WOA3, Section 4, Chapter 5, Subchapter 5M: Abyssal plains

Abyssal plains

Writing team: Moriaki Yasuhara (coordinating author), Paul V. R. Snelgrove, Daniel O. B. Jones, Malcolm R. Clark, Jeroen Ingels, Roberto Danovaro, Craig R. McClain, Yutaka Michida (co-lead member), Eric Okuku (lead member) and Jingwen Zhang.

Key points

  • Despite improvements in understanding abyssal biodiversity, knowledge gaps in abyssal evolution and biogeography, connectivity and biotic responses to natural temporal and anthropogenic change remain. The incomplete knowledge of abyssal systems reflects major challenges in sampling this vast and remote area.
  • Abyssal environments support processes underlying deep-sea and global ecosystem functioning and link closely to surface production and pelagic processes.
  • Marine carbon dioxide (CO2) removal and ocean-based carbon interventions may affect abyssal systems directly if deployed on the seafloor or affect pelagic processes that could indirectly result in changes in abyssal ecosystem function.
  • Despite knowledge improvements on abyssal biodiversity linked to deep-sea mineral exploration, full assessment of impacts of exploitation on abyssal ecosystems requires further scientific research.

1. Introduction

Abyssal plains refer to the deep-ocean floor between 3000 and 6000 m. Potentially lower alpha biodiversity characterizes abyssal relative to bathyal depths Ref 54. Yet, given their vastness, abyssal plains likely host similar or more species globally compared to other deep-seafloor habitats. Because most abyssal plains occur beyond national jurisdiction, the diversity and genetic resources they contain fall under the recently adopted Agreement under the United Nations Convention on the Law of the Sea on the Conservation and Sustainable Use of Marine Biological Diversity of Areas beyond National Jurisdiction, which aims at ensuring the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction. In addition, mineral resource presence and the abyss' potential to help mitigate climate change increase interest in abyssal habitats.

While abyssal environments were briefly described the first World Ocean Assessment, a detailed account of abyssal biodiversity was offered in the second World Ocean Assessment. Building on these efforts, the present review examines current regional abyssal biodiversity knowledge and how abyssal biodiversity may respond to future changes.

2. Environmental change since the second World Ocean Assessment

Changes in the overall status

Ongoing climate change and ocean surface warming will likely carry through to the water column and abyssal depths Ref 34. Models suggest that many abyssal ocean regions will exceed their historic range of variability over the next decade Ref 33. Sutton and others (2024) documented warming at full ocean depth east of New Zealand, driven by ocean heat gradient changes between mid and high latitudes that reduce local flows of deeper and colder sub-Antarctic water masses. These patterns continue at the global level Ref 28. In addition, there is growing recognition that shifting-and often declining-ocean productivity Ref 11 may be linked to abyssal ecosystems that depend on surface production; these changes, combined with rising temperatures, could affect abyssal biodiversity and ecosystem function Ref 41 Ref 66.

Documenting long-term changes in abyssal communities remains challenging, but some long-term abyssal observations show that climate cycles likely alter communities. At Porcupine Abyssal Plain (48º50'N 16°30'W, 4850 m depth), scavenging amphipod species composition varies with the Atlantic Multi-decadal Oscillation index Ref 25 and at Station M (34º50'N, 123° 00'W, 4100 m depth) off the California coast, megafaunal variation may link to heat anomalies Ref 31. Climate-driven changes in organic matter supply explain some changes in seafloor oxygen consumption Ref 66, feeding activity Ref 17 and benthic community structure and standing stocks Ref 25 Ref 31. Increased data resolution shows episodic carbon fluxes vary more temporally than thought Ref 26, while deep-sea fossil records over longer time scales indicate reorganization of deep-sea benthic communities with climatic changes such as reduced deep-water formation and changes in temperature and particulate organ carbon flux Ref 84 Ref 83.

Historically, most direct human-induced pressures (e.g. trawling, hydrocarbon drilling) have been focused on shelf and bathyal depths Ref 20, but anthropogenic alterations are deepening Ref 53. Recent studies document organic pollution and microplastics at abyssal depths and deeper Ref 27 Ref 1 Ref 46. Some argue that the greening of global society will require advancing blue economy initiatives, such as exploitation of abyssal mineral resources and marine CO2 removal and ocean-based carbon interventions (see below).

Understanding potential impacts from deep-sea mineral exploitation activities on abyssal habitats and their biodiversity has improved following extensive baseline surveys and experimental studies in the abyssal Clarion-Clipperton Zone in the Pacific Ocean Ref 45 Ref 29. Trials of polymetallic nodule collectors in the Clarion-Clipperton Zone spurred scientific monitoring (e.g. Gazis and others, 2025) that identified limited impacts on densities and diversity inside test tracks against a background of substantial spatial and potential temporal variation Ref 32. Gravity flow may dominate benthic sediment plumes, limiting their extent compared with previous predictions (Muñoz-Royo and others, 2022). Despite limited information on the ecological and biological effects of suspended sediment concentrations in abyssal environments Ref 23, multiple stress responses may occur Ref 68. Effects on community structure Ref 63, functioning Ref 15 Ref 75 and ecosystem services Ref 48 may persist for decades Ref 29. The timescales of recovery and species-specific responses remain an important knowledge gap and require realistic mining tests and comprehensive monitoring (e.g. Lefaible and others, 2024). Anticipated impacts of chemical toxicity, noise and light related to deep-sea mineral exploitation are equally poorly documented Ref 81. Developing restoration capacity in deep-sea and abyssal ecosystems, an area of active research Ref 13, will draw on proposed novel technologies Ref 2.

