Mangrove and Coastal Wetland Accounting

1. Outcome

This Circular provides comprehensive guidance on compiling ecosystem accounts for mangroves and coastal wetlands within an ocean accounting framework. Mangroves and coastal wetlands are among the most valuable ecosystems on Earth, providing critical blue carbon storage, coastal protection against storms and erosion, and essential nursery habitat for commercially important fish species^[1]. These ecosystems support multiple high-priority policy decisions that require robust accounting foundations: blue carbon credit verification for voluntary and compliance carbon markets requires independently verifiable measurements of carbon stocks and sequestration rates; coastal protection valuation for infrastructure planning requires quantification of avoided damages from wave attenuation and storm surge reduction; mangrove restoration prioritisation for climate adaptation and biodiversity conservation requires spatial mapping of degraded areas and expected recovery pathways; and REDD+ coastal extension for nationally determined contributions under the Paris Agreement requires systematic tracking of coastal wetland extent changes and associated emission impacts. By compiling the accounts described in this Circular, countries can provide decision-makers with the evidence needed for these applications while ensuring methodological consistency with internationally recognized statistical standards.

The accounting methodology presented here integrates SEEA Ecosystem Accounting principles with specific measurement approaches for these transitional marine-terrestrial ecosystem types. By following this guidance, practitioners will be able to compile extent accounts tracking mangrove and wetland area changes, condition accounts assessing ecosystem health through indicators such as canopy cover and hydrological connectivity, and ecosystem service accounts quantifying blue carbon sequestration, coastal protection, and nursery habitat services. The valuation methods presented enable monetary assessment of these ecosystem services using carbon pricing, avoided damage approaches, and productivity change methods. This Circular supports national ocean accounts, climate change mitigation reporting, and evidence-based coastal zone management.

This Circular connects upward to several policy and indicator circulars. TG-2.8 Climate Change Indicators draws on the blue carbon sequestration rates and carbon stock measures compiled here for SDG 13 (Climate Action) and UNFCCC reporting. TG-2.9 Disaster Risk Indicators uses the coastal protection service measurements from Section 3.4 to assess coastal vulnerability and the protective value of natural infrastructure. TG-1.8 OA and Project-Level Finance demonstrates how the carbon accounts and ecosystem service valuations compiled under this Circular support blue bonds, debt-for-nature swaps, and payments for ecosystem services mechanisms. These linkages ensure that the accounts compiled following this Circular feed directly into the policy and finance applications for which they are intended.

This Circular is one of three thematic ecosystem circulars--alongside TG-6.1 Coral Reef Accounting and TG-6.3 Seagrass Ecosystem Accounting--that apply the methodological foundations from TG-3.1 Asset Accounts and TG-4.1 Ecosystem Extent to specific coastal and marine ecosystem types. The three circulars share a consistent structure covering extent, condition, ecosystem services, and valuation, enabling cross-ecosystem comparison and aggregation within national ocean accounts. Together they provide the ecosystem-level detail that feeds into TG-6.5 Integrated Coastal Zone Accounts and TG-6.6 Marine Spatial Planning Accounts, which synthesize ecosystem information for spatial planning and policy applications.

2. Requirements

This Circular requires familiarity with:

• TG-0.1 General Introduction to Ocean Accounts -- provides foundational understanding of Ocean Accounts components and the relationship between environmental and economic accounting frameworks, establishing the conceptual context within which mangrove and coastal wetland accounts are compiled.

• TG-3.1 Asset Accounts -- for the foundational methodology on physical and monetary asset accounts, including ecosystem assets. Section 3.4 of TG-3.1 on ecosystem asset accounts is directly applicable to recording mangrove and wetland assets in the balance sheet and tracking changes through the accounting period.

• TG-1.9 Valuation -- for the general framework of valuation methods applicable to ecosystem services. This Circular applies the valuation principles from TG-1.9 to the specific context of blue carbon, coastal protection, and nursery habitat services; readers should consult TG-1.9 for foundational valuation concepts rather than expecting their full repetition here.

• TG-4.1 Ecosystem Extent -- for the general approach to ecosystem extent accounting, including spatial data requirements, classification systems, and change detection methods. Section 3.1 of this Circular applies TG-4.1 principles to the specific measurement challenges of intertidal ecosystems.

3. Guidance Material

Mangroves and coastal wetlands occupy the transitional zone between terrestrial and marine environments, forming distinctive ecosystem types that combine characteristics of both realms^[2]. These ecosystems are classified within the Marine-Freshwater-Terrestrial (MFT) transitional realm in the IUCN Global Ecosystem Typology (GET), specifically under the MFT1 Brackish Tidal Systems biome^[3]. Key ecosystem functional groups include:

• MFT1.2 Intertidal forests and shrublands (mangroves) -- characterized by salt-tolerant trees and shrubs with specialized adaptations including pneumatophores, salt excretion glands, and vivipary
• MFT1.3 Coastal saltmarshes and reedbeds -- dominated by salt-tolerant herbaceous plants and grasses in the upper intertidal zone
• MFT1.1 Coastal river deltas -- complex mosaics incorporating mangroves, saltmarshes, and other transitional ecosystem types

While the IUCN GET reference classification provides the international standard for comparability (consistent with SEEA EA Appendix A3.2), national classifications may differ substantially. Countries should map their national ecosystem classifications to the IUCN GET framework to enable international comparison, documenting the correspondence between national types and GET ecosystem functional groups. Where a one-to-one correspondence does not exist--for example, where a national system distinguishes multiple mangrove types based on dominant species or structural classes--the national types should be aggregated to the GET level for reporting purposes while retaining the finer national resolution for domestic use^[4].

