It lies at a depth of over 200 meters - and yet climate change is also threatening the ecologically important habitat of the oceanic twilight zone, researchers report. Their analyzes of sediments show that in earlier interglacial periods, this sea layer, which is now lushly populated, was largely extinct due to an undersupply of nutrients. In the course of global warming, the "Twilight Zone" is now again threatened with an enormous loss of living beings, according to the scientists' model simulations.
Above about 200 meters, the light becomes too weak for photosynthesis and thus for the build-up of biomass by marine algae. Nevertheless, the oceanic twilight zone, which extends to a depth of 1000 meters, is home to many creatures large and small. Their livelihood is formed by the nutrient-rich "snow" of organic particles that trickles down from the productive surface layers. Since the mesopelagic zone, known in technical terms, covers about a quarter of the volume of the oceans, it is a gigantic and therefore important habitat on earth.
An important habitat in sight
Despite its enormous importance, little is known about the twilight zone. However, this is now set to change: The United Nations have launched the research project "Joint Exploration of the Twilight Zone Ocean Network". “The oceanic twilight zone is poorly understood from almost every perspective. However, it harbors possibly the largest and least exploited fish population in the world and recycles about 80 percent of the organic matter that sinks from productive surface waters," reads the UN project's description.
Researchers led by Katherine Crichton from the University of Exeter are also investigating the twilight zone. In their current study, the focus was on the question of how climate change could affect this supposedly remote living environment. The scientists used paleontological sources of information: For their study, they evaluated data from investigations of marine sediments. "To understand what might happen in the future, we looked back in time," says Crichton. The researchers used the shells of tiny organisms – so-called foraminifera – as an indication of how lively the twilight zone was in past periods of the earth’s history. "We examined two interglacial periods in Earth's past, about 50 million years ago and 15 million years ago," says senior author Paul Pearson of Cardiff University.
Feed is already used up
As the researchers explain, they were able to deduce the condition of the entire environment in this sea layer at that time from the occurrence of certain foraminiferal species that only occur in the twilight zone. "What we found was that the twilight zone was not always teeming with life: there were far fewer organisms living there during the warm periods than today," says Pearson. The team attributes this to the fact that during these times hardly any food particles reached the mesopelagic. Because the warmer surface water meant that the organic material in the upper water layers was broken down before it could reach the depths. "The present rich diversity of twilight zone life evolved over the last few million years when ocean water cooled enough to act like a refrigerator, preserving food longer and improving the conditions for life to thrive," explains Crichton.
In the next step, the researchers implemented their results in Earth system models that simulated how different scenarios of further climate change development could affect the environment in the twilight zone. "Our results suggest that significant changes may already be underway," says Crichton. The model simulations indicate that warming could critically limit the food supply at depth. By the end of the century, this could result in a 20 to 40 percent decline in twilight zone life. This damage to this habitat, which is important for the earth's carbon cycle, could in turn be associated with further critical interactions.
"Even a low-emission future could have significant impacts, but they would be far less severe than medium- and high-emission scenarios," says Crichton. The researcher warns that, with current trends, there is even a risk of catastrophic damage in the twilight zone: "If we don't reduce greenhouse gas emissions quickly, much of the life in the twilight zone could disappear or become extinct within 150 years, with effects that are increasing extend over millennia,” says Crichton.
Source: University of Exeter, professional article: Nature Communications, doi: 10.1038/s41467-023-37781-6