Improving Carbon Budgets by Accounting for Inorganic Carbon in Seagrass Ecosystems

While seagrass ecosystems are acknowledged for their role as blue carbon sinks, significant uncertainties remain regarding the sequestration of sediment inorganic carbon (SIC) and its broader implications for carbon cycling. These knowledge gaps hinder a comprehensive assessment of the contribution of seagrass ecosystems to the global carbon budget. To address this gap, the drivers and sources of SIC in nine seagrass ecosystems in the tropical Indo-Pacific were analyzed using partial least squares path modeling and carbon and oxygen isotopes binding Bayesian mixing models. We found that SIC content varies regionally, ranging from 0.03% to 10.18%, and is positively correlated with seagrass biomass, temperature, salinity, and coarse-grained sediment. SIC stocks ranged from 1.53 to 203.17 Mg C ha⁻¹, confirming the role of tropical Indo-Pacific seagrass ecosystems as a significant pool for SIC. SIC content and stock beneath seagrass ecosystems are generally higher than those in unvegetated areas due to the capacity to trap particles of the seagrass canopy. SIC beneath seagrass ecosystems affected by runoff is mainly from terrestrial inputs. Conversely, SIC under seagrass ecosystems regulated by seawater is primarily derived from local production and inputs from adjacent ecosystems and fish farming wastewater. This study provides a robust method to enhance the accuracy of blue carbon accounting in seagrass ecosystems by the inclusion of inorganic carbon burial.