This section provides guidance on compiling accounts for ecosystem extent (Section 3.1), ecosystem condition (Section 3.2), blue carbon services (Section 3.3), coastal protection services (Section 3.4), nursery habitat services (Section 3.5), and the valuation methods applicable to these services (Section 3.6). Section 3.7 presents a step-by-step compilation procedure, and Section 3.8 provides a worked example with synthetic data demonstrating the full accounting sequence. The methodology builds on the asset accounting framework presented in TG-3.1 Asset Accounts and applies the ecosystem extent principles from TG-4.1 Ecosystem Extent to the specific context of coastal wetland ecosystems.

3.1 Extent Accounting

Ecosystem extent accounts for mangroves and coastal wetlands record the area of these ecosystem types within an ecosystem accounting area (EAA), typically measured in hectares or square kilometres^[5]. Extent accounts provide the spatial foundation for all subsequent condition and service accounts, making accurate mapping and change detection essential.

Ecosystem type classification

Following the SEEA EA ecosystem type reference classification based on IUCN GET, mangroves and coastal wetlands should be classified at the ecosystem functional group (EFG) level or finer national classifications^[6]. The SEEA EA states that "compilers may choose to use an EAA of smaller geographical scope, by focusing, for example, on the terrestrial or marine realm or on a subnational region"^[7]. For ocean accounting purposes, the recommended classification includes:

National classifications may disaggregate these further based on dominant species (e.g., Rhizophora-dominated vs. Avicennia-dominated mangroves) or structural characteristics (tall, medium, dwarf mangroves)^[8]. For countries in temperate regions, the relevant coastal wetland types may differ from tropical mangrove-dominated systems. Temperate coastal wetlands often include seagrass-saltmarsh mosaics, tidal freshwater marshes, and brackish reed beds that do not correspond neatly to tropical ecosystem functional groups. Compilers in these regions should identify the most appropriate GET functional groups for their national context and document any classification decisions transparently.

Remote sensing methods

Remote sensing provides the primary data source for mangrove and coastal wetland extent mapping, offering consistent, repeatable measurements across large areas^[9]. The SEEA EEA Technical Recommendations note that "when compiling the ecosystem extent account it is necessary to consider whether an existing land cover and/or land use dataset will be used or whether a new dataset with new ETs will be developed"^[10]. The remote sensing approach used here should be consistent with the methods described in TG-4.1 Ecosystem Extent, adapted for the specific spectral and structural characteristics of intertidal vegetation.

Optical satellite imagery from Landsat, Sentinel-2, and similar platforms enables classification of mangroves and wetlands based on spectral reflectance characteristics. Key spectral indices for mangrove mapping include:

• Normalized Difference Vegetation Index (NDVI) -- distinguishes vegetated from non-vegetated areas
• Normalized Difference Water Index (NDWI) -- identifies water content and intertidal zones
• Soil-Adjusted Vegetation Index (SAVI) -- reduces soil background effects in sparse vegetation

Synthetic Aperture Radar (SAR) penetrates cloud cover and provides structural information complementary to optical data^[11]. SAR is particularly valuable in tropical regions where persistent cloud cover limits optical imagery availability. C-band SAR (Sentinel-1) and L-band SAR (ALOS PALSAR) have demonstrated effectiveness for mangrove mapping.

Global datasets provide baseline information for national accounts. Global Mangrove Watch provides annual mangrove extent maps from 1996 onwards at 25-metre resolution^[12]. The Global Tidal Wetland Change Dataset tracks saltmarsh, mangrove, and tidal flat extent changes globally^[13].

Change detection

The ecosystem extent account records changes between opening and closing periods, requiring robust change detection methods^[14]. Following SEEA EA guidance, changes should be classified as:

• Managed expansions -- increases due to restoration, afforestation, or coastal management
• Unmanaged expansions -- natural colonization and succession
• Managed reductions -- conversion for aquaculture, agriculture, or urban development
• Unmanaged reductions -- natural losses from erosion, sea-level rise, or catastrophic events

The SEEA EA emphasizes that "the effects of extreme events, for example, bushfires or hurricanes, where there may be considerable loss of vegetation, soil or other ecosystem components, need not imply a change of ecosystem type"^[15]. For coastal wetlands, temporary losses from storm damage followed by regeneration should be treated as condition changes rather than extent changes.

