The Material“Source And Sink”Process Of Quanzhou Bay Under The Impact Of Typhoon Matmo(2014)

Typhoon is one of the most severe short-lived weather events,which quickly affects the migration,transformation,exchange,and transport of terrestrial materials,and the marine biological and chemical processes in the coastal system in a short time.This paper reports the results of a study carried out in the Quanzhou Bay to explore the evolution of sedimentary dynamics and the source and sink processes of the terrestrial materials under the influence of typhoon using the measurements of grain-size,sedimentary organic matter,and heavy metals and the in situ observation data(including temperature,salinity,turbidity,and flow field)collected 3~4 days and a month,respectively,after typhoon Matmo landing in 2014.The results of grain-size and water flow field observation show that the hydrodynamic force of Quanzhou Bay was mainly controlled by the Jinjiang runoff and the tidal currents in the bay.The main grain-size of sediments were silt and clay and mainly discharged by Jinjiang River in Quanzhou Bay.The sediments discharged by Jinjiang River was increased and then settled in the estuary after typhoon.The hydrodynamic force of Quanzhou Bay gradually returned to normal level a month after typhoon.The fine-grained sediments(mainly silt)were transported and diffused under the influence of tidal current,and the heavy metals and organic matter,which mainly present in the fine-grained sediments,were also spread and migrate with sediments in the bay.The distribution of total organic carbon(TOC)and total nitrogen(TN)in the sediments of Quanzhou Bay showed a gradual decreasing trend from the Jinjiang estuary to the outer bay,indicating that the sedimentary organic matter was mainly controlled by Jinjiang River input.The C/N ratio and δ13C values of sediments indicates that the sedimentary organic matter of Quanzhou Bay was mainly a mixture of terrestrial and marine sources.The terrestrial organic matter with high C/N ratio and low δ13C value was mainly distributed in the Jinjiang River estuary and gradually decreased toward the outer bay.The correlation between the content of sediment organic matter(TOC and TN)and grain-size composition indicates that hydrodynamic force was the main factor affecting the variation of sediment organic matter in Quanzhou Bay after typhoon.The tidal current transported sedimentary organic matter from the estuary to the open sea,inducing the process of dispersion and accumulation of sediment organic matter,and then resulted in the characteristics of low TN content and high C/N ratio in the sediments of the South Channel.According to the results of principal component analysis(PCA)and correlation analysis,the heavy metals in the sediments of Quanzhou Bay can be divided into 2 categories: Class I heavy metals(Cu,Zn,Pb,and Mn)were natural sources and showed a characteristics of river source that high-values were mainly occurred in estuaries and inner bay;Class Ⅱ heavy metals(V,Cr,Co,and Ni)were mainly derived from human activities,and discharged into the bay through coast or river.The high-values were mainly occurred in the estuary and coastal intertidal zone.A month after typhoon landed,the concentration of class I heavy metals gradually decreased while that of class II heavy metals increased under the effect of source change and hydrodynamic transport.However,the two types of heavy metal were both decreased along the South Channel,indicating that heavy metals in the sediments of Quanzhou Bay were transported from the bay to the open sea after typhoon.The typhoon process led to an increase in the pollution degree of class I heavy metals and a decrease of class II heavy metals.In summary,the material flux of the Jinjiang River into the Quanzhou Bay increased obviously under the influence of Typhoon Matmo,making the sediments of the Jinjiang estuary and the inner bay an important "sink" of terrestrial materials.After typhoon,the sediments of the estuary and the inner bay re-suspended and re-transported under the scouring of tidal current,and the sediments therefore transformed from a sink of terrestrial material to a diffusing source.The typhoon process is an important driving force for the transformation of material "sink" to "source" in estuaries.This study is of great significance for further understanding of the material source and sink process in small river estuaries and bays under the influence of typhoons.It is also of great significance to the environmental protection,ecological restoration and integrated management of the regional estuary and coastal system.