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

Jiacheng Li,Guangzhe Jin,Fajin Chen,Fengxia Zhou,Chao Huang,Junhui Wu,Qingmei Zhu,Shuwen Zhang,Chunqing Chen

doi:10.3390/jmse9050541

Jiacheng Li, Guangzhe Jin + Show 7 more

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https://doi.org/10.3390/jmse9050541

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Abstract

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 (Chl a), suggesting that terrigenous sources were dominant. However, in winter, the particulate organic carbon and particulate nitrogen values were correlated with the Chl a in the middle bay and bay mouth. Moreover, the δ13C values showed a significant correlation with Chl a 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.

Highlights

Environmental pollution and eutrophication are causing significant problems in estuaries and coastal waters worldwide [1]
The seasonal and spatial distributions of the POC and PN in the surface waters of Zhanjiang Bay, which is a semi-enclosed bay with concentrated human activities situated in Southern China, were investigated
The POC was not correlated with the chlorophyll a (Chl a) in winter, indicating that one source of the organic matter may be terrigenous material

Summary

Introduction

Environmental pollution and eutrophication are causing significant problems in estuaries and coastal waters worldwide [1]. The particulate organic matter (POM) in coastal waters is a complex mixture of marine and terrestrial organic compounds that are sensitive to eutrophic and artificial activities [2,3]. The stable C and N isotope compositions (δ13 C and δ15 N) and the C/N ratios have been extensively used to discriminate organic matter sources in water environments and to investigate the impacts of human activities [9,10,11]. Terrestrial plants are usually divided into C3 and C4 plants based on their carbon sequestration photosynthesis [12]. The values of terrestrial organic matter are generally lower than those of river algae because they absorb carbon with higher δ13 C values [14]. The δ13 C values of marine organic matter range from −22‰ to

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