Deep Sea and ABNJ Accounting

1. Outcome

This Circular provides emerging guidance on accounting for deep sea ecosystems and areas beyond national jurisdiction (ABNJ), representing a frontier area of ocean accounting methodology. The deep sea encompasses more than half of the Earth's surface and hosts unique ecosystems including hydrothermal vents, cold seeps, seamounts, and abyssal plains, yet remains largely unexplored and outside the scope of most national accounting frameworks. ABNJ--the high seas and the international seabed Area--fall outside the exclusive economic zones (EEZs) of coastal States and are governed by distinct international legal frameworks, creating fundamental challenges for the country-based structure of the System of National Accounts and SEEA.

Governments managing deep sea resources and ABNJ governance face critical decisions that require structured information. This Circular supports four primary decision use cases. Deep-sea mining impact assessment requires baseline ecosystem accounts to evaluate the environmental costs of seabed mineral exploitation against economic benefits, informing national positions on International Seabed Authority (ISA) licensing decisions. BBNJ marine protected area designation depends on extent and condition accounts for deep-sea ecosystems to identify priority areas for area-based management tools under the Agreement on Biodiversity Beyond National Jurisdiction, which entered into force in January 2026. Hydrothermal vent conservation necessitates accounts for chemosynthetic-based ecosystems to balance bioprospecting potential against preservation of unique genetic resources. ISA environmental management plans use ecosystem condition monitoring to set performance standards and track cumulative impacts across exploration contract areas. These decision contexts connect directly to indicator frameworks described in TG-2.10 MEA Indicators, particularly BBNJ monitoring requirements, and to governance frameworks addressed in TG-1.3 Marine Spatial Management, especially for MPAs in ABNJ.

This Circular is classified as Emerging to reflect the frontier status of deep sea and ABNJ accounting methodology. Unlike Applied circulars that provide operational guidance, this Circular acknowledges that deep sea and ABNJ methodology is still developing, and readers should anticipate that the guidance presented here will evolve significantly as scientific understanding improves, governance frameworks mature, and practical implementation experience accumulates. The entry into force of the BBNJ Agreement creates new governance structures and information requirements that will shape the development of ABNJ accounting in coming years^[1]. As implementation experience accumulates and BBNJ monitoring frameworks are established, this Circular should be revised to reflect advances in methodology, with a target for re-evaluation in 2028.

Deep sea asset accounts connect upward to multiple policy frameworks. Asset accounts for seabed minerals (described in TG-3.1 Asset Accounts) inform national natural capital budgets and provide the evidence base for depletion-adjusted income measures when mineral extraction occurs. Ecosystem extent accounts for deep-sea protected areas feed directly into MEA indicator reporting (see TG-2.10 MEA Indicators), supporting SDG 14.5.1 (marine protected area coverage) and GBF Target 3 (30x30) by documenting protection of abyssal plains, seamounts, and hydrothermal vents. Condition accounts for chemosynthetic ecosystems inform bioprospecting regulations and benefit-sharing arrangements under the BBNJ Agreement's marine genetic resources provisions, connecting to economic valuation frameworks in TG-1.9 Valuation.

By engaging with this Circular, readers will understand the jurisdictional and governance context for deep sea and ABNJ accounting, the ecological characteristics of major deep sea ecosystem types, the conceptual and practical challenges for applying SEEA methodology to these remote and data-scarce environments, and the emerging opportunities for developing ocean accounts that extend beyond national boundaries. The conceptual framework and key components of Ocean Accounts are established in TG-0.1 General Introduction to Ocean Accounts, while the methodological foundations of asset accounting are addressed in TG-3.1 Asset Accounts. For guidance on the statistical standards underpinning UNCLOS and other international legal frameworks discussed in this Circular, see TG-0.2 Overview of Relevant Statistical Standards.

2. Requirements

This Circular requires familiarity with:

• TG-0.1 General Introduction to Ocean Accounts -- provides the conceptual framework and key components of Ocean Accounts, including the relationship between accounting boundaries and jurisdictional frameworks that is essential for understanding the boundary challenges posed by ABNJ.

• TG-3.1 Asset Accounts -- for the methodological foundations of physical and monetary asset accounts, ecosystem extent and condition accounts, and the concepts of depletion and degradation that this Circular applies to deep sea and ABNJ contexts.

Readers may also benefit from familiarity with TG-0.2 Overview of Relevant Statistical Standards, which provides grounding in the international statistical standards context relevant to the extensive discussion of UNCLOS and BBNJ legal frameworks in this Circular.

3. Guidance Material

The deep sea and areas beyond national jurisdiction present unique challenges for ocean accounting. These environments are remote, largely unexplored, and governed by international frameworks that differ fundamentally from the territorial jurisdiction assumed by most national statistical systems. The SEEA Central Framework explicitly acknowledges that while its "accounts are usually compiled for countries and hence the geographic scope is limited to the economic territory of a country including its Exclusive Economic Zone (EEZ)... there is a significant amount of natural capital, in particular concerning oceans, that may be excluded"^[2].

The 2025 SNA addresses this gap by noting that "where there is interest in organising data about these types of natural capital outside of the scope of the integrated framework of the SNA in a manner that can be directly related to country based measures, the accounting definitions and treatments of the integrated framework of the SNA and the SEEA can be applied"^[3]. Paragraphs 35.128-35.129 of the 2025 SNA provide the primary conceptual justification for extending accounting frameworks to ABNJ; this is new text in the 2025 revision and represents an important opening for future development. This section provides emerging guidance on applying these frameworks to deep sea and ABNJ contexts.

