Introduction

Agriculture, including terrestrial livestock, has long been the principal protein source for many societies. While food production is an essential human activity, it does have significant environmental costs, including land degradation, climate change impacts, water consumption, deforestation, non-renewable fertilisers (inc. phosphorous), eutrophication, and chemotherapeutics. These challenges have driven innovation to change food consumption habits and produce food using more sustainable systems1. The level of nutrient emissions to surface and coastal waters has increased dramatically over the past 50 years, impairing the quality of coastal waters due to eutrophication and nuisance algal blooms2,3,4. As a result, reducing nutrient loads such as nitrogen (N), phosphorous (P), and carbon (C) from anthropogenic activities (e.g., agriculture, wastewater treatment plants and industry emissions) is urgently required to address the overall deterioration of water bodies. In the context of climate change, there is also increasing urgency to reduce carbon emissions and promote carbon sequestration practices.

Aquaculture is the activity of farming aquatic species such as finfish, crustaceans and bivalves. It has potential as a sustainable way to produce animal protein5 and is one of the food systems with a lower environmental burden supported by recent European policy, i.e., the European Green Deal6 and the Farm to Fork Strategy7. Within the aquaculture sector, farming low trophic level species, such as bivalves, could be a more sustainable approach due to its lower environmental impact compared to current food production systems, such as terrestrial animal production8.

From a farming perspective, bivalves have lower technical and labour requirements than other aquatic species, e.g., fish and crustaceans9. Bivalve farming doesn’t require feed inputs, as this group of animals are non-feeding organisms, i.e., they take up food particles from the water column or sediment (e.g., particulate organic matter, phytoplankton and zooplankton). In terms of protein content, it can be estimated that for every tonne of oyster, 109.8 kg of protein (wet weight) is produced, considering the % of protein in dry weight10,11,12. When put in the Irish oyster farming context, ~1220.7 tonnes of protein is produced per annum (calculated by 109.8 kg protein tonne oyster−1 × 11,121 tonnes oyster year−1)13. Following the same procedure, the annual protein production from Irish blue mussels and scallops can also be estimated14,15. Compared to Pacific oysters, the national protein production from blue mussels (4708.8 tonnes of protein year−1) is relatively higher. Meanwhile, in the case of scallops, 451 tonnes of protein year−1, are produced.

In addition to being a productive protein source, bivalves can provide wider ecosystem services (ES) to the aquatic environment, such as nutrient remediation and C sequestration16,17. For instance, bivalves contribute to carbon sequestration through the shell formation process (i.e., biomineralisation), making them a potential carbon sink in the short to medium term. Several studies conducted on different shellfish species (e.g., blue mussel, Pacific oyster, clams, etc.) have highlighted the potential environmental benefits and the low impacts of bivalve farming (Supplementary Tables 1 and 2 provide an extensive summary of existing literature). For instance, a Danish mussel farm estimated a potential nutrient removal of 0.6-0.9 tonne N ha−1 year−1 and 0.03-0.04 tonne P ha−1 year−1 18. A life cycle assessment (LCA) conducted in Italy showed that Manila clam (Venerupis philippinarum) and Mediterranean mussel (Mytilus galloprovincialis) production resulted in relatively low greenhouse gas emissions of 0.022 and 0.055 kg CO2-eq. kg harvested and packaged bivalves −1, respectively. This study also showed that clams and mussels can sequester 254 and 146 g of CO2 per kg of harvested and packaged bivalves, respectively19. Regarding Pacific oysters, in a previous study, it was estimated that Pacific oyster (Magallana gigas) aquaculture could remove 0.02−0.14 tonnes N ha−1 year−1 from seawater20. Another study estimated that farmed produced eastern oysters (Crassostrea virginica) releases a total of 0.13 kg CO2-eq. kg protein−1, which compared to other food sectors, was estimated to be less than 0.5% of the greenhouse gas emissions from beef, small ruminants, pork, and poultry21.

Combining ecosystem services (i.e., nutrient remediation and carbon sequestration) with the environmental impacts of bivalve farming (i.e., eutrophication potential and global warming potential) is still an emerging area of research. Considerable gaps remain in our understanding of biochemical processes and wider environmental interactions. Much of the published data have been generated in the United States, where some species are not currently farmed in the EU22,23,24,25,26,27,28. Overall, limited research has been undertaken within the EU9,18,20,29,30,31, with variability in the methodologies used, making it challenging to extract broad conclusions. In addition, most of the studies combining LCA and ES for shellfish only covered the net carbon capture potential of shellfish farming19,32,33.

