The test was originally developed by Ludwig Wilhelm Winkler, in later literature referred to as Lajos Winkler, while working at Budapest University on his doctoral dissertation in 1888. The amount of dissolved oxygen is a measure of the biological activity of the water masses. Phytoplankton and macroalgae present in the water mass-produce oxygen by way of photosynthesis.  Bacteria and eukaryotic organisms (zooplankton, fish) consume this oxygen through cellular respiration. The result of these two mechanisms determines the concentration of dissolved oxygen, which in turn indicates the production of biomass. The difference between the physical concentration of oxygen in the water (or the theoretical concentration if there were no living organisms) and the actual concentration of oxygen is called the biochemical demand in oxygen. The Winkler test is often controversial as it is not 100% accurate and the oxygen levels may fluctuate from test to test despite using the same constant sample.