Particulate organic carbon dynamics off the Yellow River Estuary revealed by in-situ hydrodynamics and carbon isotopes

The Land-to-Ocean-Aquatic-Continuum (LOAC) is a key component in global carbon cycle and budget. Delivery of organic carbon (OC) along the LOAC is impacted by coastal hydrodynamics on a variety of spatial-temporal scales. Here we provide a coupled analysis of suspended particulate OC (POC) transport and hydrodynamics in the Yellow River Estuary. Pre-aged soil POC has become the dominant component (40–50 %) in the river-estuary continuum during the flood season. Petrogenic POC, with high proportion in the river (31 %), experienced drastic deposition off the estuary due to entrapment by shear fronts and estuarine circulation, decreasing to 19–23 %. The terrestrial OC was mainly exported offshore along with the seaward extension of river plume during ebb tide, while could be pushed landward by intruding seawaters during flood tide. When the tidal effects were removed, the first-order net POC residual flux was ∼19 g/m/s in seaward direction in the near-field plume, with dominance of pre-aged soil POC at ∼8 g/m/s and petrogenic POC at ∼6 g/m/s; on the contrary, POC fluxes displayed a landward direction and decreased by two orders of magnitude in the far-field plume. Trapped by shear fronts and estuarine circulation, POC was mostly limited and temporally preserved off the river mouth during the flood season. These findings suggest that intra-tidal estuarine hydrodynamics can further influence the sources, composition, and transport of terrestrial POC, playing a significant role in regulating carbon cycling along the Land-to-Ocean-Aquatic-Continuum.