Influence of small-scale turbulence and large-scale mixing on phytoplankton primary production

To test the hypotheses that as well small scale turbulence as large scale vertical mixing cannot be neglected when quantifying primary production especially in shallow polymictic lakes, experiments were run on three different scales. (1) To achieve more natural conditions in bottles used for in situ incubation measurements of primary production, bottle stirrers were designed and tested in situ. The operation of the bottle stirrers guaranteed a homogeneous distribution of seston in the samples during incubation. Stirring increased primary production of plankton from a eutrophic lake significantly only when buoyant cyanobacteria were dominant. (2) To investigate the influence of turbulent mixing on primary production under controlled conditions, a circulating water column was maintained in large mesocosms. The comparison of static and dynamic in situ measurements of primary production revealed a distinct increase of production by mixing. (3) To find out the importance of mixing form, primary production was measured in the shallow, eutrophic, polymictic lake Muggelsee by moving water samples up and down with bottle lifts in two different ways. The two simulated motions (linear and circular) result in a different integral light supply of the enclosed phytoplankton. The higher light supply during circular movement in comparison to linear movement resulted in higher primary production in the circulating bottles compared to the bottles that were moved in a linear fashion.