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The Winkler test is used to determine the level of dissolved oxygen in water samples. The test was first developed by Lajos Winkler while working on his doctoral dissertation in 1888. The amount of dissolved oxygen is the biological activity of the water masses. Phytoplankton and macroalgae present in the water mass produce oxygen by the way of the photosynthesis. At the same time bacteria and all other organisms (zooplankton, algae, fish) consume the oxygen by respiration. The result of this two mechanisms give the concentration in dissolved oxygen.