Introduction
Mussel farming has been identified as one of the most promising food sectors that can help to meet the nutritional requirements of a growing human population while providing other ecosystem functions and services [1]. Italy, with 92,564 t in 2018, contributed to the 15% of the EU mussel production [2]. This makes Italy the third most important European producer of mussel, after Spain and France.
Mussels are filter-feeders of naturally occurring phytoplankton. As such, mussels represent the functional and trophic connection between pelagic and benthic processes, contribute to the fundamental nutrient storage and cycles, and play a key role in regulating incipient eutrophication phenomena through the top-down control on phytoplankton biomass [3].
Recently, several authors have investigated the possible contribution of farmed bivalves in mitigating the effects of climate change (i.e., increased carbon stock in seawater, CO2) [4]. Mussels build their shells through the biocalcification process, incorporating chemical carbon species, i.e., hydrogen carbonate (HCO3-), in the form of calcium carbonate (CaCO3-) while, at the same time, releasing CO2, according to the following equation: Ca22+ 2HCO3- ⇌ CaCO3 + CO2 + H2O. However, the role of mussel farming as a carbon sink is controversial and a scientific consensus on this topic is far from being achieved due to an open-ended debate about the definition of the criteria for the estimation of the biogenic carbon flux (e.g. mussel respiration) [5], [6]. Yet, according to the Kyoto Protocol, CO2 fluxes resulting from photosynthesis and animal respiration should not be taken into account when calculating greenhouse gasses (GHG) emissions, as they are part of the short C cycle and are balanced.
In this study, the Life Cycle Assessment (LCA) methodology has been applied to three case studies of mussel farming in the north Adriatic Sea, Italy. This study examines all the relevant fluxes of materials and energy across the systems and explores the potential role of biocalcification processes in sequestering carbon from the seawater during shell formation.
Materials and methods
The case studies investigated in this work are referred to as Class_A1 and Class_A2, and Class_B (one case study). Class_A case studies document the environmental performance of mussel farms located in Class A rearing areas, Class_B case study concerns the evaluation of a mussel farm located in Class B rearing area. Unlike mussels reared in Class A areas, those reared in Class B areas must undergo a depuration process before being placed on the market (Reg. EU 2017/625). The LCA approach follows the guidelines of [7]. Goal and scope: the objective of this study is to assess the environmental performance of two mussel supply chains, through the analysis of material and energy flows of the only production phase for Class_A mussels, and through both the production and treatment plant phases of Class_B mussels. In the analysis of the CO2 flows through the systems, the flows of biogenic CO2 resulting from the biocalcification process are also considered and computed. For this calculation, data on the shell:meat ratio, specific for each site, have been provided by the farmers. Environmental data are according to the literature [8]. A cradle-to-gate analysis has been carried out, considering the following processes: 1) mussel seeding and growing, 2) mussel harvesting and transport to land, and, only for Class_B, 3) mussel depuration in Italy and 4) in France. For all case studies, the system boundaries include the above processes and all material and energy inputs and outputs to and from the systems. The functional unit chosen is 1 kilogram of fresh mussels, including shell, suitable for sale. The Life Cycle Inventory is based on data provided by farmers through questionnaires and interviews (foreground data). Ecoinvent 3 database has been used to gather data about production of electricity, fuel, raw materials and transport (background data). The Life Cycle Impact Assessment has been carried out using the software SimaPro 9.1.0.7 (PRé Consultants), adopting the ReCiPe 2016 (H) method.
Results and conclusions
Carbon footprint (CF, i.e., global warming impact category) of Class_A farms amounts to 0.07 and 0.13 kg CO2 eq (case studies Class_A1 and Class_A 2, respectively). CF of Class_B farm is 0.53 kg CO2 eq. These values do not consider the carbon sequestration potential of biocalcification. The difference between Class_A and Class_B results can be attributed to the depuration process required for the sale of Class_B mussels. For all three case studies, the factor contributing most to the environmental impacts is fuel consumption.
When considering biogenic carbon fluxes, the CO2 sequestration associated with shell formation, net of the CO2 released, contributes to lowering the CF. This contribution is approximately equal to the overall GHG emissions produced by Class_A farms (CF corrected for the biocalcification process = 0.01 and 0.06 kg CO2 eq, Class_A1 and Class_A2, respectively), while it reduces the emissions of Class_B mussel production by 25% (CF corrected = 0.43 kg CO2 eq).
The CF (including the biocalcification process) associated with the production of 1 kilogram of protein is 1.06 and 1.92 kg CO2 eq for Class_A1 and Class_A2, respectively, and 9.87 kg CO2 eq for Class_B. These values are much lower than those for beef production (45-210 kg CO2 eq/kg protein) [9], and comparable or slightly higher than those of vegetable meat substitutes (6-17 kg CO2 eq/kg protein), potatoes (11.2 kg CO2 eq/kg protein) and soybean (1.9 kg CO2 eq/kg protein) [10].
In conclusion, mussel farming proves to be a sector with low environmental impacts. The positive effect of the biocalcification in incorporating CO2 within the shell allows for an annulment (Class_A) or a substantial reduction (Class_B) of farms’ GHG emissions. Given the tight correlation of shell CO2 sequestration with the ratio of shell to meat, when this ratio increases, the overall CF potential may even assume negative values.
Moreover, since the ratio between CO2 emitted and sequestered during the biocalcification process depends on both environmental (pH, salinity, temperature and partial pressure of CO2) and trophic conditions, further studies could clarify whether the choice of farming site can contribute to making this ecosystem service, typical of shellfish farming, more efficient, thus improving the sustainability of the sector.
References
[1] FAO, 2020.
[2] Eurostat (2018) Production from aquaculture excluding hatcheries and nurseries – fish_aq2a. https://ec.europa.eu/eurostat/data/database. Accessed 20.10.20.
[3] Smaal et al., 2019, Springer Nature.
[4] Alonso et al., 2021, J. Clean. Prod., 279.
[5] Munari et al., 2013, Mar. Environ. Res., 92, 264–267.
[6] Aubin et al., 2018, Int. J. Life Cycle Assess., 23 (5), 1030–1041.
[7] C. E. JRC, 2010.
[8] Frankignoulle et al., 1994, Limnol. Oceanogr., 39 (2), 458-462
[9] Nijdam et al., 2012, Food Policy, 37, (6), 760–770.
[10] González et al., 2011, Food Policy, 36, (5), 562–570.