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

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 (r 2 = 0.72 and 0.78 in July and September, respectively) and DON (r 2 = 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 (r 2 = 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. 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 not only in biogeochemical processes in the ocean but also in the carbon and nitrogen cycles 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. Thus, this carbon fixed in dissolved organic form could contribute to the sinking of atmospheric CO 2 in the ocean through the biological pump 4,5. By contrast, high and significant variations in DOC concentrations have been found in the coastal and marginal oceans owing to intense biological activities and large terrestrial inputs 6. Dissolved organic nitrogen (DON), the nitrogen component of DOM, including urea, amino acids, nucleic acids, and amino sugars, is a primary reservoir of reactive nitrogen in the oceans. When dissolved inorganic nitrogen (DIN) is depleted, DON is the dominant pool of fixed nitrogen, and can become a direct energy source for marine microorganisms 7. By the microbial carbon pump, microbial transformation of DOM from labile to recalcitrant state, nitrogen was preferentially removed relative to carbon within the DOM pool, then the recalcitrant DOM shows ultimately the high DOC to DON ratio 8. 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 world 9. These seas receive huge amounts of freshwater (0.9 × 10 12 m 3 year −1) from the Changjiang River 10 and seawater from the Kuroshio branch water, a strong western boundary current 11. Here, the mixing of freshwater driven by the Changjian...

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“…Previous research in the East China Sea showed nonconservative behavior of DOC originating from the Changjiang River due to the presence of a DOC "pulse" [22]. They suggested that microbial decomposition of POM may be the source of the DOC pulse.…”

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