Marine Litter and Plastics Accounting

1. Outcome

This Circular provides guidance on establishing stock-flow accounts for marine litter and plastics within the Ocean Accounts framework. Upon completing this Circular, readers will understand how to:

• categorise marine litter by material type and source
• compile flow accounts tracking litter from land-based and maritime sources to the marine environment
• develop stock accounts for accumulated litter in coastal zones, surface waters, the water column, and the seafloor
• attribute litter flows to economic sectors and geographic sources
• align marine litter accounts with SDG Target 14.1 indicators on marine pollution
• derive circular economy indicators linking litter accounts to plastics reduction targets under emerging international frameworks
• compile material flow accounts for synthetic materials from production through disposal to marine leakage

The accounts compiled using this guidance support derivation of indicators for environmental pressure assessment in TG-2.7 Pollution Flows, contribute to comprehensive assessment of residual flows from economy to environment in TG-3.4 Flows from Economy to Environment, inform circular economy indicators in TG-2.11 Resource Efficiency, and support reporting to the multilateral environmental agreements described in TG-2.10 MEA Indicators. Marine litter accounting also complements maritime transport activity accounts in TG-6.10 Maritime Transport where vessel-sourced litter is concerned.

2. Requirements

This Circular requires familiarity with:

• TG-0.1 General Introduction—foundational Ocean Accounts components and SIDS-specific considerations relevant to marine litter accounting in small island contexts.

• TG-3.4 Flows from Economy to Environment—residual flow accounting, solid waste accounts, and attribution of emissions and waste to economic sectors, which underpins the marine litter flow accounting approach in Section 3.3.

3. Guidance Material

Marine litter, defined as any persistent, manufactured or processed solid material discarded, disposed of, or abandoned in the marine and coastal environment, constitutes one of the most visible forms of marine pollution^[1]. Plastics are the dominant material category, accounting for approximately 80 percent of all marine debris by count^[2].

SDG Target 14.1 (preventing and reducing marine pollution) provides the primary policy anchor for this Circular^[3]. SDG indicator 14.1.1 specifically includes "floating plastic debris density" as a component of the index of coastal eutrophication and plastic debris density^[4]. An INC process to develop a legally binding instrument on plastic pollution (UNEA Resolution 5/14, March 2022) is ongoing; compilers should consult the UNEP INC website for current treaty status and emerging reporting obligations^[5][6].

Scope—microplastics. Microplastics (plastic particles less than 5mm in diameter, whether primary or secondary) are out of scope of this Circular. Detailed accounting for microplastic flows, stocks, and fragmentation is deferred pending further methodological development. This Circular addresses macroplastics (>25mm) and mesoplastics (5-25mm) only. Inline references to microplastics in the pathway descriptions are retained for completeness but are flagged as out-of-scope inputs that compilers should not attempt to quantify under this guidance. Countries developing experimental microplastic accounts should document methods separately and treat results as supplementary information.

The SEEA CF physical supply and use table structure for solid waste records waste generation by economic sector and tracks flows to collection, treatment, and disposal destinations^[7][8]. For marine litter accounting, this framework is extended to capture waste that escapes management systems and enters the marine environment, as well as litter generated directly by maritime activities. General solid waste accounting structure follows TG-3.4 Flows from Economy to Environment.

3.1 Decision Use Cases and Upward Connections

Plastics treaty reporting and compliance monitoring

Marine litter accounts link plastics material flow accounts (tracking production and use) to waste management accounts (tracking collection and disposal) to residual flow accounts (tracking marine leakage), eliminating redundant data collection across reporting obligations and ensuring consistency between national waste statistics and international environmental monitoring.

Circular economy metrics for plastics

Marine litter accounts quantify the proportion of plastic waste that escapes collection and treatment systems, providing the "leakage rate" indicator fundamental to circular economy assessment. SEEA CF paras 3.268-3.278 establish the applicable residual flow framework^[9]. These indicators feed directly into TG-2.11 Resource Efficiency.

