Abstract: BioRes Activities in the SOCAM Project — Carbon Sink Paradigm for Manila Clam Aquaculture in the Po Delta

Project Context and Deliverable Scope

This deliverable, prepared by BioRes (Research Operator, OdR) under iNEST Spoke 7 with PNRR Mission 4 Component 2 funding (grant ECS00000043), authored by Dr. Roberto Odorico, Dr. Claudio Franci, and Dr. Alessia Molinaro, presents a scenario analysis supporting the SOCAM thesis: Ruditapes philippinarum aquaculture in the Po Delta, managed according to sustainable ecology principles, operates as a net carbon sink (-0.4 kg CO₂-eq/kg clam), in contrast to the conventional perception of aquaculture as a high climate-impact sector. The analysis synthesizes three reference studies plus supplementary literature to provide ECOTAPES with a unified scientific basis for management.

The Aguiari et al. (2020) Methodological Breakthrough

Before this study, conventional LCA studies following ISO 14040/14044 systematically excluded carbon sequestration in shells, treating CaCO₃ as inert waste. Two methodological biases underpinned this exclusion: the absence of explicit regulatory guidelines for inorganic carbon inclusion, and the false perception that CaCO₃ would be "neutralized" by respiration. Aguiari et al. recognized that calcium carbonate in shells withdrawn from the biological cycle (via harvest, sediment burial, or landfill) undergoes stable diagenetic sequestration. Their innovative methodology combined three elements: (1) cradle-to-gate LCA with primary field data from Venice Lagoon farms; (2) explicit inclusion of inorganic carbon via the formula: shell weight × CaCO₃ content × (12/44) conversion factor; (3) quantification of sedimentary organic carbon through POC removal × pseudofeces deposition × long-term stability fraction.

The quantitative balance yields: emissions of +1.0 kg CO₂-eq/kg (diesel harvest 0.6 kg + transport 0.4 kg + packaging <0.1 kg) offset by sequestrations of -1.4 kg CO₂-eq/kg (shell CaCO₃ -0.9 kg + sediment POC -0.5 kg), producing a net balance of -0.4 kg CO₂-eq/kg. Sensitivity analysis confirmed robustness under ±25% parametric variation; however, removing only the shell term flips the balance to +0.5 kg CO₂-eq/kg (carbon source), underscoring the criticality of shell accounting.

Density Optimization: The Zhang et al. Study

Zhang et al. on a Chinese marine farm documented a non-linear relationship between stocking density and carbon sequestration, with differential allocation: 65-75% inorganic (shells), 25-35% organic (tissues + sediments). The analysis identified an optimal density sweet spot at 200-250 ind/m² through three phases: sub-optimal (<150 ind/m²): food is unlimited but filter feeders insufficient → 95 g C/m²/day (40% of maximum); optimal (200-250 ind/m²): adequate food + maximal individuals + metabolic efficiency → 210 g C/m²/day; excessive (>300 ind/m²): food shortage triggers physiological stress and 50-70% individual growth collapse → paradoxical drop to 150 g C/m²/day. The trade-off is critical: simply increasing density compromises the carbon sink service.

Bioenergetic Modeling: The Wang et al. DEB Framework

The Dynamic Energy Budget model integrates filtration (V_max × L² × [POC]), assimilation (p_A = 0.6-0.8), and energy allocation among tissue growth, shell calcification (cost 0.02-0.05 J/mg), and respiration. Validated on 18-24 months of empirical data from Asian farms (R² = 0.88-0.92; RMSE <10%), the model is transferable to Mediterranean systems through local parameter recalibration. Venice Lagoon quantification (92-98% similarity with Po Delta in salinity, temperature, turbidity, phytoplankton productivity) yields 50-150 g C/m²/year net sequestration, with shells representing 70% of total. Spatial optimization according to local primary productivity emerges as a key strategy.

Biofiltration and Sediment Stabilization: The Sacca di Goro Case Study

Pre-blue-crab-invasion, Sacca di Goro hosted 10,000 tonnes of clam biomass (2,000 t DW). With clearance rate of 40 L/g/day, the population filters 80 million m³/day, processing the entire 50 million m³ lagoon volume in <1 day (0.625 days turnover). At 2 mg C/L average POC, this represents 112 t C/day removed, equivalent to 41,000 t C/year = 150,000 t CO₂-eq/year, valued at €7.5-15 million/year at European carbon market prices. This service represents avoided costs of eutrophication treatment, sediment dredging, and biodiversity loss.

Three biophysical mechanisms drive sediment stabilization: pseudofeces deposition (organic "glue" effect via polysaccharides), current attenuation (-20-40% benthic velocity through shell roughness), and resuspension reduction(-30-80% erosion resistance). The tertiary ecosystem service preserves "old" buried carbon stocks against re-mineralization.

Post-Harvest Permanence and LCA Correction

Sequestered carbon permanence depends critically on shell destination in the Italian supply chain: inert landfill 55% (100+ years, <1% dissolution/century), agricultural amendment 20% (10 years, acidic reactions release CO₂), natural accumulation 15% (100+ years in brackish banks), mixed disposal 10% (0 years). Weighted mean permanence = 72 years, yielding an LCA correction factor of 0.72. Post-harvest management is therefore as critical as production: incineration instead of storage would shift the system from -0.4 to +0.5 kg CO₂-eq/kg.

Integrated Dashboard and Significance

The consolidated picture shows: 60% sequestration in shells, 15% in tissues, 25% in biofiltration + biostabilization. Parametric sensitivity identifies nutrients (85%), density (95%), and temperature (65%) as critical drivers. The conditional nature of the carbon sink function is clear: R. philippinarum is not intrinsically a carbon sink, but becomes so only when managed at optimal density (200-250 ind/m²), under monitored water quality, with sustainable post-harvest shell handling. The SOCAM project operationalizes this paradigm in the Po Delta through four integrated components: (1) field validation of carbon balance; (2) density optimization protocols; (3) automated monitoring systems; (4) post-harvest shell traceability. The BioRes scenario analysis bridges global scientific validation and local management practice, providing ECOTAPES with the foundational toolkit to align Manila clam farming with climate mitigation, ecosystem health, biodiversity conservation, water quality protection, rural livelihood support, and the European 2050 carbon neutrality commitment.