Freshwater-Marine Interaction Accounting

1. Outcome

After completing this Circular, practitioners will be able to compile accounts at the freshwater-marine interface, including catchment-coast linkages, nutrient and sediment flow accounts, and estuarine ecosystem assets. It bridges terrestrial water accounting frameworks with marine and coastal ocean accounts, addressing a critical gap in environmental-economic accounting: the systematic tracking of material and energy flows that connect inland water systems to marine environments. The freshwater-marine interface determines the condition of coastal ecosystems, the productivity of near-shore fisheries, and the extent to which land-based economic activities generate downstream environmental pressures on marine systems^[1].

The accounting methodology presented here integrates the SEEA Water framework for inland water resource accounts with SEEA Ecosystem Accounting for transitional and coastal ecosystems. SEEA Water provides the foundational structure for tracking water abstractions, returns, and quality at the river basin level, while SEEA EA extends this to the ecosystem assets and services that depend on freshwater inputs to marine systems^[2]. By following this guidance, practitioners will be able to compile flow accounts tracking nutrient, sediment, and pollutant transfers from catchments to the coast; extent and condition accounts for estuarine, deltaic, and lagoon ecosystems; and integrated accounts that reveal the economic drivers of freshwater-marine interactions and their consequences for coastal ecosystem services.

This Circular connects to several prerequisite and related circulars. TG-3.2 Flows: Environment to Economy provides the framework for recording freshwater abstraction and other resource flows from the environment to economic units within catchments. TG-3.4 Flows: Economy to Environment covers the return flows and residual discharges from economic activities back to the environment, including wastewater and agricultural runoff that constitute the primary anthropogenic pressures at the freshwater-marine interface. TG-6.2 Mangrove and Coastal Wetland Accounting addresses the coastal ecosystems that receive freshwater inputs and whose condition depends on the quantity and quality of those inputs. TG-3.11 Sub-National Ocean Accounts provides guidance on spatial disaggregation to the river basin and coastal zone scales at which freshwater-marine interactions are most meaningfully analysed.

2. Requirements

Essential prerequisites:

• 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 freshwater-marine interaction accounts are compiled.

• TG-3.2 Flows: Environment to Economy -- for the methodology on recording flows of environmental resources (including freshwater) to economic units. Freshwater abstraction within catchments directly reduces downstream flows to estuarine and marine environments, making these flow accounts essential inputs to freshwater-marine interaction accounting.

• TG-3.4 Flows: Economy to Environment -- for the framework governing return flows and residual discharges from economic activities to the environment. Nutrient loading, sediment mobilisation, and pollutant discharges recorded under TG-3.4 constitute the primary anthropogenic pressures transmitted through the freshwater-marine interface.

Helpful background:

• TG-6.2 Mangrove and Coastal Wetland Accounting -- for the ecosystem accounting methodology applied to coastal wetland systems that are directly influenced by freshwater inputs. The condition and service accounts compiled under TG-6.2 depend on the freshwater flow and quality information compiled under this Circular.

3. Guidance Material

Freshwater-marine interactions occur across a continuum of transitional environments where river systems meet the coast. These environments are classified within the IUCN Global Ecosystem Typology (GET) across several biomes and realms, reflecting their hybrid terrestrial-freshwater-marine character. The principal ecosystem types addressed in this Circular include estuaries, coastal lagoons, river deltas, and the nearshore marine environments influenced by freshwater discharge^[3]. Within the IUCN GET framework, these correspond primarily to the Marine-Freshwater-Terrestrial (MFT) transitional realm, particularly the MFT1 Brackish Tidal Systems biome, alongside elements of the Freshwater-Marine (FM) transitional realm^[4].

The SEEA Water framework provides the accounting structure for inland water resources, organising water accounts around the hydrological cycle: precipitation, evapotranspiration, runoff, groundwater recharge, and river discharge^[5]. At the freshwater-marine interface, river discharge constitutes the terminal flow in the inland water balance, delivering water, dissolved substances, and suspended materials to the coast. This terminal flow is the primary link between SEEA Water accounts and the coastal and marine ecosystem accounts compiled under SEEA EA. Current approaches suggest treating the freshwater-marine interface as a systematic accounting boundary where inland water flow accounts connect to coastal ecosystem extent, condition, and service accounts, though methods for defining and operationalising this boundary are still being refined.

