Tidal dynamics and seasonal hydrological variations influencing organic carbon distribution in the Yangtze River estuary.

The carbon cycle process in coastal areas is extremely complex due to the coupled hydrodynamic interactions between terrestrial and marine systems. To study the spatiotemporal variations of organic carbon and their influencing mechanisms, dissolved and particulate organic carbon (DOC, POC) in water, total organic carbon (TOC) in surface sediments, water quality parameters, and sediment grain size composition were measured in five coastal sites from the Yangtze River estuary (YRE). Generalized linear mixed-effects (GLMM) model was used to identify potential processes that explained these spatiotemporal patterns. The results showed that seasonal hydrological characteristics and tidal fluctuations had a significant effect on the concentrations of DOC, POC, and TOC (p < 0.001). Temporally, DOC concentrations were higher in wet season; POC concentrations were higher in dry season; TOC varied little between seasons. The maximum values of DOC and POC were observed at spring and moderate tides, respectively, whereas TOC increased significantly at neap tides. Furthermore, random forest model indicated that salinity and dissolved oxygen (DO) of water were the key drivers of DOC concentrations, while temperature, salinity and DO were the key drivers of POC, and temperature and salinity was the key driver of TOC (p = 0.01). Overall, results from this study highlight the importance of water quality and hydrodynamic factors in the variation of organic carbon, as organic carbon concentrations are influenced by multiple factors that change on both seasonal scales and tidal cycles. This study provides valuable insights into effects of environmental factors on the spatiotemporal distribution of organic carbon in coastal areas.