Extent account structure

The ecosystem extent account for mangroves and coastal wetlands follows the standard SEEA EA structure^[16]. The table below is presented as a compilation template; compilers should populate it using remote sensing change detection outputs, matching each detected change to the appropriate addition or reduction category. For a worked example demonstrating how to move from classified satellite imagery to a completed extent account, see TG-4.1 Ecosystem Extent. The ecosystem asset recording methodology in TG-3.1 Asset Accounts Section 3.4 governs how extent account entries flow into the broader asset balance sheet.

3.2 Condition Assessment

Ecosystem condition accounts record the quality of mangroves and coastal wetlands, measured through variables and indicators reflecting ecosystem composition, structure, and function^[17]. The SEEA EA defines ecosystem condition as "the quality of an ecosystem measured in terms of its abiotic and biotic characteristics"^[18].

Ecosystem condition typology application

Following the SEEA Ecosystem Condition Typology (ECT), condition variables for mangroves and coastal wetlands should be organized into six classes^[19]. The variables below are mangrove- and wetland-specific applications of the generic ECT classes defined in SEEA EA Table 5.1. Some variables (marked with RS) can be estimated from remote sensing, while others (marked with FS) require field sampling. This distinction is important for compilation planning, as remote-sensed variables can typically be compiled at lower cost and higher spatial coverage, while field-sampled variables provide greater accuracy for site-level assessment.

Physical state characteristics (Class A1) include:

• Tidal inundation regime -- frequency and duration of tidal flooding (FS)
• Sediment accretion/erosion rate -- net change in surface elevation (FS)
• Groundwater level -- depth to water table affecting root zone conditions (FS)

Chemical state characteristics (Class A2) include:

• Water salinity -- porewater and surface water salt concentration (FS)
• Nutrient concentrations -- nitrogen and phosphorus levels affecting productivity (FS)
• Dissolved oxygen -- particularly in waterlogged sediments (FS)

Compositional state characteristics (Class B1) include:

• Species richness -- number of mangrove or saltmarsh species present (FS)
• Community composition -- relative abundance of different species (FS)
• Presence of key species -- indicator or foundation species occurrence (FS)

Structural state characteristics (Class B2) include:

• Canopy cover -- percentage of area covered by tree/shrub canopy (RS)
• Canopy height -- average and maximum vegetation height (RS via LiDAR)
• Above-ground biomass -- standing crop of vegetation (RS via SAR/allometry)
• Tree density -- stems per hectare for forested wetlands (FS)

Functional state characteristics (Class B3) include:

• Net Primary Productivity (NPP) -- rate of biomass accumulation (RS via MODIS/Sentinel)
• Litterfall rate -- organic matter input to sediments (FS)
• Regeneration success -- seedling establishment and survival (FS)

Landscape and seascape characteristics (Class C1) include:

• Hydrological connectivity -- connection to tidal and freshwater flows (RS combined with FS)
• Fragmentation -- patch size distribution and isolation (RS)
• Edge effects -- proportion of edge habitat to core habitat (RS)

Key condition indicators

Based on the ECT framework, the following indicators are recommended for mangrove and coastal wetland condition assessment^[20]:

Canopy cover is a primary structural indicator readily measured through remote sensing. The SEEA EA notes that "structural state characteristics include characteristics focused primarily on the vegetation and biomass of ecosystems that reflect the amount of local living and dead plant matter"^[21]. For mangroves, canopy cover provides information on forest density and intactness, with reductions indicating degradation from selective logging, dieback, or storm damage.

Species composition reflects ecological integrity and functional capacity. The SEEA EA emphasizes that "compositional state characteristics include a broad range of 'typical' biodiversity characteristics that describe the composition of ecological communities from a biotic perspective"^[22]. For mangroves, the presence of climax species (e.g., Rhizophora in many tropical settings) indicates mature, stable ecosystems.

Hydrological connectivity is critical for coastal wetland function, affecting sediment supply, nutrient exchange, and biotic recruitment^[23]. Disruption of tidal flows through road construction, aquaculture bunds, or drainage infrastructure is a major cause of wetland degradation. Connectivity can be assessed through:

• Tidal range comparison (observed vs. reference)
• Distance to tidal inlet
• Presence of barriers to water flow

Sediment elevation change indicates whether wetlands are maintaining pace with sea-level rise^[24]. Wetlands experiencing elevation deficits (accretion rate less than relative sea-level rise) face drowning risk. Surface Elevation Tables (SETs) provide direct measurements, while remote sensing techniques can estimate broader-scale elevation trends. Given the accelerating pace of sea-level rise, sediment elevation change should be treated as a core indicator for all coastal wetland accounts in countries where relative sea-level rise exceeds 2mm per year, and as an optional but recommended indicator in other contexts. This indicator directly supports climate change adaptation planning by identifying wetlands at risk of submergence.