Beach and ocean cleanup investment justification

Stock accounts by environmental compartment (beach, floating, benthic) provide baseline estimates of litter volumes requiring removal. Flow accounts by source category enable prioritisation of interventions targeting high-contribution pathways. Time-series accounts tracking stock changes before and after cleanup programmes provide the quantitative basis for return-on-investment assessment. This use case links to environmental protection expenditure accounts addressed in TG-3.7 Governance Accounts.

Upward connections to indicator frameworks

• SDG 14.1.1 (Marine pollution)—directly supplies floating plastic debris density component^[10]
• Pollution pressure indicators (TG-2.7)—provides physical flow data for litter generation intensity, mismanaged waste rates, and marine leakage indicators
• Circular economy indicators (TG-2.11)—enables calculation of recycling rates, waste utilisation rates, and decoupling factors
• MEA reporting (TG-2.10)—supports Kunming-Montreal Global Biodiversity Framework Target 2 and UNFCCC reporting

The combined presentations approach in TG-3.8 Combined Presentations provides methods for integrating marine litter account data with economic and ecosystem condition accounts.

3.2 Marine Litter Framework: Categories, Sources, and Pathways

Material categories

Plastics—the dominant component, requiring disaggregation by:

• Polymer type (polyethylene, polypropylene, polystyrene, PET, PVC, other polymers)
• Product category (packaging, single-use items, fishing gear, consumer goods, industrial materials)
• Size class (macroplastics >25mm, mesoplastics 5-25mm, microplastics <5mm)

Bioplastics and materials marketed as biodegradable warrant careful treatment. Many do not degrade under marine conditions and should be accounted equivalently to conventional plastics. For materials with certified marine biodegradability (per ISO 18830:2016 and ISO 19679:2016 or equivalent), compilers may apply a degradation rate in the stock account's outflow rows rather than treating the material as persistent. ISO 18830 and ISO 19679 certify aerobic biodegradation potential under controlled laboratory conditions and do not directly yield field rate constants. Compilers wishing to apply a degradation outflow should derive a first-order rate constant from field studies, document the temperature, UV, and turbulence assumptions applicable to the accounting area, and record the mass outflow in a dedicated "Certified biodegradation" row separate from the general "Degradation" row. Where certified degradation rates are unavailable, conservative practice treats all plastics as persistent.

Other materials—including glass and ceramics, metal, rubber, textiles, paper and cardboard, processed wood, and other manufactured materials.

The GESAMP Guidelines for the monitoring and assessment of plastic litter in the ocean provide a harmonised categorisation framework^[11]. The European Waste Catalogue (EWC-Stat) offers additional detail for waste source identification^[12].

Source categories

Land-based sources—estimated to contribute approximately 80 percent of marine litter globally (Jambeck et al. 2015), including: municipal solid waste; industrial and manufacturing residues; construction and demolition waste; agricultural plastics; sewage-related debris; coastal tourism and recreation waste; and riverine transport from inland areas.

Sea-based sources—including: fishing activities (abandoned, lost, or discarded fishing gear—ALDFG); shipping (cargo loss, operational waste); offshore energy operations; aquaculture (equipment, floats, feed bags); recreational boating; and maritime structures and port operations.

The relative contribution of different sources varies by location. SIDS may have higher proportionate contributions from sea-based sources; see SIDS-specific considerations in TG-0.1 General Introduction^[13].

Pathways to the marine environment

The SEEA CF notes that controlled landfills are within the economy, while uncontrolled disposal to the environment represents a residual flow^[16]. For marine litter accounting, waste reaching marine waters represents a flow to the environment recorded in residual flow accounts.

Figure 6.12.1: Marine litter stock-flow relationships (see also TG-3.4 Section 3.2)

The diagram below is provided in two equivalent forms: an interactive Observable rendering for the GOAP web portal, and a Mermaid flowchart fallback that renders correctly in static PDF and print exports.

3.3 Flow Accounting: Litter Inputs to the Marine Environment

Flow accounts record the quantity of litter entering the marine environment during an accounting period, measured in mass units (tonnes) or, for monitoring purposes, item counts.

