A new study offers an unprecedented examination of oxygen loss on coral reefs around the world as oceans warm. Led by researchers at the Scripps Institution of Oceanography at UC San Diego and a large team of national and international colleagues, the study captures the current state of hypoxia — or low oxygen levels — at 32 different sites, and reveals that hypoxia is already pervasive in many corals.
The general decrease of oxygen content across the world’s oceans and coastal waters—a process known as ocean deoxygenation—is well documented, but hypoxia on corals is relatively unexplored. Loss of oxygen to the ocean is expected to threaten marine ecosystems globally, although more research is needed to better understand the biological impacts on tropical corals and reefs.
The study, published March 16 in the journal Nature natural climate change, He is the first to document anoxic conditions on coral reef ecosystems at this scale.
“This study is unique because our lab worked with a number of collaborators to compile this global oxygen data set focused specifically on coral reefs – no one has really done this on a global scale before with this number of data sets,” said marine scientist Ariel Bezner. . He is now a postdoctoral fellow at the Smithsonian Naval Station in Florida. “We were surprised to find that a lot of corals are already suffering from what we today would call hypoxia under the current conditions.”
The authors found that lower oxygen levels are already occurring in some coral reef habitats now, and are expected to get worse if ocean temperatures continue to rise due to climate change. They also used models for four different climate change scenarios to show that ocean warming and projected deoxygenation will significantly increase the duration, intensity and severity of hypoxia on coral reefs by 2100.
The analysis was led by Pezner when she was a doctoral student at Scripps Oceanography, where she worked in the Scripps Coastal and Open Ocean Biogeogenomics Research (SCOOBY) lab alongside biogeochemist Andreas Andersson.
Pezner and colleagues used independent sensor data to explore oxygen diversity and hypoxia susceptibility at 32 different reef sites across 12 sites in waters off Japan, Hawaii, Panama, Palmyra, Taiwan, and elsewhere. Many of the datasets were collected using the SeapHOx sensors, instruments originally developed by Scripps Oceanography Lab researcher Todd Martz. Deployed in various coral reef habitats, these and other independent sensors measured temperatures, salinity, pH, and oxygen levels every 30 minutes.
The SCOOBY lab and its partners collected most of the data in an attempt to characterize the seawater chemistry and metabolism of corals in different coral reef environments. International partners have been instrumental in facilitating research logistics and access to many of the study sites. Several contributors also shared data from their own studies. In Scripps Oceanography, Martz Lab, Smith Lab, and Tresguerres Lab made significant contributions to the study.
Historically, hypoxia was defined by a cutoff of a very specific concentration of oxygen in the water—less than 2 milligrams of oxygen per liter—a threshold set in the 1950s. Noting that a single universal threshold may not be applicable to all environments, all corals, or all ecosystems, the researchers 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 corals in this study experienced “weak 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 in the mid-afternoon as a result of nocturnal respiration and daytime photosynthesis, respectively. During the day when the primary producers on the corals have sunlight, Bisner said, they are photosynthesizing and producing oxygen. But at night, when there is no sunlight, there is no oxygen production and everything on the reef is heaving—breathing oxygen and exhaling carbon dioxide—leading to a less oxygenic environment, and sometimes a dip in hypoxia.
This is a natural process, said Anderson, the study’s senior author, but as the ocean warms, seawater can hold less oxygen while the biological demand for oxygen will increase, exacerbating nocturnal hypoxia.
“Imagine you’re someone who’s used to sea level conditions, and then every night you have to go to sleep somewhere in the Rocky Mountains, where the air has less oxygen. That’s similar to what these corals are experiencing,” Anderson said. “And in the future, if it worsens The duration and severity of these hypoxic events, 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 intense, low-oxygen conditions on coral reefs are likely to become more common. Using projections from climate models, the team calculated that by 2100, the total number of hypoxia observations on these reefs would increase under all warming scenarios, ranging from an increase of 13 to 42 percent under one scenario to 97 to 287 percent. Under a more extreme scenario for now.
Continuous and additional oxygen measurements on corals over different seasons and longer time scales will be “essential” to establish baseline conditions, track potential hypoxic events, and better predict future impacts on coral ecology, health, and function, the researchers said.
“Basic oxygen conditions varied widely among our coral reef habitats, which suggests that a single definition of ‘hypoxia’ may not be reasonable for all environments,” said Bezner. “Determining the relevant thresholds will be important going forward in making predictions about how coral reefs will change under warming and oxygen loss.”
This research was funded primarily by the National Science Foundation, and Pezner graduate studies were supported by a Graduate Research Fellowship from the National Science Foundation and an International Scholar Award from the Philanthropic Education Organization (PEO).
This study included a total of 22 authors representing 14 different research organizations and universities including the University of California, San Diego. University of Puerto Rico at Mayagu? ez; NOAA Pacific Island Fisheries Science Center; National Taiwan Ocean University; Georgia Southern University; University of Montana; Smithsonian Institution for Tropical Research; National Sun Yat-sen University. Okinawa Institute of Science and Technology; Sea Education Association; Monterey Bay Aquarium Research Institute; National Taiwan University; and the US Geological Survey.