Despite the wider potential benefits of bivalve aquaculture, European production of farmed mussels and oysters has been in decline. Previous studies have pointed out disease prevalence, lack of mussel and oyster seed, and low profitability as the main causes of the decline of the sector in the European Union and the EU34,35. To ensure the sustainable expansion of bivalve aquaculture, the current performance of the bivalve sector, in relation to nutrient and carbon emissions, requires further research to enable benchmark data and comparison to other sectors.

To better appraise the environmental performance, sustainability credentials and benefits of the Irish oyster-producing sector, this study evaluated the ES and environmental impacts of Pacific oyster (Magallana gigas) farming by

  1. 1.

    Assessing the nutrient remediation (i.e., N and P) and C sequestration potential ES of Pacific Oyster through morphological and elemental analysis of representative samples;

  2. 2.

    Analysing the environmental impacts of oyster aquaculture by undertaking LCAs of regional Pacific oyster farms; and

  3. 3.

    Combining ES and LCAs results to determine the benefits of Pacific oyster culture in terms of eutrophication (i.e., N and P net remediation) and global warming potential (i.e., net C sink).

The study combined ES and LCA approaches to provide a more holistic evaluation of the ecological potential of bivalve aquaculture and the environmental impact of their production.

Results

Ecosystem services

The morphological analysis results of the three size classes of Pacific oyster at each production site studied are presented in Supplementary Table 3. Results of the nutrient (i.e., %N and %P) and carbon content (i.e., %C) analysis for each site and size investigated did not show significant differences in %N and %C values between sites and sizes (Supplementary Table 4). %P values followed a similar trend as %C and %N, with homogeneous values between sites and sizes. Results to farm scale were extrapolated in terms of a fresh whole product to compare them across the entire Irish sector for Pacific oyster farming. Since no significant differences in nutrient and carbon % were observed between size categories for a given site (Supplementary Table 4), nutrient and carbon content values were averaged per site to express the kg of N, P and C removed from the sea per tonne of fresh product per site (Fig. 1). The results showed increased removal of nutrients (77.3% more N removed per tonne) and carbon (13.4% more C removed per tonne) in Site 2 compared to Site 1 (Fig. 1a, c). In terms of P removed, Site 2 removed 50% more P per tonne of fresh product than Site 1 (Fig. 1b).

Fig. 1: Nutrients and carbon removed from the sea per tonne of Pacific oyster produced at the sites studied.
figure 1

a Nitrogen removed per tonne of fresh Pacific oyster (Magallana gigas) product harvested per site investigated. Values are mean ± standard deviation, n = 3. b Phosphorous removed per tonne of fresh Pacific oyster (Magallana gigas) product harvested per site investigated. Values are mean ± standard deviation, n = 3. c Carbon removed per tonne of fresh Pacific oyster (Magallana gigas) product harvested per site investigated. Values are mean ± standard deviation, n = 3.

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Extrapolating the oyster data at the national scale (i.e., Irish production), the results showed that 834.3 tonnes of C, 33.9 tonnes of N and 3.9 tonnes of P could potentially be removed from the Irish coastal waters per annum (Table 1). From the economic perspective, this would be equivalent to a nutrient removal value of €1.9 million year−1, of which N removal accounts for the most significant proportion (i.e., 97.67%). In terms of population equivalent, the amounts of N removed annually from the investigated sites are comparable to the N emitted by populations of 51 (Site 1) and 230 (Site 2). Based on the national extrapolation, the Irish Pacific oyster sector could potentially remove the nitrogen content equivalent to the wastewater generated by 10,285 people.

Table 1 Extrapolated results on quantities of carbon, nitrogen, and phosphorous removed annually per site from average annual production and estimated nutrient removal values
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Life cycle assessment

The LCA results for 1 tonne of Pacific oysters produced in 2019 (Supplementary Table 7; Fig. 2a) showed an estimated GWP of 373.86 kg CO2 eq. The single most significant contributor to GWP was grading and packing at 38%. This was driven by the use of electricity to operate the various grading machines, hoppers and shaking tables for processing and grading the oysters. The second largest contributor to GWP was diesel production and combustion at 20% of GWP. Electricity data was provided as an annual figure for the whole farming site but was not measured for individual equipment. Therefore, it was not possible to differentiate between activities such as grading, processing, or stock deployment. Trestles accounted for 17% of GWP, followed by depuration at 16% of GWP. The remaining inputs (i.e., bags and seed production) contributed 10% to GWP.

