Seagrass Ecosystem Accounting

1 Outcome

This circular provides comprehensive guidance on compiling ecosystem accounts for seagrass meadows, addressing the distinct challenges of accounting for these critically important yet difficult-to-monitor marine ecosystems. Seagrass meadows represent one of the most valuable coastal ecosystem types, providing essential services including carbon sequestration (blue carbon storage), sediment stabilisation, coastal protection, nutrient cycling, and nursery habitat for commercially important fish species.^[1]

Seagrass ecosystem accounts support decision-making across multiple policy domains. Countries implementing this guidance will be able to measure and value carbon sequestration for blue carbon policy and climate commitments, quantify coastal protection services for disaster risk reduction planning, assess nursery habitat contributions to fisheries management, and track water quality through seagrass condition indicators. These accounts provide the evidence base for marine spatial planning decisions that balance competing uses of coastal waters, for evaluating the effectiveness of seagrass restoration investments, and for monitoring progress toward SDG Target 14.2 on sustainable management and protection of marine and coastal ecosystems.^[2]

Upon implementation, countries will be able to:

a) Compile extent accounts for seagrass ecosystems using appropriate mapping methodologies that address subtidal visibility constraints, consistent with the geospatial methods described in TG-4.1 Remote Sensing and Geospatial Data;

b) Develop condition accounts incorporating seagrass-specific indicators including shoot density, species composition, sediment characteristics, and light availability, building on the condition accounting framework in TG-3.1 Asset Accounts and the biophysical indicator selection guidance in TG-2.1 Biophysical Indicators;

c) Quantify and record ecosystem services flows from seagrass meadows, including provisioning, regulating, and cultural services, with particular emphasis on carbon sequestration, nursery habitat, and coastal protection;

d) Apply appropriate valuation methods to estimate the monetary value of seagrass ecosystem services and assets, drawing on methods described in TG-1.9 Valuation; and

e) Integrate seagrass ecosystem accounts within broader national ocean accounting frameworks consistent with SEEA EA principles and the GOAP framework introduced in TG-0.1 General Introduction to Ocean Accounts.^[3]

This circular enables TG-7.1 Blue Carbon Accounting, which will build on the carbon stock and sequestration measurement methods described here, and TG-7.4 Coastal Protection Valuation, which will draw on the wave attenuation and sediment stabilisation services quantified in this guidance. The remote sensing methods described in TG-4.1 Remote Sensing and Geospatial Data provide the foundation for seagrass extent mapping, while the climate indicators developed in TG-2.8 Ocean and Climate Linkages connect seagrass carbon services to national climate accounting systems.

2 Requirements

2.1 Prerequisite Knowledge

This Circular requires familiarity with:

• TG-0.1 General Introduction to Ocean Accounts -- for the conceptual framework, terminology, and key components of Ocean Accounts, including the relationship between environmental and economic accounting frameworks that underpin ecosystem-level analysis.

• TG-3.1 Asset Accounts -- for the methodology of physical and monetary asset accounts, including the treatment of ecosystem assets as described in Sections 3.3 and 3.4 of that circular. The extent accounting structure (Section 3.4.1) and condition accounting framework (Section 3.4.2) provide the templates that this circular applies to seagrass ecosystems.

• TG-4.1 Remote Sensing and Geospatial Data -- for satellite imagery sources (Section 3.1), ecosystem extent mapping methodologies (Section 3.2), and quality assurance procedures applicable to marine ecosystem extent mapping (Section 3.4). Seagrass extent mapping relies on the optical remote sensing platforms and water column correction techniques described in that circular, supplemented by the acoustic survey methods introduced in Section 3.1.3 below.

Readers should note that TG-1.9 Valuation is NOT a direct prerequisite for seagrass accounting, though countries intending to compile monetary accounts should consult that circular for detailed valuation guidance. This is because seagrass ecosystems may be monitored and accounted for in physical terms without requiring monetary valuation as a first step.^[4]

2.2 Institutional Requirements

Countries implementing seagrass ecosystem accounts shall:

a) Establish coordination mechanisms between the National Statistical Office, marine environmental agencies, fisheries authorities, and relevant research institutions, consistent with governance arrangements described in TG-0.1;

b) Designate technical responsibility for seagrass monitoring to agencies with appropriate marine survey and remote sensing capabilities;

c) Develop data sharing arrangements for integrating field survey data with satellite-derived extent estimates, following the data integration principles in TG-4.1 Section 3.4;

d) Maintain quality-assured databases of seagrass extent, condition, and associated ecosystem service flows, applying the quality framework from TG-0.7; and

e) Participate in international seagrass monitoring networks where available, including the Global Seagrass Monitoring Network.^[5]