This section provides guidance on catchment-coast accounting linkages (Section 3.1), nutrient and sediment flow accounts (Section 3.2), estuarine and deltaic ecosystem asset accounts (Section 3.3), integration with SEEA Water accounts (Section 3.4), and spatial delineation of the land-sea interface (Section 3.5). The methodology builds on the flow accounting frameworks in TG-3.2 Flows: Environment to Economy and TG-3.4 Flows: Economy to Environment, and applies the ecosystem accounting principles from SEEA EA to the specific context of transitional water ecosystems.

3.1 Catchment-Coast Accounting Linkages

Catchment-coast accounting establishes the systematic connection between economic activities within river basins and their downstream effects on coastal and marine ecosystems. The fundamental accounting principle is that material flows through hydrological pathways link upstream economic pressures to downstream environmental outcomes. These linkages are bidirectional: tidal and storm-driven marine influences also penetrate upstream, affecting freshwater quality, sediment dynamics, and ecosystem condition in lower catchments^[6].

River basin as accounting unit

The river basin (or catchment) is the natural spatial unit for freshwater-marine interaction accounting. SEEA Water identifies the river basin as the primary spatial framework for water resource accounting, consistent with integrated water resources management (IWRM) principles^[7]. For ocean accounting purposes, the river basin provides the upstream boundary of the accounting area, while the coastal zone and nearshore marine environment provide the downstream boundary. The connection between these two spatial domains occurs at the river mouth, estuary, or coastal discharge point.

Compilers should delineate river basins using hydrological boundaries (watersheds) rather than administrative boundaries, though administrative disaggregation may be required for linking to economic statistics. TG-3.11 Sub-National Ocean Accounts provides guidance on reconciling hydrological and administrative spatial units. Where multiple river basins discharge into a single coastal zone, the accounting framework should aggregate basin-level flows to derive total catchment inputs to the receiving marine environment.

The following table identifies the principal linkage types between catchment activities and coastal outcomes:

Temporal dynamics

Freshwater-marine interactions exhibit strong temporal variability at multiple scales. Seasonal discharge patterns determine the annual cycle of freshwater input to coastal waters, while episodic flood events can deliver the majority of annual sediment and nutrient loads within a few days^[8]. Accounting periods should be aligned with hydrological years where possible, and compilers should document whether flow data represent annual averages, seasonal totals, or event-based measurements. For nutrients and sediments, load estimates based on continuous monitoring will differ substantially from those based on periodic grab sampling, and the methodology used should be recorded in account metadata.

Longer-term trends in catchment hydrology--driven by land use change, climate change, and water infrastructure development--affect the baseline against which annual changes are measured. Compilers should establish a reference period for baseline flows and document any significant changes in catchment characteristics (dam construction, land use conversion, urbanisation) that alter the flow regime during the accounting period.

Accounting for regulated flows

Many river systems are heavily regulated by dams, diversions, and inter-basin transfers. These interventions alter the natural timing, magnitude, and quality of freshwater delivery to the coast. In accounting terms, dam regulation creates a distinction between natural (unregulated) flow and actual (regulated) flow. The SEEA Water framework records actual abstractions and returns, but for freshwater-marine interaction accounting, it may also be useful to estimate the counterfactual natural flow to quantify the extent of hydrological alteration, though robust estimation of unregulated baselines remains methodologically challenging^[9].

Key accounting entries for regulated systems include:

The hydrological alteration index may provide a useful summary measure of catchment modification relevant to coastal ecosystem condition, though its application in an accounting context is still emerging. Values close to 1.0 indicate near-natural flow regimes, while values significantly below 1.0 indicate substantial flow reduction. Values above 1.0 may occur where inter-basin transfers augment natural flows. This index can inform the condition assessment of estuarine ecosystems (Section 3.3), where freshwater inflow is widely recognised as a primary determinant of ecosystem health.

3.2 Nutrient and Sediment Flow Accounts

Nutrient and sediment flows through river systems to the coast represent critical material transfers that shape coastal ecosystem condition and productivity. These flows are recorded as physical flow accounts within the SEEA framework, tracking the movement of specific substances from economic sources through the environment to receiving coastal waters^[10].

Nutrient flow accounting

Nutrient loading to coastal waters, particularly nitrogen (N) and phosphorus (P), is the primary driver of coastal eutrophication worldwide. Excess nutrients stimulate algal growth, which upon decomposition depletes dissolved oxygen, creating hypoxic or anoxic "dead zones" that degrade fisheries, tourism, and coastal ecosystem services^[11]. The SEEA framework for recording residual flows from the economy to the environment (TG-3.4 Flows: Economy to Environment) provides the accounting structure for these nutrient discharges.

