Strong heterogeneity in laterally exchanged particulate organic matter across tidal marshes in a large river delta

Understanding the lateral exchange of particulate organic matter (POM) between wetlands and estuaries is essential for understanding coastal carbon budgets and dynamics. We studied tidal and seasonal changes in the concentration of POM ([POC]) and its optical properties in three marsh creek systems along a salinity gradient in one of the world's largest river deltas — the Yangtze River Delta. Results reveal strong variability, with tides and among sites, in both [POC] and compositions of POM. The maximum tidal POC export occurred in fall with brackish marshes exporting [POC] rich (9 times higher) with a lower chromophoric signal (i.e., lower light-absorbing capacity) compared to the freshwater marsh (avg. [POC] of 1708 (brackish) and 192 (freshwater) μmol L⁻¹; avg. normalized POM absorption a∗_(350)p of 4.3 (brackish) and 16 (freshwater) L mol⁻¹ cm⁻¹). Creek POM in the brackish marshes were dominated by materials having visible fluorescence (avg. proportion of 54 %), likely due to a higher contribution from suspended solids and wind-induced soil erosion. Contrary, in the freshwater marsh the tidal POM was consistently enriched in materials strongly fluorescing in the UVA range (avg. proportion of 84 %), likely caused by extreme droughts and higher aboveground biomass. The primary driver of the observed differences among the three marshes were differences in hydrodynamics, with the resulting high spatial heterogeneity complicating delta-wide assessments of carbon flows and stocks. This study highlights the importance of [POC] in carbon budgets and the necessities of integrating site-specific lateral POC monitoring to improve estimates on carbon budgets in large deltas.