Fig. 2: Life Cycle Assessment results for 1 tonne of Pacific oysters produced in 2019.
figure 2

a The contribution of the environmental burden from the farming processes to produce a tonne of fresh Pacific oysters (Magallana gigas) to market. b Impact assessment of the Cumulative Energy Demand for a tonne of fresh Pacific oysters.

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The AP for 1 tonne of oysters were estimated to be 1.33 kg SO2 eq. Diesel combustion and production accounted for 41% of the AP. The steel that was used in the production of the trestles contributed to 18% of AP. The remaining contributors were those relying on the use of energy. Grading, packing, and depuration combined accounted for 31% of AP. The remaining 10% of AP arose from oyster bag production (6%) and seed production (4%). The EP for a tonne of oysters was estimated to be 0.4 kg PO4 eq. Trestle production accounted for 33% of the EP. The combustion of diesel was the second largest contributor at 27%, followed by depuration at 13%. CED was 4,757.5 MJ tonne−1 of oysters. The contribution of each process followed a similar pattern to the other impact categories, except for diesel production, which accounted for 22% of energy demand. In contrast with other impact categories, the contribution from oyster bag production was higher, accounting for 16% of CED. When assessed across the different energy categories, the primary energy source for oyster production comes from non-renewable fossil fuels (Fig. 2b).

Life cycle assessment and ecosystem services

The quantification of oyster ES (i.e., nutrient remediation and carbon sequestration) resulted in improved EP and GWP emissions. When characterised to PO4 eq., oyster shells were able to sequester 2.36 kg PO4 eq. tonne−1 (combining N and P values) and 1.28 kg PO4 eq. tonne−1 (only N values) (Fig. 3a). When compared to the EP of oyster production at this site (0.39 kg PO4 eq. tonne−1, Supplementary Table 7), the results indicate that Pacific oyster production has a high nutrient remediation potential. Considering only the N remediation potential, Pacific oysters sequestered 228.2% more kg PO4 eq. tonne−1 than emitted, resulting in a negative eutrophication potential (i.e., −0.89 kg PO4 eq. tonne−1). When C was characterised as CO2 eq., results show that 273.54 kg CO2 eq. tonne−1 was bound in the shell (Fig. 3b). When compared to the estimated GWP for Pacific oyster production to farmgate (373.86 kg CO2 eq. tonne−1, Supplementary Table 7), carbon emissions from Pacific oyster farming are reduced by 73.17%, resulting in a balanced emission of 100.32 kg CO2 eq. tonne−1.

Fig. 3: Quantification of Pacific Oyster ecosystem services.
figure 3

a kg of PO4 eq. sequestered (i.e., nutrient remediation), emitted (i.e., Life Cycle Assessment emissions from oyster farming) and balanced [i.e., Kg of PO4 eq. emitted (LCA)- Kg of PO4 eq. sequestered (N)] per tonne of Pacific oyster harvested. b kg CO2 eq. sequestered (i.e., carbon sequestration), emitted (i.e., Life Cycle Assessment emissions from oyster farming) and balanced [i.e., Kg of CO2 eq. emitted (LCA)- Kg of CO2 eq. sequestered] per tonne of Pacific oyster harvested. N nitrogen, P phosphorous, LCA life cycle assessment.

