VERTICAL MIXING IN A SHALLOW, EUTROPHIC LAKE - POSSIBLE CONSEQUENCES FOR THE LIGHT CLIMATE OF PHYTOPLANKTON

Profiles of temperature-gradient microstructure are used to define the size and location of mixing regions, the intensity of turbulence, and the potential exposure of phytoplankton to fluctuating irradiance in a shallow, turbid, productive lake. The part of the water column which was mixing tended to be subdivided into two regions with different dy namics, one in which the turbulence was active and one in which it was constrained by buoyancy. Generally the upper layer, which ranged from 0.3 to 1.5 m deep, was actively mixing. Energy dissipation rates were on the order of 10(-7) m2 s-3, vertical eddy diffusivities ranged fro m 10(-3) to 10(-5) m2 s-1, and overturns mixed on a time scale of minu tes. Phytoplankton could become well mixed before turbulent transport within overturns ceased and, while the wind persisted, were likely to experience continuous fluctuations in irradiance. In one of the larges t overturns, phytoplankton could circulate between the 90% light level and the 5% light level in 3-4 min. Where buoyancy affected turbulence , energy dissipation rates ranged from 10(-9) to 10(-7) m2 s-3 and ver tical eddy diffusivities from 10(-7) to 10(-4) m2 s-1. Mixing times ba sed on these diffusivities exceeded pi/N, the time scale for turbulent transport, indicating overturns would mix only partially. Phytoplankt on could still experience large fluctuations in irradiance, but the fl uctuations probably were not continuous.