Pelagic primary production and related parameters in the River Danube near Vienna (Austria)
Riedler, P.; Schagerl, M.
In the River Danube downstream of Vienna (river kilometer 1901.5) pelagic primary production and algal class distribution were observed in course of a year. Special importance was attached to the key factors hydrology and light climate. All along the study time centric diatoms predominated in the phytoplankton. In winter reduced solar irradiance and low temperature caused chlorophyll-a concentrations below 10 µg · 1-1, similar quantities were measured after spates due to flushing and dilution. As abiotic conditions became appropriate chlorophyll-a reached the maximum of 67 µg · 1-1. For measuring primary production a vertical light gradient was simulated in the laboratory, therefrom originating data were fitted into a nonlinear regression model. Fundamental parameters defining the P-I function indicate an algal community generally adapted to low light conditions. With regard to water level fluctuations, hourly solar radiation and vertical light attenuation primary production was applied to the field. The results indicate a positive daily net production on an average. In spring, summer and fall mostly autotrophic conditions were calculated, but in winter the pelagic community experienced heterotrophic states over longer periods.