Dynamic Regulation of Nutrient Stoichiometry and Greenhouse Gas Mitigation through Blue-Carbon Restoration in Xiaohai Lagoon, Hainan—China's Largest Tropical Lagoon

Marine nutrient cycling, particularly of carbon (C), nitrogen (N), phosphorus (P), and silicon (Si), is intricately linked to phytoplankton metabolism, with the Redfield ratio (106:16:1, extended to 15–20 for Si) traditionally serving as a benchmark for nutrient stoichiometry. However, tropical coastal ecosystems experience significant spatial and temporal heterogeneity due to anthropogenic activities, geographic variability, and seasonal shifts, exacerbating imbalances in carbon and nutrient dynamics.

Blue-carbon ecosystems, as a "natural solution," offer the potential to mitigate eutrophication and acidification. These highly productive systems can transform CO₂ sources into carbon sinks, contributing to carbon neutrality and improving coastal ecosystem resilience. Xiaohai Lagoon, the largest lagoon in Hainan, China, represents a successful case study of blue-carbon restoration. Over three years of comprehensive restoration measures, including large-scale seagrass and seaweed planting, the lagoon achieved Class I water quality through substantial government investment.

Using high-resolution field surveys and real-time water quality monitoring, this study demonstrates how blue-carbon ecosystems dynamically regulate lagoon health through in situ metabolism. During the rainy season (October–December), blue-carbon species rapidly absorbed excess nutrients from land sources, and by November, shifted nutrient dynamics from nitrogen (N) limitation to phosphorus (P) limitation. This transformation converted the lagoon from a CO₂ emission source to a CO₂ sink through photosynthesis. During this process, the combined CO₂ equivalents of three typical greenhouse gases—CO₂, CH₄ (methane), and N₂O (nitrous oxide)—turned negative, −617 g CO₂e m⁻² annually under mean conditions and up to −1,800 g CO₂e m⁻² annually under optimal conditions, underscoring the substantial role of blue-carbon systems in mitigating climate change. In addition, dissolved oxygen (DO) levels increased (107%–136%), and acidification was alleviated (pH 8.41 ± 0.14). However, the decomposition of organic matter from declining blue-carbon species disrupted stoichiometry and caused water quality to deteriorate again, underscoring the critical need for sustained ecological governance.

Our findings highlight the pivotal role of blue-carbon restoration in regulating offshore nutrient stoichiometry, mitigating greenhouse gas fluxes, and enhancing coastal ecosystem health.  Scaling these results to 10 Hainan lagoons reveals a mitigation potential of ~310,000–500,000 tons CO₂e annually. These insights provide a scientific foundation for advancing Hainan’s ecological civilization pilot zone and offer practical strategies for global coastal management and achieving carbon neutrality.