Observations of turbulence during a zooplankton migration in a small lake

A pilot field experiment was conducted to assess the importance of zooplankton generated mixing in stratified lakes. The objective of this work is to improve understanding of thermocline mixing in the lake interior, which is crucially important for better predictions of transport of dissolved substances, and consequent impacts upon lake ecosystem functioning. In this experiments, turbulence was directly measured in the thermocline of a lake during a vertical migration of crustacean zooplankton (Daphnia). Profiles of turbulence were measured with a temperature microstructure instrument in Monkswood Reservoir (UK), a small manmade lake with small wind fetch, steep sides, and a flat bottom with a depth of approximately 11 m. Twenty-seven profiles were measured over the course of three hours during sunset on 1 September 2014, during which there was no wind. Zooplankton tows were conducted before and after the turbulence profiling to verify the distribution of Daphnia before and after sunset. The zooplankton tows showed an increase in daphnia above 6 m depth and a decrease below 6 m after sunset. There was also an increase in the dissipation rate of turbulent kinetic energy to 10^-7 W/kg (two orders of magnitude above the background levels) at 6 m depth over the course of the time series. Given the uncertainty in measuring the length scales of turbulence associated with small zooplankton, it is not certain if the observed turbulence during the time of migration was due the migration or due to other causes, such as the onset of penetrative convection associated with nighttime cooling.