Covering over half of the planet, and comprising 95% its habitable volume, the deep ocean (>200 m) is critical to any analysis of the role of the ocean in climate mitigation and adaptation. Beyond its capacity to absorb excess heat, the deep ocean has a predominant role in sequestering carbon and removing it from the atmosphere. The deep sea hosts a broad range of pelagic and benthic ecosystems, which provide services that are vitally important to the entire ocean and biosphere, ranging from nutrient cycling to habitat provision, including greenhouse-gas regulation, support to biodiversity (including genetic diversity), food supply and energy production.

Today, significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems are being recorded at great depth. Recent projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep- water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L-1 by 2100. Bathyal depths (200–3000 m) worldwide are predicted to undergo the most significant reductions in pH in all oceans by the year 2100 (0.3 to 0.4 pH units). O2 concentrations may also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century.

However, how these changes will affect deep-sea ecosystem (both benthic and pelagic) functions and the ecosystem services the deep sea provided are just starting to be inventoried. There are, in particular, still large gaps in our knowledge of deep hydrology, hydrography, pelagic and seafloor ecology that must be filled to anticipate how these changes may impair ecosystems at depth and potential feedbacks to surface waters. The growing imprint of human activities at great depths, including contaminant accumulation, overfishing, and disturbances from seafloor extractive activities, further justifies the need for a better understanding of how direct impacts will interact with climate stressors.

New knowledge is critical to improve predictions and assess societal impacts, and requires the expansion of deep-water observing programs with experimental capacities, to support the design of marine protected areas encompassing vulnerable regions in deep waters, and to inform environmental management of industrial activities and development of new policies addressing deep national and international waters. There is also an unprecedented need to integrate the deep ocean into ocean science and policy. New international regulations (e.g, for mining) and treaties (e.g. for biodiversity), environmental management, and spatial planning also must incorporate climate change impacts on deep ocean-processes.

In this session, we invite presentations that describe how climate stessors may alter deep-ocean ecosystems, as well as their combination with other occurring anthropogenic stressors (e.g., fishing, mineral mining), and what the possible societal implications may be. Current initiatives and observing programs, scientific and policy advances and technological developments that will contribute to this effort are also welcome.


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