Abstract. Dissolved organic carbon (DOC) 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. CDOM and FDOM exhibited negligible seasonal
variations, while DOC displayed a significant seasonality, with the average
concentration being highest in May (156 µmol L−1), lowest in
November (87 µmol L−1), and comparable between January (118 µmol L−1) and August (112 µmol L−1). Although DOC, CDOM, and
FDOM in surface water were generally higher than in bottom water, the
difference between the two layers was statistically insignificant. 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 on the east
side. All three variables showed rapid drawdowns in the head region of the
estuary (salinity <5); their dynamics in the main estuary were
primarily controlled by conservative mixing, leading to linearly declining
or relatively constant (for DOC in May and November only) contents with
increasing salinity. The decrease in FDOM with salinity was 5 %–35 % faster
than that of CDOM, which in turn was 2–3 times quicker than that of DOC.
Salinity and CDOM absorption coefficients could serve as indicators of DOC
in August and January. Freshwater endmembers in all seasons mainly contained
fresh, protein-rich DOM of microbial origin, a large part of it likely being pollution-derived. Protein-like materials were preferentially consumed in
the head region but the dominance of the protein signature was maintained
throughout the estuary. Exports of DOC and CDOM (in terms of the absorption
coefficient at 330 nm) into the South China Sea were estimated as 195×109 g and 266×109 m2 for the PRE and
362×109 g and 493×109 m2 for the entire
Pearl River Delta. The PRE presents the lowest concentrations and export
fluxes of DOC and CDOM among the world's major estuaries. DOM delivered from
the PRE is, however, protein-rich and thus may enhance heterotrophs in the
adjacent coastal waters. Overall, the PRE manifests lower abundance and
smaller spatiotemporal variability of DOM than expected for a sizable
estuary with a marked seasonality of river runoff due supposedly to the
poorly forested watershed of the Pearl River, the rapid degradation of the
pollution-derived DOM in the upper reach, and the short residence time of
freshwater.
Bio- and photo-lability of dissolved organic matter in the Pearl River (Zhujiang) estuary