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Discussion

The present study appraised Ireland’s Pacific oyster production system, its contribution to ES, and its environmental impact through LCA. The designed experimental protocol produced data on the nutrients and C sequestration potential of one of the most farmed shellfish species in the country and globally36. The results obtained in the present study showed similar %C, %N, and %P in Pacific oyster shells and tissues compared to other shellfish species17. In terms of C sequestration and nutrient removal ES, the current Irish oyster sector may have a C sequestration potential of 834.3 tonnes year−1 and N and P removal potential of 33.9 and 3.9 tonnes year−1, respectively. Comparison with other ES studies in shellfish species is difficult due to differences in the metrics used. Compared to the present results for Pacific oyster farming (i.e., 3.05 kg N tonne oyster−1, 0.35 kg P tonne oyster−1 and 70.52 kg C tonne oyster−1), a study conducted on blue mussels showed higher nutrient removal and carbon sequestration potential (i.e., 5.0–8.5 kg N tonne mussel−1, 0.43–0.95 kg P tonne mussel−1 and 74.7-77.5 kg C tonne mussel−1)37. Due to resource limitations, only nutrient removal and C sequestration processes associated with oyster farming were investigated in this study. Hence, the net nutrient removal and C sequestration potential of oyster farming could change if nutrient and C emissions from shellfish were assessed within the experimental boundaries. The accumulation of faeces and pseudo-faeces under the oyster trestles results in bio-deposition, a process where seabed sediments are enriched with organic matter, N and P bio-deposits. Studies on the effects of oyster reefs on microbial diversity and ecosystem processes38,39 reported increased CO2 fluxes in habitats covered by Pacific oysters, possibly related to increased microbial activity from the decomposition of organic matter supplied in oyster biodeposits. Therefore, enriched sediments could be used as a potential energy and food source for invertebrate consumers, thus stimulating primary productivity and creating geological modifications of the underlying sediment40. On the other hand, several studies41,42 have pointed out that the calcification process of bivalve shells releases CO2 into the environment, thus altering the CO2 fluxes of oyster cultivation. However43,44, discussed that because shell production can be considered a by-product of the main ecosystem value of bivalve aquaculture, partitioning of the CO2 respired between the soft tissue and shell could be justified when including bivalve shells in the carbon trading system. In addition, a recent study introduced a new potential negative emission technology concept, namely, Carbon Sequestration via bivalve Shellfish Farming (CSSF) from the ecosystem perspective, with a net carbon sequestration ratio of 13.64% for oysters when compared to a natural ecosystem (i.e., ~1%)45.

Regarding the nitrogen flux, bivalves can also contribute to N removal through denitrification mechanisms. Denitrification is the microbial conversion of reactive N to inert nitrogen gas (N2). Oyster production can stimulate denitrification in three ways: (1) by enhancing denitrification through increased deposition of organic matter in sediments; (2) through denitrifying bacteria present in their bodies; and/or (3) by providing habitat for other filter-feeding macrofaunal communities46. A meta-analysis on oysters’ impact on coastal biogeochemistry compared the effect of an oyster ecosystems on N fluxes and denitrification potential using data from 45 studies. The meta-analysis highlighted the potential for denitrification as an N removal mechanism in oyster habitats and aquaculture sites47. Despite the positive evidence of denitrification effects from oyster habitats, its consideration as a nutrient remediation process must be done cautiously, as it may vary according to the site characteristics, species studied, water quality, benthic habitat, nutrient fluxes and the methodology used to measure denitrification47,48. Therefore, in order to include shellfish aquaculture in carbon trading schemes and footprint calculations, a deeper understanding of the regional impact and interaction of oysters to the nutrient and carbon fluxes is needed. This would include the frequent analysis of the nutrients and carbon removal in the water column and the oysters, as well as a mass balance approach under laboratory conditions to gauge the impact and interactive effects.

Due to limited data, several assumptions were made for the extrapolation of results. To extrapolate the shellfish individual results to farm level, it was assumed that shellfish from across the farm uptake carbon and nutrients the same way as the average performance obtained from the sampled individuals. This assumption is justified since the most affecting parameters (i.e., shellfish species, cultivating condition and water quality) on nutrients and C sequestration potential were considered constant at the farm level49,50. In the national extrapolation, it was assumed that shellfish from all farming sites across Ireland would perform at the same level as the ones investigated. The farms investigated in this study were located on the west coast of Ireland, where water quality, environmental conditions, and cultivation practices differ from those on the southern and eastern Irish coasts. Therefore, future research should investigate other shellfish-producing areas to confirm the present results and expand the number of oyster samples assessed to increase the resolution of N, P, and C bioaccumulation datasets.

In terms of value, in 2022, the Irish oyster farming industry recorded an income of ~ €64.45 million, with production costs of ~ €41.19 million13. Although oyster farming is the second largest industry in the Irish aquaculture sector, several constraints exist to promote its growth. It has been shown that farmers experience a lack of capital for investment, mainly related to the problems of Ireland’s licensing system. Market expansion is limited by these licensing problems, as they can hinder the implementation of innovative farming techniques to make the industry more efficient and sustainable51. Therefore, if a nutrient credit programme were implemented at the national and international level, nutrient removal from oyster production would represent a potential benefit of €1.9 million annually to the Irish shellfish sector. This benefit could be used to reduce production costs, as capital for investment, and to support marketing efforts to expand the sector and make it more profitable.