Land-based source flows

Land-based litter flows require estimation of:

• Mismanaged waste—waste not formally collected or improperly disposed after collection
• Leakage rates—the proportion of mismanaged waste that reaches marine waters (dependent on proximity to coast, hydrological connectivity, and waste characteristics)

The methodology developed by Jambeck et al. (2015) estimates plastic waste input from land to ocean based on coastal population, waste generation rates, waste management infrastructure, and modelled leakage^[17]:

Plastic marine debris = Coastal population ×
                        Waste generation rate ×
                        Plastic content of waste ×
                        Inadequately managed waste share ×
                        Leakage factor to ocean

Lau et al. (2020) extended this framework for scenario analysis of waste management interventions and material substitution^[18]. Borrelle et al. (2020) projected plastic waste growth trajectories^[19].

For ocean accounting purposes, this estimation approach should be disaggregated by economic sector where possible, geographically referenced to identify priority coastal areas, and time-series consistent to track trends.

The SEEA CF physical supply and use table for solid waste provides the structural framework^[20]:

For marine-specific accounting, "waste to environment" is disaggregated to distinguish:

• Waste to marine and coastal waters
• Waste to inland waters (with subsequent marine pathway)
• Waste to land (coastal littering, beach deposits)

Maritime source flows

Fishing gear losses—ALDFG estimation uses fishing and aquaculture activity data from TG-6.7 and TG-6.8. Abandoned, lost, or discarded fishing gear is estimated at 5.7 percent of fishing nets, 8.6 percent of traps, and 29 percent of fishing lines lost globally each year^[21]. These global average rates may not reflect conditions in artisanal fisheries; compilers should apply gear-specific and fishery-specific loss rates where available from national or regional fisheries management organisations^[22]. Accounting requires data on fishing fleet composition, gear types, estimated loss rates, and conversion to mass units.

Vessel operational waste—MARPOL Annex V regulates discharge of garbage from ships^[23]. Accounting draws on port reception facility records, MARPOL incident reporting, and vessel surveys.

Offshore operations—Waste from offshore energy platforms, aquaculture installations, and maritime structures. Cross-reference to TG-6.9 Offshore Energy for offshore accounting context.

Cargo losses—Container losses and other cargo overboard incidents, reported under maritime incident databases.

Riverine inputs

Rivers transport substantial quantities of litter from inland areas to the marine environment, with a small number of rivers contributing a disproportionate share of global riverine plastic input^[24]. Accounting requires:

• Monitoring of litter concentrations in river outflows
• Estimation of total transport based on flow volumes
• Attribution to upstream catchment sources where possible

For national ocean accounts, riverine inputs represent the cumulative effect of upstream waste management and should be linked to inland waste generation and management accounts. Hydrological data used for water flow accounting may serve as a basis for estimating litter transport volumes where direct litter monitoring data are limited.

3.4 Stock Accounting: Marine Litter Accumulation

Stock accounts record the quantity of litter accumulated in the marine environment at the opening and closing of an accounting period. The following template provides a compilation framework.

Table 3.4.1: Marine litter physical stock-flow account (tonnes)

The "primary entry" notation indicates the compartment where each source type is first recorded; subsequent redistribution is captured through transfer rows. Inter-compartment transfers balance: total outflows as transfers (215 tonnes) equal total inflows from transfers (215 tonnes), maintaining mass conservation. Column arithmetic: for each compartment, opening stock + total inflows - total outflows = closing stock. The "import via ocean transport" row records transboundary litter flows arriving via ocean currents; receiving-coastline countries (e.g., Pacific island states in North Pacific Gyre currents) should populate this row to distinguish externally-sourced accumulation from domestic inputs (see Section 3.5).

Note on terminal sinks: The seabed compartment is treated as a terminal sink; no resuspension or redistribution from seabed to other compartments is recorded. Where monitoring data indicate significant resuspension (e.g., in high-energy shallow-water environments), compilers should add a transfer outflow row for the seabed compartment and a corresponding inflow row for the water column.

Beach and coastal stocks

Beach litter is the most accessible stock for monitoring, with established survey methodologies^[25]. Stock accounts record mass or item counts per unit area (beach length, surface area), aggregated to total coastal stock and disaggregated by material category.