Nutrient flow accounts should track loads (mass per unit time) rather than concentrations alone, as the ecological impact on receiving waters depends on the total mass of nutrients delivered. The principal sources and their typical accounting treatment are:

The total nutrient load delivered to the coast is the sum of all source contributions, minus in-stream retention (nutrient uptake and denitrification within rivers, wetlands, and floodplains). In-stream retention can be substantial--riparian wetlands and floodplains may remove 20-80% of nitrogen load through denitrification--and should be recorded as an ecosystem service (water purification) in the accounts^[12]. This retention service is considered an intermediate ecosystem service that may directly connect to the ecosystem condition of the receiving coastal environment, though quantifying retention rates at catchment scale remains an active area of methodological development.

Compilers should distinguish between gross nutrient discharge (total load entering waterways from all sources) and net nutrient delivery to coast (load reaching the river mouth after in-stream processing). The difference represents the water purification service provided by freshwater ecosystems within the catchment. This distinction is critical for policy, as it reveals both the pressure from economic activities and the mitigating role of intact freshwater ecosystems.

Sediment flow accounting

Sediment transport from catchments to the coast governs coastal morphology, determines the fate of deltas and estuaries, and provides the substrate for mangrove and wetland accretion. Globally, dam construction has reduced sediment delivery to the coast by an estimated 25-30%, contributing to accelerated coastal erosion and delta subsidence^[13]. Sediment flow accounts track the supply, transport, and delivery of particulate material through river systems to receiving coastal waters.

The sediment budget for a river basin can be expressed as:

Sediment delivery to coast = Erosion supply - In-channel storage - Reservoir trapping - Floodplain deposition

Each component should be recorded in the physical flow account:

Sediment delivery directly affects the extent and condition of coastal ecosystems covered by TG-6.2 Mangrove and Coastal Wetland Accounting. Mangroves and saltmarshes require sediment accretion to maintain elevation relative to sea level; reduced sediment supply from upstream dam construction or sand mining can lead to submergence and ecosystem loss^[14]. Conversely, excessive sediment delivery from land clearing or mining can smother coral reefs and seagrass beds. The sediment flow account thus provides essential context for interpreting changes in coastal ecosystem extent and condition accounts.

Pollutant and contaminant tracking

Beyond nutrients and sediments, freshwater systems transport a range of chemical contaminants to coastal waters, including heavy metals, persistent organic pollutants, microplastics, and pharmaceuticals. While comprehensive contaminant accounting across all substances is beyond the scope of most initial compilations, compilers should identify the priority contaminants for their national context and establish monitoring and accounting protocols for those substances^[15].

A tiered approach is recommended:

• Tier 1: Account for total nitrogen and total phosphorus loads (minimum requirement for all freshwater-marine interaction accounts)
• Tier 2: Add suspended sediment load, biochemical oxygen demand (BOD), and priority metals (e.g., mercury, cadmium, lead)
• Tier 3: Include emerging contaminants (microplastics, pharmaceuticals, per- and polyfluoroalkyl substances) where monitoring data exist

The pollutant flow account should be structured consistently with the residual flow accounts in TG-3.4 Flows: Economy to Environment, attributing loads to source industries and tracking delivery to the receiving coastal environment.

3.3 Estuarine and Deltaic Ecosystem Asset Accounts

Estuaries, deltas, and coastal lagoons are the ecosystems that occupy the freshwater-marine interface. These transitional water bodies support high biological productivity, serve as nursery habitat for fisheries, provide water filtration services, and are centres of human settlement and economic activity^[16]. Accounting for these ecosystems as assets requires extent accounts, condition accounts, and ecosystem service accounts following the SEEA EA framework.

Ecosystem type classification

Transitional water ecosystems span the boundary between freshwater and marine realms, creating classification challenges. Within the IUCN GET reference classification used by SEEA EA, the relevant ecosystem functional groups include^[17]:

Compilers should note that the ecosystem types at the freshwater-marine interface often form complex spatial mosaics. A single estuary may contain elements of several GET functional groups (tidal channels, mangrove fringes, saltmarsh flats, subtidal basins) that function as an integrated system. For accounting purposes, compilers may either: (a) map and account for each component ecosystem type separately, consistent with the SEEA EA recommendation to compile accounts at the ecosystem type level; or (b) define composite estuarine or deltaic ecosystem assets that encompass the full mosaic, documenting the internal composition. The choice depends on data availability and policy needs; approach (a) provides greater analytical resolution while approach (b) may better capture the functional integrity of transitional systems. Experience with both approaches is still limited, and emerging pilot implementations suggest that the optimal strategy may vary substantially by estuary type and data environment.

