Drainage ditch erosion history as recorded in the varved sediment of a small lake in East Finland

Short core stratigraphy of total N, P, Zn, Hg and diatoms were investigated in the varved sediment of Lake Heinaelampi, a small 8 m deep pond in East Finland. The sediment has recently changed from organic mud to minerogenic silt. The presence of varved sediment offers independent control for super(210)Pb and super(137)Cs dates and these are in good agreement: the sediment quality initially changed around the year 1930 due to increased field erosion, and the inflow of mineral matter was further increased by ditching of paludified forests in 1980. Diatom analysis reveals that eutrophication has occurred before and concurrent with the sedimentary change. The causes of eutrophication and the increased levels of Zn and Hg are discussed.