Beach stock accounts recognise that beaches are dynamic interfaces:

• New litter deposits during each tidal cycle and storm event
• Removal occurs through clean-up activities, burial, and resuspension
• Opening and closing stocks relate through: Closing stock = Opening stock + Deposits - Removals - Degradation

Clean-up activities should be recorded as economic flows (environmental protection expenditure under CEPA 3) and their effect on beach stocks quantified^[26]. This links to environmental expenditure accounts in TG-3.7 Governance Accounts.

Floating litter stocks

Surface and near-surface floating litter is measured through visual surveys, net tows, or remote sensing. SDG indicator 14.1.1 specifies floating plastic debris density as a monitoring parameter^[27]. Stock accounts for floating litter face specific challenges:

• Highly dynamic distribution influenced by currents, winds, and convergence zones
• Variable vertical distribution (surface to depth)
• Difficulty in scaling point observations to total stock estimates

Global accumulation zones (ocean gyres) represent areas of elevated floating debris concentration. Stock accounts may record density estimates (items or mass per km2) for defined ocean areas, total estimated stock based on density and area, and uncertainty ranges. Litter density may also serve as an ecosystem condition indicator; the relationship between accumulation and ecosystem condition is addressed in TG-2.1 Biophysical Indicators.

Water column stocks

Litter suspended in the water column is less well characterised than surface or benthic stocks. Accounting approaches are necessarily more experimental and may rely on depth-stratified sampling data, modelled vertical distribution based on particle characteristics, or extrapolation from limited survey data.

Seafloor stocks

Benthic litter accumulates on the seabed, particularly in areas of high sedimentation, submarine canyons, and deep-sea plains. Monitoring uses trawl surveys, remotely operated vehicle (ROV) transects, and baited camera systems. Stock accounts should record density by seabed type and depth zone, estimated total stock within national waters, and composition by material category. The SEEA EA addresses ecosystem condition, including pollutant presence as a condition variable^[28].

3.5 Source Attribution and Material Flow Accounts

Effective marine litter policy requires attribution of litter to the economic units and locations responsible for its generation. The SEEA CF principle of recording residual flows by source industry applies^[29].

Industry source attribution

Industry sources of marine litter, classified by ISIC:

Where direct industry-level data on marine litter contributions are unavailable, compilers may use waste composition data from beach litter surveys to attribute litter to source industries through product-to-industry concordances. For example, packaging items in beach surveys can be mapped to the manufacturing industry that produced them (ISIC 10-12 for food packaging) and to the retail or hospitality industry that distributed them (ISIC 47, 55-56). Detailed guidance on industry classification is in TG-3.3 Economic Activity.

Household source attribution

Households are major sources of marine litter through consumer product disposal, littering, inadequate waste storage, and microplastic releases (laundry, personal care products). Household marine litter is recorded in the household sector column of the physical supply table.

Geographic allocation

Coastal zone delineation—Coastal zone definitions should align with administrative boundaries while recognising functional connectivity to marine waters^[30]. See TG-1.2 Marine Spatial Planning.

Catchment-based accounting—River catchments as accounting units enable tracking of riverine litter pathways.

Maritime zone accounting—Recording of sea-based source litter by location (territorial sea, EEZ, high seas) supports jurisdictional analysis.

Transboundary litter flows. Litter originating in foreign jurisdictions and transported by ocean currents should be recorded separately in physical flow accounts using the "import via ocean transport" row in Table 3.4.1. Failure to do so will distort domestic source attribution. The three principal estimation approaches are: (a) ocean debris transport modelling (using circulation models to back-calculate probable source regions); (b) source-marker or polymer-signature analysis (comparing polymer types and additive profiles to known source regions); and (c) branded-product origin analysis from beach surveys. Each approach carries substantial uncertainty; transboundary import estimates should always be reported with a sensitivity range and a method tag. Attribution of transboundary litter to source countries requires international coordination through regional seas conventions (e.g., SPREP for the Pacific)^[31]. For SDG 14.1.1 reporting, domestic source attribution takes precedence; transboundary flows should be reported as supplementary information.

The SEEA CF supports sub-national disaggregation where data permit^[32]. Priority geographic units include coastal municipalities, river catchments with marine outlets, port areas and shipping lanes, and marine protected areas.

3.6 Material Flow Account for Synthetic Marine Litter: Worked Example

This section presents a worked example tracking plastics from production through consumption, waste generation, collection and treatment, to marine leakage.

