Palynological evidence of natural and anthropogenic impacts on aquatic environmental changes over the last 150 years in Dongping Lake, North China

Abstract

Dongping Lake is located in Dongping County, west of Shandong Province, North China. It is the only large reservoir for flood control and detention in the lower reaches of the Yellow River, and an important regulator for the Eastern Route of South-to-North Water Diversion Project in China. In order to retrieve past historical changes of aquatic vegetation in Dongping Lake, a sediment core was drilled from the lake. Based on the high-resolution chronological sequence which was established according to 210Pb and 137Cs methods, changes in physical conditions and water quality associated with the disturbance of the Yellow River, as well as the reactions of aquatic species to environmental conditions and human activities in Dongping Lake are discussed. The data suggest that the lake has undergone considerable aquatic environmental change, resulting in remarkable change of aquatic vegetation (development period, mass disappearing period, restoration and development period, thriving period), which are attributed to both natural and anthropogenic impacts over the past 150 years. From 1855 to 1865, aquatic pollen was in great abundance, attributed to occasional disturbance by floods of the Yellow River during this period. Frequent influence of the Yellow River (high water level, turbid water, low transparency and strong hydrodynamic condition) could be responsible for the near absence of Typha and Myriophyllum from 1865 to 1940. The high and stable accumulation rate of 0.29 g cm−2 y−1 during this period also supports this. The Yellow River captured the Huai River and discharged to the Yellow Sea in 1938. Without disturbance of the Yellow River, the improved growing conditions in the lake from 1938 led to a partial recovery of aquatic vegetation. The rapid decline of the sedimentary flux also supports this. The lake was disturbed by the Yellow River again in 1947, and the decreasing sediment concentration was recorded by the continuing decreased sedimentary flux. As the water level rose, a noticeable increase of Myriophyllum and a decrease of Typha were observed. High abundance of Myriophyllum and Typha and frequent appeared Nymphoides and Trapa/Nymphea from 1963 could be related to a change in sediment supply, stable environmental conditions, and eutrophication-related changes. The increasing abundance of Pediastrum and low stable sedimentary flux also support this. However, the remarkable rise of sedimentary flux and decrease of Pediastrum from 1995 might be a reflection of enhanced human activities in recent years.