Reference conditions

Deriving condition indicators from raw variable measurements requires reference conditions against which to assess current state, as specified in SEEA EA Section 5.3^[25]. For mangroves and coastal wetlands, three approaches to establishing reference conditions are available:

• Historical baselines -- using pre-disturbance condition documented through historical records, early remote sensing imagery, or sediment core analysis to establish what the ecosystem looked like before significant human modification
• Minimally disturbed reference sites -- identifying protected or relatively undisturbed examples of the same ecosystem type within the national territory or ecoregion as contemporary benchmarks
• Expert-defined targets -- establishing reference levels through expert assessment where historical data and reference sites are unavailable, using published literature on optimal ecosystem characteristics for the relevant ecosystem type

Compilers should document which reference approach is used for each indicator, as the choice of reference condition affects the resulting condition index values and their interpretation.

Condition account structure

The condition indicator account compiles selected indicators by ecosystem type^[26]:

3.3 Blue Carbon Services

Blue carbon refers to carbon captured and stored by coastal and marine ecosystems, particularly mangroves, saltmarshes, and seagrasses^[27]. These ecosystems are among the most carbon-dense on Earth, storing carbon in both living biomass and accumulated sediments over millennia.

Carbon stock accounting

Mangroves and coastal wetlands store carbon in multiple pools requiring separate accounting^[28]. The following table summarizes the principal carbon pools, their measurement methods, typical ranges, and accounting treatment:

Above-ground biomass carbon includes all living vegetation above the soil surface. For mangroves, this comprises tree trunks, branches, leaves, and pneumatophores. Carbon content is typically estimated at 45-50% of dry biomass weight^[29].

Below-ground biomass carbon includes root systems. For mangroves with extensive stilt root and pneumatophore systems, below-ground biomass may equal or exceed above-ground biomass. Root:shoot ratios of 0.5-1.5 are commonly applied^[30].

Sediment carbon represents the largest carbon pool in most coastal wetlands, with organic-rich sediments accumulating over centuries to millennia. The standard accounting depth is 1 metre, consistent with IPCC 2013 Wetlands Supplement methodology^[31]. While deeper deposits exist and may be significant for total carbon stock estimation, the 1-metre standard provides comparability across sites and countries. Where comprehensive assessments are desired--for example, in high-value conservation or carbon market contexts--compilers may additionally report deeper sediment carbon stocks (e.g., to 2-3 metres) as a supplementary item, clearly distinguishing these from the standardized 1-metre figures. For consistency with climate-related accounting approaches used in other ocean accounting circulars, see TG-2.8 Climate Change Indicators.

The SEEA EA notes that "for the measurement of carbon retention service, the scope is limited to biocarbon in ecosystems (excluding geo-carbon stored in subsoil assets such as oil and gas) and restricted to what the Intergovernmental Panel on Climate Change calls long-lived biomass"^[32].

Carbon sequestration measurement

Carbon sequestration represents the ongoing removal of CO2 from the atmosphere, distinct from carbon storage^[33]. The SEEA Valuation publication states that "the carbon sequestration component is measured by the net ecosystem carbon balance, which takes all changes in carbon stocks (e.g. respiration, timber harvest, forest fires) into account"^[34].

For mangroves and coastal wetlands, carbon sequestration occurs through:

• Biomass accumulation -- net growth of above- and below-ground vegetation
• Sediment burial -- incorporation of organic matter into accreting sediments

Global average sequestration rates for mangroves are approximately 1.5-2.5 tonnes CO2 per hectare per year, though rates vary considerably with species, age, and environmental conditions^[35]. Saltmarshes typically sequester 0.5-2.0 tonnes CO2 per hectare per year^[36].

Measurement approaches include:

• Biomass change methods -- repeated measurements of vegetation structure and allometric equations
• Sediment core dating -- radioisotope dating (210Pb, 137Cs) to establish accretion rates and carbon burial
• Eddy covariance -- direct measurement of CO2 exchange for intensive monitoring sites

Emissions from wetland degradation

Degraded or disturbed coastal wetlands may become carbon sources rather than sinks, releasing stored carbon to the atmosphere through oxidation of exposed sediments, decomposition of dead biomass, and conversion of organic soils^[37]. When accounting for blue carbon services, compilers should record emissions from wetland conversion and degradation as negative entries in the carbon flow account. The principal scenarios generating emissions include conversion of mangroves to aquaculture ponds, drainage of coastal wetlands for agriculture or development, and erosion exposing previously buried sediment carbon. These emissions can be estimated using IPCC 2013 Wetlands Supplement emission factors, applied to the area of wetland lost or degraded as recorded in the extent account (Section 3.1). This treatment ensures that carbon accounts reflect both the sequestration benefits of intact wetlands and the emission costs of their degradation, providing balanced information for policy decisions on coastal land use.

Carbon service account structure

The ecosystem service account for blue carbon services should distinguish carbon retention (stock-based) and carbon sequestration (flow-based) following SEEA EA recommendations^[38]. This two-component approach is consistent with the treatment of carbon services in TG-6.1 Coral Reef Accounting and TG-6.3 Seagrass Ecosystem Accounting, enabling aggregation across coastal ecosystem types.