Extent accounting for transitional ecosystems

Extent accounts for estuarine and deltaic ecosystems follow the standard SEEA EA structure (see TG-6.2 Mangrove and Coastal Wetland Accounting Section 3.1 for the general approach)^[18]. However, transitional water ecosystems present specific measurement challenges:

Dynamic boundaries. The spatial extent of estuaries fluctuates with tidal cycles, river discharge, and seasonal variation. The accounting boundary should be defined using a consistent reference state (e.g., mean high water for the landward boundary, mouth cross-section for the seaward boundary) and documented in metadata. Changes in the estuary mouth configuration--such as barrier beach opening/closing in intermittently open systems--should be recorded as condition changes rather than extent changes unless they result in permanent loss or gain of the ecosystem type.

Submerged extent. Unlike terrestrial ecosystems where extent is measured as surface area, estuarine ecosystems include significant submerged components (subtidal channels, basins). Extent may be reported as either surface area or water volume, depending on the ecosystem type. For subtidal estuarine basins, water volume at a reference tidal state may be more informative than surface area alone.

Multi-dimensional classification. Estuarine extent accounts should distinguish between intertidal and subtidal components, and between vegetated (mangrove, saltmarsh, seagrass) and unvegetated (mudflat, sand flat, open water) areas. This disaggregation enables linkage to the vegetation-specific accounts compiled under TG-6.2 Mangrove and Coastal Wetland Accounting.

The extent account template for transitional ecosystems:

Condition assessment for transitional ecosystems

The condition of estuarine and deltaic ecosystems depends critically on the freshwater inputs they receive, making condition assessment inseparable from catchment-coast flow accounting^[19]. Condition variables should be organised according to the SEEA Ecosystem Condition Typology (ECT), with particular attention to variables that reflect the freshwater-marine interaction:

Physical state (Class A1):

• Freshwater inflow volume and timing (deviation from reference hydrograph)
• Salinity gradient (longitudinal and vertical stratification patterns)
• Tidal prism (volume exchanged per tidal cycle)
• Water residence time (flushing rate)
• Sedimentation/erosion rates

Chemical state (Class A2):

• Dissolved oxygen (spatial and temporal patterns, frequency of hypoxia)
• Nutrient concentrations (N, P; linked to catchment loading accounts in Section 3.2)
• Turbidity and light availability
• pH and carbonate chemistry
• Priority contaminant concentrations

Compositional state (Class B1):

• Fish species richness and community composition
• Benthic invertebrate diversity
• Phytoplankton community structure (including harmful algal bloom species)
• Presence of key/indicator species (e.g., diadromous fish, filter-feeding bivalves)

Structural state (Class B2):

• Habitat heterogeneity (diversity of substrate types, depth classes)
• Vegetation cover (emergent, submerged, riparian)
• Biogenic reef structures (oyster reefs, mussel beds)

Functional state (Class B3):

• Primary productivity (phytoplankton and benthic)
• Nutrient cycling rates (denitrification, nutrient uptake)
• Fish recruitment (juvenile abundance and growth)

Landscape and seascape characteristics (Class C1):

• Connectivity to upstream catchment (barriers, flow modification)
• Connectivity to marine environment (mouth openness, tidal exchange)
• Surrounding land use (proportion of catchment under natural vegetation)
• Fragmentation of estuarine habitats

The condition account should record opening and closing values for each selected variable, with reference conditions established using historical baselines (pre-disturbance state), minimally disturbed reference estuaries, or expert-defined targets, consistent with the approach described in SEEA EA Section 5.3. For estuarine systems, the reference condition for freshwater inflow-dependent variables should reflect the natural (pre-regulation) flow regime.

Ecosystem services of transitional waters

Estuarine and deltaic ecosystems provide a distinctive suite of ecosystem services that depend on the maintenance of freshwater-marine interactions^[20]:

Water purification and nutrient regulation. Estuaries process and attenuate nutrient loads from upstream catchments through denitrification, sedimentation, and biological uptake. This intermediate ecosystem service reduces the nutrient load reaching offshore marine waters, protecting coral reefs, seagrass beds, and pelagic ecosystems from eutrophication. One promising approach to measuring the physical quantity of this service is to estimate the mass of nitrogen or phosphorus removed between the head and mouth of the estuary (tonnes N or P per year), derived from the difference between upstream delivery (Section 3.2) and downstream export, though accurately attributing removal to specific processes remains methodologically challenging.

