The authors found that low oxygen levels are already happening in some reef habitats now, and are expected to get worse if ocean temperatures continue to warm due to climate change. They also used models of four different climate change scenarios to show that projected ocean warming and deoxygenation will substantially increase the duration, intensity, and severity of hypoxia on coral reefs by the year 2100.
The analysis was led by Pezner while she was a PhD student at Scripps Oceanography, where she worked in the Scripps Coastal and Open Ocean BiogeochemistrY Research (SCOOBY) lab alongside biogeochemist Andreas Andersson.
Pezner and colleagues used autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 diverse reef sites across 12 locations in waters off Japan, Hawaii, Panama, Palmyra, Taiwan, and elsewhere. Many of the datasets were collected using SeapHOx sensors, instruments originally developed by the lab of Scripps Oceanography researcher Todd Martz. These and other autonomous sensors were deployed in different coral reef habitats, where they measured temperature, salinity, pH, and oxygen levels every 30 minutes.
The SCOOBY lab and partners collected most of the data in an effort to characterize seawater chemistry and reef metabolism in different coral reef environments. The international partners were instrumental in facilitating research logistics and access to many study sites. Several contributors also shared data from their own studies. At Scripps Oceanography, the Martz Lab, Smith Lab, and Tresguerres Lab all made significant contributions to the study.
Historically, hypoxia has been defined by a very specific concentration cutoff of oxygen in the water—less than two milligrams of oxygen per liter—a threshold that was determined in the 1950s. The researchers note that one universal threshold may not be applicable for all environments or all reefs or all ecosystems, and they explored the possibility of four different hypoxia thresholds: weak (5 mg/L), mild (4 mg/L), moderate (3 mg/L), and severe hypoxia (2 mg/L).
Based on these thresholds, they found that more than 84 percent of the reefs in this study experienced “weak to moderate” hypoxia and 13 percent experienced “severe” hypoxia at some point during the data collection period.