2.3 Technical Requirements

Countries shall ensure technical capacity for:

a) Processing and analysing satellite imagery at resolutions appropriate for seagrass detection (typically 10-30m for broadscale mapping, higher resolution for detailed habitat delineation), as specified in TG-4.1 Section 3.1;

b) Conducting underwater visual surveys or acoustic mapping for ground-truthing and detailed extent assessment;

c) Measuring seagrass condition indicators including shoot density, canopy height, and species composition, following the biophysical indicator selection framework in TG-2.1;

d) Estimating carbon stocks in seagrass biomass and underlying sediments, which will be further developed in TG-7.1 when available; and

e) Modelling ecosystem service flows using biophysical models appropriate for seagrass ecosystems, consistent with the SEEA EA guidelines on biophysical modelling.^[6]

3 Guidance Material

3.1 Extent Accounting

Ecosystem extent accounts record the area of seagrass meadows at the beginning and end of each accounting period, along with changes in extent during the period.^[7] The general methodology for ecosystem extent accounts is described in TG-3.1 Section 3.4; this section provides seagrass-specific guidance. Seagrass extent mapping presents unique challenges compared to terrestrial ecosystems due to their subtidal location, the optical properties of the water column, and the dynamic nature of seagrass distributions.^[8]

3.1.1 Classification Framework

Seagrass meadows are classified within the IUCN Global Ecosystem Typology as ecosystem functional group M1.1 (Seagrass meadows) within the M1 Marine Shelf biome.^[9] This classification is consistent with the marine ecosystem typology presented in TG-4.1 Section 3.2.1. The IUCN GET describes seagrass meadows as:

"Seagrass meadows are important sources of organic matter, much of which is retained by seagrass sediments. Seagrasses are the only subtidal marine flowering plants and underpin the high productivity of these systems."^[10]

The IUCN GET further specifies that seagrass ecosystems "have a higher abundance and diversity of flora and fauna, compared to surrounding unvegetated soft sediments and comparable species richness and abundances to most other marine biogenic habitats." This high biodiversity, combined with the three-dimensional structure that provides shelter and binds sediments, distinguishes seagrass meadows as a distinct functional group warranting separate accounting.^[11]

For national ecosystem accounts, countries may develop more detailed classifications based on:

• Dominant species composition (e.g., Posidonia, Zostera, Thalassia, Halophila meadows)
• Depth zone (intertidal versus subtidal meadows)
• Density class (sparse, moderate, dense)
• Associated habitat (meadows on sand, mud, or mixed substrates)

Such national classifications should be designed to crosswalk to the IUCN GET Level 4 (ecosystem functional groups) to ensure international comparability.^[12] Detailed crosswalk tables between species assemblages and national classification schemes are expected to emerge as more countries compile seagrass accounts; in the interim, countries are encouraged to document their classification decisions to support future harmonisation.

3.1.2 Mapping Challenges

Seagrass mapping faces several methodological challenges that distinguish it from terrestrial ecosystem mapping:

Subtidal visibility constraints. Unlike terrestrial vegetation, seagrass meadows are submerged and their detectability from optical sensors depends on water depth, turbidity, and water column properties. Light attenuation in the water column limits optical remote sensing to depths of approximately 10-15 metres in clear waters, and significantly less in turbid coastal environments.^[13] Deeper seagrass meadows, which may extend to 40-60 metres in exceptionally clear waters, cannot be detected using standard optical methods. TG-4.1 Section 3.1.3 provides guidance on satellite-derived bathymetry techniques that can assist with depth correction but do not resolve this fundamental limitation.

The IUCN GET specifies that "minimum water depth is determined mainly by wave orbital velocity, tidal exposure and wave energy (i.e. waves disturb seagrass and mobilise sediment), while maximum depth is limited by the vertical diminution of light intensity in the water column."^[14] This depth-light relationship creates a detectability gradient that compilers must account for when interpreting extent estimates.