Due to the lack of European-derived data sets, the monetary benefit of the present study was estimated based on the nutrient removal valuation methodology of a previous U.S. based study29. Therefore, to accurately reflect the current European and Irish oyster farming status, more studies are needed on the valuation of ecosystem services in Europe. It is now recognised that bivalve production provides not only ecosystem services but also cultural and economic services. The high amounts of C, N and P in oyster shells give them great potential for use in various applications. A study conducted in Korea found that oyster shell meal used as a liming agent for agricultural fields significantly increased soil pH and improved soil nutritional status, i.e., available phosphate and organic matter mass52. Shellfish shells could also be used as calcium supplements for livestock. The addition of venus shells (Venus gallina) to a limestone supplement significantly improved the egg production performance of laying hens53. Oyster shells could also be a sustainable alternative to traditional building materials (e.g., mortar sand). A study conducted in South Korea showed that small oyster shell particles (2-0.074 mm) were a potential substitute for conventional mortar sands in terms of compressive strength54. Additionally, waste oyster shells are a potential hard substrate for preparing artificial reefs for coral and oyster reef restoration55,56,57,58. Hence, the reuse of oyster shells and their variety of applications could represent a new income stream for the oyster industry, while allowing its transition towards a blue circular economy.

In the Irish Pacific oyster farming sector, fuel use (i.e., use of tractors for oyster harvesting) and energy use (i.e., grading and packaging) were the main drivers of the environmental burden. Infrastructure and equipment played a secondary role in environmental impact. The low service life of the oyster bags and trestles influenced all impact categories. Recent LCA studies on shellfish aquaculture have also reported similar findings, with 39% of GWP for mussels farmed in Italy arising from equipment and infrastructure19. To reduce the operation-associated impacts, operators could apply alternative approaches to lower fuel and energy use, such as using renewable energy sources (e.g., biofuels and green hydrogen) or investing in more efficient engines. According to the Renewable Energy Directive, by 2030, EU countries must ensure that the share of renewables in final energy consumption in transport is at least 14%, including a minimum share of 3.5% of advanced biofuels59. On the other hand, extending the service life of the farming equipment could reduce infrastructure-associated impacts in oyster farming.

In food LCAs, there have been increasing calls for studies to present the results as a function of the system60,61. For food production systems, that is its contribution to human nutrition and food security and thus the results of this study were also considered in terms of protein production, their relative impacts, and ecosystem services. To produce 1 kg of protein from oyster production, there is a need for 28.9 kg of shelled oysters (based on an edible yield of 32% and a protein content of 10.8%). This would result in GWP of 10.8 kg CO2 eq./kg of oyster protein, AP of 0.04 kg SO2 eq., 0.01 kg PO4 eq., and CED of 137.7 MJ. The resultant shell material from 28.9 kg of oysters would result in 19.68 kg of oyster shell, which would have bound 5.11 kg CO2 eq. Subtracting this sequestered CO2 from the emitted CO2 would result in 5.71 kg CO2 eq./kg of oyster protein. This would also result in an edible protein energy return (ep-EROI) on investment of 8.2% (based on the assumption that 1 kg of protein is equal to 16.73 MJ) which is within the ranges of previously published values62.

Overall, oyster farming has shown relatively low global impacts on the environment compared to other seafood production sectors (e.g., wild catch fisheries and aquaculture) or livestock farming (Supplementary Table 2). However, within LCAs, it is difficult to compare the results of one study with those of another confidently due to differences in scope, system boundaries, data availability, assumptions, and methodology. The main environmental drivers in producing animal proteins from aquaculture, fisheries, and terrestrial farming are feed production, energy use, and fuel use. Furthermore, these food production systems are more complex in their life cycle stages, requiring sophisticated infrastructure, more labour, complex technologies (e.g., recirculating aquaculture systems, flow-through aquaculture systems), and additional processing steps (e.g., feed production, slaughtering, meat processing). In contrast, oyster farming has a less complex lifecycle than other farmed animal species and is typically undertaken using traditional techniques (i.e., non-fed rack-and-bag culture on intertidal areas) that do not require feed input and sophisticated infrastructures. There may be opportunities in the bivalve sector for value-added or novel food products targeted at environmentally conscious consumers. These strategic prospects may exist in high-growth food sectors, such as sports nutrition and snacks, for marketing it as a proteinous and nutritious food. Additional opportunities exist in pairing bivalves’ nutritional density and environmental performance to inform consumers better how they can meet their nutritional requirements while limiting their environmental impact63. While the present study demonstrates a low ecological impact, future LCAs should aim to increase the sample size of farms to obtain more solid results. In addition, there is a need to expand the scope of the study higher up the value chain and look at value-added products and the valorisation of circular economy opportunities for bivalve waste and shells in particular.

