Fluorescence characteristics of dissolved organic matter and its association with the nepheloid layer in the northern South China Sea

The northern South China Sea (SCS) is characterized by multiple oceanographic phenomena and has developed diverse patterns of distribution, migration and transformation of dissolved organic carbon (DOC). To better understand the DOC behavior and its relationship with the nepheloid layer, UV-visible absorption spectra and three- dimensional fluorescence spectra of dissolved organic matter (DOM) were obtained in the northern SCS, as well as beam attenuation (BA), major physicochemical parameters and chlorophyll 𝑎 (Chl-𝑎) data. DOC and chromophoric dissolved organic matter (CDOM) gradually decrease from the surface to deeper layers, with high-low alternations occurring in the euphotic zone. The fluorescence intensity of DOM is primarily attributed to protein-like components, followed by humic-like components (24.6%). CDOM exhibits a typical marine origin and is produced mainly through bacterial production in situ. The spatial and temporal distributions of DOC and humic-like components are influenced by major physicochemical factors (such as temperature, salinity, and nutrients) and Chl-𝑎. In contrast the protein-like components might be closely associated with bacterial activity. The distributions of DOC and humic-like components are significantly correlated with the presence of the nepheloid layer. In the euphotic zone, phytoplankton particulates are the primary source of humic-like components, while suspended particles affect the distribution of humic-like components below the euphotic zone. The results presented direct evidence for the function of the marine nepheloid layer in the organic carbon cycle.