3.1 Jurisdictional Framework

The United Nations Convention on the Law of the Sea (UNCLOS) establishes the legal framework governing ocean jurisdiction, defining distinct maritime zones with different rights and obligations^[4]. Understanding this framework is essential for determining accounting boundaries and the attribution of assets and flows. For countries that have not ratified UNCLOS, many of its key provisions are considered to reflect customary international law and remain relevant to accounting boundary determination.

Maritime zones under UNCLOS

Territorial Sea extends up to 12 nautical miles from baselines. Coastal States exercise full sovereignty over the territorial sea, including the seabed and subsoil^[5].

Exclusive Economic Zone (EEZ) extends up to 200 nautical miles from baselines. Coastal States have sovereign rights for exploration, exploitation, conservation and management of natural resources, both living and non-living, in the waters, seabed and subsoil^[6]. The SEEA CF notes that "following article 57 of the United Nations Convention on the Law of the Sea of 10 December 1982, a country's EEZ may extend up to 200 nautical miles from the country's normal baselines"^[7].

Continental Shelf may extend beyond 200 nautical miles to the outer edge of the continental margin, subject to limits defined in UNCLOS Article 76. Coastal States exercise sovereign rights over the continental shelf for the purpose of exploring and exploiting natural resources, including sedentary species^[8]. For extended continental shelf areas, the Commission on the Limits of the Continental Shelf (CLCS) reviews submissions from coastal States.

The High Seas comprise all parts of the sea not included in the EEZ, territorial sea, or internal waters of a State^[9]. The high seas are open to all States for navigation, overflight, fishing, and scientific research, subject to obligations for conservation of living resources^[10]. Approximately 64% of the ocean surface lies beyond national jurisdiction.

The Area designates the seabed, ocean floor and subsoil beyond the limits of national jurisdiction^[11]. UNCLOS declares the Area and its resources to be "the common heritage of mankind"^[12]. The International Seabed Authority (ISA) administers the Area and its mineral resources on behalf of all States^[13].

Implications for accounting boundaries

The country-based structure of national accounts creates a fundamental boundary issue for ABNJ resources. The SEEA CF states that "the SEEA Central Framework includes all natural resources, cultivated biological resources and land within a country of reference (including resources within a country's exclusive economic zone)"^[14]. Resources in ABNJ are explicitly excluded from individual country asset boundaries.

However, several mechanisms create accounting connections to national economies:

1. Quota-based fish stocks: Where international agreements allocate fishing quotas in ABNJ, countries may record these rights as assets. The SEEA CF notes that "if economic ownership can be established for natural resources, for instance via internationally agreed quotas, these resources are within scope of the integrated framework of national accounts"^[15]. See TG-6.7 Fisheries Stock Assessment for guidance on recording quota-based fish stocks.

2. Seabed mining contracts: Entities holding exploration or exploitation contracts from the ISA conduct activities in the Area on behalf of sponsoring States. These activities generate economic flows attributable to national economies. See TG-3.10 Offshore Energy Accounts for methodological parallels with offshore mineral extraction.

3. Flagged vessels: Production by vessels flagged to a country is attributed to that country's economy regardless of where in the ocean the activity occurs^[16]. See TG-3.5 Maritime Transport Accounts for guidance on attributing marine economic activity.

Additional mechanisms may emerge as BBNJ implementation proceeds. The treatment of research vessels, environmental monitoring activities, and non-extractive uses in ABNJ--including marine scientific research and bioprospecting--remains an area requiring further methodological development, and compilers should monitor developments in this space.

The 2025 SNA suggests that accounts could potentially be compiled for areas like "the Pacific Ocean" applying "the accounting definitions and treatments of the integrated framework of the SNA and the SEEA"^[17]. Such regional or global ocean accounts would complement national accounts by capturing natural capital currently outside country-based frameworks.

Figure 6.6.1: ABNJ governance framework and accounting implications^[18]

BBNJ Agreement implications

The BBNJ Agreement, which entered into force in January 2026, establishes new frameworks for conservation and sustainable use of marine biodiversity in ABNJ^[19]. The first Conference of the Parties is expected in late 2026, and accounting implications will become clearer as implementing decisions are made. Key provisions relevant to ocean accounting include:

• Marine genetic resources: Mechanisms for access to and benefit-sharing from marine genetic resources of ABNJ, including digital sequence information^[20]. This creates potential for recording benefit flows as resource rent equivalents.
• Area-based management tools: Including marine protected areas (MPAs) in ABNJ, requiring delineation and condition monitoring^[21]. MPA boundaries could serve as ecosystem accounting units.
• Environmental impact assessments: Mandatory assessments for activities affecting ABNJ environments^[22]. These generate baseline data potentially usable for accounting purposes.
• Capacity-building and technology transfer: Supporting developing States' participation in ABNJ research and governance^[23].

These provisions will generate new data flows and governance requirements that may enable more comprehensive accounting for ABNJ ecosystems as implementation proceeds. The development of monitoring frameworks for BBNJ-designated MPAs could provide baseline data for ecosystem extent and condition accounts in ABNJ. This Circular should be updated following COP1 to reflect agreed monitoring and reporting frameworks.

