Distribution and bioavailability of dissolved and particulate organic matter in different water masses of the Southern Yellow Sea and East China Sea

Abstract

As the largest marginal seas, the Southern Yellow Sea (SYS) and the East China Sea (ECS) are important areas to study the biogeochemical cycles of organic matter from different sources. Seawater samples were collected from the SYS and ECS to determine the composition of organic carbon and major biochemicals (amino acids and carbohydrates) making up the dissolved (DOM) and particulate organic matter (POM). Concentrations of dissolved organic carbon (DOC) in the surface waters exhibited a latitudinal gradient, with high values appearing in the Yellow Sea Coastal Current Water (YSCCW). As the labile fraction of DOM, the concentrations of total hydrolysable amino acids (THAA) and total dissolved carbohydrates (TDCHO) were variable and tended to decrease from inshore to offshore sites. The DOC, THAA, and TDCHO concentrations were negatively correlated with salinity, suggesting that physical mixing and lateral transport played an important role in the distribution of DOM. In contrast, phytoplankton production directly affected the particulate amino acid (PAA) concentrations. High concentrations of PAA in the bottom water along a transect were probably due to resuspension by strong currents. DOC-normalized yields of THAA and TDCHO (THAA-C% and TDCHO-C%) were used to characterize the bioavailability of DOM in different water masses. Elevated THAA-C% values occurred in regions with high phytoplankton biomass, while high TDCHO-C% values were widespread in the Yellow Sea Warm Current Water (YSWCW) and the YSCCW. THAA-C% and TDCHO-C% had similar depth distributions, higher in the surface and middle layers than in the bottom layer, suggesting that organic matter undergoes diagenetic alterations as it sinks. The degradation index (DI) of DOM was lower than that of POM, which could be explained by their different sources and degradation states. The sporadic occurrence of hot spots likely represented important sites of biological activity. • Concentration, chemical composition, and bioavailability of OM were studied. • Physical mixing and lateral transport affect the distribution of DOM. • Amino acid and carbohydrate yields decreased with water depth. • Hot spots of bioavailable DOM occur sporadically in the surface water.