3.4 Coastal Protection Services

Coastal protection services are the ecosystem contributions of mangroves, saltmarshes, and other coastal ecosystems in protecting shorelines from erosion, wave damage, and storm surge impacts^[39]. The SEEA Valuation publication defines these as "the ecosystem contributions of linear elements in the seascape - coral reefs, sand banks, dunes or mangrove ecosystems along the shore - in protecting the shore and thus mitigating the impacts of tidal surges or storms on local communities"^[40]. This definition applies equally to the coral reef coastal protection services described in TG-6.1 Coral Reef Accounting, and the measurement approach below is designed to be consistent with the treatment there, enabling comparison and aggregation of coastal protection services across ecosystem types.

Wave attenuation

Mangroves and saltmarshes reduce wave energy through friction with vegetation structure^[41]. Wave attenuation rates depend on:

• Vegetation density and structure
• Forest width (distance inland from shoreline)
• Wave characteristics (height, period, direction)
• Water depth and tidal stage

Studies demonstrate wave height reductions of 13-66% per 100 metres of mangrove forest, with higher attenuation rates in denser vegetation and lower water depths^[42]. Saltmarshes provide similar protection, with wave attenuation rates of 1.5-3% per metre of marsh width reported^[43].

Storm surge reduction

Beyond wave attenuation, coastal wetlands reduce storm surge heights by increasing surface roughness and providing water storage^[44]. Mangroves can reduce storm surge heights by 4-50 centimetres per kilometre of forest width, depending on storm intensity and forest characteristics^[45]. This protection is particularly valuable during tropical cyclones and typhoons when storm surge causes the majority of coastal damages.

The SEEA Valuation publication notes that "World Bank (2016) provides specific guidance on the measurement of the coastal protection by mangroves and coral reefs"^[46]. The World Bank methodology, published in Managing Coasts with Natural Solutions, provides the detailed technical procedures for quantifying coastal protection services. Rather than reproducing that methodology in full, this Circular identifies the key factors determining supply:

• Forest width and density
• Species composition (different root structures provide varying resistance)
• Bathymetry and coastal slope
• Storm characteristics

Compilers with access to hydrodynamic modelling capacity should follow the World Bank expected damage function approach. Where such capacity is limited, the simplified benefit transfer approach described in Section 3.6 below provides a pragmatic alternative.

Physical measurement

Quantifying coastal protection services requires linking ecosystem structure to physical protection provided and the assets protected^[47]. Recommended metrics include:

• Wave attenuation rate (% reduction per 100m width)
• Storm surge reduction (cm reduction per km width)
• Shoreline erosion prevention (m/yr erosion avoided)
• Protected coastline length (km of coastline fronted by wetlands)

The identification of beneficiaries is essential for service accounting. Beneficiaries include:

• Residential and commercial property owners
• Infrastructure (roads, utilities, ports)
• Agricultural land
• Coastal communities at risk

Coastal protection service account

3.5 Nursery Habitat Services

Nursery population and habitat maintenance services are "the ecosystem contributions necessary for sustaining populations of species that economic units ultimately use or enjoy, either through the maintenance of habitats (e.g., for nurseries or migration) or the protection of natural gene pools"^[48]. For mangroves and coastal wetlands, the nursery function supports commercial and subsistence fisheries by providing critical juvenile habitat.

Ecological basis

Mangroves and coastal wetlands provide nursery habitat through^[49]:

• Structural complexity -- roots, stems, and vegetation provide refuge from predators
• Food resources -- detritus, algae, and invertebrate prey support juvenile fish
• Environmental moderation -- reduced wave energy and thermal buffering
• Connectivity -- links between coastal nurseries and offshore adult habitats

The SEEA EA recognizes that "nursery population services supplied by seagrass meadows are an input to the supply of fish biomass provisioning services, which in turn contribute to the benefit of marketed fish"^[50]. The same logic applies to mangrove nursery services. Nursery services are classified as intermediate ecosystem services in SEEA EA (para 6.15), meaning they contribute to the final biomass provisioning services recorded in fisheries accounts. To avoid double counting, nursery service values should not be added to the value of fisheries production they support. Instead, they should be recorded separately as an intermediate input, documenting the ecosystem contribution to fisheries without summing the two. The supply-use accounting framework described in SEEA EA Chapter 7 provides the structure for recording these relationships consistently. If a fisheries accounting circular is compiled (e.g., TG-6.7), the linkage between nursery services here and biomass provisioning there should be explicitly documented.

Species-habitat relationships

Documenting the linkage between mangrove/wetland habitat and fisheries productivity is essential for accounting^[51]. Key relationships include:

• Juvenile density surveys quantifying fish and invertebrate use of wetland habitats
• Tagging and genetic studies tracing adult fish to juvenile nursery origins
• Correlations between wetland extent and fisheries catch at regional scales

Studies consistently demonstrate positive relationships between mangrove area and catches of mangrove-associated species including penaeid shrimp, mud crabs, barramundi, and snappers^[52]. The SEEA Valuation publication cites research finding "that the marginal effect of mangroves on technical efficiency of the commercial marine fishery is 1.86t/ha fringe mangrove per year"^[53].

