The Agiturb laboratory turbulence generation system and its application to plankton studies: zooplankton and phytoplankton

Plankton species live in a turbulent flow and are fully adapted to it. They have specific behaviour and responses related to turbulence characteristics and intensities, that are still largely unknown. Turbulence systems in the laboratory are needed to perform controled experiments with different zooplankton and phytoplankton species. Here we present the Agiturb turbulence generation system and some first results using different plankton species.

 

In the Agiturb system, the turbulent flow is produced using four contra-rotating agitators that are place under a cubic tank. The model for such flow is the so-called “four-roll mill” proposed by G.I. Taylor in 1934 to generate a statistically stationary, spatially inhomogeneous flow with compression and stretching. In our experiment,  the flow close to the agitators is a free flow similar to the four-roll mill, without the cylindrical rolls. The injection of the energy in the flow is produced by 4 stirring bars activated by 4 magnetic stirrers situated at symmetric positions, the centers being placed at one-fourth of the width of the tank. The cubic tank is almost half-full with 15 liters of sea water. For each experiment, the magnitude of the rotation rate of each agitator was identical, with two agitators rotating clockwise and two anti-clockwise, the same directions being along the diagonal. Different values of the rotation rate were chosen to reach different turbulence levels, characterized by the microscale Reynolds number R_λ  going from 130 to 360.

 

We present the result of two different experiments: the first one is a record, using a high speed camera in the infrared, of copepods trajectories, at different turbulent intensities, in order to see an optimal Reynolds number for copepods swimming activities (Acartia tonsa). The second one is a systematic study of the proliferation of diatoms under different turbulent intensities (Pseudo-nitzschia). In both cases different rotation rates of the system are considered, and an optimal turbulence level has been found, with maximum swimming activity for copepods and maximum growth rate for diatoms.