Browsing by Author "Hirvonen, Saara"

Lake sediment study plays an important part in paleolimnology and environmental history research since element concentration fluctuations in sediments can be used to describe the changes that have been taking place in lakes and their catchments. This Master's thesis is an appointment from Someron vesiensuojelu ry., and it aims to find out how Lake Oinasjärvi and its catchment have changed over time. The study is focused on the changes in land-use (wih particular regard to agriculture), lake status and sedimentation rate. Possible indications of meromixis are also searched. The study was carried out by analysing magnetic susceptibility, and Al, K, Na, Mg, C, N, P, S, Mn, Fe, Ni, Cu, Pb, Co, V, Ba, Zn, Sn and Sb concentrations from a 280 cm section of sediments that had been cored from Lake Oinasjärvi in 2009. Organic content of sediments was concluded by means of C-concentrations. C/N ratio was calculated in order to express possible lake eutrophication, and Fe/Mn as well as Cu/Zn ratios in order to depict redox conditions in near-bottom water during sedimentation. The most interesting sediment layers were dated using 14C method, and pollen analysis was carried out to support it and implicate vegetational history. Litterature and map interpretation was used to clarify land-use change patterns, and the dependencies between variables were tested by means of simple linear regression analysis. The study revealed that the status of Lake Oinasjärvi and its catchment area has changed significantly after the establishment of permanent settlements from the 1300's onwards. The change was most pronounced in mineral matter increase and organic matter decrease in sedimentation and in consolidation of human activity indicative pollen in sediments. Radiocarbon dating of surface samples failed because of high clay content of sediments, but illustrative sedimentation rates could be calculated using other results and literature. Today, sedimentation rate exceeds natural sedimentation rate by more than 400 %. The varves observed in sediments with the Fe/Mn and Cu/Zn ratios suggest that Lake Oinasjärvi has a tendency to at least periodic and partly meromixis, which can be strengthened by human activity. This would, however, require further research of the near-bottom water. The lake is also, on the basis of C/N ratio, eutrophicated from its natural status, but the majority of the elemental concentrations of the sediment surface layer are, however, quite close to the reference level. Only those elements that spread mostly as atmospheric pollutants show high concentrations in surface sediments. Especially the reduction of agricultural loads and erosion are crucial tasks to be done in the catchment area in order to improve the status of the lake. Maintenance and development of the municipal wastewater treatment plant is also important.