The ocean’s so-called “twilight zone” is located between 200 and 1,000 meters deep, and although it gets very little light, is home to a diversity of organisms and billions of tons of organic matter.
A new study published in the journal Nature Communications has found that life in the twilight zone could decline dramatically due to climate change, which could lead to a 20-40 percent reduction in organisms inhabiting this ocean region. Moreover, in a high-emissions scenario, life in this zone could be severely depleted within 150 years, and may not recover for millennia.
By using records from preserved microscopic shells in ocean sediments, the researchers examined how abundant life was in the twilight zone in past warm climates. “We still know relatively little about the ocean twilight zone, but using evidence from the past we can understand what may happen in the future,” said study lead author Katherine Crichton, a postdoctoral fellow in Earth System Modelling at the University of Exeter.
“We looked at two warm periods in the Earth’s past, about 50 million years ago and 15 million years ago,” explained senior author Paul Pearson, a professor of Environmental Sciences at Cardiff University. “We found that the twilight zone was not always a rich habitat full of life. In these warm periods, far fewer organisms lived in the twilight zone, because much less food arrived from surface waters.”
Animals in the twilight zone feed on particles on organic matter sinking from the ocean surface. Since in warmer waters, this matter is degraded much faster by bacteria, less food is available, thus contributing to significant population declines. The rich variety of life that we are witnessing today in this zone evolved over millions of years, when ocean waters had cooled enough to preserve the food for longer and allowing life to thrive. Unfortunately, due to current global warming, this situation may once more change dramatically.
“Unless we rapidly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of much twilight zone life within 150 years, with effects spanning millennia thereafter,” Crichton explained.
“Even a low-emissions future may have a significant impact, but that would be far less severe than medium- and high-emissions scenarios. Our study is a first step to finding out how vulnerable this ocean habitat may be to climate warming.”