Spectral confusion. The spectral signature of seagrass can be confused with other benthic features including macroalgae, coral, and unvegetated sediments, particularly in mixed habitats. This challenge is compounded by the overlying water column, which absorbs and scatters light differentially across wavelengths.^[15]

Temporal variability. Seagrass meadows exhibit seasonal variation in biomass and spatial extent, particularly in temperate regions. Mapping must account for this variability through appropriate timing of imagery acquisition and, where feasible, multi-temporal analysis.^[16]

Fragmented distributions. Many seagrass meadows occur as patchy or fragmented distributions rather than continuous stands, requiring higher resolution imagery to accurately delineate boundaries.

Because TG-4.1 focuses primarily on satellite and bathymetric data sources, this circular provides the primary guidance on acoustic survey methods for seagrass mapping. Section 3.1.3 below describes these methods in the context of the Tier 3 integrated approach.

3.1.3 Mapping Methods

Countries should adopt tiered approaches to seagrass extent mapping based on available resources and the specific characteristics of their seagrass ecosystems, consistent with the tiered approach framework applied throughout the GOAP Technical Guidance:^[17]

Tier 1: Global data products.

At the most basic level, countries may utilise existing global seagrass distribution datasets as starting points. Key resources include:

• UNEP-WCMC Global Distribution of Seagrasses dataset
• Allen Coral Atlas benthic habitat maps (which include seagrass classes, as referenced in TG-4.1 Section 3.2.2)
• Regional seagrass mapping initiatives

Tier 1 approaches provide indicative extent estimates but typically lack the temporal consistency and national verification required for formal ecosystem accounts.^[18]

Tier 2: Satellite imagery classification.

Moderate resolution optical satellite imagery (Sentinel-2, Landsat) can be used to map seagrass extent in suitable conditions. Recommended approaches include:

• Supervised classification using training data from field surveys
• Object-based image analysis to delineate meadow boundaries
• Water column correction algorithms to compensate for depth effects
• Multi-temporal compositing to reduce cloud contamination and capture seasonal variation

Sentinel-2 imagery is particularly suitable for seagrass mapping due to its 10m resolution in visible bands, coastal aerosol band, and 5-day revisit time.^[19] Platform specifications and data access are described in TG-4.1 Section 3.1.1. However, satellite-derived seagrass maps require ground-truthing with in-situ data.

Tier 3: Integrated multi-source mapping.

The most rigorous approach integrates satellite imagery with additional data sources:

• Aerial photography or UAV imagery for high-resolution mapping of accessible areas
• Acoustic surveys (sidescan sonar, multibeam echosounder) for mapping beyond optical depth limits
• Underwater video transects for habitat verification and species identification
• Diver surveys for detailed condition assessment at representative sites

Acoustic methods are particularly valuable for mapping deeper seagrass beds that are beyond optical detection limits. Sidescan sonar can detect the acoustic contrast between seagrass and bare sediment at depths exceeding 30 metres.^[20] Because TG-4.1 does not currently address acoustic survey techniques, this circular serves as the primary GOAP reference for acoustic methods in the context of seagrass extent mapping. Countries employing acoustic surveys should document their protocols and calibration procedures to enable reproducibility across accounting periods.

3.1.4 Recording Extent Changes

The ecosystem extent account records opening stock, additions, reductions, and closing stock of seagrass area, following the structure defined in TG-3.1 Section 3.4.^[21] Changes in seagrass extent may result from:

Managed expansion: Deliberate seagrass restoration or transplanting efforts Managed reduction: Physical removal for dredging, coastal development, or infrastructure Natural expansion: Natural colonisation of suitable substrates, recovery from disturbance Natural reduction: Natural mortality, storm damage, disease, grazing pressure Catastrophic losses: Mass mortality events from marine heatwaves, sediment burial, toxic algal blooms, or oil spills

For accounting purposes, reductions due to eutrophication-driven decline (where excessive nutrient loading causes epiphyte overgrowth and light limitation) should be classified as degradation attributable to pollution pressures rather than natural reduction.^[22] The IUCN GET notes that "in eutrophic waters, high nutrient availability can lead to the overgrowth of seagrasses by epiphytes and shading by algal blooms, leading to ecosystem collapse."^[23] This ecological mechanism provides the conceptual basis for distinguishing anthropogenic degradation from natural variability.