3.2 Deep Sea Ecosystems

The deep sea encompasses the vast majority of the ocean by volume, extending from approximately 250 metres depth to the deepest trenches at nearly 11 kilometres. The IUCN Global Ecosystem Typology (GET) organises deep sea ecosystems within two biomes: M3 Deep Sea Floors (benthic ecosystems) and M2 Pelagic Ocean Waters (water column ecosystems extending to depth)^[24]. This classification aligns with the SEEA EA recommendation to use the IUCN GET as a reference classification for ecosystem types. See TG-0.2 Overview of Relevant Statistical Standards for guidance on ecosystem classification systems.

M3 Deep sea floors biome

The deep sea floor biome comprises benthic ecosystems below approximately 250 metres where insufficient light reaches for photosynthesis^[25]. These are predominantly heterotrophic systems dependent on organic matter flux from surface waters or, in the case of chemosynthetic ecosystems, on chemical energy sources.

M3.1 Continental and island slopes are large sedimentary environments extending from the shelf break to abyssal basins, characterised by strong depth gradients in pressure, temperature, light and food availability^[26]. The IUCN GET notes that "these aphotic heterotrophic ecosystems fringe the margins of continental plates and islands, extending from the shelf break (~250 m deep) to the abyssal basins (4,000 m)"^[27]. Continental slopes within EEZs fall within national accounting boundaries; slopes on the extended continental shelf may also be attributable to coastal States.

M3.2 Submarine canyons function as "dynamic flux routes for resources between continental shelves and ocean basins" and are "one of the most productive and biodiverse habitats in the deep sea"^[28]. Canyons enhance nutrient transport, provide refuge and nursery habitats, and may support dense communities of cold-water corals and sponges. For canyons within EEZs, see TG-6.4 Other Marine Ecosystem Accounts for guidance on accounting for these features.

M3.3 Abyssal plains represent "the largest group of benthic marine ecosystems, extending between 3,000 and 6,000 m deep and covered by thick layers... of fine sediment"^[29]. Despite low productivity, abyssal plains host high biodiversity, with "many species... so far... sampled only as singletons"^[30]. These systems account for approximately 76% of the total seafloor area. The vast majority of abyssal plains lie in ABNJ.

M3.4 Seamounts, ridges and plateaus are major geomorphic features characterised by hard substrates and elevated topography. Approximately 171,000 seamounts have been documented worldwide, covering 2.6% of the sea floor, while ridges cover approximately 9.2%^[31]. These features support elevated productivity and may act as stepping stones for dispersal or barriers between adjacent basins. Many seamounts are targets for fishing and potential mining activities.

M3.5 Deepwater biogenic beds are formed by sessile suspension-feeders such as cold-water corals, sponges, and bivalves that create structurally complex habitats^[32]. These slow-growing systems are critical for local biodiversity but highly vulnerable to physical disturbance. Cold-water coral ecosystems share some characteristics with shallow-water coral reefs addressed in TG-6.1 Coral Reef Accounts, though they differ fundamentally in energy source and growth dynamics.

M3.6 Hadal trenches and troughs represent the deepest ocean systems, extending from 6,000 to 11,000 metres depth and comprising 27 disjoint deep-ocean trenches and 20 additional features^[33]. These isolated environments exhibit high endemism and unique adaptations to extreme hydrostatic pressure.

M3.7 Chemosynthetic-based ecosystems (CBEs) include hydrothermal vents, cold seeps, and organic falls (whale falls, wood falls). These systems derive primary productivity from chemoautotrophy using reduced compounds such as hydrogen sulphide and methane^[34]. The IUCN GET describes these as characterised by "high faunal biomass" but "low diversity and high endemism"^[35]. Hydrothermal vents occur on mid-ocean ridges, back-arc basins and active seamounts; cold seeps on continental margins; food-falls along cetacean migration routes. CBEs are of particular interest for bioprospecting and marine genetic resources, and the BBNJ Agreement's marine genetic resources provisions may have significant implications for how these assets are valued--an area requiring substantial methodological development.

M2 Pelagic ocean waters (deep component)

The pelagic realm extends throughout the water column. Deep pelagic ecosystems include:

M2.3 Bathypelagic ocean waters (1,000-4,000 m depth) are characterised by permanent darkness, low temperatures (1-3C), and dependence on organic matter flux from surface waters^[36].

M2.4 Abyssopelagic ocean waters (4,000 m to near the seafloor) represent the deepest water column ecosystems with extremely low biological productivity^[37].

The IUCN GET notes that these deep pelagic ecosystems receive "less than 1% of the primary production of the euphotic zone" and support "sparse populations of heterotrophic bacteria, fauna and fish"^[38]. For shallower pelagic ecosystems, see TG-6.5 Pelagic and Open Ocean Accounts.

3.3 Extent and Condition Accounting

Accounting for the extent and condition of deep sea ecosystems presents formidable data challenges. These environments remain poorly mapped, with limited baseline information on ecosystem distribution, structure, and function. This section provides a target framework for structuring accounts as data become available; compilers should not interpret this as operational guidance but rather as a direction for future development. The Emerging badge reflects the significant implementation gap between the account structures described and the data currently available to populate them.

Mapping challenges

The IUCN GET observes that "less than 1% of the seafloor has been investigated biologically"^[39]. High-resolution bathymetric mapping exists for only a small fraction of the ocean floor, limiting the ability to delineate ecosystem extent with precision. Key challenges include:

1. Inaccessibility: Most deep sea ecosystems cannot be observed directly except through expensive deep-sea submersibles and remotely operated vehicles (ROVs).

2. Scale: The deep sea floor covers approximately 300 million square kilometres--more than twice the Earth's land surface--making comprehensive mapping impractical with current technology.