Given the potential of oyster farming in nutrient uptake, there are opportunities to use or include oyster aquaculture in integrated catchment management. With many Irish rivers failing to meet the requirements of the Water Framework Directive (2000/60/EC)64 and the national herd increasing, the pairing or co-location of these food production systems as complementary activities can mitigate the excess of nutrients in coastal and transitional waters, while producing a low carbon food product. This integrated approach may be limited regarding suitable sites, but novel and emerging oyster culture systems may address this.

In addition, uncertainty exists regarding the balanced EP results presented in this study.

Determining the balanced EP of oyster farming by combining EP-emitted values by LCA approaches with the results of nutrient extraction is challenging. To date, the researchers of the present study are not aware of an approved methodology for estimating the net EP from bivalve production systems. Consequently, the authors used a first-principles approach similar to the estimation of net GWP, where the GHG contribution is considered as CO2 equivalents to an atmospheric pool, whereas EP does not necessarily consider a common pool estimate. In addition, the life cycle impact assessment methodology used was the CML method65. This is one of the oldest LCIA methodologies, but it is still one of the most widely used in fisheries and aquaculture LCAs66,67. The methodology is limited as it does not include fate and exposure, time horizon and geographical scale65. Furthermore, it presents EP as a worst-case scenario, assuming that 100% of emissions contribute to impacts68. On the other hand, results may also vary if more recent EP characterisation factors are used to convert N and P content in shells to PO4 eq. As can be seen, there is a distinct gap in the tools available to model the nutrient removal potential of bivalves under an LCA framework. However, there have been recent advances in the field of understanding the biogeochemical processes of shellfish denitrification and their potential in nutrient management and removal46,47,48,69. Therefore, considering these limitations, future work should focus on developing an LCA methodology that can accurately model and include the nutrient remediation processes more completely as part of LCAs to estimate the net EP of bivalve production systems.

The present study also shows the carbon sequestration potential of oyster production (274 kg CO2 eq. tonne harvested oysters−1), which is higher compared to clams (254 kg CO2 eq. tonne harvested clams−1) and mussel (146 kg CO2 eq. tonne harvested mussels−1) farming19. With carbon farming being included in the EU’s new Common Agriculture Policy Strategic Plan 2023-2027, there is also an opportunity for bivalve aquaculture to aid and play an active role in this form of environmental management70. Although acidification potential was calculated for the LCA process, this impact category wasn’t considered in the ecosystem services due to the resource limitation of this study. However, if included, the sulfur cycle should also be considered in the calculations, as oysters may promote hydrogen sulfide production in anaerobic conditions that can occur beneath the oyster trestles71.

Ecosystem influences such as changes to the benthos community, introduction of invasive species, pests and diseases, creation of novel habitats from the farming structures, alteration of nutrient cycling, effect on higher trophic level animals, etc, should be considered under sustainability assessments. The scope of this study didn’t allow for measurement of these ecosystem influences; however, previous research has highlighted their importance and consideration to improve the sustainability of oyster aquaculture72,73,74. In addition, point-of-harvest impacts such as mechanical stress (i.e., benthos compaction by workers and tractor pathway) could also affect the benthos macrofauna in oyster farming sites74. Another study conducted in a salmon hatchery also showed the importance of supplementing (where relevant) life cycle assessments with broader ecosystem assessments75. Therefore, to have a comprehensive understanding of the environmental impacts and benefits of oyster farming, future research should measure ecosystem and points-of harvest impacts, ideally with comparison to a similar virgin site over a suitable time frame.

The seafood sector, including aquaculture producers, processors, wholesalers, retailers and food certification bodies, is facing a growing demand for information on the environmental footprint of their products from customers, investors and government agencies76. The present study provides a scientific basis to meet these informational demands and contribute to the imminent introduction of a science-based metric such as the Product Environmental Footprint77. This study brings an innovative and valuable approach with positive results, thus serving as a reference point for future research on the sustainable potential of the shellfish sector.

