Hydrological and Biogeochemical Controls on Absorption and Fluorescence of Dissolved Organic Matter in the Northern South China Sea

Abstract

The Kuroshio intrusion from the West Philippine Sea (WPS) and mesoscale eddies are important hydrological features in the northern South China Sea (SCS). In this study, absorption and fluorescence of dissolved organic matter (CDOM and FDOM) were determined to assess the impact of these hydrological features on DOM dynamics in the SCS. DOM in the upper 100 m of the northern SCS had higher absorption, fluorescence, and degree of humification than in the Kuroshio Current of the WPS. The results of an isopycnal mixing model showed that CDOM and humic‐like FDOM inventories in the upper 100 m of the SCS were modulated by the Kuroshio intrusion. However, protein‐like FDOM was influenced by in situ processes. This basic trend was modified by mesoscale eddies, three of which were encountered during the fieldwork (one warm eddy and two cold eddies). DOM optical properties inside the warm eddy resembled those of DOM in the WPS, indicating that warm eddies could derive from the Kuroshio Current through Luzon Strait. DOM at the center of cold eddies was enriched in humic‐like fluorescence and had lower spectral slopes than in eddy‐free waters, suggesting inputs of humic‐rich DOM from upwelling and enhanced productivity inside the eddy. Excess CDOM and FDOM in northern SCS intermediate water led to export to the Pacific Ocean interior, potentially delivering refractory carbon to the deep ocean. This study demonstrated that DOM optical properties are promising tools to study active marginal sea‐open ocean interactions.

Microbial transformation of distinct exogenous substrates into analogous composition of recalcitrant dissolved organic matter

“…Slit widths were 10 nm and 5 nm for excitation (Ex) and emission (Em), respectively. The EEM spectra of all samples were blank corrected by pure water EEMs measured on the same day (Guo et al ., 2014, Wang et al ., 2017). EEMs were decomposed into individual components by widely used parallel factor analysis (PARAFAC) using MATLAB 2012 and the DOM Fluor toolbox (Stedmon and Bro, 2008).…”

Molecular characteristics of microbially mediated transformations of Synechococcus‐derived dissolved organic matter as revealed by incubation experiments

“…Excitation–emission matrices (EEM) were measured using a 1‐cm quartz cuvette and a Varian Cary Eclipse spectro‐fluorometer, as described elsewhere (Wang et al ., ). Briefly, excitation (ex) wavelengths ranging from 240 to 450 nm were scanned in 5‐nm intervals, while emission (em) spectra were scanned every 2 nm between wavelengths of 280‑600 nm.…”

Dissolved Organic Matter Dynamics in the Epipelagic Northwest Pacific Low‐Latitude Western Boundary Current System: Insights From Optical Analyses

“…Fluorescence excitation‐emission matrices (EEMs) were measured using a Varian Cary Eclipse spectrofluorometer according to Wang et al. (2017). Briefly, emission scans from 280 to 600 nm were recorded at 2‐nm intervals with excitation wavelengths ranging from 240 to 450 nm at 5‐nm intervals using a 1‐cm pathlength quartz cuvette.…”

“…Various biological (planktonic population and productivity), chemical (nutrient level, apparent oxygen utilization [AOU]), and photochemical factors were posited to regulate the production‐degradation dynamics of the epipelagic DOM pool (Catalá et al., 2016; Mopper et al., 2015; Rochelle‐Newall & Fisher, 2002). Hydrodynamic processes, such as lateral, vertical (e.g., mesoscale eddies) and frontal mixing between different currents, may further modulate epipelagic DOM quantity/composition through regulation of the aforementioned biogeochemical factors (Wang et al., 2017; Wu et al., 2015). However, owing to the lack of data with high spatial resolution, the combined hydrological and biogeochemical regulation of oceanic DOM cycling remains poorly understood.…”

“…In addition, the confluence of different water currents in this region propagates the quasi‐persistent cold Mindanao Eddy and warm Halmahera Eddy (Kashino et al., 2013; Zhou et al., 2010). This mesoscale eddy pair could affect surface DOM dynamics and its vertical transport by generating the upwelling and subduction of water masses, thereby influencing nutrient supply, primary production, and microbial DOM production/degradation (Jiao et al., 2014; Moutin & Prieur, 2012; Wang et al., 2017). Seasonal and interannual variations of the Mindanao and Halmahera eddy pair could regulate the lateral DOM flux through modulating fluctuations in lateral transport volumes of the associated currents (Kashino et al., 2013).…”