The effects of forest ditching and water level changes on sediment quality in a small lake, Perhonlampi, Central Finland

Abstract

Sediments of Lake Perhonlampi were studied by loss on ignition (LOI), total element and simultaneous extraction element analyses in order to examine the effects of forest ditchings and water level lowerings on sediment quality. 14C-dating and b-activity measurements and historical data were used for dating the sedimentation rate. Principal components analysis (PCA) was used in order to examine variations within the data and intercorrelation between the variables.

The water level lowering of a larger water course, including Perhonlampi, around 5 000 BP is detected in the sediment as a silt rich layer and as a drop in LOI values. The artificial water level lowerings at the end of the 19th century and at 1964 are not clearly visible in the LOI values, because shorelines were at that time surrounded by floating peat margin. The artificial water level lowerings only caused a slight increase in total Al, Fe, K, Mg, Na, Zn and in acid soluble Fe, Mn and Zn.

The clearest geochemical and sedimentological changes are caused by forest ditchings at 1971 and 1980. Sedimentation of mineral matter has increased and the overall sedimentation rate has changed from 0.62 mm·a-1 over the last 4300 years to 6.5 mm·a-1 during the last 24 years. The natural sediments are covered by 15 cm thick silty deposits. The concentration of total Al, Ca, Cu, Fe, K, Mg, Na, Ni and Pb has increased after the forest ditchings. The clearest increase can be seen in the concentrations of erosion indicators K, Mg and Na. Cd, S and P concentration falls towards the surface of the sediment because of dilution by mineral matter. Simultaneous extraction showed that deposition of silty material has increased the concentration of acid soluble forms of Fe, Cu, Mn and Zn and diluted the labile fractions in the surface parts of the sediment.

Accelerated erosion and sedimentation has lasted over 20 years, far longer than usual in Finland. Due to the erosion sensitive sediments of the ditched areas and abundant ground water discharge in the headwaters of the drainage basin, the disturbances will most probably continue. If the sedimentation rate remains at the current level the lake will fill up within some 150 years.