Seasonal variations of organic matter characteristics in the central-eastern Yellow Sea

Abstract

The central-eastern Yellow Sea serves a crucial role in the transfer of organic matter (OM) to the Pacific Ocean. However, detailed information on the characteristics and origins of OM in this region is limited. This study investigated the concentrations and stable isotopic compositions of carbon (δ13C) and nitrogen (δ15N) in particulate and sedimentary organic matter from the central-eastern Yellow Sea across different seasons from 2019 to 2023. Additionally, the physicochemical properties, size-fractionated phytoplankton biomass (Chla), and the concentration and fluorescence characteristics of dissolved organic matter (DOM) were determined. Given the frequently observed high concentration of suspended solids in the bottom layer, sediment resuspension due to tidal currents might significantly contribute to the properties of organic matter (OM) in coastal areas. The δ13C values of particulate organic matter (POM) in coastal areas were higher than the end-member values typically reported for terrestrial sources. In addition, the lowest δ13C values were recorded in the central part of our study area, where small phytoplankton were predominantly abundant. Our findings suggest that in the coastal regions, phytoplankton-derived OM and/or resuspended OM might be more crucial than terrestrial-derived OM, with depleted δ13C value of OM possibly originating from picophytoplankton. The nature of DOM was linked to biotic processes involving phytoplankton and bacteria, as well as the resuspension of sedimentary organic matter. No significant influx of terrestrial OM into coastal sediments was observed. In general, the source and nature of OM appear to be influenced by hydrodynamic conditions and the distribution of lower trophic level organisms in the central-eastern Yellow Sea.