3.2 Condition Assessment

Ecosystem condition accounts complement extent accounts by recording the quality or state of seagrass ecosystems relative to a reference condition.^[24] SEEA EA recommends a three-stage approach to condition accounting: recording condition variables, deriving condition indicators, and optionally aggregating into composite condition indices. The general framework is described in TG-3.1 Section 3.4; this section provides seagrass-specific indicators. The principles for selecting biophysical indicators that are policy-relevant, scientifically defensible, and cost-effective to monitor are addressed in TG-2.1 Biophysical Indicators.

3.2.1 Condition Variables

Key condition variables for seagrass ecosystems include:

Structural characteristics:

• Shoot density (shoots per square metre)
• Canopy height (centimetres)
• Percent cover within meadow boundaries
• Above-ground and below-ground biomass

Species composition:

• Dominant species identity
• Species richness (number of seagrass species present)
• Presence of indicator species (both positive indicators of good condition and negative indicators of degradation)

Sediment characteristics:

• Organic carbon content in sediments (percent or tonnes per hectare) -- relevant to carbon accounting in TG-7.1
• Sediment grain size distribution
• Sediment accumulation rates
• Sulphide concentrations (elevated levels indicate stress)

Water quality and light availability:

• Light attenuation coefficient (Kd)
• Depth of light penetration (Secchi depth)
• Nutrient concentrations in overlying waters
• Chlorophyll-a concentrations (elevated levels indicate eutrophication pressure)

Associated biota:

• Epiphyte loading on seagrass leaves
• Abundance of mesograzers (amphipods, gastropods that control epiphytes)
• Fish community composition and abundance within meadows^[25]

The IUCN GET emphasises that "mesograzers, such as amphipods and gastropods, play an important role in controlling epiphytic algal growth on seagrass," and that "mutualisms with lucinid molluscs may influence seagrass persistence."^[26] These ecological relationships provide the functional basis for including associated biota as condition indicators, not merely as biodiversity metrics.

To support international comparability, Table 3.2.1 presents a recommended minimum set of condition variables that all countries should endeavour to record. Countries with greater monitoring capacity are encouraged to measure additional variables from the full list above.

Table 3.2.1: Recommended minimum condition variables for seagrass ecosystem accounts

This minimum set balances practicality with the need for consistent condition reporting. Percent cover and shoot density capture structural condition; canopy height reflects growth vigour; epiphyte load serves as a pressure indicator for eutrophication; and species diversity provides a biodiversity dimension. Countries should record reference condition values specific to their dominant seagrass species and environmental settings, following the guidance in Section 3.2.2 below.

3.2.2 Reference Conditions

For condition accounting, variable measurements are compared against reference conditions representing ecosystems with minimal anthropogenic disturbance.^[27] For seagrass meadows, establishing reference conditions is challenging because:

• Few pristine seagrass meadows remain globally
• Natural variability in seagrass condition across environmental gradients (depth, latitude, substrate) complicates definition of a single reference state
• Historical baselines are often unavailable due to lack of long-term monitoring

SEEA EA recommends that reference conditions be based on expert assessment of natural or historical conditions, drawing on the best available scientific evidence.^[28] For seagrass, sources of reference condition information include:

• Protected marine areas with limited human disturbance
• Historical photographs, records, or traditional ecological knowledge
• Palaeoecological evidence from sediment cores
• Scientific literature on seagrass ecology in relatively pristine systems

3.2.3 Condition Indicators

Condition indicators express the state of each ecosystem characteristic relative to the reference condition, typically as a proportion or index value between 0 (complete degradation) and 1 (reference condition).^[29] Example seagrass condition indicators include:

The example above shows a seagrass meadow in moderate condition, with structural indicators suggesting some degradation while carbon stocks remain relatively intact.

3.3 Ecosystem Services

Seagrass meadows generate a diverse suite of ecosystem services that can be recorded in ecosystem service flow accounts.^[30] Following the SEEA EA classification, services are organised as provisioning, regulating and maintenance, and cultural services. The ecosystem service identification and measurement framework in TG-2.4 Ecosystem Goods and Services provides the general methodology for quantifying these flows.

3.3.1 Provisioning Services

Biomass provisioning. Seagrass meadows support artisanal fisheries through their role as nursery habitat and foraging grounds. The service is measured as the contribution of seagrass ecosystems to fish catch, typically estimated through:

• Catch data from fishing grounds associated with seagrass meadows
• Bio-economic models linking seagrass extent to fisheries productivity
• Meta-analysis of seagrass-fishery relationships^[31]

It is important to attribute only the ecosystem contribution to catch, excluding the contribution of human inputs (labour, capital, technology). The SEEA EA describes methods for estimating ecosystem contributions to biomass provisioning, including residual value and simulation approaches.^[32] Detailed guidance on fisheries valuation may be found in TG-1.9.

