Distribution, seasonality, optical characteristics, and fluxes of dissolved organic matter (DOM) in the Pearl River (Zhujiang) estuary, China

Abstract

<p><strong>Abstract.</strong> Dissolved organic carbon concentration in the Pearl River estuary (PRE) of China was measured in May, August, and October 2015 and January 2016. Chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the latter three seasons were characterized by absorption and fluorescence spectroscopy. Parallel factor analysis of the fluorescence spectra identified two protein-like, two humic-like, and one oxidized quinone-like FDOM components. The seasonality of average DOM abundance varied as follows: DOC: May (156 μmol L<sup>−1</sup>) > January (114 μmol L<sup>−1</sup>) ≈ August (112 μmol L<sup>−1</sup>) > November (86 μmol L<sup>−1</sup>); CDOM absorption at 330 nm: August (1.76 m<sup>−1</sup>) > November (1.39 m<sup>−1</sup>) ≈ January (1.30 m<sup>−1</sup>); FDOM expressed as the sum of the maximum fluorescence intensities of all FDOM components: November (1.77 R.U.) > August (1.54 R.U.) ≈ January (1.49 R.U.). Average DOM abundance in surface water was higher than in bottom water, their difference being marginal (0.1–10 %) for DOC in all seasons and for CDOM and FDOM in November and January, and moderate (16–21 %) for CDOM and FDOM in August. DOC showed little cross-estuary variations in all seasons while CDOM and FDOM in January were higher on the west side of the estuary than in the middle and on the east side. All three variables exhibited large variations and/or rapid drawdowns at the head of the estuary (salinity < 5) due to multiple freshwater endmembers and/or biotic losses. In the saltier zone, they declined linearly with salinity except relatively constant DOC in May and November. The decrease in FDOM was 5–35 % faster than that in CDOM, which in turn was 2–3 times faster than that in DOC. Salinity and CDOM absorption coefficients can serve as indicators of DOC in August and January. Absorbance- and fluorescence-based indices demonstrate that freshwater endmembers in all seasons mainly contained fresh, protein-rich DOM of microbial origin, though the proportion of humic-like components was somewhat higher in August. Protein-like materials were preferentially consumed in the low-salinity section but the dominance of the microbial signature was maintained throughout the saltier zone. Exports of DOC and CDOM (in terms of <i>a</i><sub>330</sub>) into the South China Sea were estimated as 195 × 10<sup>9</sup> g and 266 × 10<sup>9</sup> m<sup>2</sup> for the PRE, and 362 × 10<sup>9</sup> g and 493 × 10<sup>9</sup> m<sup>2</sup> for the entire Pearl River Delta. Compared to other world major estuaries, the PRE presents the lowest concentrations and export fluxes of DOC and CDOM. Nonetheless, DOM delivered by the PRE is protein-rich and thus may significantly impact the local ecosystem.</p>

Depth‐Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

“…[DOC] and CDOM absorbance were determined using the same instrument and procedures reported by Li et al (2019). DOC samples were acidified immediately after sample collection to pH ~2 using 2‐N hydrochloric acid (Reagent grade, Merck).…”

“…Inner filtering effects were negligible due to the samples' low absorption coefficients ( a CDOM (254) < 15 m −1 ). The procedures for PARAFAC modeling, performed over the wavelength ranges from 260–450 nm for excitation and 300–600 nm for emission, were identical to those reported by Li et al (2019), except that the PARAFAC analysis in the present study was fed with far more EEMs (total 832), which included vertical profiling samples covering the entire northern SCS.…”