Fluorescent Dissolved Organic Matter (FDOM) in the East Sea (Japan Sea): Distributions, Sources, and Sinks

Abstract

We measured the north–south transactional distributions of nutrients, dissolved organic carbon (DOC), and fluorescent dissolved organic matter (FDOM) in the East Sea (Japan Sea) in November 2019. The humic-like (C peak and M peak) and protein-like (T peak) components were identified using the parallel factor analysis model. The concentrations of C peak, M peak, and nutrients increased with depth, while T peak and DOC concentrations decreased with depth. However, unusually higher DOC concentrations were observed in the deep layer of some stations near the Ulleung Basin, relative to those in the other stations, perhaps by internal oceanic waves driven by a combination of the anti-cyclonic eddy and the unique bottom topography. The humic-like FDOM in the deep layer was found to be produced mainly by aerobic degradation of sinking organic matter in the water column and additionally by anaerobic processes in the sediments, based on its plot against apparent oxygen utilization. Although the concentrations of M peak were almost constant in the entire depth, C peak decreased gradually from 1000 m to the surface, suggesting relatively ineffective production of M peak in the deep ocean and/or more effective UV degradation of C peak in the surface layer. The labile portion of FDOM, T peak, was observed only in the euphotic layer, indicating its rapid removal before reaching the deep ocean. Our results observed in the East Sea, a miniature of the global ocean, provide new insights into the production and removal mechanisms regarding the FDOM distributions in the ocean.