Methods

Site selection

The oyster production sites (i.e., oyster producers or buyers for further processing) used in this study were located on a sheltered bay northwest of the Republic of Ireland. Samples from two Pacific oyster-producing sites (i.e., Site 1 and Site 2) were collected for morphological and elemental analysis, while the LCAs were modelled using operational data from three Pacific oyster farms located along the West coast, i.e., Site 1, Site 3 and Site 4, (Fig. 4).

Fig. 4: Study overview for the data collection on life cycle assessment and ecosystem models.
figure 4

Solid boxes represent procedures. Dashed boxes represent outcomes. L large size, M medium size, S small size, N nitrogen, P phosphorus, C carbon, LCA life cycle assessment, GWP global warming potential, EP eutrophication potential, AP acidification potential, CED cumulative energy demand. Created in BioRender. Wan, A. (2024) BioRender.com/a57b755

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Ecosystem services methodology

Morphological and elemental analyis

For morphological and elemental analysis, farmers at each production site randomly harvested 15 individuals per market size category (i.e., small, 67.4–112.5 mm length; medium, 89.4–119.3 mm length; and large, 94.8–120.7 mm length) during different times of the winter season (i.e., February and March). Thus, there were a total of 45 samples per site. The following morphometric measurements were undertaken per oyster:

  1. a.

    Total shell length, width, and depth; mm oyster−1;

  2. b.

    Total wet weight; g oyster−1;

  3. c.

    Shell wet weight and tissue wet weight; g oyster−1,

  4. d.

    Dry tissue and shell weights; g oyster−1.

Among the 15 individuals morphologically assessed per site and size category, sets of 6 individuals were randomly selected and pooled for the elemental analysis37. Thus, for each site, 18 individuals were selected for elemental analysis. Tissue and shells from pooled individuals were dried in a fan-assisted oven at 80 °C until a constant weight was achieved. Dried tissue and shells from pooled individuals were crushed using a mortar and pestle for dried tissue and a mill for dried shells. The pooled tissue and shell samples were then analysed for N and C content through an elemental CHN analyser (Flash smart elemental analyser, Thermo Fisher, Waltham, Massachusetts, United States). P content was measured through Inductively Coupled Plasma Optical Emission Spectrometry (700 series ICP-OES, Agilent, Santa Clara, California, United States). The results obtained as %C, %N, and %P in the dried tissue and shell samples were used to calculate: (i) the average %C, %N and %P per individual oyster (and separately the tissue and shell for each oyster), size category and site investigated; (ii) the average mass of C, N, and P removed per fresh individual oyster; and (iii) the average mass of C, N, and P removed per tonne of oysters harvested. Differences in the elemental analysis (i.e., %C, %N, %P) of Pacific oyster between the three size classes and the sites investigated were analysed using two-way ANOVA tests. A post-hoc Tukey’s test was conducted on each dataset to discern significant differences between sizes and sites. Statistical significance was assigned when P < 0.05.

National ecological impact

The morphological and elemental analysis results were then extrapolated to farm and national scale to obtain: (a) the quantities of nutrients and carbon removed annually on each production site using the average annual production for the period 2015–2020, i.e., average annual production (tonne year−1) × N, P or C removed per tonne of fresh product (kg tonne−1); and (b) national extrapolation of nutrients and carbon removed using the most recent estimated total annual production of Irish Pacific oysters13, i.e., N, P or C removed per tonne of fresh product (kg tonne−1) × national production of Pacific oysters (tonne year−1).

An ecosystem services analysis of Pacific oyster farming was undertaken to associate a monetary value with the nutrient remediation potential. This valuation of nutrient removal ES was calculated using the following median values for the removal of N (€18.9 kg−1) and P (€33.9 kg−1)29. These monetary values represent the theoretical cost of upgrading a wastewater treatment plant to remove one kg of N and P. Such values can vary between treatment plants depending on existing load, plant technology, discharge limits, plant size, etc. Nutrient valuation was also extrapolated nationally by applying the national production of Pacific oysters for 202213. Results were also equated to wastewater treatment plant performance in terms of population equivalent for N removal. A wastewater treatment plant, with secondary treatment, was estimated to remove, on average, 3.3 kg N person−1 year−1 25. This figure was applied to calculate the population equivalent where a wastewater treatment plant would remove the amount of N remediated (extrapolated as per the above) by Pacific oyster farming in Ireland.

