Utility of radium quartet for evaluating porewater-derived carbon to a saltmarsh nearshore water: Implications for blue carbon export

Saltmarshes are global hotspots of carbon sequestration and storage and are known as effective blue carbon ecosystems. However, the role of porewater exchange in saltmarshes as a source of carbon to the nearshore waters is still poorly constrained. Herein, we examined the radium quartet, dissolved inorganic (DIC) and organic (DOC) carbon in the porewater and nearshore surface water of Chongming Dongtan saltmarsh, China. Multiple methods based on the radium quartet were applied to estimate the porewater exchange, including the three-endmember model, mass balance model and time series observation. All methods revealed that the porewater exchange rate in Chongming Dongtan saltmarsh equaled 3.37 ± 1.23 cm d⁻¹. The porewater-derived DIC and DOC fluxes were then estimated to be (1.51 ± 0.64) × 10⁷ and (9.97 ± 6.96) × 10⁵ mol d⁻¹, respectively, which correspondingly made up 64.6% and 35.6%, of the total inputs into the Chongming Dongtan saltmarsh nearshore water. Considering the intertidal area covered by saltmarsh vegetation, carbon export through the porewater exchange was 3.87 ± 1.55 g C m⁻² d⁻¹, and was 1.2-fold greater than the carbon burial rate, accounting for approximately 29% of carbon outwelling in Chongming Dongtan saltmarsh. This study highlights the significance of porewater exchange for evaluating carbon sequestration capacity, and suggests that porewater exchange should not be overlooked in blue carbon assessments of saltmarshes.