Bacterial metabolism in the River Danube: Parameters influencing bacterial production

1. Microbial parameters were determined at five sampling sites in the River Danube up- and downstream of Vienna, Austria, twice monthly over an annual cycle. Bacterial production (BP) was estimated from thymidine and leucine incorporations; additionally, the effect of turbulence on BP and the conversion factors for converting incorporation rates into bacterial cell production were determined using the cumulative approach. 2. BP under turbulent conditions was not significantly different from that under stagnant conditions. For thymidine, a mean annual conversion factor of 3.2 x 10(18) cells mol(-1) thymidine incorporated was calculated. For leucine, the corresponding factor was 0.07 x 10(18) cells mol(-1) leucine. Average annual BP calculated by thymidine incorporation was significantly higher than BP calculated from leucine incorporation and ranged from 47.2 to 77.5 mu g Cl-1 day(-1) depending on the tracer and the conversion factor used. 3. Bacterial growth rates ranged from 0.1 day(-1) during winter to 1.7 day(-1) in the summer. A strong correlation was found between temperature as well as chlorophyll n and bacterial growth when temperature was greater than 5 degrees C; a major spring phytoplankton bloom at a temperature below 5 degrees C did not increase BP. 4. Dissolved organic carbon (DOG) concentrations varied between 2 and 7.2 mg Cl-1 and comprised between 50 and 92% of the total organic carbon pool in the River Danube. Based on the DOC concentration and an assumed bacterial growth yield of 20% we calculated mean DOC turnover times of around 69 days in the winter and less than 8 days during the summer.