| DongPing Lake is a large-scale shallow lake at the lower reaches of the Yellow River.As a major regulator of the Eastern Route of South-to-North Water Diversion Project and awater source reservoir to send water from west to east of Shandong Province, formationand evolution of DongPing Lake is deeply effected by the Yellow River flooding, andworsening water quality conditions also directly endanger the water quality of thetransferred-water. In this study, on the basis of precise dating, high-resolutionenvironmental proxies analyses of pollen, particle size, magnetic susceptibility, and formsof phosphorus, et al, combined with the economic development information of DongpingCounty, evolutionary sequence of pollen, nutrients, et al are established, and theevolutionary process and mechanism of DongPing Lake over the past150years arediscussed, which would not only provide a basis for the next further studying flooddynamic mechanism of the Yellow River, but scientific evidence for the water qualitysafety assurance of South-to-North Water Diversion Project and water transfer from westto east of Shandong Province. Based on analyses of high-time resolution sporopollen,grain-size, magnetic susceptibility, and forms of phosphorus of a55-cm-long sedimentcore of Dongping Lake, the results show that:(1)19,482spores and pollen grains were obtained from55samples, spanning morethan120genera plant types. Approximately354pollen grains per sample were counted,with the average concentration of2150grains/gram. Herb and tree pollen dominate in thepalynoflora, of which up to78.5%are herbs,14.2%trees.4.7%are fern spores, and2.5%are shrub pollen. Aquatic plant pollen and pre-Quaternary reworked sporopollen fossilsappear occasionally.(2) Transport of rivers (such as the Yellow River, Dawen River and agriculturaldrainages) and lakeshore erosions are the main sedimentary sources of Dongping Lake.Grain-size analysis shows that particle size of Dongping Lake are mainly clay (diameter＜4μm), fine silt (diameter4-16μm) and coarse silt (diameter16-64μm). These threefractions account for more than98%of the total grains: clay content (~13.9-51.2%), finesilt (~42.7-56.2%), and coarse silt (~4.9-40.1%). Sand content is extremely low, generally less than2%.(3) Magnetic susceptibility of the sediments changed serrated, high in the center andlow at each end, and with a sudden change sometimes. The maximum value of magneticsusceptibility is53.3×10-8m3kg-1, the minimum is19.7×10-8m3kg-1, and an average of37.4×10-8m3kg-1.(4) The SMT sequential extraction method was applied to determine content of TP, IP,OP, NaOH-P and HCl-P in the core sediments of Dongping Lake. Results indicate thatchanges of TP and other phosphorus with depth can be divided into two stages: below5cmis relatively stable; above5cm changes greatly and frequently, an upward trend inNaOH-P and OP, a downward trend in HCl-P, IP and TP. The pollution history reflected bythe phosphorus of the sediments is accordance with the economic development history ofDongping County. Before1960s, Dongping County mainly based on agriculturalproduction and the economic growth is very slow. After1960s, the economic andpopulation began to grow slowly, but human pollution was still light, which can beindicated by the stable phosphorus during this stage. From the mid-1990s, due to the rapiddevelopment of industrial enterprises and increased population, there’s markable changesin the phosphorus in the core sediment of Dongping Lake. The higher content of OP andNaOH-P in the sediments also indicated the human contamination since mid-1990s.(5) Based on analyses of high-time resolution sporopollen, grain-size, magneticsusceptibility, and forms of phosphorus of a55-cm-long sediment core of Dongping Lake,we show that the sedimentary environment of Dongping Lake have seen significantchanges over the past150years.1) From1855to1865, the lake is in a hydrostaticdepositional environment, where submerged plants were in great abundance, emerged andfloating-leaved aquatic plants were comparatively fewer, and reworked sporopollen fossilswere not observed. Magnetic susceptibility was low. Fluctuation of Mz, amounts of clayand silt were present, which may be due to high volumes of sediments round DongpingLake basin transported into Dongping Lake via the Yellow River during the earlylake-forming stages.2) From1865to1922, the lake is in a backswamp depositionalenvironment, where aquatic plants are nearly nonexistent, appearing occasionally in some layers with relatively low abundance. Fern spores show a two-fold increase in occurrenceand reworked sporopollen fossils are frequently identified. All of these indicate a stronghydrodynamic condition and frequent environmental change. Magnetic susceptibility andgrain-size characteristics also support this interpretation.3) From1922to1961, aquaticplants appeared at the upper layers of this zone at relatively low amounts, and reworkedsporopollen fossils were frequently identified. Grain-size characteristics show an obviouschange in grain-size distribution, indicating a strong hydrodynamic condition and unstabledepositional environment. Besides, magnetic susceptibility was also high during this stage.We attribute these relationships to a backswamp depositional environment.4) From1961to2007, Dongping Lake is in a hydrostatic depositional environment, where thehydrodynamic condition is weak and stable. Evidence for this is shown by a remarkableincrease in submerged plants, a rapid decrease in fern spores, and occasional appearancesof reworked sporopollen fossils in the lower part of the zone. However, grain-sizecharacteristics reveal that fluctuation of Mz, amounts of clay, silt and sand is noticeableand frequent, which may be mainly related to impacts of ever-enhancing human activitiesin recent years. Magnetic susceptibility was also relatively low during this stage. |
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