A new study provides an unprecedented look at the loss of oxygen in coral reefs around the world due to warming oceans. Led by researchers at UC San Diego’s Scripps Institution of Oceanography and a large group of national and international colleagues, the study documents the current state of hypoxia — or low oxygen levels — at 32 different locations and reveals that hypoxia is already widespread in many reefs.
The overall decline in oxygen content in the world’s oceans and coastal waters — a process known as ocean deoxygenation — has been well documented, but hypoxia in coral reefs has been relatively underexplored. Ocean oxygen loss is predicted to threaten marine ecosystems worldwide, although more research is needed to better understand the biological effects on tropical corals and coral reefs.
The study was published March 16 in the journal Nature Climate Change, is the first to document oxygen conditions in coral reef ecosystems at this scale.
“This study is unique because our lab worked with several collaborators to compile this global oxygen dataset specifically focused on coral reefs — no one has really done this on a global scale before with this number of datasets,” said marine scientist Ariel Pezner; , now a postdoctoral fellow at the Smithsonian Naval Station in Florida. “We were surprised to find that many coral reefs are already experiencing what we would define as hypoxia today under current conditions.”
The authors found that low oxygen levels are already occurring in some reef habitats now and are expected to worsen if ocean temperatures continue to rise due to climate change. They also used models of four different climate change scenarios to show that projected ocean warming and deoxygenation will significantly increase the duration, intensity and severity of coral reef hypoxia by the year 2100.
The analysis was led by Pezner while she was a doctoral student at Scripps Oceanography, where she worked in the Scripps Coastal and Open Ocean BiogeochemistY Research (SCOOBY) laboratory with biogeochemist Andreas Andersson.
Pezner and colleagues used autonomous sensor data to investigate oxygen variability and hypoxia exposure at 32 different reef sites at 12 locations in waters off Japan, Hawaii, Panama, Palmyra, Taiwan and elsewhere . Many of the data sets 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 colleagues collected most of the data in an effort to characterize seawater chemistry and reef metabolism in different coral reef environments. International partners were instrumental in facilitating research logistics and access to multiple study sites. Several contributors also shared data from their own studies. At Scripps Oceanography, the Martz Lab, the Smith Lab, and the Tresguerres Lab contributed significantly to the study.
Historically, hypoxia has been defined by a very specific cutoff of oxygen concentration in water — less than two milligrams of oxygen per liter — a threshold established in the 1950s. The researchers note that a universal threshold may not apply to all environments or all reefs or all ecosystems and investigated 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 “mild to moderate” hypoxia, and 13 percent experienced “severe” hypoxia at some point during the data collection period.
As the researchers expected, oxygen was lowest in the early morning at all sites and highest at midday as a result of nocturnal respiration and daytime photosynthesis, respectively. During the day when the main producers on the reef have sunlight, they photosynthesize and produce oxygen, Pezner said. But at night, when there’s no sunlight, there’s no oxygen production, and everything on the reef respires — breathing in oxygen and breathing out carbon dioxide — resulting in a less oxygenated environment and sometimes a dip in hypoxia.
This is a normal process, said Andersson, the study’s senior author, but as ocean temperatures rise, seawater can hold less oxygen, and biological demand for oxygen will increase, exacerbating this nocturnal hypoxia.
“Imagine being a person who is used to sea level conditions, and then every night you have to sleep somewhere in the Rocky Mountains, where the air has less oxygen. This is similar to what these corals experience at night and early morning when they experience hypoxia,” Anderson said. “And in the future, if the duration and intensity of these hypoxic events worsens, then it could be like sleeping on Mount Everest every night.”
The researchers found that as global temperatures continue to rise and marine heat waves become more frequent and severe, low-oxygen conditions on coral reefs are likely to become more frequent. Using projections adopted from climate models, the team estimated that by the year 2100, the total number of hypoxic observations on these reefs would increase under all warming scenarios, ranging from an increase of 13 to 42 percent in one scenario to 97 to 287 percent under a more extreme scenario than now.
The researchers said that continuous and additional measurements of coral reef oxygen over different seasons and longer time scales will be “imperative” to define baseline conditions, monitor potential hypoxic events and better predict future impacts on ecology, health and the functioning of reefs.
“Basic oxygen conditions varied widely among our reef habitats, suggesting that a single definition of ‘hypoxia’ may not make sense for all environments,” said Pezner. “Determining what thresholds are relevant will be important for making predictions about how reefs might change due to warming and oxygen loss.”
This research was primarily funded by the National Science Foundation, and Pezner’s graduate studies were supported by a National Science Foundation Graduate Research Fellowship and a Philanthropic Educational Organization (PEO) International Scholars Award.
A total of 22 authors representing 14 different research organizations and universities, including UC San Diego, participated in this study. University of Puerto Rico at Mayagu?ez; NOAA Pacific Islands Fisheries Science Center. National Taiwan Ocean University; Georgia Southern University; University of Montana; Smithsonian Tropical Research Institute; National Sun Yat-sen University; Okinawa Institute of Science and Technology. Maritime Education Association; Monterey Bay Aquarium Research Institute; National Taiwan University; and the US Geological Survey.