Evolution Of The Mangrove Coast And The Succession Of Mangrove Community Within The Last Hundred Years In Yingluo Bay,Guangxi Province

In the present study,210Pb measurement, grain size analysis and palynological analysis were employed to analyze samples which were collected from three cores in Yingluo Bay, Guangxi province. The purpose of this study is to (1) understand the evolution of sedimentary environment in the mangrove coastal zone on the basis of grain size characteristic and palynological assemblage,(2) investigate the historical development period of mangrove community according to the mangrove pollen assemblage characteristics and palynological concentration,(3) discuss the evolution of the mangrove coastal zone and the succession of mangrove community over the last hundred years in Yingluo Bay, Guangxi province by comparing our results with the meteorological data under the age frame establish by210Pb measurement.The age frame was established by testing210Pb, and then the CIC model was used to calculate the average sedimentation rate of core YLW03, YLW02and018. After being calibrated by compaction, the average sedimentation rate are0.22cm/a,0.21cm/a and0.44cm/a, respectively. The ages at the bottom of each cores are about1812AD,1677AD and1816AD.The results of grain size analysis indicate that the core YLW03and YLW02was in a strong hydrodynamic environment before the eighteenth century. From then on, the water power became weaker. Core O18was located far away from mangrove community, its grain size parameters are very volatile, which means a complex hydrodynamic environment at this position.Palynological analysis shows that core YLW02and YLW03had extremely low palynological concentration before1891AD and1975AD, respectively. This may reflect that the two places were located at tidal channels at that time. After1891AD, the mangrove pollen increased to a high level, indicating that the mangrove community appeared after1891AD. The palynological concentration and mangrove pollen increase rapidly in core YLW03after1975AD, which gives us a hint that the mangrove community appeared at this place after1875AD. Besides, mangrove community was likely to grow at the regions where core O18was collected between1894AD and1914AD. The coastline migrated landward after1914AD, leading to an evolution from high tidal zone to low tidal zone, and as a result, the mangrove community disappeared.Comprehensive analysis of the palynological assemblage and grain size data, shows that before the mangrove community appeared, the north and east areas of the Yingluo Bay was located in tidal channels whose hydrodynamic environment was strong. Gradually, the hydrodynamic conditions of the tidal zone started to be weaker with the increase of beach elevation. Thus, the mangrove community developed under this special condition. In addition, since no big rivers exist around the Yingluo Bay, the evolution from low tidal zone to middle tidal zone may be caused by tidal creek lateral swing.The succession of mangrove communities was affected by the local beach surface elevation in the Yingluo Bay. The mangrove community succession process can be roughly divided into three stages according to the variation of beach surface elevation. At the first stage, there was the pioneer species Avicennia marina community; and then Aegiceras corniculatum, Kandelia candel and Rhizophora stylosa were the dominant species; At the last stage, Bruguiera gymnorhiza and Excoecaria agallocha were dominanted. In generally, mangrove community succession shows a gradual process with a transition community consisting of two or more species.