Stable Carbon And Nitrogen Isotopes Of Particulate Matter In The Northwest South China Sea

Carbon is the main element of life,which have different forms in estuaries,oceans and other aquatic biological systems.It have been the hot issue in the biosphere's material and energy cycles.Nitrogen is an indispensable nutrient for the marine organisms.Carbon and nitrogen are two key elements in marine ecosystems.Particulate organic matter?POM?is one of the main forms of carbon and nitrogen in marine.Compared with dissolved organic matter,particulate organic matter is more bioavailable to organisms,maintaining a high level of biological activity,and playing an important role on the global carbon and nitrogen cycle.With the emergence and application of isotope methods,stable carbon and nitrogen isotopes,and C/N are used to track the source and transformation of organic matter in estuaries,near shores,and open ocean systems.The marginal sea is affected by various physical processes and human activities,which makes the migration and transformation of particulate organic matter more complex.Therefore,it is necessary to study the biogeochemistry of particulate organic matter in the marginal sea.In the northwestern South China Sea,there are physical processes such as Qiongdong upwelling,western Guangdong coastal currents,freshwater from the Pearl River estuary,and some subtropical bay influenced by human activity.These processes make it difficult to study the migration and transformation of the particulate organic matter in the marginal sea.Stable isotopes have the advantage of recording the carbon and nitrogen sources and their biogeochemical processes.In this study,?¹³C,?¹⁵N and the C/N ratio of particulate matter in the upwelling area in the east of Hainan Island,the Pearl River estuary and its adjacent shelf area?Western Guangdong?,Zhanjiang Bay were investigated to explore the source and biogeochemical behavior of particulate matter in different physical fields and subtropical bay.It aims to reveal the influence of upwelling,coastal currents,and marginal sea on the source and migration of particulate matter.The main results of this dissertation were as followings:?1?In the upwelling area of eastern Hainan Island,the high concentrations of particulate organic carbon?POC?and particulate nitrogen?PN?,enriched?¹³C and?¹⁵N in the surface water of the nearshore area in transects S1,indicated POC and PN were controlled by the coastal current and freshwater discharge.However,the characteristics of low-temperature and high-salt on the nearshore surface water from transects S2 and S3,indicate the existence of upwelling.Coastal upwelling played an more important role in determining the sources of POM in the surface water of the nearshore area,and the decomposition and modification of in-situ production may also influenced the?¹³C and?¹⁵N of particulate matter.Depleted?¹³C and high C/N ratios in the offshore area surface water from transects S1 and S3 suggested that nearshore terrestrial organic matter was transported offshore due to tidal movement.Nitrogen fixation has resulted in lower?¹⁵N in the surface water of the offshore area.The lower C/N in the middle water and bottom water indicates that the particulate organic matter is mainly derived from the marine organic matter,the selective decomposition and regeneration of POM were the primary processes in the mid-and bottom water.?2?In the Pearl River Estuary and its adjacent waters,the spatial distribution of POC and PN is similar in summer and spring.However,the insignificant correlation between?¹³C and?¹⁵N and the different distributions indicate that biological processes have different effects on the fractionation of carbon and nitrogen.In spring,?¹³C generally decreased from land to sea,and elevated?¹³C occurred at the nearshore stations,suggesting that the eutrophic level,phytoplankton biomass and microbial activity played a more important role in determining the sources of POM.In summer,?¹³C in the Pearl River Estuary generally increased seawards,reflecting the increasing contribution of marine organic matter.Compared with?¹³C,?¹⁵N values display a complicated spatial distribution.The enriched?¹⁵N in the bottom water can be influenced by the degradation and mineralization of suspended matter.Domestic sewage and industrial wastewater were responsible for higher?¹⁵N in the Pearl River Estuary.?¹⁵N can be easily modified by a series of complex biogeochemical processes.?3?The OM in the surface sediment of Zhanjiang Bay was a mixture of terrestrial and marine sources,and was dominated by marine OM?54.9%±15.2%?.Compared to the central Zhanjiang Bay,the channel and coastal Zhanjiang Bay areas had higher?¹³C and?¹⁵N values,higher TOC and TN concentrations,and lower TOC/TN ratios,indicating higher primary productivity and higher percentages of marine OM in the latter two subregions.Mariculture activities,sewage inputs,and dredging were responsible for these phenomena.Clear seasonal variations in OM were observed in Zhanjiang Bay.The average proportions of terrestrial OM in summer increased by 10.2%in the Zhanjiang Bay channel and 26.0%in the coastal Zhanjiang Bay area compared with those in spring.Heavy rainfall brought a large amount of terrestrial OM into the channel and coastal Zhanjiang Bay areas,leading to the increase of the terrestrial OM fraction in these two subregions in summer.In summary,anthropogenic influences had a significant influence on the spatial and seasonal variations of sedimentary OM in Zhanjiang Bay.