Other provisioning. In some regions, seagrasses themselves are harvested for traditional uses (thatching, fertiliser, crafts), though such uses are typically minor in economic terms.

3.3.2 Regulating and Maintenance Services

Global climate regulation (carbon sequestration). Seagrass meadows are highly efficient blue carbon ecosystems, sequestering atmospheric CO2 in both above-ground biomass and, more importantly, in accumulating sediments.^[33] This service will be addressed in detail in TG-7.1 Blue Carbon Accounting when that circular is developed. The carbon sequestration service is measured as the annual carbon flux into long-term storage, typically in tonnes of CO2-equivalent per hectare per year. Key characteristics of seagrass carbon sequestration include:

• Sequestration rates of 83-450 g C m-2 yr-1 (significantly higher per unit area than many terrestrial forests)
• Long-term storage in anaerobic sediments where decomposition is inhibited
• Carbon stocks in sediments that may represent centuries to millennia of accumulation
• Risk of carbon release if meadows are degraded or destroyed^[34]

For accounting purposes, the climate regulation service should record the annual sequestration flow (addition to stock) and, where seagrass is being lost, the carbon emissions associated with degradation (reduction in service provision or release of stored carbon). The linkage between seagrass condition and carbon service capacity can be quantified through regression models relating shoot density or percent cover to carbon accumulation rates, providing the biophysical model required by SEEA EA.^[35]

Coastal and sediment stabilisation. Seagrass canopies attenuate wave energy and currents, reducing coastal erosion and protecting shorelines.^[36] This service is relevant to coastal protection valuation addressed in TG-7.4 when developed. The IUCN GET notes that seagrass ecosystems "bind sediments and, at fine scales, dissipate waves and currents," providing the ecological foundation for this service.^[37]

The sediment stabilisation service can be measured through:

• Wave attenuation rates across meadows (percentage reduction per metre of meadow width)
• Sediment trapping and retention rates
• Comparison of erosion rates between protected and unprotected coastlines

This service provides direct economic benefits by reducing infrastructure damage and avoiding the need for engineered coastal protection structures.

Water purification and nutrient cycling. Seagrass meadows remove nutrients from the water column through uptake and by promoting denitrification in sediments.^[38] This service helps maintain water quality and can reduce eutrophication impacts on other coastal ecosystems including coral reefs. However, the IUCN GET notes that "seagrass growth can be limited by nitrogen and phosphorous availability, but in eutrophic waters, high nutrient availability can lead to the overgrowth of seagrasses by epiphytes and shading by algal blooms, leading to ecosystem collapse."^[39] This non-linear response to nutrient loading presents challenges for accounting and requires careful treatment when quantifying water purification services.

Nursery population and habitat maintenance. Seagrass meadows provide critical nursery habitat for juveniles of many commercially and ecologically important fish and invertebrate species.^[40] The IUCN GET specifies that "the complex three-dimensional structure of the seagrass provides shelter and cover to juvenile fish and invertebrates," and that seagrass systems "support infauna living amongst their roots, epifauna, and epiflora living on their shoots and leaves, as well as nekton in the water column."^[41]

Species dependent on seagrass nursery habitat include:

• Commercially important fish (snapper, sea bream, mullet species)
• Crustaceans (prawns, blue swimmer crabs)
• Molluscs (scallops, queen conch)
• Megafauna (dugongs, green sea turtles)

The nursery service is distinct from the biomass provisioning service: nursery habitat maintenance supports the recruitment and survival of juvenile fish, while biomass provisioning captures the subsequent harvest of adult fish from seagrass-associated fisheries.^[42] The IUCN GET notes that "grazing megafauna, such as dugongs, manatees and turtles, can contribute to patchy seagrass distributions, although they tend to 'garden' rather than deplete seagrass," highlighting the reciprocal relationship between habitat provisioning and herbivore pressure.^[43]

3.3.3 Cultural Services

Recreation and tourism. Seagrass meadows support recreational activities including snorkelling, diving, recreational fishing, and wildlife viewing (particularly for charismatic megafauna such as dugongs, manatees, and sea turtles).^[44] These services can be measured through:

• Visitor numbers to seagrass-associated recreation sites
• Expenditure on seagrass-dependent recreational activities
• Travel cost or revealed preference studies

Education and research. Seagrass ecosystems serve as sites for marine education programs and scientific research. While challenging to quantify, these services contribute to knowledge generation and environmental awareness.