Effects of change and interaction with other components of the abyssal marine system

Marine CO2 removal and ocean-based carbon interventions incorporate diverse potential approaches, including artificial iron fertilization, which will likely change the quantity and quality of carbon flux to the abyss Ref 30. A fossil study shows that not only warming but also changes in dust flux and resulting iron fertilization alter deep-sea ecosystems Ref 83. Some deep-ocean carbon technologies may directly alter abyssal communities, indicated in small-scale experiments Ref 4 and natural analogue studies of organic falls Ref 21. Precautionary management of marine CO2 removal and ocean-based carbon interventions should also consider potential second-order effects on abyssal seafloor systems and often-overlooked interactions and feedback between carbon reservoirs Ref 30. Importantly, even if these climate mitigations stabilize global temperatures, climate trends and changes in global ocean circulation and shallow-water and deep-sea marine systems may continue (e.g. Sigmond and others, 2020).

3. Region-specific changes in benthic biodiversity knowledge

Arctic Ocean

A recent synthesis revealed high diversity of deep Arctic geomorphological features and a paucity of biodiversity data in deep basins (Ramirez-Llodra and others, 2024). Abyssal Arctic fauna is highly sensitive to sea ice melt and the resulting enhanced algal bloom and flux Ref 6. Ongoing projects are investigating the Arctic seafloor response to changing ice conditions to identify changes at species and community levels but also in functional characteristics across the abyssal Arctic Ocean. Biogeographic distribution information over large spatial scales merits further investigation.

Baltic Sea, Northeast Atlantic Ocean, Mediterranean Sea

The Baltic Sea does not reach abyssal depths and the Mediterranean Sea's abyss (Calypso Deep, Ionian Sea) remains poorly investigated. Hence, the focus is on comparatively well-described and studied Northeast Atlantic Ocean abyssal systems, including newly discovered rocky habitats and biodiversity Ref 55. Integrated taxonomic study demonstrates the importance of knowledge on habitat heterogeneity for inferring large-scale biodiversity patterns (e.g. Schmidt and others, 2024). Understanding of deep-water mass properties and how they influence biodiversity and biogeography of Northeast Atlantic abyssal ecosystems has improved, including the role climatic changes play in these processes Ref 50. Improved understanding of deep-sea change critically requires abyssal temporal observations. Despite the fact that there are only a few deep-sea long-term monitoring stations, those that exist have provided crucial information. These stations include Porcupine Abyssal Plain in the Northeast Atlantic, operational since the mid-1980s Ref 25. Recent insights from Porcupine Abyssal Plain highlight regional oxygen differences over decadal time scales (Ruhl and others, 2024). Although funding cuts threaten these time series, assessing long-term abyssal ecosystem changes requires extended temporal observations. Paleoecological data from deep-sea sediment cores such as the International Ocean Discovery Programme provide historical biodiversity time-series spanning much longer durations Ref 82 Ref 73 and causally link deep-sea diversity to deep-sea temperature Ref 16 as an important driver of abyssal biodiversity. A better understanding of abyssal biodiversity dynamics and its drivers will require more scientific effort.

South Atlantic Ocean and the wider Caribbean

Bridges and others (2023) recently emphasized the paucity of recent abyssal biodiversity studies in the Central and South Atlantic region. Shallow coastal studies dominate Caribbean Sea biodiversity research. The scarce available information shows some regional megafauna variation in the Caribbean abyss Ref 10 and reduced biodiversity in the continental rise-abyssal zone relative to slope depths Ref 24.

Indian Ocean, Arabian Sea, Bay of Bengal, Red Sea, Gulf of Aden and Persian Gulf, Strait of Malacca and South China Sea

Noting that the Gulf of Aden, the Persian Gulf and the Strait of Malacca do not reach abyssal depths and the limited Red Sea abyssal areas, these seas are omitted here and the focus is on the Indian Ocean (including the Bay of Bengal) and the South China Sea.

Despite some abyssal Indian Ocean biodiversity records in the Ocean Biodiversity Information System Ref 74 data remains very limited Ref 57. The presence of deep-sea minerals motivated Indian-led abyssal research since the 1980s, but efforts have slowed, reinforcing a need for wider Indian Ocean biodiversity research.

In the deep South China Sea, biodiverse communities on large plastic debris dumps demonstrate plastics as habitats for deep-sea epibenthos Ref 67. South China Sea biodiversity knowledge generally remains limited for abyssal depths Ref 47 Ref 36. Deep-sea biological investigations in the South China Sea in the last decade have emphasized bathyal cold seep habitats (e.g. Wang and others, 2022).