Life cycle assessment methodology

Goal and scope

LCA studies were undertaken on three Pacific oyster sites along Ireland’s West coast. A cradle-to-gate system boundary was used for each site’s farming and on-site processing activities. The systems boundaries included aquaculture infrastructure, seed procurement, consumable materials, energy production (electric and diesel), culture and harvesting, processing, and packaging. Waste management and treatment of waste materials and packaging are also included within the system boundaries. The functional units applied were one tonne of live oyster product (meat and shell). Each studied site produced, on average, 111 tonnes of oysters for the market annually. All sites used bags and trestles to grow their oysters, and oyster seed was purchased domestically (Fig. 5).

Fig. 5: Cradle-to-gate system boundary and schematic overview of the farming materials used for the production of Pacific oyster in the north-west of Ireland
figure 5

a System boundaries used in the Life Cycle Assessment (LCA) of Pacific oyster production in the north-west of Ireland. The solid boxes represent processes (foreground and background). The dot-dashed box indicates the system boundaries. Solid arrows depict direct mass flows, and dashed arrows indicate indirect mass flows. b Plan and longitudinal overview of the layout and dimensions of oyster trestles. c Typical trestle setup and oyster bags used on the farming sites.

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Life cycle inventory

The life cycle inventory used primary data from the partner farms. Primary data was collected through questionnaires, interviews, and site visits. Energy, fuel, and consumables values were validated against bills and invoices where possible. Secondary data was collected from established life cycle databases such as Ecoinvent v3.10, and Agribalyse 3.0.1 to populate the life cycle inventories. The life cycle inventory of the present study covers all farm-based activities, infrastructures, and use of resources (Supplementary Tables 5 and 6). The main transport vehicles used for daily farming activities at each site were a fleet of tractors and trailers. The trestles at each site were manufactured from 25 mm reinforced steel bars and weighed 18 kg per segment. The service life of the trestles was estimated to be 15 years. Oyster bags were made of high-density polyethene and weighed approximately 800 g per bag, with an average service life of 8 years.

Life cycle assessment

The life cycle impact assessment methodology was undertaken through the CML method65. The following impact assessment categories were included: 100-year global warming potential (GWP, kg CO2 eq.), Acidification potential (AP, kg SO2 eq.), Eutrophication potential (EP, kg PO4 eq.), and Cumulative energy demand (CED, MJ), which assesses the degree of energy consumption associated with a production system78. As previously reported, these impact categories are the most concerning for aquaculture and shellfish production systems66,79,80.

Life cycle assessment and ecosystem services

In this study, the elemental analysis results (i.e., N, P, and C content in Pacific oyster shells) were adapted to LCA impact categories to estimate the eutrophication potential and global warming potential equivalents of nutrient uptake provided by Pacific oyster aquaculture in Ireland. N and P content in the shell were converted to PO4 eq., a compatible form under the EP impact category. Characterisation factors of 0.42 and 3.07 were applied to convert N and P to PO4 eq., respectively81. To determine the balanced PO4 eq. emissions, only the N content in shells converted to PO4 eq. was considered, as N is typically the limiting nutrient in the marine environments68. In addition, it should be noted that the CML methodology used for this life cycle impact assessment doesn’t distinguish between freshwater and marine eutrophication. Therefore, the balanced PO4 eq emissions were calculated as follows: balanced PO4 eq = emitted PO4 eq (LCA)—sequestered PO4 eq (N), where emitted PO4 eq (LCA) are the PO4 eq emissions from the LCA results and sequestered PO4 eq (N) are the PO4 eq. converted values from N content in oyster shells. To determine the net GWP of Pacific oyster farming, C content in oyster shells (i.e., the amount of CO2 sequestered in the shell during biocalcification) was converted to CO2 eq32. The same calculation was applied to determine the net CO2 eq emissions: balanced CO2 eq = emitted CO2 eq (LCA)—sequestered CO2 eq., where emitted CO2 eq (LCA) is the CO2 eq emissions from the LCA results and sequestered CO2 eq are the CO2 eq. converted values from C content in oyster shells. The N, C, and P contained in the soft tissue were not included within the ES calculations as they are considered a short stage of the biogenic carbon cycle. On the contrary, shells can sequester nutrients for extended periods19,21.