Environment Evolution And Its Response To Human Activities Around Dali Lake During The Last Two Centuries

Based on a sediment core sampled in Dali Lake, the chronostratigraphy of sedimentary strata was reconstructed during the last two centuries by radioactive nuclide (210Pbex). Meanwhile, the particle size, total nitrogen (TN), total phosphorus (TP), total organic matter content (TOC),and the change of the sedimentary rate were analyzed as well. The indications of particle size were discussed by means of its distribution and the interaction with nutrient salt. The sources, pollution status and level of nutrient salt in different historical stages were analyzed.210Pbex CRS model was used to estimate the sedimentary rate with respect to the environment change during the last two centuries. The results showed that the top horizon was mainly composed of sandy clay (sC), sandy mud (sM), sandy silt (sZ), and silt (Z), which belonged to two particle size groups according to R cluster analysis. Overall, the sediment could be named as silt clay with a particle size distribution of poor sorting, positive skewness, and flat and secondary peak. The particle size of 15.2μm could be the most sensitive grade to the environment.The concentrations of TN, TP, and TOC had been increasing since 1965, which had been leading the lake water to be heavy eutrophicated in the recent years. Similar with the change stages of the environment evolution, the change of the concentrations of TN, TP, and TOC in the sediment had three change stages as well in the past two centuries. The concentrations varied spatially in the lake sediment. The. concentration of TN was higher in the south central area, TP and TOC decreased from the south to the north area, and all elements decreased from the top downward. This indicated that the concentrations of the nutrients in the sediment were resulted by both inner sources and human activities around the lake.The average value of C/N was 12.21 with an error of 6-14 meaning that the main source of the organic matter and nutrient salt in the sediment was aquatic organisms during the past two hundred years. Meanwhile, the organic matter had significant correlation with both the concentrations of TN and TP. The results also suggested that N and P in the sediment were from biological activities. On the other hand, the concentrations of TN, TP, and organic matter were positive correlation to the particle size that was less than 4μm and negative correlation to the particle size that was greater than 4μm. This indicated that the concentrations of N and P were also affected by the content of clay in the sediment to some extent.