Dilute soda lakes are among the world's most productive environments and are usually dominated by dense blooms of cyanobacteria. Up to now, there has been little information available on heterotrophic bacterial abundance, production, and their controlling factors in these ecosystems. In the present study the main environmental factors responsible for the control of the heterotrophic bacterial community in five shallow soda pools in Eastern Austria were investigated during an annual cycle. Extremely high cyanobacterial numbers and heterotrophic bacterial numbers up to 307 x 10(9) L(-1) and 268 x 10(9) L(-1) were found, respectively. Bacterial secondary production rates up to 738 micro g C L(-1) h(-1) and specific growth rates up to 1.65 h(-1) were recorded in summer and represent the highest reported values for natural aquatic ecosystems. The combination of dense phytoplankton blooms, high temperature, high turbidity, and nutrient concentration due to evaporation is supposed to enable the development of such extremely productive microbial populations. By principal component analysis containing the data set of all five investigated pools, two factors were extracted which explained 62.5% of the total variation of the systems. The first factor could be interpreted as a turbidity factor; the second was assigned to as concentration factor. From this it was deduced that bacterial and cyanobacterial abundance were mainly controlled by wind-induced sediment resuspension and turbidity stabilized by the high pH and salinity and less by evaporative concentration of salinity and dissolved organic carbon. Bacterial production was clustered with temperature in factor 3, showing that bacterial growth was mainly controlled by temperature. The concept of describing the turbid water columns of the shallow soda pools as "fluid sediment" is discussed.