North Pacific Ocean

Since the publication of the second World Ocean Assessment, considerable new abyssal knowledge has accrued for the North Pacific Ocean, in particular the Clarion-Clipperton Zone with its abyssal polymetallic nodule fields. Higher species richness may characterize some Clarion-Clipperton Zone fauna compared to other abyssal habitats Ref 69. Regional Clarion-Clipperton Zone animal species richness is estimated at 6,000 to over 8,000, 88% to 92% of which are undescribed Ref 52. Although experts describe new abyssal species each year Ref 52 and target new geographic locations Ref 8, limited taxonomic expertise constrains descriptions of new species. Regional-scale syntheses have enabled better understanding of biodiversity and identified knowledge gaps and standardization issues Ref 22 Ref 65 Ref 69 Ref 61. Findings include linking basin-scale productivity gradients and a major biogeographic discontinuity related to the carbonate compensation depth Ref 61. High environmental heterogeneity characterizes abyssal environments Ref 77, noting the role of topography (Simon-Lledó and others, 2019a), substratum Ref 18 Ref 64, episodic food supply Ref 62 and ecosystem engineers Ref 14 Ref 51.

Despite increasing studies of abyssal connectivity, understanding remains limited and inconsistent among faunal groups Ref 72, an important gap in the context of potential deep-sea mineral exploitation. Most abyssal studies note the high proportion of rarely sampled species Ref 40, but population genetic studies at multiple locations suggest well-connected populations over broad regions Ref 69 Ref 71. At the global scale, biogeophysical environments broadly define deep-sea biogeographic provinces Ref 78 Ref 39, although Watling and Lapointe (2022) found weaker support using lower bathyal data for anthozoans. Brandt and others (2019) noted that the hadal Kuril Kamchatka trench potentially limited connectivity and created faunal discontinuities in some abyssal faunal groups in the northwest Pacific Ocean.

South Pacific Ocean

Outside areas of mineral resource interest, few studies have addressed South Pacific abyssal environments. However, in 2024, the Ocean Census Programme sampled the Bounty Trough east of New Zealand to 5000 m depth, focusing on biodiversity discovery Ref 43 and documented numerous new species and extended many distributional records.

The Hunga Tonga-Hunga Ha'apai eruption off Tonga in 2022, among the largest in recent history Ref 49 Ref 59, catalysed several scientific voyages to map changes in topography and faunal communities. Few epifauna and macro-infauna survived the ash that blanketed hydrothermal vent fields more than 80 km from the volcano Ref 5 and adjacent abyssal plains, except where topographic highs such as knolls and hills provided shelter from direct sediment flows Ref 59. Surveys from 2024 and 2025 are investigating the recovery of benthic communities from this eruption. In addition, efforts to map and prioritize hydrothermal vent ecosystem protection at a global scale continue Ref 42.

Southern Ocean

Noting limited advances in knowledge regarding biodiversity and biogeography on Southern Ocean abyssal plains, researchers nonetheless report new species and habitats. Sampling of a Ross Sea seamount at 3,380 m depth revealed seven new species Ref 37. Numerous hydrothermal vents reported from the South Sandwich Arc in the Southern Ocean Ref 35 lack characteristic vent or seep megafauna, although taxa in the vicinity varied greatly.

4. Implications for achieving the targets of the 2030 Agenda for Sustainable Development and its Sustainable Development Goals

Society must determine the appropriate balance between economic and socioclimatic gains and the environmental consequences of human activities, including deep-sea mineral exploitation and marine CO2 removal.

Long-term research of abyssal ecosystems demonstrates community responses to temporal changes in organic carbon flux, underscoring their critical role in global ocean processes. The increase in environmental baseline studies - particularly in the context of mineral exploration in the Clarion-

Clipperton Zone, the Central Indian basin and the Northwest Pacific Ocean - has accelerated abyssal taxonomic and ecological research, as reflected in a stock-taking report on the contribution of the International Seabed Authority (ISA) to the scientific objectives of the United Nations Decade of Ocean Science for Sustainable Development. However, the need for biological research, environmental monitoring and evidence-based decision-making to guide sustainable management and conservation of these ecosystems remains, reflecting requirements described under the United Nations Convention on the Law of the Sea and the regulatory mandate of ISA.

5. Key remaining knowledge and capacity gaps and new gaps

Despite increased biological sampling in areas of interest for mineral resources, considerable knowledge gaps remain Ref 3. These gaps include understanding of trophic relationships, life histories, population and ecosystem connectivity and natural temporal variability of communities. Roest and others (2024) prioritized ecosystem-level functioning as a key management focus, recognizing the impracticality of obtaining detailed species-level data.

Key knowledge gaps include:

  • A global paucity of abyssal biodiversity knowledge, particularly for hard-bottom habitats;
  • Knowledge of species geographic ranges, connectivity patterns and resilience of assemblages to climate stressors and human activities in the abyss;
  • Emerging geoengineering technologies may present new opportunities to combat global change, yet may also pose risks of impact to abyssal ecosystems. Sustained monitoring, fundamental ecological research and evidence-based governance are needed.

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