3. Ecosystem delineation: Unlike coastal ecosystems with distinct boundaries (e.g., coral reefs, seagrass meadows), many deep sea ecosystems grade into one another along environmental gradients, complicating boundary definition.

4. Temporal variability: Some deep sea features, particularly hydrothermal vents, are geologically ephemeral, with active venting occurring on timescales of years to decades.

For guidance on remote sensing and spatial data sources relevant to deep sea mapping, see TG-4.1 Remote Sensing Data. The Seabed 2030 initiative aims to complete global bathymetric mapping by 2030, which would support improved ecosystem extent estimation.

Deep sea data priority assessment

Table 1 presents an indicative assessment of data priorities for deep sea accounting, considering both economic significance and current data availability. This assessment is intended to guide investment in data collection and research.

Table 1: Indicative data priorities for deep sea accounting

Extent account structure

For deep sea ecosystem types within national jurisdiction (EEZ and extended continental shelf), the extent account structure follows SEEA EA methodology as described in TG-3.1 Asset Accounts. For ABNJ, similar structures could be applied at regional or global scales.

Table 2: Structure of deep sea ecosystem extent account

For hydrothermal vents and other ephemeral features, "natural expansion" and "natural reduction" may be significant as geological processes create and extinguish active vent sites. Anthropogenic reductions may occur through seabed mining, bottom trawling (on seamounts), or other human activities. The accounting treatment of ephemeral ecosystems raises conceptual questions not explicitly addressed in the SEEA EA. When a vent ceases activity and the associated community dies, this should be recorded as "natural reduction" rather than catastrophic loss, since it reflects geological processes rather than human disturbance. Methodological guidance on geologically ephemeral ecosystems is an area for future development.

Condition variables

The SEEA EA framework for condition accounts can be adapted to deep sea contexts. Relevant condition variables include:

Physical state:

• Hydrostatic pressure (depth-dependent)
• Temperature (including thermal anomalies at vents)
• Substrate type and stability
• Sedimentation rates

Chemical state:

• Oxygen concentration (including oxygen minimum zones)
• pH and carbonate saturation (relevant for cold-water corals)
• Nutrient concentrations
• Pollutant concentrations (including from mining activities)

Compositional state:

• Species richness and endemism
• Community composition
• Presence/absence of indicator taxa
• Microbial diversity (particularly for CBEs)

Structural state:

• Biomass of megafauna
• Structural complexity (e.g., coral framework, vent chimneys)
• Bioturbation indicators (lebensspuren)

Functional state:

• Chemosynthetic productivity (for CBEs)
• Organic matter flux and remineralisation rates
• Carbon sequestration in sediments

For guidance on selecting condition variables and constructing condition indicators, see TG-2.1 Biophysical Indicators.

For deep sea ecosystems, establishing reference conditions is particularly challenging given the limited historical baseline data. The SEEA EA concept of ecosystem integrity--"the ecosystem's capacity to maintain its characteristic composition, structure, functioning and self-organization over time within a natural range of variability"^[40]--may be operationalised by reference to undisturbed areas, where available. Research on "natural laboratories"--areas protected from human impact--could inform reference condition development, and compilers should prioritise identification of such reference sites within their marine jurisdictions.

3.4 Compilation Procedure for Deep Sea Accounts

This section outlines the step-by-step procedure for compiling deep sea ecosystem accounts, from data identification through account entry. Due to the Emerging status of deep sea accounting, this procedure is aspirational, documenting the workflow that would be followed as data become available rather than prescribing immediate action.

Step 1: Identify accounting area and jurisdictional boundaries

Define the spatial scope of the accounting area, distinguishing:

• Deep sea areas within the EEZ (depths >250m)
• Extended continental shelf areas subject to CLCS claims
• Adjacent ABNJ areas (if compiling experimental accounts for Areas Beyond National Jurisdiction)

Document the jurisdictional status of each spatial unit. For EEZ deep sea areas, accounting boundaries follow standard national accounting principles. For extended continental shelf, verify the status of CLCS submissions and outer limits. For ABNJ experimental accounts, document the basis for aggregation (e.g., regional seas, biogeographic provinces, or ISA contract areas).

Step 2: Acquire bathymetric and geomorphology data

Obtain the best available bathymetric data for the accounting area. Priority sources include:

• National hydrographic surveys (where available)
• GEBCO (General Bathymetric Chart of the Oceans) global compilation
• Multibeam sonar surveys from research expeditions
• Satellite-derived bathymetry for depths <1000m

Use geomorphometric analysis to delineate ecosystem types following IUCN GET categories:

• Identify continental slopes (depth gradient analysis)
• Detect seamounts and ridges (topographic prominence algorithms)
• Delineate abyssal plains (slope <0.1 degrees, depth 3000-6000m)
• Map submarine canyons (convergence of bathymetric contours)

For guidance on geospatial data processing, see TG-4.3 Geospatial Data Integration.

Step 3: Compile ecosystem extent baseline

For each ecosystem type identified in Step 2, measure opening extent in km2. Record the measurement date and data source. Assign a data quality rating following TG-0.7 Quality Assurance protocols:

• High: Multibeam survey with ground-truthing
• Medium: Satellite-derived bathymetry or predictive modelling validated by samples
• Low: GEBCO compilation or unvalidated predictive models

Populate the opening stock row of the extent account (Table 2 structure).

