Environmental Changes And Their Responses To Anthropogenic Activities In The Dashahe Reservoir Watershed

Sporopollen assemblages and grain-size characteristics in sediments from the Dashahe Reservoir, were analyzed to explore the impacts of human activities, such as Eucalyptus plantation, livestock farming and land-use change, on the reservoir ecosystem. The study aids to understand the anthropogenic effects on reservoir environments, and to provide constructive suggestions on water resource management and land-use planning within and around the Dashahe Reservoir.A chronological framework was established using 210 Pb and 137 Cs dating methods. Sporopollen assemblages and grain size data, together with historical documents, were synthesized to reconstruct the vegetation change in the catchment and eutrophication process in Dashahe Reservoir. Moreover, the ecological change patterns and the related driving factors were further discussed.Major results of the analyses are as follows:1. The mean sedimentation rate was 0.86 cm/a in Core S4 of the Dashahe Reservoir.2. Sixty-two families and 128 genera were identified in the 45 sediment samplessectioned at 1-centimeter interval, and Pinaceae, Gramineae, Dicranopteris andGleicheniaceae dominated throughout the core. Note that Scenedesmus was an importantindicator of eutrophication in Dashahe Reservoir.3. An end-member analysis of the grain size changes revealed that the reservoirunderwent three stages of environmental change, i.e., impoundment in the early stage of thereservoir(1962 ~ 1983 a), transitional period(1984 ~ 2004 a) and limnetic depositionperiod(2005 ~ 2013 a). During the early stage after the reservoir-building, an elevation inwater level flooded the reservoir area, and thus the exposed, coarse surface sediments werewashed down and deposited, ending up with the dominance by the end-member 3(21.9 ~196.7 μm). The improved forest cover and weakened erosion led to the dominance byend-member 2(5.1 ~ 65.6 μm) in the transition period. As the reservoir environment wasgradually stabilized, end-member 1(2.4 ~ 15.2 μm) became the dominant component.4. Core S4 was divided into 3 stratigraphic zones using CONISS analysis(constrained incremental sum of squares cluster) of spores and pollen abundance data, consistent with the division with grain-size analysis. ZONE I corresponds to Year 1962 ~ 1983, a period right after the large-scale deforestation for steel-making movement in 1958 that presumably had resulted in low forest coverage and heavy soil erosion. Both environmental changes may have contributed to the low sporopollen abundances and the dominance of the coarse end-member 3 in sediments. Meanwhile, the low total nitrogen and total phosphorus concentrations, together with rare occurrence of Scenedesmus, indicated that the Dashahe Reservoir remained oligotrophic. ZONE II corresponds to Year 1984 ~ 2004 with increased sporopollen abundances due to afforestation. During this period, the catchment was mainly covered by secondary forest of pine and pteridophytes. Moreover, the increase in Gramineae and Eucalyptus, and the appearance of Scenedesmus and Pediastrum, probably reflect enhanced nutrient loading from agricultural non-point sources. ZONE III corresponds to Year 2004 ~ 2013. During this period, the Dashahe Reservoir and its catchment had undergone rapid socioeconomic development, which was characterized by enhanced manufacture industry, livestock farming, and Eucalyptus plantation. The decreased sporopollen abundances and C/N values but increased TN and TP levels implied that algal blooms occurred in response to the eutrophication process in the reservoir, which was possibly caused by a large amount of raw sewage discharge from factories and poultry farms.