Compilation procedure overview

1. Identify synthetic material production and imports—from industrial production statistics and trade data
2. Estimate synthetic material consumption—using supply-use balancing (domestic production + imports - exports)
3. Record waste generation by sector—from waste surveys and composition studies
4. Estimate waste collection and treatment—from municipal and industrial waste management records
5. Model mismanaged waste—as the difference between waste generated and waste formally collected
6. Estimate marine leakage—applying leakage factors to mismanaged waste based on proximity to marine environment

Step 1: Synthetic material production and imports

From industrial production statistics (ISIC 20, 22) and customs trade data (HS chapters 39, 54, 55):

Step 2: Synthetic material consumption by sector

Step 3: Waste generation

Material balance closure. The 46,200-tonne difference between consumption (151,000 tonnes) and waste generation (104,800 tonnes) represents the net addition to the stock of plastic products in use (durable goods with remaining service life), together with minor losses from degradation during the use phase. The SEEA CF material balance principle requires that all material inputs be accounted for in outputs, stock changes, or dissipative losses. This in-use stock change should be recorded in a separate physical balance sheet:

Opening stock of plastic products in use
  + Consumption
  - Waste generation
  - Use-phase dissipative losses
  = Closing stock of plastic products in use

The recycling rate is calculated against waste generated, not total consumption: 18,500 / 104,800 = 17.7% (see Step 4). Compilers should clearly state the denominator when reporting recycling rates to ensure comparability with international reporting.

Step 4: Waste collection and treatment

Step 5: Mismanaged waste estimation

Step 6: Marine leakage estimation

Apply Jambeck et al. (2015) leakage factors as defined in Section 3.3. The values below are illustrative central estimates; compilers should calibrate factors to national monitoring data or published country-level estimates where available.

Note on the maritime operational waste factor. MARPOL Annex V requires all ships to deliver garbage to port reception facilities (PRFs) before discharge; compliance is monitored through Garbage Record Books and Port State Control (PSC) inspections. The 0.60 central value is an upper-bound assumption that does not reflect MARPOL Annex V compliance rates and should be replaced with a country-specific estimate based on PRF utilisation data (IMO GISIS database) and PSC inspection statistics. Where empirical national data are unavailable, compilers should report a sensitivity range rather than a single point estimate and flag the assumption as requiring expert peer review.

Compilers should: cite the source for each leakage factor used; report a low--high sensitivity range alongside the central estimate; calibrate factors to national monitoring data where these exist; and document any departures from published source-methodology values.

Interpretation

• Collection gap: 24.6% of plastic waste is mismanaged, representing the primary intervention target
• Coastal concentration: 57% of mismanaged waste is in coastal zones where leakage probability is highest (25% vs 8% for inland)
• Sector priorities: Fishing, retail/hospitality, and households together account for 62% of total plastic waste generation
• Marine leakage rate: 6,050 / 104,800 = 5.8% of plastic waste generated ultimately enters marine environment

Policy implications:

• Expanding waste collection coverage in coastal zones from 75% to 85% would reduce coastal mismanaged waste and marine leakage from this source by an estimated 1,480 tonnes annually
• Extended producer responsibility for packaging (ISIC 10-11, 47) could address approximately 48,000 tonnes of plastic waste at source
• Fishing gear deposit-return schemes could recover a portion of the 1,200 tonnes of gear losses

Quality assurance for this worked example is addressed in TG-0.7 Quality Assurance.

3.7 SDG 14.1 Alignment and Multi-Target Linkages

SDG Target 14.1 states: "By 2025, prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution"^[33]. SDG indicator 14.1.1 comprises an index of coastal eutrophication and floating plastic debris density^[34].

Indicator components

Floating plastic debris density is measured as items per square kilometre of sea surface or mass (grams or kilograms) per square kilometre, using beach litter surveys, visual surveys, net-based sampling (manta trawls, neuston nets), or remote sensing approaches.

Index of coastal eutrophication (ICEP) addresses nutrient pollution separately and is not the focus of this Circular; see TG-3.4 Flows from Economy to Environment for nutrient accounting.