3.4 Valuation Methods

Monetary valuation enables aggregation of seagrass ecosystem services and assets for incorporation in extended national accounts and policy analysis.^[45] Following SEEA EA principles, valuation should be based on exchange values where possible, though complementary approaches may be required for services not exchanged in markets. For comprehensive guidance on valuation methods, see TG-1.9 Valuation.

3.4.1 Valuing Carbon Sequestration

The global climate regulation service from seagrass carbon sequestration can be valued using:

Social cost of carbon (SCC). The SCC represents the economic damage caused by an additional tonne of CO2 emissions, or equivalently, the benefit of avoiding that emission. Current SCC estimates typically range from USD 50-200 per tonne CO2, depending on the discount rate, damage functions, and scope of impacts considered.^[46] Countries should use current authoritative SCC estimates from recognised sources such as the US EPA Interagency Working Group on the Social Cost of Greenhouse Gases or equivalent national assessment bodies, as these values are subject to periodic revision as scientific and economic understanding advances.

Carbon market prices. Where carbon credits from blue carbon projects are traded, market prices provide observable exchange values. However, seagrass is currently underrepresented in carbon markets compared to mangroves and salt marshes, and verified seagrass carbon credits remain rare.^[47]

For accounting purposes, the annual carbon sequestration flow is valued by multiplying physical sequestration (tonnes CO2-eq) by the selected price. Consistency with the carbon price used for other blue carbon ecosystems (mangroves, salt marshes) and terrestrial carbon sinks is essential for coherent national accounts, as described in TG-2.8 Ocean and Climate Linkages.

3.4.2 Replacement Cost Approaches

For services where direct valuation is difficult, replacement cost methods estimate the cost of providing equivalent services through engineered alternatives:^[48]

Coastal protection. The replacement cost for seagrass coastal protection services can be estimated as the avoided cost of constructing seawalls, breakwaters, or beach nourishment to achieve equivalent wave attenuation and erosion control.

Water purification. The replacement cost for nutrient removal can be estimated from the cost of wastewater treatment infrastructure required to achieve equivalent nutrient load reduction.

Replacement cost approaches should be applied with caution, as they assume the engineered alternative would actually be constructed in the absence of the natural service, and that the replacement provides genuinely equivalent benefits.^[49]

3.4.3 Fisheries Contribution

Valuing the contribution of seagrass nursery habitat to fisheries requires apportioning total fishery value between the ecosystem contribution and human inputs. Approaches include:

Production function methods. Econometric models estimate the statistical relationship between seagrass extent and fishery catch, controlling for fishing effort and other inputs. The estimated coefficient represents the marginal contribution of seagrass to production.^[50]

Resource rent approaches. The resource rent (gross value of catch minus all costs of fishing including normal returns to capital) represents the surplus attributable to the natural resource. This rent can be apportioned between fish stocks and supporting habitat (including seagrass nursery areas) based on ecological evidence or model estimates. See TG-3.1 Section 3.3.2 for the treatment of fish stocks in asset accounts.

Benefit transfer. Where primary valuation is not feasible, values from existing studies may be transferred to the site of interest with appropriate adjustments for context.^[51]

3.4.4 Asset Valuation

The monetary value of seagrass ecosystem assets is estimated as the net present value of expected future ecosystem service flows, following the methodology described in TG-3.1 Section 3.2:^[52]

Asset Value = Sum of (Annual Service Value / (1 + r)^t) for t = 0 to T

Where r is the discount rate and T is the asset life. For seagrass assets under effective protection and management, an indefinite asset life may be assumed, simplifying the calculation to:

Asset Value = Annual Service Value / r

The choice of discount rate significantly affects asset values, particularly for services with long time horizons such as carbon storage. Countries should apply discount rates consistent with those used for other natural assets in their national accounts.^[53]

3.5 Compilation Procedure

This section provides step-by-step guidance for compiling seagrass ecosystem accounts, integrating the extent, condition, and services guidance from Sections 3.1-3.4 into an operational workflow.

