Lake Illawarra is a wave-dominated barrier estuary (after Roy et al. 2001) located approximately 100 km south of Sydney (Fig. 1). The Lake (max depth ~ 3.2 m) is characterised by a sand barrier at the entrance, where energetic swells on the coast occasionally closed the entrance from the sea entirely. As of 2007, the entrance of the Lake was permanently opened, which increased the intrusion of marine waters and altered the transport of sediments within the Lake. Before the permanent opening of the entrance, freshwater input was limited within the estuary. Lake hydrodynamics were influenced predominantly by entrance condition and tides (when the Lake was open) (Kumbier et al. 2018). The new equilibrium imposed by the built structures has yet to be reached, with changes to the entrance morphology expected to increase into the future (Couriel et al. 2013).

Figure 1.  

Location of Lake Illawarra in New South Wales (NSW) Australia (inset) and the basic spatial units (1 km² grid, n = 73) used to measure mangrove, tidal marsh and seagrass in ecosystem accounting and (c) the seven level 2 statistical areas (SA2s) used in Australian Bureau of Statistics (ABS) census data used to estimate full-time equivalent employment (FTE).

Ocean Accounts extend existing accounting standards, where the present study draws upon SEEA-EA and employment from census methods, described in part within the SNA. Environmental and economic components within the Lake Illawarra ‘system’ could therefore be organised into environmental assets (ecosystem extent and condition), their flows (ecosystem services) and employment within related sectors of the ocean economy (Fig. 2a). The Ocean Accounts align with the structure, concepts and definitions described within the SEEA-EA, which is a spatially explicit approach to measure ecosystem extent, condition and services in both physical (e.g. litres, tonnes) and monetary terms (Fig. 2b). The production of the fisheries sector was explored separately to ecosystem services, with additional analyses performed to partition the contribution of coastal ecosystems to harvested biomass. Ocean employment was defined as employment in ocean-related sectors, with data sourced from census data.

a

b

Figure 2.

The Ocean Accounts framework, adapted from the technical guidance (GOAP 2021a), with specific focus on ecosystem accounts and their links to the ocean economy.

a: An overview of the Ocean Accounts framework, subset to three accounting table groups relevant to the Lake Illawarra study, namely: (1) environmental assets, (2) flows to the economy and (3) the ocean economy. *Tables from framework that were not compiled for this study.  
b: The table groups from the Ocean Accounting framework may be disaggregated to ecosystem accounts, following guidance from the System of Environmental-Economic Accounting Ecosystem Accounts (SEEA-EA) and measures of the ocean economy, aligned with the Ocean Accounts technical guidance. In aligning with the Ocean Accounts Framework, Ecosystem Accounts encompassed: (1) environmental assets and (2) flows to the economy, with tables concerning (3) the Ocean Economy compiled separately.  

The present study observed an expansion of mangroves and contraction of seagrass extent in Lake Illawarra between 2010 and 2020, increasing by 2 ha (1197%) and decreasing by 82 ha (-9%), respectively (Table 3). Mangrove expansion occurred primarily near the entrance channel, with single trees and shoots establishing to the west and south of the estuary (Fig. 3). Of the seagrass lost between 2010 and 2020, 76% occurred adjacent to the entrance channel and flood-tide delta, while the marginal expansion of tidal marsh is concentrated at the southern foreshore of the Lake.

Figure 3.  

Ecosystem extent for the (A) mangrove, (B) tidal marsh and (C) seagrass coastal ecosystems for the 2010, 2015 and 2020 accounting periods. Basic spatial units used in ecosystem accounting overlayed. The (D) change at the Lake entrance and flood-tide delta is shown for all three ecosystems, with the location indicated in the red within (A).

Figure 4.  

The amount of catch landed in (A) physical (tonnes) and (B) monetary (A$ thousands) values, apportioned amongst seagrass, tidal marsh and other ecosystems for crabs, mullet and prawn species. Trophic modelling of stable isotope data in published studies was used to estimate the energy transfer from producers to consumers. Raw data tables for the Figure is presented in Table SM 3.2 of Suppl. material 3.

