The mesopelagic zone of the ocean, also called “the twilight zone,” refers to the ocean at an approximate depth of 200m to 1,000m. Beginning just below the sunniest parts of the ocean, the twilight zone spans from where only 1% of light reaches, to the depth of the ocean where there is no light. It contains the largest quantity of fish stocks in the world’s oceans.
Researchers have studied the geological past of the twilight zone and their findings suggest that the rising temperature of the ocean due to climate change could cause a 20-40% reduction in life in the twilight zone by the end of the century.
The study, published in Nature Communications, was funded by the Natural Environment Research Council. By studying microscopic shells found in ocean sediments, the authors were able to reconstruct the abundance of life in the twilight zone during warmer periods in Earth’s history. "We looked at two warm periods in the Earth's past, about 50 million years ago and 15 million years ago," said co-author Professor Paul Pearson of Cardiff University. These periods are the early Eocene and mid-Miocene, respectively.
They found that plankton were less abundant and less diverse during these warm periods. The authors hypothesize that warmer oceans produce less food supply for inhabitants of the twilight zone, a result of organic matter being broken down more quickly by bacteria when in warmer waters. This effect causes less organic matter to sink to the twilight zone, a process which also affects the oxygen availability at this depth, thus limiting habitat availability for inhabitants as well.
Although the twilight zone currently contains an abundance of life, lead study author Dr. Katherine Crichton noted that "the rich variety of twilight zone life evolved in the last few million years, when ocean waters had cooled enough to act rather like a fridge, preserving the food for longer, and improving conditions allowing life to thrive.”
Using the cGENIE Earth system model, they made projections for the future of the twilight zone in the warming ocean caused by climate change.
Current projections suggest the “disappearance or extinction of much twilight zone life” within the next 50-150 years, according to Dr. Crichton, given the current rate of greenhouse gas emissions and their contribution to climate change.
Beyond being home to the ocean’s largest fish stock, the twilight zone plays a crucial role in the larger ecosystem of the ocean — recycling approximately 80% of the organic material that descends from the ocean’s layers above.
"Our findings suggest that significant changes may already be under way," Dr. Crichton said. "Our study is a first step to finding out how vulnerable this ocean habitat may be to climate warming."