Fisheries nursery and production. Estuaries serve as critical nursery habitat for many commercially important marine fish and invertebrate species, as well as supporting resident estuarine fisheries. This service is closely linked to the nursery habitat services described in TG-6.2 Mangrove and Coastal Wetland Accounting Section 3.5, and compilers should ensure consistent treatment to avoid double counting between estuarine and mangrove/wetland nursery accounts.

Sediment regulation. Estuaries and deltas trap, redistribute, and export sediment, affecting coastal morphology and the sediment supply to adjacent beaches, mangroves, and wetlands. This service is closely linked to the sediment flow accounts in Section 3.2 and the sediment accretion requirements of coastal wetlands documented in TG-6.2.

Flood attenuation. Estuarine floodplains and wetlands provide natural flood storage, attenuating both riverine and coastal storm surge flooding. This service is valued using avoided damage methods, consistent with the coastal protection valuation approach in TG-6.2 Mangrove and Coastal Wetland Accounting Section 3.6.

3.4 Integration with SEEA Water Accounts

The SEEA Water framework provides the standardised accounting structure for inland water resources, covering water supply and use tables, physical water flow accounts, water quality accounts, and water asset accounts^[21]. Freshwater-marine interaction accounting extends SEEA Water to the coast by treating river discharge as the terminal outflow in the inland water balance and the opening inflow in the coastal/marine water balance.

Water supply and use tables at the basin-coast interface

SEEA Water supply and use tables record the volume of water abstracted from the environment by economic units and the volume returned after use^[22]. For freshwater-marine interaction accounting, the key entries are:

Net abstraction within catchment. Total water abstraction by agriculture, industry, and households minus return flows. Net abstraction reduces the volume of freshwater reaching the coast, directly affecting estuarine salinity, flushing, and ecosystem condition.

Environmental flows. Many countries have established environmental flow requirements specifying minimum discharge levels to maintain downstream ecosystem health. Environmental flows represent a managed allocation of water to the environment, recorded in the supply and use table as a flow from the economy to the environment. Compliance with environmental flow requirements should be tracked in the accounts, as shortfalls directly impact estuarine and coastal ecosystem condition.

Return flow quality. The quality of return flows (wastewater discharges, agricultural drainage, cooling water) determines the nutrient and contaminant load delivered to downstream environments. Linking SEEA Water quality accounts to the nutrient flow accounts in Section 3.2 requires matching discharge point locations and volumes with concentration data to derive mass loads.

The following table illustrates the linkage between SEEA Water entries and freshwater-marine interaction accounts:

Water quality accounts

SEEA Water quality accounts complement physical flow accounts by recording the quality characteristics of water resources^[23]. For the freshwater-marine interface, water quality accounts should track key parameters at the river-estuary boundary and at the estuary-ocean boundary, enabling calculation of nutrient and contaminant loads as the product of flow volume and concentration.

The water quality account at the freshwater-marine boundary should include:

Compilers should establish monitoring stations at the upstream boundary of the estuarine ecosystem (typically the tidal limit or head of salt intrusion) and at the downstream boundary (estuary mouth or offshore reference point). The difference in loads between these two points may approximate the net processing (retention, transformation, or export) occurring within the estuarine ecosystem, which can then be recorded as an ecosystem service flow, though disentangling estuarine processing from tidal mixing effects remains an area of active research.

Linking water asset accounts to ecosystem asset accounts

SEEA Water includes water asset accounts that record the stocks of water in surface water bodies, groundwater, and soil moisture^[24]. For transitional ecosystems, the water asset is inseparable from the ecosystem asset: the estuary is simultaneously a water body (recorded in SEEA Water) and an ecosystem asset (recorded in SEEA EA). Compilers should ensure consistency between these two accounting treatments, avoiding double counting while recognising the complementary information each provides.