The flow (annual sequestration) of carbon (C), nitrogen (N) and phosphorus (P) into the biomass of mangroves, tidal marsh and seagrass for 2020 are presented in Table 4, where only C flows could be estimated across all three ecosystems. For 2020, the three ecosystems were estimated to sequester 174.07 tonnes C, with a monetary value of A$2783. The stock of C, N and P within the biomass of the three coastal ecosystems are presented in Table 5. An estimated 845 tonnes C was estmated across the three ecosystems in 2020, with seagrasses accounting for 43% of total carbon. The net carbon balance between 2010 and 2020 was a gain of 142.67 tonnes C within biomass, despite a loss of 38 tonnes C captured within biomass due to the contraction of seagrass.

The capture of N and P into biomass was estimated to increase by 13% and 9%, respectively between 2010 and 2020 (Table 5). The contraction of seagrass decreased capture of N by 2.37 tonnes N, which was offset by the expansion of mangroves and tidal marsh, leading to a net change of 3.87 tonnes N. Similarly, P capture in biomass increased by 0.35 tonnes P, with mangroves estimated to increase P capture by 1110% between 2010 and 2020. The largest net change in N and P capture was observed in the woody component of mangrove biomass (Fig. 5, see Table SM2.7 of Suppl. material 2).

Figure 5.  

Capture (in tonnes) of (A) nitrogen and (B) phosphorus into the biomass of three coastal vegetation ecosystem types (mangroves, tidal marsh and seagrass) for Lake Illawarra across the three accounting years. Note that seagrass is presented as Ruppia sp. and Zostera sp. Raw data tables for the Figure are presented in Table SM 2.7 of Suppl. material 2.

The monetary value of ecosystem services for coastal vegetation were estimated for both stock (lifetime of the asset) and flow during the accounting period. The monetary value of carbon stock within Lake Illawarra was estimated at A$13,522 in 2020, increasing by 20% for the accounting period. The monetary value of N and P stock was estimated at A$70 million and A$50 million, respectively Table 5.

Significant variations in catch, by total volume and composition were observed between 2010 and 2020. The highest catch by weight and value was observed in 2010, landing over 200 tonnes with a value estimated at A$1.14 million (Table 6). The composition of the catch shifted between accounting periods, with almost 60 tonnes of prawns landed in 2010, relative to 12.5 tonnes in 2020 and 27.8 to 6.1 tonnes of crab between 2010 and 2020. The net change in total catch within the accounting period was 74 tonnes, with prawn catch decreasing by 47 tonnes, while crab and finfish decreased by 22 tonnes and 26 tonnes, respectively.

In estimating the contribution of ecosystems to the diet of target species of commercial size, the biomass of crab, prawn and mullet species showed reasonably strong attribution to seagrass ecosystems (based on food web modelling from other seagrass-dominated systems) (Fig. 4). Using diet as an indicator of energy flow, the 60 - 70% of the composition of crab, mullet and prawn biomass were traced to seagrass biomass (see Suppl. material 3), where the value of seagrass supporting crab, prawn and mullet catch was estimated at A$451,425 and A$255,152 in 2010 and 2020, respectively. Tidal marsh contribution to commercial catch was valued at A$135,228 and $55,744 in 2010 and 2020, respectively. While biophysical conditions can influence fisheries productivity, changes to value are also influenced by catch composition, fishing intensity and market price.

Ocean employment in Lake Illawarra increased by 112% from 34 to 72 FTE employees between 2011 and 2016 (Table 7). Almost all sectors increased in employment, with water transport support services increasing by 227%, which included jobs such as stevedoring, water freight transport and terminal operations and other support services. Over the 5-year period, fishing only increased by 1 FTE (10%). The census data reported ‘boat building’ and ‘ship building’ for 2011 and 2016, respectively, where aggregating both activities identified a 67% increase in employment.

Changes in coastal vegetation within Lake Illawarra were observed between 2010 and 2020, inclusive. Mangrove extent increased by 2 ha, tidal marsh extent increased by 5 ha, whilst seagrass contracted by 82 ha (Fig. 3, Table 3). The change in seagrass extent occurred predominantly around the flood-tide delta near the entrance of the estuary, whilst the increase in mangrove extent was observed predominantly near the Lake entrance.