Composite index construction. SDG 14.1.1 is currently reported as two parallel sub-indicators rather than a single numerical composite. Countries should report both sub-indicators with consistent temporal and spatial reference units, following the UNSD SDG 14.1.1 metadata document. Compilers should coordinate their marine litter and nutrient pollution accounting programmes to ensure temporal and spatial consistency between the two sub-indicator datasets. The UNSD SDG 14.1.1 metadata should be consulted directly before publication, as the composite methodology continues to be refined.

Multi-target linkages

• SDG 12.4 (Environmentally sound management of wastes)—Marine litter flow accounts quantify waste escaping management systems.
• SDG 12.5 (Reduce waste generation)—Time-series accounts track progress in material circularity as reflected in declining leakage rates.
• SDG 6.3 (Improve water quality)—Riverine litter transport estimates link land-based pollution to downstream marine impacts.

The multi-indicator framework for deriving ocean policy indicators is addressed in TG-2.7 Pollution Flows.

Indicator derivation from accounts

Guidance on indicator derivation and interpretation is in TG-2.7 Pollution Flows.

4. Data Sources and Compilation

Compilation of marine litter and plastics accounts draws on multiple data sources. General guidance on data collection methods is in TG-4.1, TG-4.8, and TG-4.9; administrative data sources are addressed in TG-4.3 Administrative Data.

Key data sources include:

Waste management statistics—National waste statistics and composition surveys; municipal solid waste data; recycling and recovery rates by material type; industrial waste reporting.

Environmental monitoring programmes—Beach litter surveys (national programmes, OSPAR, HELCOM, regional seas conventions); marine litter monitoring; microplastics monitoring; citizen science and clean-up organisation data.

Maritime and fisheries data—Port reception facility records (MARPOL Annex V); fishing gear inventories and loss reporting; maritime incident databases; vessel tracking data (AIS).

Economic statistics—Industry production data for plastic-intensive sectors; trade data for plastic products; tourism statistics.

Modelling and estimation—Waste leakage models (Jambeck methodology and derivatives); river transport models; ocean circulation and debris transport models; fragmentation and degradation models.

International reporting—UNEP marine litter databases; regional seas convention assessments; Global Partnership on Marine Litter data.

Compilation pathways and phased implementation

Phase 1: Foundation—Beach litter survey data are the most widely available starting point. Countries with established beach monitoring programmes (OSPAR, HELCOM, or national programmes) can compile initial stock accounts for coastal compartments and use survey composition data to estimate source attribution.

Phase 2: Flow estimation—Building on waste management statistics, compilers estimate land-based litter flows using the Jambeck methodology. Maritime source flows are estimated from fisheries and port data using the physical supply and use table framework (Section 3.3).

Phase 3: Comprehensive accounts—Integration of floating and benthic stock data and refined geographic disaggregation produces comprehensive stock-flow accounts, requiring investment in monitoring infrastructure and modelling capacity.

Data gap assessment and prioritisation guidance is in TG-4.6 Data Harmonisation. Quality assurance considerations are addressed in TG-0.7 Quality Assurance.

5. 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: [To be confirmed]

Reviewers: [To be confirmed]

6. References and Further Reading

This Circular should be read in conjunction with:

• SEEA Central Framework, Chapter III: Physical flow accounts, particularly section 3.6.5 on solid waste accounts^[35]
• GESAMP (2019). Guidelines for the monitoring and assessment of plastic litter in the ocean
• UNEP (2021). From Pollution to Solution: A Global Assessment of Marine Litter and Plastic Pollution
• SDG Framework, Target 14.1 and Indicator 14.1.1 methodology
• Regional seas convention marine litter monitoring protocols (OSPAR, HELCOM, Barcelona Convention, NOWPAP)
• TNFD disclosure recommendations for waste and pollution metrics^[36]
• Lau, W.W.Y. et al. (2020). Evaluating scenarios toward zero plastic pollution. Science
• Borrelle, S.B. et al. (2020). Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science
• SPREP (2018). Pacific Regional Action Plan: Marine Litter 2018-2025. Secretariat of the Pacific Regional Environment Programme.
• ISO 18830:2016 and ISO 19679:2016, Marine biodegradability test methods for plastic materials.