Distribution, spectral characteristics, and seasonal variation of dissolved organic matter in the northern Beibu Gulf, South China Sea

Abstract

Dissolved organic matter (DOM) in the land–ocean interface plays a critical role in the global carbon cycle, yet its dynamic is not well understood. Dissolved organic carbon (DOC) in the northern Beibu Gulf was measured seasonally from April 2020 to January 2021. Chromophoric and fluorescent DOM (CDOM and FDOM) were synchronously characterized by absorption and fluorescence spectroscopy. Three fluorescent components were identified from 597 samples using parallel factor analysis, including two humic-like and one protein-like component. DOC displays a significant seasonality with the average level being highest in summer (177 μmol/L) and lowest in winter (107 μmol/L). CDOM and FDOM levels in summer are also higher than those in winter. Although the variation ranges of DOC, CDOM, and FDOM in surface water are generally greater than in bottom water, the difference between the two layers is statistically insignificant. River discharge and anthropogenic input are important external sources of DOM in the low-salinity nearshore waters (i.e., estuaries and coast), whereas DOM in offshore waters is mainly produced by in situ biological activity. The mixing behavior of DOM in the northern Beibu Gulf varies spatially but minimally on a seasonal scale. Two distinct conservative mixing behaviors of DOC, CDOM, and humic-like FDOM occur in the nearshore waters from Tieshangang Bay and excluding Tieshangang Bay, respectively. The removal of protein-like FDOM along the salinity gradient and the negative correlation between protein-like FDOM intensity and apparent oxygen utilization are pronounced in the nearshore waters excluding Tieshangang Bay, jointly indicating that oxygen is consumed by microbial communities. In contrast, a net addition of DOC occurs in the high-salinity offshore waters, whereas the CDOM and humic-like FDOM undergoes quasi-conservative mixing. Overall, this study reveals notable spatial and seasonal variations in the concentration, source, and mixing of DOM at the land–ocean interface and highlights the importance of sources and processes in shaping the amount and composition of DOM exported to the ocean margin.

Source, distribution, and transformation of dissolved and particulate organic matters in Qinzhou Bay, Northern Beibu Gulf

“…Terrestrial C3 plants -25.9 ± 1.2 >20 (Meyers, 1994;Sarma et al, 2014) Terrestrial C4 plants -13.1± 1.2 >20 (Meyers, 1994;Sarma et al, 2014) Soil organic matter -19.2± 2.4 10-14 (Kendall, 1998;Sarma et al, 2014) Sewage -24.8 ± 3.2 7-8 (Andrews et al, 1998;Sarma et al, 2014) Marine organic matter -20.0 ± 2.0 5-9 (Middelburg and Herman, 2007;Ye et al, 2017) Freshwater phytoplankton -30.0 ± 2.0 5-14.6 (Thornton and Mcmanus, 1994;Ye et al, 2017) Cai removal pattern) (Stedmon et al, 2000;Guo et al, 2007;Guo et al, 2014). Previous studies have demonstrated that optical parameters could provide valuable information concerning the source, composition, and transformation of DOM in marine environments (Yang et al, 2015;Zhu et al, 2018;Zhu et al, 2022a). Here, optical parameters clearly revealed the major source of DOM in the surface waters of Qinzhou Bay.…”

Mid-Long-Term Prediction of Surface Seawater Organic Carbon in the Southern South China Sea Based on Multi-Applicability CNN-LSTM Prediction Model

“…CDOM has no apparent seasonality and has the lowest concentration in the summer [94]. CDOM in seawater can be divided into two categories according to their sources: in situ degradation of organisms in the ocean and dissolved organic matter in terrestrial soil brought into the ocean through river runoff [29]. Compared with nearshore and estuarine areas, the Nansha Sea area is not influenced by land-based sources and belongs to Class I water bodies with low CDOM content [95].…”

“…The flux and degradation of organic carbon play a significant role in shaping microbial communities within the ocean and have far-reaching implications for the carbon cycle in the SCS [2]. Our understanding of carbon cycling in coral reef systems in the SCS is currently limited due to the lack of research on the distribution and seasonal variations of organic carbon in this region, as emphasized by Zhu et al [29]. In the face of increasingly serious ocean acidification, coral reef bleaching is severe, and nearly two-thirds of coral reefs are at risk of extinction [30,31].…”