The largest inhabited space on the planet is located more than 200 m below the ocean surface, where darkness is almost total. These ocean depths play a major role in mitigating climate change through heat and anthropogenic CO2 sequestration. In addition to undergoing gradual warming and acidification, deep waters are less well ventilated, which reduces oxygen availability. Changes in surface phytoplankton production also affect the quantity and quality of nutrient resources available in the deep ocean. What will be the consequences of these disturbances on this vast and largely unexplored environment? Models set the framework and predict 50 to 80-year trends, but struggle to provide answers about the near future. Since observations reveal faster changes than model predictions do, there is a pressing need to adapt human activities to address potential risks. Many ecosystem services are linked to exchanges between the seabed and ocean surface ecosystems. These ecosystems play a role in long-term CO2 and CH4 sequestration by trapping carbon in the form of carbonates or organic matter (living organisms, debris, particles, or compounds dissolved in water). Increasing temperature, and decreasing oxygen and pH affect species distribution and, more generally, the entire nutrient cycle on which sustainable economic activities, such as artisanal fisheries, are based. Without a better understanding of these phenomena in space and over time, anticipating the consequences of climate change on biodiversity and deep ecosystems remains very difficult, as does assessing the impacts of new industrial activities combining with climate change consequences. Implementing key climate change adaptation measures must be based on an unprecedented effort to acquire the new knowledge needed to establish a legislative framework and effective management tools.
The concept of ecosystem services (ES) includes the ecological functions and the economic value of ecosystems which contribute to human well-being. This approach is already applied to coastal water management, but it is rarely applied to the deep sea although it represents 97% of the ocean’s volume. Deep-sea ES include provisioning services such as fish catch or industrial agents, regulation services such as carbon storage, and cultural services such as inspiration for the arts. However, the deep sea is facing increasing pressures in the form of direct and indirect human activities. This synergy of impacts is widely unknown and the lack of regulation regarding certain parts of the ocean requires great caution.
The deep ocean (200m below the surface to 11,000m) represents over 98% of marine waters in volume. The image of a stable and homogeneous environment over vast areas, with low biological activity, does not actually reflect the diversity of deep-sea ecosystems nor their sensitivity to climate change. Even on the abyssal plains, variations in abundance of key species have been attributed to changes in the photosynthetic productivity at the surface of the ocean. Moreover, many biodiversity and productivity ‘hot-spots’ of the deep seafloor, and their foundation species such as deep-sea corals could be particularly vulnerable to the already observable changes at great depths, such as local or regional warming deep water, acidification and deoxygenation and modifications of the circulation of water masses. This vulnerability questions our ability to anticipate the consequences of climate change on poorly known ecosystems and the services they provide.