Browsing by Author "Rantala, Marttiina"

Multiple anthropogenic stressors on lake ecosystems demand effective measures towards improved protection and management of water bodies. The Water Framework Directive defines a common goal for sound water management and obliges EU member countries to monitor and protect the ecological status and water quality of all relevant surface waters. However, major problems hindering the attempts for effective water management are the lack of long-term observational data on reference status and an inadequate understanding of the responses of lake ecosystems to environmental pressures. With this regard, paleolimnological techniques are invaluable as they provide means to seek for past analogies of lake-environment interactions. Long-term development in the water quality and ecological status of Lake Storträsk, located in southern Finland, was assessed using a variety of paleolimnological proxies. The aim was to determine the reference status of this dystrophic lake, and to attain holistic understanding of late Holocene environmental changes and their influences on the lake's status. The principal hypothesis of the study was that late Holocene climate changes, catchment development and contemporary human activities have affected the status of the lake leaving records in the abiotic and biotic features of the sediment deposits. A 43-cm sediment core was obtained from the lake basin and studied for its physicochemical and biological attributes. The core was dated with radiometric methods, and a time frame of ca. 4500 years was established for the sediment sequence. Loss-on-ignition and magnetic susceptibility were measured and the elemental content of the sediment was assessed by ICP-MS- and CNS-analyses. Fossil diatom assemblages were studied to reconstruct long-term development in lake-water pH. Ordination techniques and diversity indices were applied to identify temporal patterns and relationships in the bioassemblages, and Spearman's correlation coefficients were calculated to assess statistical relationships between the studied abiotic and biotic parameters. The results reveal that, whereas long-term climate changes have had the most profound impact on the water quality and ecology of Lake Storträsk, the status of the lake has also been altered by early catchment disturbances and historical human activities as well as more intensive anthropogenic disturbances after the establishment of intensive agriculture in the area. The base of the sediment core reflects the transition from the warm and dry Holocene Thermal Maximum to the cooling and increasingly humid late Holocene. After the initial phase, a more stable development took place, disrupted by possible signs of forest fires and early clearance periods in the area. From the late Middle Ages onwards, human activities within the vicinity of the lake became more intensive leaving highly distinct marks in the sediment, but were receded towards the present. Regardless of the current location of Lake Storträsk in a conservation area, and the apparent inability of the barren catchment to support any intensive agricultural practices, it is clear that human influences on the lake have been significant. The results of this study highlight the importance of long-term perspective in the assessment of lake reference conditions since lake ontogeny is often far from linear. Furthermore, this study emphasizes the importance of combining information from multiple paleolimnological proxies as they provide a more robust and holistic basis for understanding lake-environment dynamics particularly in humic boreal lakes which often respond to environmental change in distinct ways.