The environmental signal of diatoms in recent (0-90 years) sediments from the Lærdalsfjord, Western Norway
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Two sediment cores were retrieved from the oxic Inner and Outer Lærdalsfjord, Western Norway. The fossil diatom record of these cores was used to reconstruct environmental and climate change over the past 90 years, providing a basis for future ecosystem management of the Lærdalsfjord. Two relationships have been observed between climate factors and the fossil diatom composition. These are (a) a positive relationship between precipitation levels and the relative abundance of total freshwater diatoms, and (b) a negative relationship between decreased nutrient upwelling driven by increasing temperatures and the relative abundance of the marine benthic diatom species Paralia sulcata. These two relationships have also been used to date the sediment cores. The gradually decreasing sedimentation rates in the Inner Lærdalsfjord after the construction of a first hydropower plant at Borgund in 1974 and a second hydropower plant at Stuvane in 1988 indicate an impact of the two hydropower plants on the river flow pattern. This impact has also been observed in decreasing relative abundance of freshwater planktonic diatoms in the Inner Lærdalsfjord. The relatively stable sedimentation rates and stable relative abundance of freshwater planktonic diatoms in the Outer Lærdalsfjord suggest that the Outer Lærdalsfjord is impacted less by the changes in flow pattern of the partially regulated river Lærdalselv compared to the Inner Lærdalsfjord. After the construction of the second hydropower plant in 1988 and the loss of the natural delta between 1995 and 2000, the already existing impact of hydropower plants on the resulting reduction in flow velocity of the river Lærdalselv has been enhanced. As a result, the low river flow velocity caused the reduction in nutrients transported out to the Inner and Outer Lærdalsfjord, which furthermore led to the decreasing relative abundance of marine planktonic diatoms in the Inner and Outer Lærdalsfjord after 1988. This enhanced effect has also been shown in an increase in the freshwater benthic diatoms in the Inner Lærdalsfjord and their decrease in the Outer Lærdalsfjord after 2000. This thesis shows that fossil diatom records in recent (0-90 years) Lærdalsfjord sediments contain environmental signals and can help reconstruct environmental and climate change.