Beneath the ocean’s surface lies a greater unknown than outer space – the ‘twilight zone’. The zone is named to capture its position at 200m – 1000m below the surface, where sunlight decreases rapidly through depth, a zone most of us know only through the wonders of ecological life here, such as bioluminescent species. Yet, its distance from the surface does not mean separation from human influence. We need to incorporate the zone into discussions of anthropogenic climate change.

A recent paper in ‘Nature’ highlighted that without a strong emission reduction policy, significant ecological change to the twilight zone would likely occur within the next century. This change is because temperature is a critical determinant of oxygen and food availability through the ocean’s depth, controlling where and what marine ecosystems are. Studies like this recognise the urgency of acknowledging the twilight zone’s role within the Earth system before catastrophic damage occurs.

Science’s broader ability to identify what the exact changes will be and how they will happen is severely constrained by a lack of existing research. Without this, policy efforts are hampered. Such unanswered questions include: How biodiverse is the twilight zone? Will species be able to migrate to new depths to survive? 

A lack of recognition

The USA’s National Oceanic and Atmospheric Association (NOAA) highlights that 80 per cent of the ocean is unexplored. The twilight zone likely makes up a substantial proportion of this. The juxtaposition between our knowledge of marine life zones and terrestrial life is stark. Twelve men have stood on the moon while only three people have descended to the bottom of the Mariana Trench.

One problem is that adapting research methods from terrestrial ecosystems to use in marine ecosystems is often not feasible. Measuring environmental and ecological characteristics in situ is comparatively tricky in marine environments. Even scuba diving, which enables ecological research and sampling in the sunlight zone, is limited to around 40m below the surface. Hence, novel technologies, such as remote sensing using vessels, are fundamental to understanding the life and significance of the twilight zone.

Despite evident technological challenges, the importance of the twilight zone to global ocean health and carbon sequestration should not be underestimated. The ‘ecosystem services’ provided by this zone benefit us all, whether directly or indirectly, and are often expressed in terms of financial benefits.

Notably, the ecological life of the twilight zone plays a critical role in the ‘biological carbon pump’. According to a Wood Hole Oceanographic Institute report, an estimated two to six billion tonnes of carbon are sequestered annually. An essential process in carbon sequestration is ‘diel vertical migration’ – the daily cycle of species dependent on the depth of the sunlit layer to consume photosynthesising plankton from upper waters. The collective significance cannot be questioned despite the process operating at the scale of individual organisms’ movement. Six billion tonnes is enough to have sequestered all anthropogenic carbon emissions in 1950 – emphasising how natural mechanisms may hold answers for an effective carbon reduction policy.

Wood Hole Oceanographic Institute estimates that the value of carbon storage of this size is $300-900 billion dollars. Carbon storage offsets economic impacts by regulating atmospheric carbon dioxide concentration and reducing the rate of global warming. Through this, excess damages that are projected to occur in a warmer climate, such as increased storm frequency leading to expensive infrastructure damage, are reduced in the short term. Assigning monetary value to nature alludes to an entirely different discussion, but it is necessary to acknowledge the ‘financial’ benefit provided by the twilight zone.

Anthropogenic influences

The relationship between ocean and atmosphere warming is already well illustrated. According to the Intergovernmental Panel on Climate Change (IPCC), the rate of ocean warming since 1993 has likely doubled. Research so far highlights that many positives are unlikely for twilight zone species. Mitigating climate change through emissions reduction policies remains equally essential for ocean species to limit the triple threat of deoxygenation, acidification and temperature rise. If ecosystems are changed, processes such as the biological carbon pump may become altered, impacting the efficiency of carbon storage through the twilight zone.

Governance and Policy in the Twilight Zone

The nature of the interplay between politics and governance further limits the protection of the twilight zone. Most of the twilight zone is within the ‘high seas’, which describes the region beyond 200 nautical miles of a country, not considered under the control of any single territory or nation. The lack of a unifying treaty or national-level stakeholders within the high seas has led to a lack of global-scale policy implementation to conserve the twilight zone.

However, progress has been made with the United Nation’s 2023 Biodiversity Beyond National Jurisdiction Treaty. The treaty, resulting from decades of negotiation, will forefront conservation and sustainability across the marine high seas, including the twilight zone. Significantly, enabling large-scale marine protected areas (MPAs) will help reach global-scale targets (such as the Convention on Biodiversity’s “30 by 30” target, which aims for 30 per cent of the ocean to be protected by 2030). The resumed conference in June will enable further discussion on implementing the treaty and how to adopt it in practice.

A multi-dimensional perspective

Although MPAs offer a viable strategy to protect the biodiverse twilight zone, there are still pitfalls. The dynamic nature of the marine environment means static management strategies, which assume these ecosystems are constant throughout depth, can restrict MPAs’ efficiency. 

Deep sea zones are critical to protecting marine biodiversity and establishing regions of ‘refugia’. Instead of static MPAs focused solely on longitude and latitude, shifting to a 4D approach could incorporate ocean depth and time alongside longitude and latitude to offer a dynamic approach to marine management. 

The twilight zone deserves more of our attention. Although widely unknown, the significance of the species it hosts is emerging as fundamental to mitigating climate change and providing other ecosystem services. Seemly small-scale processes like daily vertical migration are critical to the entire Earth System. There is a need for interdisciplinary & collaborative efforts beyond Western science to implement policy at a global scale, engaging with both financial and cultural services offered via the twilight zone. Protecting a world beyond our vision has never been so urgent.