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

To test the hypotheses that as well small scale turbulence aslarge scale vertical mixing cannot be neglected whenquantifying primary production especially in shallowpolymictic lakes, experiments were run on three differentscales. (1) To achieve more natural conditions in bottles usedfor in situ incubation measurements of primaryproduction, bottle stirrers were designed and tested in situ. The operation of the bottle stirrers guaranteed ahomogeneous distribution of seston in the samples duringincubation. Stirring increased primary production of planktonfrom a eutrophic lake significantly only when buoyantcyanobacteria were dominant. (2) To investigate the influenceof turbulent mixing on primary production under controlledconditions, a circulating water column was maintained in largemesocosms. The comparison of static and dynamic in situmeasurements of primary production revealed a distinctincrease of production by mixing. (3) To find out theimportance of mixing form, primary production was measured inthe shallow, eutrophic, polymictic lake Müggelsee bymoving water samples up and down with bottle lifts in twodifferent ways. The two simulated motions (linear andcircular) result in a different integral light supply of theenclosed phytoplankton. The higher light supply duringcircular movement in comparison to linear movement resulted inhigher primary production in the circulating bottles comparedto the bottles that were moved in a linearfashion.