Quantitative organic carbon source in suspended particulates from the Dagu River-estuary-Jiaozhou Bay system (North...

The small and medium-sized rivers play an important role in the transport and burial of organic carbon (OC) in coastal zones. However, the small- and medium-sized rivers discharging into the marine environment have received less attention regarding their OC source. Understanding sources of OC in coastal zones is important to better evaluate the processes controlling the carbon cycle. In this study, the Dagu River-Estuary-Jiaozhou Bay system, an important carbon sink and a small bay heavily affected by human activities, was selected as the study area. We conducted comprehensive analyses of bulk parameters and biomarkers in the suspended particulate matter (SPM) of the Dagu River and Jiaozhou Bay. The Spearman correlation coefficients between bulk parameters, stable isotopes, biomarkers and environmental parameters in the SPM verified that the Dagu River represents the primary fluvial input to this marine system, delivering enhanced terrestrial organic matters and nutrients. The particulate organic carbon (POC), particulate nitrogen (PN), stable isotopes (δ¹³C) and biomarkers (C₂₆+C₂₈+C₃₀n-alkanols and the ΣPB (sum of phytoplankton biomarkers)) were used in this study to elucidate the sources of POC, respectively. The three endmember mixing model (based on δ¹³C and C/N) was applied to quantify OC into river, wetland and phytoplankton OC fractions. The results revealed distinct spatial partitioning: river OC dominated riverine SPM (58 %), while wetland OC prevailed in the inner bay (55 %), and phytoplankton OC prevailed in the estuarine SPM (58 %). Biomarkers (n-alkanes and coprostanol) were used to further assess the impact of the anthropogenic activities such as petroleum contamination and the sewage discharge. The results indicated that the SPM in the Jiaozhou Bay has been widely affected by human activities. This study employs an integrated three-method approach to comprehensively characterize the sources and composition of OC across the river-estuary-bay system.