The change in biophysical characteristics of Lake Illawarra, such as mangrove expansion, sediment erosion and deposition, have been linked to the permanent opening of the Lake entrance in 2007 (Regena 2016). The permanent opening increased the influence and retention of marine waters within the estuary, which provides ideal conditions for the establishment of mangroves (Woodroffe et al. 2016, Rodríguez et al. 2017). In parallel, the permanent opening increased the velocity of tidal currents, especially within the entrance channel, leading to scouring (i.e. erosion) and transport of sediment into the estuary basin, burying seagrass beds (Regena 2016). The entrance continues to deepen and scour, suggesting an equilibrium has not yet been reached, with increased sediment deposition and seagrass burial expected to continue (Young et al. 2014).

Accounts detailing fisheries production, in terms of the amount landed and its value, could be used to identify feedbacks with changes to ecosystems that are supporting exploited species. Within Lake Illawarra, the accounting period saw a change in prawn and crab catch, at -79% and -78%, respectively, between 2010 and 2020. Crustacean landings are generally highly variable and 2010 happened to represent a particularly productive year for Lake Illawarra, so the catch values reported for 2010 could be considered atypical. Year-to-year variability in crustacean fisheries can arise from fishery decisions and market values, variability in temperature, spawning and recruitment processes, growth and survival, amongst other things. Factors such as rainfall (and drought) can have a substantial influence on prawn biomass, growth and survival (Dall et al. 1990). Prawn species that comprise the majority of catch within the Lake may benefit from the permanent lake opening as they have a juvenile phase within the estuary, but are ocean spawners (Dall et al. 1990). Thus, while it may be tempting to attribute this difference to changes identified within the ecosystem accounts, there are several other contextual factors that need to be considered.

Changes to the value of gross value product (GVP, Fig. 4) are also impacted by unrelated factors to ecosystem extent and condition, including market forces impacting demand, supply, imports and exports. For example, while finfish decreased by 26 tonnes landed, the value of the catch increased by A$ 54 thousand. As such, while catch in physical and monetary terms represents benefits supported by ecosystems, the trends over time need to be interpreted with caution and are not a direct measure of the supply of ecosystem services (GOAP 2021a).

An alternate method is the use of diet (measured via stable isotopes) to link the biomass of ecosystems to catch landed. Diet indicators have been suggested as an indirect means to measure dependency, where commercial species may depend on ecosystem biomass, although may not intersect spatially (Taylor et al. 2018a). For example, 65% of the biomass of mullet were dependent on tidal marsh, while 86% of the biomass of crab species were dependent on seagrass. There is a clear spatial disconnection between ecosystem dependency and residence of commercially-important species. Crabs are found largely along intertidal areas although they demonstrate a dependency on seagrass. Conversely, subtidal fish species were observed to be dependent on tidal marsh, which are rarely submerged within the region and thus limited in providing habitat for such species (Saintilan 2009).

Ocean-related employment within Lake Illawarra was estimated to grow from 34 to 72 Full-time equivalent (FTE) jobs between 2011 and 2016. Despite the significant reduction in prawn and crab catch within the accounting period, FTE employment within the overall fisheries sector remained stable (Table 7). A decline in the employment of a specific sub-fishery (e.g. prawn, crab fisheries) could not be observed directly due to aggregation to the general ‘fishing’ classification in both 2011 and 2016 censuses and estuary catch of all species examined within this study were mostly managed and reported under a single fishery. The increase in FTE employment was attributable to the ‘water transport’ sector, which could be linked to the expansion and maintenance of water transport infrastructure (e.g. docks and jetties) (Fletcher et al. 2020). The permanent opening of the Lake entrance increased navigability, which may be a driver of increased water transport and boating traffic, with observed increases in visitation (Baxter and Daly 2010).

The immediate use of ocean accounts is in the monitoring and evaluation of trends across the environment and economy, to provide information for management interventions. Accounts could be used to demonstrate the impacts of changes to ecosystem extent, to flows such as climate change and eutrophication mitigation. If accounts are maintained over longer timescales, accounts could trace relationships with economic sectors, to better demonstrate trade-offs between ecosystem change and impacts to their services and their benefits to society and the economy. For example, large amounts of seagrass were lost, which would have significantly reduced carbon, nitrogen and phosphorus capture, but this is somewhat offset by the expansion of mangroves. Seagrasses, however, were identified as an important source of primary production, supporting the food webs in which exploited species fed (Fig. 4), where mangroves and their services may not necessarily act as a substitute (c.f. nutrient removal) for supporting the production of these species. Such accounts provide evidence for the contribution of these cosatal ecosystems and could be used as evidence towards their role as nature-based solutions in providing carbon a carbon sink and addressing excess nutrients within the water column (Nesshöver et al. 2017).