Fish recruitment metrics

Quantifying nursery services requires measurement of^[54]:

• Juvenile fish density -- individuals per unit area or volume of habitat
• Species composition -- diversity and proportion of commercially important species
• Growth rates -- productivity of the nursery habitat
• Survival rates -- proportion of juveniles surviving to recruit to adult populations
• Recruitment contribution -- proportion of adult population derived from wetland nurseries

These metrics connect habitat extent and condition to fisheries production, enabling attribution of fisheries benefits to ecosystem services. For initial accounts where field data on these metrics are limited, simpler proxy indicators may be used as a starting point. The extent of suitable nursery habitat (defined as mangrove or wetland area with demonstrated fish use, based on published studies for the relevant biogeographic region) provides a Tier 1 proxy. As monitoring programmes develop, compilers can progressively incorporate the more sophisticated metrics listed above.

Nursery service account structure

3.6 Valuation Methods

Monetary valuation of mangrove and coastal wetland ecosystem services enables integration into economic decision-making, comparison with development alternatives, and inclusion in comprehensive wealth accounts^[55]. This section addresses valuation approaches for each service type, applying the general valuation framework from TG-1.9 Valuation to mangrove- and wetland-specific contexts.

Carbon pricing

For blue carbon services, valuation requires an appropriate price per tonne of carbon (or CO2 equivalent)^[56]. The SEEA Valuation publication identifies three approaches:

Social cost of carbon estimates the marginal damage cost of carbon emissions based on integrated assessment models. The SEEA Valuation notes that "based on a review of the different models, the document gives a range of USD14.9-80.5/ton CO2 in 2020"^[57], though subsequent analyses have revised these estimates substantially upward. Because carbon prices change rapidly, compilers should obtain current prices from authoritative sources--the World Bank State and Trends of Carbon Pricing report provides annual updates on compliance market prices globally, and national carbon pricing instruments should be referenced where applicable. The social cost of carbon approach aligns with the avoided damages framing for carbon retention services.

Compliance market prices from emissions trading systems provide market-based valuations. The SEEA Valuation recommends "for the carbon sequestration component, the use of a compliance market price where they are available"^[58]. Prices vary significantly across markets and over time, reinforcing the need for compilers to specify the price source, date, and any adjustments applied.

Voluntary carbon market prices for blue carbon credits offer another reference point, with mangrove and coastal wetland credits typically commanding premium prices due to co-benefits^[59].

The two-component approach recommended by SEEA EA values:

• Carbon retention using social cost of carbon applied to opening stocks, converted to annual flow through an appropriate rate of return
• Carbon sequestration using compliance or voluntary market prices applied to net annual carbon uptake

Avoided damage valuation

Coastal protection services are most appropriately valued using avoided damage cost methods^[60]. The SEEA Valuation publication states that "World Bank (2016) recommends using the avoided damage cost method applying an expected damage function (EDF), which equates the value of an ecosystem asset that provides coastal protection services with the expected avoided damages"^[61].

The EDF approach requires:

1. Hazard modelling -- probability distribution of storm events and associated surge heights/wave energy
2. Exposure assessment -- identification and valuation of assets at risk (property, infrastructure, lives)
3. Vulnerability functions -- relationship between hazard intensity and damage extent
4. Counterfactual analysis -- comparison of damages with and without coastal wetland protection

Menendez et al. (2020), cited in SEEA Valuation, "computed the benefits of flood risk protection provided by mangroves worldwide...estimating the global value of flood protection benefits to exceed US$65 billion annually"^[62].

Replacement cost methods provide a Tier 2 alternative, estimating the cost of engineered structures (seawalls, breakwaters) providing equivalent protection^[63]. While conceptually simpler, this approach may over- or under-estimate value depending on whether engineered alternatives would actually be constructed.

Benefit transfer for data-limited contexts: Where countries lack the data or modelling capacity for primary EDF analysis, benefit transfer provides a pragmatic Tier 1 approach. Compilers can draw on published global and regional valuations--such as the Menendez et al. (2020) estimates or the Natural Capital Project's InVEST Coastal Vulnerability model outputs--adjusting for local conditions including coastline exposure, asset density, and purchasing power parity. The Global Ocean Accounts Partnership maintains a database of valuation studies that can support benefit transfer applications.

Productivity change methods

Nursery habitat services are appropriately valued using productivity change methods that estimate the contribution of habitat to fisheries production^[64]. The SEEA Valuation publication describes this approach:

"Nursery services support various provisioning services and can be valued in terms of the contribution to the market value of the latter...Changes in those nursery services impact on the provisioning services and the link between the two can be estimated using the residual value method or productivity change method"^[65].

Implementation requires:

1. Production function estimation -- statistical relationship between wetland area/condition and fisheries catch
2. Marginal productivity calculation -- additional catch attributable to marginal wetland area
3. Market value application -- multiplication by appropriate fish prices

The SEEA Valuation cites Anneboina and Kumar (2017), who "use the productivity change method to value the nursery function of mangroves for commercial marine fisheries in India...Gross value of output is multiplied by average marginal contribution per hectare mangrove to obtain an estimate of 146,000 Rs/ha yr (approximately US$1,900/ha yr)"^[66].

