Oyster farming and hydrodynamic conditions regulate composition and sources of sedimentary organic matter in a typical river-estuary-bay continuum

Estuaries and bays provide favorable conditions for mariculture, which in turn influences regional carbon cycling. However, a key challenge lies in comprehending the role of mariculture in sedimentary organic matter (SOM) dynamics during the transport of organic carbon (OC) along river-estuary-bay continuums. To address this knowledge gap, surface sediments were collected from a typical oyster farming area, i.e. the lower reaches of Dafeng River (LDR) to Dafeng River Estuary (DRE) and adjacent Qinzhou Bay (QZB). The sediments were analyzed for total OC (TOC) and nitrogen (TN), as well as stable isotopic composition (δ¹³C and δ¹⁵N) to determine the source and fate of SOM. The results revealed higher TOC and TN contents in DRE than LDR and QZB due to an increased proportion of fine-grained components, which was caused by weak hydrodynamic sorting in the estuary. Bayesian mixing model based on δ¹³C and TOC/TN ratio indicated that shellfish biodeposition was an important source of SOM, with an annual mean contribution of 36.6 ± 8.0 %. Notably, despite the small scale of oyster farming in QZB, shellfish biodeposition contributed 39.3 ± 6.2 % to SOM, suggesting that intensive oyster farming in the estuary reshaped the composition of SOM in the bay. Furthermore, terrestrial input made a significant contribution to SOM during summer in QZB compared to DRE due to remineralization of labile shellfish-derived OC during transport from the estuary to bay. Overall, this study highlighted the substantial impact of oyster farming and hydrodynamics conditions on SOM dynamics in coastal waters.