short-term exposures on adult corals. Almost nothing is known about hypoxia in younger juvenile corals, at their most vulnerable life stages, yet this is critical information for understanding how deoxygenation may impact the capacity of dwindling coral populations to recover.

In 2022, thanks to a University of Glasgow -Smithsonian Institute knowledge exchange research award and a Smithsonian Institution 6-month post-doctoral fellowship, I was able to set up some experiments at the Smithsonian Marine Station to investigate the impact of ocean deoxygenation on coral larval settlement, early recruit survival, and adult coral metabolism. We found that deoxygenation impacted the survival of early coral recruits in Orbicella faveolata and C. natans, but not in Pseudodiploria strigosa. For more information you can find the results published in Frontiers of Marine Science, via this link or in the pdf below. 

Thank you to the Coral Conservation Society for additional funding to support my visit to the Smithsonian Marine Station!

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