Step 1: Define the ecosystem accounting area and classify seagrass ecosystem types

Delineate the geographic boundary of the national EEZ or sub-national coastal region to be covered by the account, following the spatial boundary guidance in TG-4.1 Remote Sensing and Geospatial Data Section 3.2.6. Within this area, classify seagrass meadows to the IUCN GET functional group M1.1, and where national circumstances permit, disaggregate by dominant species or depth zone as described in Section 3.1.1 above. Document the crosswalk between any national classification and the IUCN GET to ensure international comparability.

Step 2: Map seagrass extent using tiered approach

Select the appropriate mapping tier (1, 2, or 3) based on available data and technical capacity (Section 3.1.3). For Tier 1, download UNEP-WCMC global seagrass datasets and clip to the national EAA. For Tier 2, acquire Sentinel-2 or Landsat imagery for the accounting period, apply water column correction algorithms, and classify seagrass using supervised classification with field training data. For Tier 3, integrate satellite imagery with acoustic surveys, UAV imagery, and diver transects. Validate all maps against ground-truth data and document accuracy using confusion matrices as specified in TG-4.1 Section 3.5.1.

Step 3: Detect changes in extent and populate extent account

Compare opening and closing extent maps for the accounting period using change detection techniques. Classify detected changes as managed or natural expansions and reductions following Section 3.1.4, consulting with national marine agencies to distinguish managed interventions (restoration, dredging) from natural processes (storm damage, recovery). Record extent changes in the standard SEEA EA extent account table format (Table 4.1 of SEEA EA), ensuring that additions and reductions sum correctly to the net change in extent.

Step 4: Measure condition variables and derive indicators

Select a representative sample of seagrass meadows stratified by species type, depth, and pressure exposure. At each site, measure the minimum condition variables specified in Table 3.2.1 (percent cover, shoot density, canopy height, epiphyte load, species diversity) using standardised field protocols. Establish reference condition values from protected sites, historical records, or scientific literature following Section 3.2.2. Calculate condition indicators for each variable using the linear transformation formula: Indicator = (V - VL) / (VH - VL), where V is the observed value, VH is the high reference level, and VL is the low reference level. Optionally aggregate indicators into a composite condition index using equal weights or expert-weighted averages.

Step 5: Quantify ecosystem service flows in physical units

For carbon sequestration, measure sediment carbon accumulation rates using dated sediment cores (210Pb or 137Cs) and combine with extent data to calculate total annual carbon sequestration in tonnes CO2-eq/yr. For coastal protection, model wave attenuation across meadow width using hydrodynamic models or empirical attenuation coefficients from the literature, and map the length of coastline protected. For nursery habitat, conduct juvenile fish density surveys at representative seagrass sites and apply productivity change models linking habitat area to fisheries recruitment. For each service, record both the physical quantity and the spatial unit (hectares, kilometres, tonnes) following the SEEA EA ecosystem services flow account structure (Table 7.1 of SEEA EA).

Step 6: Apply monetary valuation (optional)

For countries compiling monetary accounts, apply the valuation methods in Section 3.4 to convert physical service flows into monetary values. Use current carbon market prices or social cost of carbon for sequestration services (Section 3.4.1), replacement costs or avoided damage approaches for coastal protection (Section 3.4.2), and production function or resource rent methods for fisheries contribution (Section 3.4.3). Ensure consistency with carbon prices used in other blue carbon accounts and with discount rates used in national accounts. Record monetary values in the monetary ecosystem services supply-use table following SEEA EA Chapter 9 guidance.

Step 7: Compile asset valuation and degradation accounts

Calculate the monetary value of seagrass ecosystem assets as the net present value of expected future service flows using the formula in Section 3.4.4. For the opening asset value, use the previous period's closing value or, if this is the first compilation, calculate the opening NPV from expected baseline services. For the closing asset value, recalculate NPV based on revised extent and condition. Record ecosystem degradation as the difference between opening and closing asset values after adjusting for revaluations due to price changes, following SEEA EA paragraphs 10.71-10.80. Where significant losses have occurred, disaggregate degradation by driver (eutrophication, storm damage, dredging) to support targeted policy responses.

Step 8: Document methods and quality assurance

Prepare metadata documentation following ISO 19115 standards, describing data sources, processing methods, accuracy assessments, and assumptions for each account component. Apply the quality assurance framework from TG-0.7 Quality Assurance, including coherence checks between extent, condition, and services accounts. For example, verify that reductions in seagrass extent are reflected in reduced carbon sequestration capacity, and that condition declines are consistent with observed pressures such as eutrophication. Publish accounts with accompanying quality statements that transparently report limitations and fitness-for-purpose assessments.

