Current Fluctuations in the Surface Waters of Small Lakes

Abstract

The transfer of momentum from the wind to the surface of a lake creates fluctuations in velocity which are an expression of turbulent eddies, surface and internal waves. To know the scaling from the wind stress to the stress at the surface of a lake is important for budgets of energy transfer and for models of mixed layer dynamics. These processes have not been examined in shallow, naturally eutrophic lakes with small fetches. An omnidirectional warmbead thermistor-flowmeter was used to measure fluctuations in current speeds throughout the upper mixed layer of a shallow lake in equatorial Africa. The flow sensor resolves speeds of less than 0.1 cm sec⁻¹ and frequencies of ca. 5 Hz. The first four statistical moments, probability density functions, and energy spectra have been computed from the current records, and Richardson’s numbers have been calculated. Current speeds within 1.5 cm of the surface fluctuated rapidly from 0.3 to 16 cm sec⁻¹. The mean speeds were ca. 5 cm sec⁻¹ and the turbulent intensities (ratio of rms speed to mean speed) were greater than 0.4. There was a graded shift from high frequency, high amplitude fluctuations at the surface to lower frequency, smaller amplitude fluctuations at 10 cm depth. This shift occurred in the absence of thermal stratification. At 10 cm the turbulent intensity was still high (0.25). These data illustrate the extreme activity and variety of responses to wind mixing in low energy lacustrine environments.