Response Of Diatom Assemblages From Qi'ao Island To The Climate Changes In Pearl River Delta Over The Past 300 Years

An estuarine delta is an important interface connecting the land and the sea,and its environmental changes are believed to have been driven by both natural and human forces.The Pearl River Delta(PRD),located in the low latitudes of the West Pacific,is under the persistent influence of prevailing monsoons and thus has unique climatic conditions.In addition,as a foreland of China's Reform and Opening-up,the PRD has experienced rapid social and economic developments for decades and is currently facing problems such as ecological fragility and environmental degradation.Investigation of the long-term environmental changes in PRD,especially under the context of ongoing global climate changes,provide important implications in environmental protections and resource managements,and therefore has become one of the hot research topics in recent years.In this study,sediment cores(S1,S2 and B1)were obtained in September 2019 from the mangrove wetlands of Qi'ao Island,and multiple sediment proxies,including diatoms,grain size and nutrient indicators(TN and TP),were used to reconstruct the?300-year history of environmental and climate changes in PRD.The main findings are as follows:1.According to the results of ²¹⁰Pb and ¹³⁷Cs dating,the average sedimentation rate of Core S1 is?0.29 cm/a,and through comparison of magnetic susceptibility in Cores S1 and S2 with Core S1,the average sedimentation rates for two intervals(0-10 cm;11-57 cm)of Core S2 are derived to be?0.27 cm/a and?0.21 cm/a,respectively.Subsequently,the chronological frameworks of the two cores are established:Year 1759-2019 for S1,and Year 1745-2019 for S2.2.Results of the particle size analysis in Core S1 show that the percentage of clay,silt and sand are ranged from 10.3%-24.1%(mean=17.0%),49.5%-72.0%(mean=64.3%)and8.7%-37.6%(mean=18.7%),respectively,indicating that the silt is the most dominant component of the sediment particles in PRD.The increase in silt and sand contents since1920 may be attributed to the increased flood events caused by climate changes,and the intensified soil erosion from human activities over the past 100 years.3.A total of 138 epiphytic diatom taxa belonging to 2 classes,6 orders,11 families,34genera,and 169 benthic diatom species from 2 classes,6 orders and 13 families and 49genera,were identified in Qi'ao Island.The dominant epiphytic species included Diadesmis contenta and Denticula neritica,while the dominant benthic taxa were Nitzschia clausii,D.contenta,Gyrosigma eximium and Cyclotella striata.A non-metric multidimensional scaling(NMDS)ordination was used to reveal the distribution patterns of diatom communities from different mangrove species and sampling seasons,and the results showed that both the epiphytic and benthic communities differ from those of the Sonneratia apetala and Kandelia obovata mangroves.Moreover,the diatom communities in summer were different from those in winter,which is presumably due to the seasonal temperature difference.4.A total of 574 diatom taxa in 2 classes,7 orders,14 families and 67 genera were identified in Core S2,and the dominant species contained C.striata,Achnanthidium pusillum,Paralia sulcata and Cocconeis euglypta.5.A CONISS cluster analysis suggested that the sediment diatom assemblages can be divided into 3 zones:Zone 1 from 1745-1870,Zone 2 from 1870-1922,and Zone 3 from1922-2019.According to the various preferences of salinity,the sediment diatoms can be grouped into four categories:marine,brackish to marine,brackish to freshwater and freshwater diatoms,and these environmental prefences can be applied into the interpretation of salinity gradient changes in environments.Throughout the Core S2,the relative abundance of marine diatoms has continued to rise,whereas that of freshwater diatoms has gradually declined.The succession of diatoms with higher-salinity preference replacing the ones with lower-salinity preference,probably indicating joint influence of sea level rise and salt tide intrusion in PRD under the context of ongoing global warming.