Step 4: Identify and record extent changes

Where time-series data are available, identify changes in ecosystem extent. Sources of change data include:

• Repeat bathymetric surveys (rare for deep sea areas)
• ISA environmental monitoring data for contract areas
• ROV/submersible survey reports

Classify changes as:

• Natural expansion: New hydrothermal vent fields discovered and confirmed active
• Natural reduction: Documented cessation of vent activity, seamount colonisation retreat
• Anthropogenic reduction: Seabed disturbance from mining, trawling damage to seamounts, cable/pipeline impacts

Record changes in the extent account. For data-scarce environments, extent changes will commonly be zero or unobservable during typical accounting periods (1 year).

Step 5: Compile condition variable measurements

For ecosystem types where condition monitoring data exist (typically limited to ISA contract areas, protected areas, or research expedition sites), compile condition variables following the SEEA EA Ecosystem Condition Typology structure. Priority variables by ecosystem type:

Seamounts (M3.4):

• Physical: Substrate type, current speed
• Chemical: Dissolved oxygen at summit depth
• Compositional: Megafaunal taxa richness (from ROV transects)
• Structural: Cold-water coral cover (%), sponge density
• Functional: Not typically measured

Hydrothermal vents (M3.7):

• Physical: Vent fluid temperature, flow rate
• Chemical: H2S concentration, Fe/Mn concentrations
• Compositional: Indicator taxa presence (vent-endemic species)
• Structural: Chimney structure area, tubeworm aggregation density
• Functional: Chemosynthetic productivity (where measured)

Measure condition variables against reference values. For pristine deep sea ecosystems, the reference condition is the undisturbed state. Where baseline data are insufficient to establish reference values, expert judgment or regional comparisons may be used, noting the limitation.

Step 6: Calculate condition indices

Transform condition variables into standardised indicators following TG-2.1 Biophysical Indicators methodology. For each variable:

Indicator = (V - VL) / (VH - VL)

Where V is the observed value, VH is the reference (high) value, and VL is the degraded (low) value. For inverse variables (e.g., pollutant concentrations), invert the formula.

Aggregate indicators into composite condition indices using equal weighting or expert-weighted aggregation. Document the aggregation method.

Step 7: Populate extent and condition accounts

Enter compiled data into the account structures presented in Section 3.3. For extent accounts, ensure the closing stock = opening stock + additions - reductions. For condition accounts, present raw variable values alongside calculated indicators.

Document all data sources, measurement dates, quality ratings, and estimation methods in metadata. For deep sea accounts, comprehensive metadata are essential given the data scarcity and reliance on limited observations.

Step 8: Link accounts to policy frameworks

Cross-reference compiled accounts to decision use cases:

• For deep-sea mining assessment, highlight extent and condition for areas subject to ISA exploration contracts
• For BBNJ MPA designation, identify ecosystem types with high endemism or low condition scores as priority conservation areas
• For MEA reporting (via TG-2.10 MEA Indicators), extract protected area coverage and condition trends for SDG 14 and GBF indicators

This compilation procedure will evolve as deep sea data infrastructure improves. Compilers should treat this as a framework for structuring available data rather than a specification of minimum data requirements.

3.5 Ecosystem Services from the Deep Sea

Deep sea ecosystems provide services that, while often poorly quantified, may be substantial at global scales. These include both provisioning services with current economic value and regulating services with future or indirect value. This section outlines the conceptual framework for ecosystem service accounting in deep sea contexts, acknowledging that practical implementation remains distant for most services. The services identified here are based on scientific understanding of deep sea processes rather than established accounting practice, and compilers should treat this section as a foundation for future development rather than as a basis for immediate compilation.

Provisioning services

Genetic resources: Deep sea organisms, particularly from extreme environments such as hydrothermal vents, harbour unique genetic resources with potential applications in biotechnology and pharmaceuticals^[41]. The BBNJ Agreement establishes frameworks for access and benefit-sharing related to marine genetic resources from ABNJ, which may enable future monetary valuation of these resources. The CEPA classification includes "Scientific, cultural and educational services" (CEPA division 15) which may encompass genetic resources, though specific guidance for marine genetic resources is limited.

Fisheries resources: Some commercial fish stocks occur on or near deep sea features, particularly seamounts. The SEEA CF notes that for migratory stocks, attribution to countries occurs "during the period when those stocks inhabit its EEZ"^[42]. High seas fish stocks subject to regional fisheries management organisations may be attributed based on quota allocations. See TG-6.7 Fisheries Stock Assessment and TG-3.6 Fish and Fisheries Accounts for detailed guidance.

Mineral resources: The Area contains significant mineral deposits, including polymetallic nodules, cobalt-rich ferromanganese crusts, and seafloor massive sulphides^[43]. UNCLOS defines "polymetallic nodules" as "one of the resources of the Area consisting of any deposit or accretion of nodules, on or just below the surface of the deep seabed, which contain manganese, nickel, cobalt and copper"^[44]. The ISA has issued exploration contracts covering approximately 1.5 million square kilometres of seabed, though no commercial exploitation has yet occurred. See TG-3.10 Offshore Energy Accounts for methodological parallels.

Regulating services

Carbon sequestration: Abyssal sediments are a major long-term carbon sink. The biological carbon pump transports organic carbon from surface waters to the deep sea, where a fraction is buried in sediments and sequestered for geological timescales. Deep sea carbon sequestration contributes to global climate regulation but is not currently quantified in ecosystem service accounts. For carbon-related accounting approaches, see TG-3.7 Carbon Accounts.

Climate regulation: Deep water formation and thermohaline circulation influence global climate patterns. These ocean-atmosphere interactions are global in scale and not readily attributed to specific ecosystem assets.