Aggregation and double counting

When aggregating values across multiple services, care must be taken to avoid double counting^[67]. Key considerations include:

• Carbon services and coastal protection services are largely independent and additive
• Nursery services supporting fisheries are intermediate inputs to biomass provisioning services; their values should be reported separately and not summed with fisheries production values, consistent with the supply-use accounting structure in SEEA EA Chapter 7
• Carbon stored in fish biomass is typically excluded from blue carbon accounting
• Cultural services (tourism, recreation) are additional to the three services emphasized here

The SEEA EA supply-use framework ensures consistency through recording each service flow once at the appropriate user^[68].

Valuation summary table

3.7 Compilation Procedure

This section provides a step-by-step procedure for compiling mangrove and coastal wetland ecosystem accounts, from data collection through account entry and integration.

Step 1: Data Collection and Source Identification

Compile the following data sources:

1. Spatial data: Satellite imagery (Landsat, Sentinel-2, SAR), national vegetation maps, protected area boundaries
2. Extent data: Remote sensing classifications, national wetland inventories, global datasets (Global Mangrove Watch)
3. Condition data: Field surveys (tree density, species composition, water quality), remote sensing variables (canopy cover, NDVI)
4. Carbon data: Biomass measurements, sediment core samples, allometric equations, literature values for carbon density
5. Service data: Wave attenuation studies, storm protection assessments, fisheries catch statistics, juvenile fish surveys
6. Valuation data: Carbon prices, property values in coastal zones, fish market prices, engineering cost estimates

Coordinate data collection across agencies: environmental ministries (field data), national statistical offices (economic data), meteorological agencies (storm data), fisheries authorities (catch statistics).

Step 2: Classification and Mapping

1. Map national classifications to IUCN GET functional groups (MFT1.1, MFT1.2, MFT1.3)
2. Define ecosystem accounting area (EAA) boundaries, including all mangrove and wetland areas within national jurisdiction
3. Classify ecosystems using remote sensing, distinguishing mangroves, saltmarshes, and delta mosaics
4. Validate classifications using ground-truth surveys and accuracy assessment

Step 3: Extent Account Compilation

1. Generate baseline extent map using remote sensing classification for opening period
2. Detect changes using multi-date imagery and change detection algorithms
3. Classify changes as managed/unmanaged expansions or reductions
4. Populate extent account with opening extent, additions, reductions, closing extent
5. Validate account balancing: Closing extent = Opening extent + Additions - Reductions

Step 4: Condition Assessment

1. Select condition variables representing all six ECT classes (A1, A2, B1, B2, B3, C1)
2. Measure variables using field surveys and remote sensing
3. Establish reference conditions using historical baselines, reference sites, or expert targets
4. Normalize variables into indicators scaled against reference conditions
5. Compile condition account recording variables, reference levels, opening/closing values

Step 5: Ecosystem Service Quantification

Blue carbon services:

1. Measure carbon stocks by pool (above-ground, below-ground, sediment)
2. Calculate sequestration rates from biomass change or sediment core dating
3. Estimate emissions from degraded/converted areas using IPCC emission factors
4. Compile carbon stock account and carbon flow account

Coastal protection services:

1. Map mangrove/wetland fringe and calculate forest width
2. Identify protected coastline length and beneficiary populations
3. Estimate wave attenuation rates from literature or hydrodynamic models
4. Quantify storm surge reduction using expected damage functions or replacement cost

Nursery habitat services:

1. Delineate nursery habitat areas based on field surveys or literature
2. Measure juvenile fish density and species composition
3. Estimate recruitment contribution to commercial fisheries
4. Link nursery extent/condition to fisheries production using production functions

Step 6: Monetary Valuation

1. Select valuation methods appropriate to each service type (see Section 3.6)
2. Apply carbon pricing to sequestration flows and retained stocks
3. Calculate avoided damages or replacement costs for coastal protection
4. Estimate productivity contributions for nursery habitat
5. Document assumptions, price sources, and uncertainty

Step 7: Account Integration and Balancing

1. Reconcile accounts: Ensure extent, condition, and service accounts are mutually consistent
2. Check balancing identities: All stock-flow accounts should balance
3. Link to broader accounts: Connect to national ocean accounts, climate accounts, fisheries accounts
4. Prepare metadata: Document sources, methods, quality, limitations
5. Compile time series: Enable tracking of changes over multiple accounting periods

3.8 Worked Example

This worked example demonstrates the compilation of mangrove and coastal wetland ecosystem accounts for a hypothetical coastal zone in a Southeast Asian setting. The example follows the extent-condition-services-valuation sequence presented in Section 3 and illustrates the key accounting entries and calculations. All data are synthetic and illustrative.

