Dynamics of bottom currents in a small lake1

Abstract

The dynamics of bottom currents in Baldeggersee, Switzerland, were investigated with a specially designed recording current meter sensitive to current velocities of 0.3–2.5 cm s−1. Supporting information was obtained from thermistor‐chain temperature recordings and from records of atmospheric boundary‐layer parameters measured in the center of the lake. During 1980–1981 bottom currents recorded for 1‐week periods on the central bottom plateau of the lake (65‐m water depth) were highly variable. The mean speed was slightly > 1 cm s−1 with no seasonal trend. Speeds >2.5 cm s−1 occurred after periods of high wind; quiescent periods were rare and seldom lasted more than 1 h. Spectral analysis shows that the primary mechanism generating bottom currents during stratification is internal gravity waves in the thermocline. During winter mixing when chemical stratification is observed below 30 m, the wind stress distribution is directly reflected in the bottom current pattern. In both periods wind‐induced disturbances were felt in the bottom layer with little time delay, indicating the event‐dominated structure of bottom current dynamics. Bursts of particle transport in the bottom layer are observed during periods of high wind, suggesting episodic particle resuspension. Such events may be important for the control of mass exchange at the sediment‐water interface.