Nutrient cycling: Deep sea ecosystems play critical roles in global biogeochemical cycles, including nitrogen, phosphorus, and silica cycling. Remineralisation of organic matter in the deep sea returns nutrients that eventually upwell to support surface productivity.

Challenges in service valuation

Monetary valuation of deep sea ecosystem services is at an early stage of development. The SEEA EA valuation chapters (8-11) are designated as "internationally recognized recommendations" rather than full statistical standard, reflecting outstanding methodological concerns; for deep sea ecosystems, these concerns are amplified by data scarcity and conceptual challenges. Key challenges include:

1. Spatial attribution: Many deep sea services operate at global scales (climate regulation, carbon sequestration) that cannot be attributed to discrete ecosystem assets. See TG-2.2 Macro Dependencies for approaches to macro-scale service flows.

2. Temporal scales: Carbon sequestration benefits may extend over millennia, challenging the discounting approaches used in NPV valuation. See TG-1.9 Valuation for discussion of discount rate selection.

3. Non-use values: Much of the value of deep sea ecosystems may be non-use value (existence, bequest, option values) not captured in market transactions.

4. Irreversibility: Deep sea ecosystems are slow to recover from disturbance, creating potential for irreversible losses that standard economic models may not adequately capture.

Monetary valuation of deep sea ecosystem services should accordingly be considered highly experimental. Compilers exploring such valuation should document their methods transparently and present results with appropriate caveats.

3.6 Accounting Challenges and Methodological Gaps

Deep sea and ABNJ accounting faces multiple challenges that reflect both the frontier status of the field and fundamental questions about how accounting frameworks should be extended to global commons. This section catalogues these challenges to inform future methodological development and to set appropriate expectations for compilers. As is appropriate for an Emerging circular, this section transparently acknowledges methodological gaps to support appropriate expectations and help prioritise research needs.

Attribution challenges

The country-based structure of national accounts assumes that assets can be assigned to specific countries. For ABNJ, this assumption breaks down:

1. The commons problem: ABNJ resources are not owned by any country. While activities in ABNJ can be attributed to countries based on flag state, sponsoring state, or quota allocation, the underlying assets remain collective.

2. Migratory species: Highly migratory species transit between EEZs and high seas areas, complicating stock attribution. Current conventions attribute stocks to countries "during the period when those stocks inhabit its EEZ"^[45], but this creates discontinuous records.

3. Transboundary flows: Ecosystem services from ABNJ (e.g., carbon sequestration, nutrient cycling) benefit all countries but cannot be attributed to specific beneficiaries.

Governance gaps

Current governance frameworks for ABNJ are fragmented across multiple international bodies:

• ISA: Mineral resources of the Area
• Regional fisheries management organisations (RFMOs): Specific fish stocks in specific regions
• International Maritime Organization (IMO): Shipping and pollution
• International Whaling Commission (IWC): Cetacean management
• BBNJ Conference of the Parties: Biodiversity conservation (as of 2026)

This fragmentation creates data gaps and inconsistencies that complicate comprehensive accounting. There is no single body responsible for integrated environmental-economic accounting for ABNJ.

Data scarcity

Data scarcity is the most fundamental challenge for deep sea accounting. Unlike other ocean accounting domains where methodology may exceed data availability, for the deep sea both methodology and data are severely limited. Compilers should not attempt to produce deep sea accounts without first addressing data availability. Key gaps include:

1. Baseline gaps: For most deep sea ecosystems, no baseline inventory of extent or condition exists.

2. Monitoring gaps: Ongoing monitoring of deep sea ecosystems is minimal, making it difficult to track changes over time.

3. Taxonomic gaps: Many deep sea species remain undescribed, limiting biodiversity assessment.

4. Economic data gaps: There is limited information on the economic activities occurring in ABNJ or their values.

Emerging opportunities

Despite these challenges, several developments may enable progress in deep sea and ABNJ accounting:

1. BBNJ implementation: The BBNJ Agreement's monitoring, reporting, and assessment requirements will generate new data on ABNJ ecosystems.

2. Seabed mapping initiatives: Programmes such as Seabed 2030 aim to map the entire ocean floor by 2030, providing improved baseline data for extent accounts.

3. Remote sensing advances: New satellite and autonomous vehicle technologies are expanding observation capabilities for the deep ocean. See TG-4.1 Remote Sensing Data.

4. ISA reporting requirements: Environmental impact assessments and monitoring under ISA contracts generate data on areas subject to mineral exploration.

5. Digital sequence information: Databases of genetic information from deep sea organisms may enable valuation of genetic resources.

The development of operational deep sea and ABNJ accounts will require concerted international effort to address these challenges. This Circular will be updated as methodology advances and as implementation experience from early-mover countries and international organisations becomes available. This Circular should be reviewed and updated on a 2-year cycle (next review: early 2028), with key triggers for interim updates including the adoption of monitoring frameworks at BBNJ COP, approval of a first commercial seabed mining contract by ISA, and publication of significant deep sea mapping advances.

3.7 Worked Example: Deep-Sea Mining Impact Assessment

This worked example demonstrates the compilation of deep sea ecosystem accounts for a hypothetical 200,000 km2 deep-sea area (depths exceeding 1,000 m) within a coastal State's extended continental shelf and adjacent ABNJ. The example follows the extent-condition-services-valuation sequence presented in Section 3 and illustrates the key accounting entries and calculations. Given the Emerging status of this Circular and the severe data limitations for deep sea environments, compilers should treat the methods and values below as highly illustrative. This example is intended to demonstrate the account structure and decision-support workflow rather than to provide realistic magnitudes.

Setting and decision context

Accounting area: A deep-sea accounting area of 200,000 km2 spanning the continental slope and adjacent abyssal plain, with bathymetry ranging from 1,000 to 4,500 m. The area includes 12 mapped seamounts (M3.4, total area 3,200 km2), 3 confirmed active hydrothermal vent fields (M3.7, total area 8 km2), and extensive abyssal plain sediments (M3.3). Of the total area, 140,000 km2 lies within the State's extended continental shelf claim (approved by CLCS), and 60,000 km2 lies in ABNJ (the Area). One seamount hosts an ISA exploration contract for polymetallic sulphides.

Decision context: The sponsoring State is evaluating whether to support progression of the ISA exploration contract to the exploitation phase. The decision requires assessment of: (1) the economic value of mineral resources; (2) the environmental cost of ecosystem degradation from mining; and (3) the opportunity cost of foregone ecosystem services. Ecosystem accounts provide the evidence base for this assessment.

Step 1: Extent account (year t to t+1)

Extent data compiled from multibeam bathymetry (seamounts, abyssal plains) and ROV surveys (hydrothermal vents). Extent changes reflect the discovery of a new vent field through research expeditions (natural expansion), cessation of one known vent (natural reduction confirmed by temperature monitoring), and seabed disturbance from mineral exploration test mining (anthropogenic reduction).

Table 3: Extent account for deep-sea accounting area

Interpretation: The 3 km2 abyssal plain reduction represents sediment disturbance from environmental baseline surveys and small-scale test mining conducted under the ISA exploration contract. The 0.5 km2 seamount reduction reflects localised disturbance from geological sampling on the seamount hosting the mineral deposit. These anthropogenic reductions are small relative to total extent but may be significant locally. The +1 km2 net change in vent extent reflects discovery of a larger new vent field (2 km2) offset by cessation of a smaller known vent (1 km2), consistent with the geological dynamism of hydrothermal systems.

Step 2: Condition account

Condition indicators derived from ROV surveys (structural variables), water column sampling (chemical variables), and environmental monitoring data associated with the ISA exploration contract. Reference conditions drawn from undisturbed control sites within the same depth range and biogeographic province.

Table 4: Condition account for deep-sea accounting area

Calculation notes: For benthic disturbance (inverse variable), the indicator is calculated as: (VL - V) / (VL - V H) = (15 - 0.8) / (15 - 0) = 0.95. For species discovery rate, the indicator is: (V - VL) / (VH - VL) = (12 - 3) / (18 - 3) = 0.60. Species discovery rate serves as a proxy for biodiversity intactness in data-sparse deep sea environments, with high rates indicating substantial undocumented diversity and ecosystem integrity.

Composite condition index: Simple average across four indicators: (0.95 + 0.60 + 0.74 + 0.64) / 4 = 0.73

Interpretation: The composite condition index of 0.73 indicates that the deep-sea area is in relatively good condition (73% of reference condition), reflecting the limited extent of human disturbance to date. However, the species discovery rate indicator (0.60) suggests that biodiversity may be less intact than other condition dimensions, potentially due to historical fishing impacts on seamounts or natural variability in deep-sea communities. The high benthic disturbance indicator (0.95) reflects that physical impacts remain localised. Compilers should note that even small reductions in condition may be effectively irreversible on human timescales due to the extremely slow recovery rates of deep-sea ecosystems.

Step 3: Ecosystem services (annual flows)

Ecosystem service flows estimated from scientific literature, ISA environmental baseline reports, and expert judgment. Values are highly uncertain and should be interpreted as illustrative orders of magnitude rather than precise estimates.

Table 5: Ecosystem service flows from deep-sea accounting area

Valuation notes:

• Genetic resources: Valuation is highly experimental, based on estimated research and licensing value of compounds isolated from deep-sea organisms during exploration activities. The BBNJ Agreement's benefit-sharing provisions may alter how these values are recorded and distributed in future accounts. Eight novel compounds represents cumulative discoveries across multiple research expeditions; the annual flow of new discoveries may be lower.

• Carbon sequestration: Estimated at 3.0 t CO2/km2/yr based on sediment trap studies in similar deep-sea environments. Total sequestration = 200,000 km2 × 3.0 t CO2/km2/yr = 600,000 t CO2/yr. Valued conservatively at USD 20/t CO2 (social cost of carbon); the deep-sea sediment carbon sink operates over geological timescales, and attribution to discrete ecosystem assets is methodologically uncertain.

• Mineral potential: Recorded qualitatively only, as no commercial exploitation has commenced. Geologic surveys estimate 8 million tonnes of polymetallic sulphide ore containing copper, zinc, gold, and silver, with gross in-situ value of approximately USD 3-4 billion at current commodity prices. However, the net present value to the State depends on extraction costs, ISA benefit-sharing arrangements, and discount rates, requiring detailed feasibility assessment beyond the scope of this account.

• Scientific value: Approximated by research expedition expenditure attributable to the area (vessel time, ROV operations, analytical costs). This represents use value from research activities rather than the intrinsic information value of the ecosystem.

Step 4: Asset valuation

Asset values calculated as the net present value of expected future ecosystem service flows, using a 4% real discount rate over a 25-year projection horizon (appropriate for long-lived natural assets under conservative management). Two scenarios presented to illustrate sensitivity to valuation assumptions.

