Changes in sedimentary organic carbon sources from rivers to bays and their responses to human activities

Human activities have significantly altered the sources and concentrations of organic carbon (OC) in sediments of rivers and bays, and small-to medium-sized rivers are particularly sensitive to these changes. This study investigates the sources and sequestration of OC in sediments across Laizhou Bay and its riverine inflows under varying human influences. Analyses of total organic carbon (TOC), total nitrogen (TN), δ¹³C, δ¹⁵N, and C/N ratios indicate that rivers along the southern coast deliver substantial OC fluxes to the ocean. Bayesian stable isotope mixing model analysis reveals that organic matter (OM) in the lower Yellow and Xiaoqing Rivers is mainly derived from soil OM, with minor contributions from sewage. In contrast, OM in the Bailang, Wei, and Jiaolai Rivers predominantly originates from sewage and phytoplankton, reflecting the significant impact of human activities. In Laizhou Bay, terrestrial and marine contributions to OC are 57 % and 43 %, respectively, with shellfish biodeposition playing a crucial role. Overall, human activities such as sewage discharge and shellfish farming have markedly reshaped the sources and contributions of OC across the river-to-bay continuum. Industrial, agricultural, and marine aquaculture activities play critical roles in shaping the river-bay OC system and should not be overlooked.