Origin of the Particulate Organic Matter in a Monsoon-Controlled Bay in Southern China

• Main Authors: Jiacheng Li, Fajin Chen, Shuwen Zhang, Chao Huang, Chunqing Chen, Fengxia Zhou, Junhui Wu, Guangzhe Jin, Qingmei Zhu
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Journal of Marine Science and Engineering
• Subjects: particulate organic matter; stable isotopes; C/N ratio; isotope mixing model; Zhanjiang Bay
• Online Access: https://www.mdpi.com/2077-1312/9/5/541

In this study, the isotopic composition (δ¹³C and δ¹⁵N), total organic carbon content, total nitrogen content, and C/N ratios of suspended particulate organic matter (POM) in Zhanjiang Bay, which is a semi-enclosed bay with concentrated artificial activities in Southern China, were analyzed in order to investigate the seasonal variations in the principal POM sources in the monsoon region. In summer, the δ¹³C and δ¹⁵N values showed a weak correlation with the chlorophyll a (<i>Chl a</i>), suggesting that terrigenous sources were dominant. However, in winter, the particulate organic carbon and particulate nitrogen values were correlated with the <i>Chl a</i> in the middle bay and bay mouth. Moreover, the δ¹³C values showed a significant correlation with <i>Chl a</i> during the winter, indicating that the contribution of the in situ phytoplankton was relatively important and was affected by the monsoon in winter. Compared with the corresponding δ¹³C values, the δ¹⁵N values exhibited a complex spatial distribution. By using a Bayesian mixing model, in the upper bay, the source of POM was mainly from marine organic matter (49%) in summer, and almost an equilibrated contribution of all sources in winter. In the middle bay and bay mouth, the POM contribution mainly originated from marine organic matter (53%) during the winter. In contrast, the POM source was mainly soil organic matter (63%) in summer, suggesting that the POM was sourced from the runoff from the upstream basin. Our results suggest that the seasonal shifts of the source of POM should be taken into account when estimating C or N mass balance in the monsoon-controlled bay.