Factors controlling the distributions of dissolved organic matter in the East China Sea during summer

Jeonghyun Kim,Hyuk Je Lee,Jang Kyun Kim,Sang Rul Park,Tae-Hoon Kim

doi:10.1038/s41598-020-68863-w

Abstract

To determine the distribution of dissolved organic matter (DOM) in the East China Sea (ECS) during the summer, we measured the dissolved organic carbon (DOC) and nitrogen (DON), fluorescent dissolved organic matter (FDOM), and chlorophyll a (Chl. a) in the upper 100-m layer of this region during July and September 2015. The DOC (r2 = 0.72 and 0.78 in July and September, respectively) and DON (r2 = 0.43 and 0.33) were significantly correlated with salinity, suggesting that the river is the primary origin of DOM. However, we found that at a DOC “pulse” under a salinity ranging from 24 to 35, the extrapolating DOC values (304 ± 11 μM) were twice higher than those with a salinity of close to 0, as found in a previous study. The excess DOC concentration seemed to be attributed to the microbial metabolism during transport from the estuary based on the good relationships between DOC and marine humic-like FDOM (r2 = 0.42 and 0.47), as well as the fluorescence, humification, and biological indexes, but showed no correlation with Chl. a. Thus, the results of our study indicate that microbial activities can be a significant factor controlling the distribution of DOM in the ECS during summer.

Highlights

Dissolved organic matter (DOM) in the oceans is one of the largest reservoirs of carbon and nitrogen on Earth, and its distribution and behaviour play crucial roles in biogeochemical processes in the ocean and in the carbon and nitrogen c ycles[1]
Some water patches with relatively lower salinity (i.e., Changjiang Diluted Water (CDW)) showed in the East China Sea and the southern sea of Korea
The significant negative correlation between dissolved organic matter (DOM) and salinity suggests that the river is the primary source of DOM in the East China Sea (ECS)

Summary

Introduction

Dissolved organic matter (DOM) in the oceans is one of the largest reservoirs of carbon and nitrogen on Earth, and its distribution and behaviour play crucial roles in biogeochemical processes in the ocean and in the carbon and nitrogen c ycles[1]. Dissolved organic carbon (DOC), the carbon component of DOM, can be preserved for thousands of years in the deep open oceans with relatively lower and uniform concentrations because more than 90% of DOC is recalcitrant to microbial utilization in the water column[2,3]. The northwestern Pacific marginal seas, including the East China Sea (ECS) and the southern sea off Korea, are among the largest continental shelves in the w orld[9] These seas receive huge amounts of freshwater (0.9 × 1012 m3 year−1) from the Changjiang River[10] and seawater from the Kuroshio branch water, a strong western boundary c urrent[11]. The objectives of this study were to identify the optical properties of DOM as a tracer of the DOM source and to evaluate the factors controlling the distributions of DOM in the northern ECS and the southern sea off Korea during the summer

Objectives

Methods

Results

Discussion

Conclusion