Setting: A national ecosystem accounting area (EAA) containing 8,000 hectares of mangrove forest (MFT1.2 Intertidal forests and shrublands) and 3,000 hectares of salt marsh (MFT1.3 Coastal saltmarshes and reedbeds), totalling 11,000 hectares of coastal wetland ecosystem. The mangrove system is dominated by Rhizophora and Avicennia species, with the salt marsh occupying the upper intertidal fringe.

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

Interpretation: The coastal zone experienced net loss of 100 hectares of mangroves during the accounting period, with managed reductions (aquaculture conversion) exceeding managed expansion (restoration). Saltmarshes showed no net change, with additions exactly offsetting reductions.

Step 2: Condition account

Condition indicators are derived from field survey and remote sensing data using reference levels established from minimally disturbed sites in the same biogeographic region:

Composite condition index (equal weights): (0.74 + 0.60 + 0.70 + 0.70) / 4 = 0.69

The indicator score is calculated using the formula: Indicator = (Observed - VL) / (VH - VL), where VH is the reference value and VL is the degraded value. For example, canopy cover indicator = (72 - 20) / (90 - 20) = 52 / 70 = 0.74.

Step 3: Ecosystem services (annual flows)

Carbon sequestration calculation:

• Mangrove: 8,000 ha × 2.0 t CO2/ha/yr = 16,000 t CO2/yr
• Salt marsh: 3,000 ha × 1.0 t CO2/ha/yr = 3,000 t CO2/yr
• Monetary value: 16,000 × $80 = $1,280,000; 3,000 × $80 = $240,000

Coastal protection calculation: Expected damage function approach, comparing modelled storm damages with and without mangrove/wetland protection. Total avoided damages across storm probability distribution = $6.4 million/yr.

Nursery habitat calculation: Productivity change method. Estimated marginal contribution of 1 ha mangrove to fisheries = 0.25 tonnes catch/yr. Total nursery area = 8,400 ha (mangrove + salt marsh). Contribution = 8,400 × 0.25 = 2,100 tonnes/yr. At average fish price of $1,857/tonne, value = $3,900,000.

Step 4: Asset valuation

Applying a 4% social discount rate over a 25-year projection horizon with stable service flows:

Asset value = Annual service value × present value annuity factor (4%, 25 years) Asset value = 12,770,000 × 15.62 = 199,500,000 USD

The present value annuity factor for 4% over 25 years is calculated as: [1 - (1 + 0.04)^-25] / 0.04 = 15.62.

This asset value represents the net present value of expected future ecosystem service flows, discounted to the present. The perpetual annuity approach (dividing annual flow by discount rate) would yield $12,770,000 / 0.04 = $319,250,000, but the 25-year horizon approach is more conservative and commonly applied in ecosystem accounting.

Step 5: Upward connections to policy circulars

This worked example demonstrates how mangrove and coastal wetland accounts feed into the policy and indicator circulars identified in Section 1:

TG-2.8 Climate Change Indicators: The carbon sequestration rate of 19,000 t CO2/yr and carbon stocks of approximately 1,200,000 tonnes C (estimated at 150 t C/ha × 8,000 ha mangrove) feed into national blue carbon reporting for NDCs under the Paris Agreement and SDG 13 monitoring.

TG-2.9 Disaster Risk Indicators: The coastal protection service value of $6.4 million/yr and the 85 km of protected coastline feed into vulnerability assessments and natural infrastructure indicators for disaster risk reduction.

TG-1.8 OA and Project-Level Finance: The asset value of $199.5 million and the annual service flows provide the measurement foundation for structuring blue bonds, debt-for-nature swaps, or payments for ecosystem services mechanisms. The carbon account data support blue carbon credit issuance under voluntary carbon standards.

This worked example illustrates the full accounting sequence for mangrove and coastal wetland ecosystems. Actual compilations will require country-specific data, locally calibrated reference levels, and detailed valuation studies for each service type. The carbon price of USD 80/t CO2 is illustrative and should be replaced with the prevailing compliance market price or social cost of carbon applicable in the compiler's jurisdiction. The example values are illustrative and should not be used as benchmarks for specific national contexts.

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: [Names and affiliations]

Reviewers: [Names and affiliations]

5. References and Further Reading

• SEEA Ecosystem Accounting, Chapters 4 (Extent), 5 (Condition), 6 (Services), 7 (Supply-Use), 10 (Valuation)
• SEEA Valuation, Sections 4.2.9 (Carbon), 4.2.13 (Coastal Protection), 4.2.14 (Nursery Services)
• SEEA EEA Technical Recommendations, Chapter 3 (Spatial Data)
• IUCN Global Ecosystem Typology, Biome MFT1 (Brackish Tidal Systems)
• IPCC 2013 Wetlands Supplement (Blue Carbon Methodology)
• World Bank (2016), Managing Coasts with Natural Solutions
• Global Mangrove Watch (www.globalmangrovewatch.org)
• Anneboina and Kumar (2017), Valuing Mangrove Nursery Function for Commercial Marine Fisheries
• Menendez et al. (2020), Global Flood Protection Benefits of Mangroves