A strength of ocean accounting is the ability to support integrated coastal decision-making (and evaluate the outcomes of decisions), which requires knowledge derived from multiple domains. Ocean accounts facilitate this process by providing a ‘common set of facts,’ relevant to several coastal policy processes, such as supporting:

1. the evaluation of policies and management interventions and
2. providing information for planning processes.

Accounts that are maintained over time may trace the impacts of policy across ecosystems, society and the economy (Ruijs et al. 2019), where the accounts support the framing of the system through a Driver-Pressure-State-Impact-Response (DPSIR) approach (see Grondard et al. 2021). When combined with empirical studies, accounts may be used to identify drivers and the potential impacts of management interventions. The efficacy and unintended impacts of the ‘response’ may be monitored by ocean accounts, through both data and the production of statistics and indicators (Fenichel et al. 2020, Farrell et al. 2021).

Ocean accounts further support planning processes, including the development of ocean-based sectors (i.e. blue economy) or area-based planning (Gacutan et al. 2022). The last decades have seen the widespread adoption of Integrated Coastal Zone Management (ICZM), where some jurisdictions have further extended area-based management to both the coastal and marine domain through Coastal and Marine Spatial Planning processes (CMSP, Halpern et al. 2012). Both ICZM and CMSP may utilise an ecosystem-based approach, which endorses consideration of both social and ecological components of the system. Similar sentiments are found within the Lake Illawarra Coastal Management Program (2019 – 2029), which aims to ‘protect and enhance natural processes’, while also considering social, cultural and economic values (BMT 2019). The accounts compiled within this study directly contribute to the monitoring of ecosystem state and future account extensions may consider ecosystem services and social indicators that explore the links between ecosystems (and their health) and amenity and recreational values. In summary, accounts provide a standardised means of monitoring multiple system components.

Integrated and standardised assessment of ecosystems and their services pose a significant conceptual and data challenge. Even for a data-rich and well-studied area such as Lake Illawarra, the compilation of several accounts required several iterations to refine the classifications used. The process also required a multi-disciplinary collaboration across the fields of coastal ecology, geographical information systems (GIS), ecosystem services, environmental-economic and national accounting. During account compilation, it was clear that ecosystem condition accounts could not be compiled and several ‘condition’ and ecosystem service ‘factors’ identified within literature could not be applied directly to Lake Illawarra and warranted further testing against empirical data.

Ecosystem condition and how it affects services, is often overlooked due to complexity and data limitations. It is vital, however, in refining estimates of ecosystem service supply, by considering the functioning of biotic and abiotic ecosystem components. For example, the emergence of young (< 5 years) mangrove ecosystems within Lake Illawarra may significantly increase the rate at which carbon is incorporated into biomass (Lunstrum and Chen 2014), although the lack of structures (i.e. woody biomass, aerial roots) may significantly limit the provisioning of nursery habitats for commercial fish species at the ‘mangrove fringe’ (sensu Aburto-Oropeza et al. 2008) and hence their omission from the present study. Another challenge was the accuracy and reliability of the parameters (i.e. ecosystem service ‘factors’) that translate the extent (and condition) of ecosystems to their provisioning of services. The parameters were sourced from Lake Illawarra where possible, but other estimates were sourced from studies of different estuaries (see Suppl. material 2). This highlights the need for a robust compilation of ‘factors’ at local scales, to increase the accuracy and reliability of ecosystem service estimates. It demonstrates, however, that the accounting exercise provides a useful means of identifying the priority knowledge and data gaps for future research and investment.

Ocean accounting and environmental-economic accounting, generally, have several limitations that should be considered when managing the coastal domain, namely the biases in selecting the contents within accounts and the identification of tipping points. As explored by Chen et al. (2020) and Perkiss et al. (2022), the choice of system components for compilation will bias the values represented within the accounting area. For example, the omission of mangroves as a relatively limited ecosystem would have excluded the estimated contributions to nutrient and carbon capture and further limited a key impact linked to the permanent lake opening. Accounts are also limited in predicting rapid changes in the system (i.e. tipping points), related to measures of thresholds and irreversibility (Chen et al. 2020). For example, accounts compiled prior to the Lake opening would have been limited in predicting any rapid changes to the Lake system (e.g. ecosystem extent and employment).