The recommended approach is:

1. Record the physical water stock (volume) in SEEA Water asset accounts
2. Record the ecosystem asset (extent, condition, services) in SEEA EA accounts
3. Link the two through the physical flow accounts, where changes in water volume affect ecosystem condition and service provision
4. In monetary terms, avoid summing the water asset value (based on water supply services) and the ecosystem asset value (based on ecosystem services) where these draw on the same underlying resource

This integration is intended to ensure that the accounts provide a more complete picture of the freshwater-marine interface while minimising internal inconsistency, though practical experience with this reconciliation is still developing. TG-3.2 Flows: Environment to Economy provides further guidance on avoiding double counting between resource flow accounts and ecosystem service accounts.

3.5 Land-Sea Interface Spatial Delineation

Spatial delineation of the freshwater-marine interface is a prerequisite for compiling all accounts described in this Circular. The land-sea interface is not a single line but a zone of transition, and the boundaries adopted for accounting purposes will affect the scope and content of the resulting accounts^[25].

Delineation principles

Three spatial boundaries must be defined to delineate the freshwater-marine interaction accounting area:

Upstream (catchment) boundary. The watershed divide of all river basins draining to the coastal zone of interest. This boundary is typically well-defined by topography and can be derived from digital elevation models. For large continental river basins, it may be appropriate to focus on the lower catchment (e.g., the last 50-100 km upstream of the coast) rather than the entire basin, depending on data availability and policy relevance. Where the full basin is included, sub-basin disaggregation following TG-3.11 Sub-National Ocean Accounts enables attribution of pressures to specific upstream areas.

Landward (coastal) boundary. The inland limit of marine influence, typically defined by one of: the tidal limit of the river, the extent of salt intrusion, or a fixed distance inland from the shoreline. For consistency with coastal ecosystem accounts in TG-6.2 Mangrove and Coastal Wetland Accounting, the landward boundary should encompass all intertidal and supratidal ecosystems associated with the freshwater-marine interface.

Seaward boundary. The offshore limit of significant freshwater influence. This can be defined by salinity thresholds (e.g., the 30 ppt isohaline), river plume extent (detectable from satellite ocean colour imagery), or a fixed distance offshore. The seaward boundary should ideally extend far enough to capture the zone where river-derived nutrients and sediments materially affect marine ecosystem condition and productivity, though standardised methods for delineating this zone are still emerging.

Spatial data requirements

Delineation of the freshwater-marine interface requires the following spatial datasets:

Compilers should document all spatial boundary decisions in account metadata, including the criteria used, the reference state adopted (e.g., mean or spring tide), and any adjustments made for local conditions. Consistency of spatial boundaries across accounting periods is essential for tracking changes in ecosystem extent and condition.

Integration with marine spatial planning

The spatial delineation of the freshwater-marine interface has direct relevance to marine spatial planning (MSP) and integrated coastal zone management (ICZM). The accounts compiled under this Circular provide the evidence base for identifying zones where upstream catchment management is critical for maintaining downstream marine ecosystem health. Compilers should coordinate with MSP and ICZM processes to ensure that accounting boundaries align with planning zones where possible, and that account outputs are formatted for use in spatial decision-support tools.

The linkage between river basin management plans and marine spatial plans is a key policy application of freshwater-marine interaction accounts. By quantifying the material flows connecting catchments to the coast, these accounts may enable more integrated management decisions that optimise outcomes across the land-sea interface rather than treating terrestrial and marine management as separate domains. Practical demonstrations of this integration in policy settings are still limited, and further pilot applications will help clarify best practices.

Implementation Considerations

For minimum institutional capacity, data infrastructure, and human skills requirements for compiling these accounts, see TG-0.8 Implementation Readiness Assessment. For guidance on adapting these methods to sub-national scales, see TG-3.11 Sub-National Ocean Accounts.

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: [To be confirmed]

Reviewers: [To be confirmed]

5. References

• SEEA Water (2012), System of Environmental-Economic Accounting for Water
• SEEA Ecosystem Accounting (2021), Chapters 4 (Extent), 5 (Condition), 6 (Services), 7 (Supply-Use)
• IUCN Global Ecosystem Typology, Biome MFT1 (Brackish Tidal Systems), Biome FM1 (Freshwater-Marine)
• SEEA EEA Technical Recommendations, Chapter 3 (Spatial Data)
• Syvitski et al. (2005), Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean
• UNEP/GPA (2006), The State of the Marine Environment: Trends and Processes
• Kennish (2002), Environmental Threats and Environmental Future of Estuaries
• Barbier et al. (2011), The Value of Estuarine and Coastal Ecosystem Services
• Howarth et al. (2011), Coupled Biogeochemical Cycles: Eutrophication and Hypoxia in Temperate Estuaries