Horizontal and vertical variations in sedimentation and resuspension rates in a stratifying lake -: effects of internal seiches

The temporal and spatial variations in the rate of sedimentation and sediment resuspension in Lake Rehtijarvi (southern Finland) were studied by sediment traps deployed in the stratifying and non-stratifying regions of the lake. Both the gross sedimentation rate and the resuspension rate recorded by near-bottom traps were significantly higher in the stratifying region (> 20 m depth) than in the shallow part of the lake (2 m depth). These rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m(-2) day(-1) in July 2005, whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m(-2) day(-1) throughout the study. The elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish, but was due to the occurrence of internal seiches. The seiche amplitude was on average the highest in July, although the wind velocities were low; this was because winds temporarily were blowing along the longitudinal axis of the lake. Due to the steep slopes, the sediments of Lake Rehtijarvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations.