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 dynamics, 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 from 10(-3) to 10(-5) m2 s-1, and overturns mixed on a time scale of minutes. 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 largest 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 vertical eddy diffusivities from 10(-7) to 10(-4) m2 s-1. Mixing times based on these diffusivities exceeded pi/N, the time scale for turbulent transport, indicating overturns would mix only partially. Phytoplankton could still experience large fluctuations in irradiance, but the fluctuations probably were not continuous.