3.6 Worked Example

This worked example demonstrates the compilation of seagrass ecosystem accounts for a hypothetical 12,000-hectare seagrass meadow system spanning temperate and tropical waters. The example follows the extent-condition-services-valuation sequence presented in Section 3 and illustrates the key accounting entries and calculations.

Setting: A national ecosystem accounting area (EAA) containing 12,000 hectares of seagrass meadow classified as M1.1 Seagrass meadows. The system comprises temperate Posidonia beds (7,500 ha, to 15 m depth) and tropical Thalassia-Halophila meadows (4,500 ha, to 12 m depth), distributed across two coastal embayments.

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

Step 2: Condition account

Condition indicators are derived from field survey data using species-specific reference levels from protected reference sites:

Note: For epiphyte load, a lower value indicates better condition. The indicator formula is inverted: Indicator = (VL - V) / (VL - VH).

Composite condition index (equal weights): (0.60 + 0.57 + 0.67 + 0.71) / 4 = 0.64

Step 3: Ecosystem services (annual flows)

Step 4: Asset valuation

Applying a 4% social discount rate over an indefinite time horizon (assuming effective management and protection):

Asset value = Annual service value / discount rate Asset value = 10,928,000 / 0.04 = 273,200,000 USD

Alternatively, over a 25-year finite horizon:

Asset value = 10,928,000 x 15.62 = 170,700,000 USD

This worked example illustrates the full accounting sequence for seagrass ecosystems. Actual compilations will require country-specific data, species-appropriate reference levels, and primary 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 eutrophication-related decline is classified separately from natural reduction to support pressure-specific policy analysis, consistent with the guidance in Section 3.1.4. The example values are illustrative and should not be used as benchmarks for specific national contexts.

4 Acknowledgements

This guidance draws on the conceptual framework and methodological recommendations of the System of Environmental-Economic Accounting -- Ecosystem Accounting (SEEA EA) and its supporting technical materials. The IUCN Global Ecosystem Typology provides the ecosystem classification framework. Scientific understanding of seagrass ecosystem services draws on extensive research literature, including foundational works by Costanza et al., Duarte et al., and the global seagrass research community.^[54]

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Reviewers: [Names and affiliations]

5 References

De Boer, W.F. (2007). 'Seagrass-sediment interactions, positive feedbacks and critical thresholds for occurrence: A review'. Hydrobiologia 591: 5-24.

Duarte, C.M., Middelburg, J.J., Caraco, N. (2005). 'Major role of marine vegetation on the oceanic carbon cycle'. Biogeosciences 2: 1-8.

Fourqurean, J.W., Duarte, C.M., Kennedy, H., et al. (2012). 'Seagrass ecosystems as a globally significant carbon stock'. Nature Geoscience 5: 505-509.

Keith, D.A., Ferrer-Paris, J.R., Nicholson, E., Kingsford, R.T. (eds.) (2020). IUCN Global Ecosystem Typology 2.0: Descriptive profiles for biomes and ecosystem functional groups. Gland, Switzerland: IUCN.

Larkum, W.D., Orth, R.J., Duarte, C.M. (eds.) (2006). Seagrasses: Biology, Ecology and Conservation. The Netherlands: Springer.

NCAVES and MAIA (2022). Monetary valuation of ecosystem services and ecosystem assets for ecosystem accounting: Interim Version 1st edition. United Nations Department of Economic and Social Affairs, Statistics Division, New York.

Orth, R.J., Carruthers, T.J., Dennison, W.C., et al. (2006). 'A global crisis for seagrass ecosystems'. BioScience 56(12): 987-996.

United Nations (2021). System of Environmental-Economic Accounting -- Ecosystem Accounting (SEEA EA). New York: United Nations.

United Nations (2014). System of Environmental-Economic Accounting 2012 -- Central Framework. New York: United Nations.

United Nations (2022). Guidelines on Biophysical Modelling for Ecosystem Accounting. New York: United Nations Department of Economic and Social Affairs, Statistics Division.

Van der Heide, T., Govers, L.L., de Fouw, J., et al. (2012). 'A three-stage symbiosis forms the foundation of seagrass ecosystems'. Science 336(6087): 1432-1434.

Cross-Reference Summary