Scenario A -- Conservative (carbon sequestration and scientific value only):

Annual service flow (excluding genetic resources) = USD 15,200,000 Present value annuity factor (4%, 25 years) = 15.62 Asset value = 15,200,000 × 15.62 = USD 237,400,000

Scenario B -- Including genetic resource potential:

Annual service flow (including genetic resources) = USD 17,600,000 Present value annuity factor (4%, 25 years) = 15.62 Asset value = 17,600,000 × 15.62 = USD 274,900,000

Sensitivity: The USD 37.5 million difference between scenarios (16% of total value) illustrates the sensitivity of deep-sea asset valuation to the inclusion of poorly quantified services. Genetic resource flows are highly uncertain both in physical terms (discovery rates may vary significantly) and monetary terms (commercial value of compounds depends on uncertain pharmaceutical development outcomes). Compilers should present such results with scenario analysis and clearly communicate uncertainty ranges to users.

Step 5: Decision support analysis

Mining impact projection: If the ISA exploration contract progresses to commercial exploitation, projected impacts over a 20-year mining operation include:

• Extent loss: 85 km2 of abyssal plain directly disturbed (0.043% of total area), plus 120 km2 indirect disturbance from sediment plume
• Condition decline: Composite condition index projected to decline from 0.73 to 0.52 in mined area, and to 0.68 in indirectly affected area
• Service flow reduction: Annual service losses of USD 620,000 (carbon sequestration in disturbed sediments) + USD 240,000 (lost scientific value from altered community composition) = USD 860,000/yr

Ecosystem degradation cost: The decline in condition from 0.73 to 0.52 in the 85 km2 directly disturbed area represents a 29% condition loss. Applying this to the proportional asset value:

Degradation = (85 km2 / 200,000 km2) × USD 237.4M × 0.29 = USD 100,800

This is the capital loss from mining impacts. Additionally, the annual service flow reduction of USD 860,000/yr valued in perpetuity (USD 860,000 / 0.04) represents a further capital loss of USD 21.5M. Total ecosystem degradation cost from mining is approximately USD 22.6 million (using the conservative valuation scenario).

Economic benefit from mining: Gross in-situ ore value of USD 3-4 billion minus extraction costs (estimated 70% of gross value for deep-sea mining) yields net value of USD 900M-1.2B. ISA benefit-sharing arrangements allocate 40% to the Area (distributed to developing countries) and 60% to the contractor State. State benefit = USD 540M-720M over 20-year operation.

Trade-off assessment: The ecosystem degradation cost (USD 22.6M) is small relative to the mining revenue (USD 540M-720M), suggesting mining is economically efficient under standard cost-benefit analysis. However, this comparison omits several factors:

1. Irreversibility: Deep-sea recovery timescales exceed 100 years; the degradation is effectively permanent.
2. Option value: Delaying mining preserves future options for genetic resource discovery or alternative uses.
3. Existence value: Non-use values (existence, bequest) are not captured in service flow valuation.
4. Risk: Sediment plume impacts may exceed projections; actual disturbance area could be larger.

The accounting framework transparently presents the trade-offs but does not prescribe the decision. Policy-makers must weigh economic benefits, environmental costs, and non-economic values in light of national sustainability commitments and international obligations under the BBNJ Agreement.

Step 6: Upward connections to policy frameworks

This deep-sea account supports multiple decision contexts:

TG-1.1 National Ocean Budgets: The USD 22.6M degradation cost informs natural capital budgeting. If mining proceeds, this cost should be deducted from gross mineral revenues when calculating environmentally adjusted net domestic product (NDP). The degradation cost represents consumption of natural capital that does not contribute to sustainable income.

TG-2.10 MEA Indicators: Extent accounts feed SDG 14.5.1 (MPA coverage) if the deep-sea area includes protected zones. The 0.8% benthic disturbance metric supports SDG 14.2.1 (ecosystem-based management) by tracking human pressures. Condition index trends inform BBNJ monitoring requirements for area-based management tools.

TG-1.3 Marine Spatial Management: The spatial distribution of high-condition areas (seamounts, vent fields) versus disturbed areas guides MPA designation and mining exclusion zones. The connectivity index (0.65) suggests moderate larval dispersal potential, informing network design for deep-sea MPAs.

TG-3.1 Asset Accounts: Mineral resources (USD 3-4B gross value) are recorded as non-renewable natural assets under the SEEA CF framework. The ecosystem asset value (USD 237M-275M) is recorded separately. Both are included in national balance sheets as components of ocean wealth, enabling comparison of natural capital stocks across asset classes.

This example demonstrates how deep-sea ecosystem accounts structure decision-relevant information for mining impact assessment. The account structure--extent, condition, services, valuation--provides a systematic framework for organizing diverse data sources and maintaining accounting identities that ensure internal consistency. The worked example can be adapted to other deep-sea decision contexts (BBNJ MPA designation, hydrothermal vent conservation, ISA environmental management) by adjusting the spatial scope, ecosystem focus, and decision criteria while maintaining the same accounting structure.

4. Acknowledgements

This Circular has been approved for public circulation and comment by the GOAP Technical Experts Group in accordance with the Circular Publication Procedure.

Authors: Jordan Gacutan (GOAP Secretariat), Mitchell Lyons (GOAP Secretariat)

Reviewers: [To be confirmed -- expert review should include deep sea science expertise, such as consultation with the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) or the Joint Programming Initiative on Healthy and Productive Seas and Oceans (JPI Oceans), and international law expertise, such as consultation with the IUCN Environmental Law Centre or relevant academic institutions]