A study from NYU Abu Dhabi reveals that brief nighttime drops in ocean oxygen increase energy expenditure in small coral reef fish, potentially affecting reef ecosystems. Researchers examined the Gulf blenny in the Arabian Gulf, the world's hottest sea, where such conditions are already extreme. The findings highlight additional stress from warming oceans beyond just higher temperatures.
In the Arabian Gulf, where summer water temperatures often exceed 36 degrees Celsius (97 degrees Fahrenheit), coral reefs endure some of the planet's most intense heat. This warmth reduces oxygen solubility in seawater, pushing fish like the Gulf blenny to their physiological limits. A new study by scientists at NYU Abu Dhabi investigates how nighttime oxygen declines exacerbate this challenge.
Oxygen levels on reefs fluctuate daily: photosynthesis from plants boosts oxygen during daylight, but at night, respiration by animals causes levels to fall without replenishment. Warming oceans worsen this by limiting oxygen diffusion and water mixing. The research, conducted at the NYU Abu Dhabi Marine Biology Laboratory, simulated these nighttime drops in controlled tanks using oxygen levels observed on local reefs.
During hypoxia, Gulf blennies slightly lowered their energy use. However, upon oxygen recovery, their aerobic metabolic rate rose by 8.67 percent over the next six hours, leading to a 2.87 percent overall daily increase in energy expenditure. The fish's critical oxygen threshold, or Scrit, stands at 47 percent air saturation—below which they cannot sustain normal metabolism. Reef measurements showed oxygen dipping under this level on more than half of summer days.
At the genetic level, low oxygen triggered genes for oxygen sensing, aiding short-term survival, but no significant lactate accumulation indicated reliance on anaerobic metabolism. Instead, increased post-hypoxia activity likely drove the energy rebound.
"These fish are already coping with some of the warmest ocean conditions on Earth," said lead author Daniel Ripley. "Our findings show that repeated nighttime drops in oxygen add a hidden layer of stress that could make it harder for them to grow and survive in the long-term."
Small fish like the Gulf blenny underpin reef food webs, serving as prey for larger species. John Burt, head of the NYUAD Marine Biology Lab, noted: "These findings have important implications for the ecology and fisheries associated with coral reefs worldwide. The Gulf blenny is representative of all small, cryptic fish that serve as the base of the food web on coral reefs worldwide, and Gulf-like conditions are going to become more common on global reefs in the coming decades."
The Arabian Gulf offers a glimpse of future ocean stressors under climate change, where even minor daily energy costs could accumulate, limiting growth and reproduction. The study appears in Functional Ecology.