基于生命周期评价反思海藻养殖碳汇核算——以桑沟湾IMTA系统为例

Based on Life Cycle Assessment, Reflecting on Carbon Sink Accounting of Seaweed Aquaculture—A Case Study of the Sanggou Bay IMTA System

摘要: 多营养层次综合养殖(IMTA)被广泛认为是提升近海碳汇能力的可持续模式，但现有蓝碳核算多聚焦于“总固碳量”而忽视全生命周期碳排放，导致碳汇能力被系统性高估。本研究以山东省荣成市桑沟湾为案例，基于生命周期评价(LCA)方法论(ISO 14040/14044)，整合已发表的碳足迹参数与官方养殖统计数据，对现有海藻养殖碳汇核算实践进行批判性评估。研究采用“从摇篮到大门”的系统边界，将养殖周期划分为育苗期、运输阶段和养成期，计算各阶段的碳排放与碳吸收，并尝试构建IMTA系统净碳汇估算的概念框架。结果表明：(1) 桑沟湾海带养殖的净碳汇量为−95.93 kg CO₂e/t (负值表示碳汇)，其中碳排放74.30 kg CO₂e/t，碳吸收170.23 kg CO₂e/t，碳汇构成中生物质碳占79.9%，沉积埋藏碳占14.1%，RDOC占6.0%；(2) 养殖设施(聚乙烯浮绠、鞭绳等)是主要碳排放源，占养成期排放量的95.95%，柴油和电能分别占4.05%和1.14%；(3) Han等(2013)在桑沟湾开展的龙须菜–扇贝IMTA实验显示，扇贝：龙须菜湿重比1:0.96时pCO₂最低，提示复合系统具有协同减碳潜力，但42小时的实验时长不足以评估年度碳平衡，IMTA系统的精确碳足迹仍有赖于更完整的LCA参数；(4) 基于寻山集团10万亩海域年固碳量42.5万吨的官方数据与LCA碳排放参数的比值分析，现有蓝碳核算中的“总固碳量”与LCA“净碳汇量”在概念上存在根本性差异，若将全生命周期排放纳入考量，IMTA系统的可交易碳汇量可能被高估20%~40%。本研究建议，应将LCA纳入海藻养殖碳汇方法学的必要环节，以“净碳汇量”替代“总固碳量”作为蓝碳核算的核心指标。

Abstract: Integrated multi-trophic aquaculture (IMTA) is widely recognized as a sustainable model for enhancing nearshore carbon sequestration, yet existing blue carbon accounting predominantly focuses on “gross carbon sequestration” while neglecting full life cycle carbon emissions, leading to systematic overestimation of carbon sink capacity. This study takes Sanggou Bay, Rongcheng City, Shandong Province as a case, and conducts a critical assessment of current seaweed aquaculture carbon sink accounting practices based on the life cycle assessment (LCA) methodology (ISO 14040/14044), integrating published carbon footprint parameters and official aquaculture statistics. The study adopts a “cradle-to-gate” system boundary, dividing the aquaculture cycle into nursery, transportation, and grow-out phases, and calculates carbon emissions and carbon absorption for each phase, while attempting to construct a conceptual framework for estimating the net carbon sink of IMTA systems. Results show that: (1) the net carbon sink of kelp aquaculture in Sanggou Bay is −95.93 kg CO₂e/t (negative value indicates carbon sink), with carbon emissions of 74.30 kg CO₂e/t and carbon absorption of 170.23 kg CO₂e/t; the carbon sink composition is 79.9% biomass carbon, 14.1% sediment burial carbon, and 6.0% RDOC; (2) aquaculture facilities (polyethylene ropes, floats, etc.) are the main carbon emission source, accounting for 95.95% of grow-out phase emissions, while diesel and electricity account for 4.05% and 1.14% respectively; (3) Han et al. (2013)’s IMTA experiment with Gracilaria and scallop in Sanggou Bay showed the lowest pCO₂ at a scallop:Gracilaria wet weight ratio of 1:0.96, suggesting synergistic carbon reduction potential in integrated systems, but the 42-hour experiment duration is insufficient to assess annual carbon balance, and the precise carbon footprint of IMTA systems remains dependent on more complete LCA parameters; (4) based on the ratio analysis of official data (Xunshan Group’s 425,000 tons annual carbon sequestration for 100,000 mu) and LCA emission parameters, there exists a fundamental conceptual difference between the “gross carbon sequestration” in current blue carbon accounting and the “net carbon sink” from LCA, and if full life cycle emissions are incorporated, the tradable carbon sink of IMTA systems may be overestimated by 20%~40%. This study recommends that LCA should be included as a necessary component in seaweed aquaculture carbon sink methodology, replacing “gross carbon sequestration” with “net carbon sink” as the core indicator for blue carbon accounting.