Contribution of Picocyanobacteria to total primary production and community respiratory losses in a backwater system

The contribution of autotrophic picoplankton (APP) to phytoplanktonic prima ry production, investigated during the phytoplankton growing season (March- September) in a macrophyte-dominated backwater system near Vienna, showed t hat APP mainly consisted of rod-shaped and coccoid cyanobacteria. Two stati ons were examined, exhibiting similar seasonal patterns in the development of picocyanobacteria, although the two sites differed in picocyanobacterial cell numbers and biomass by a factor of 1.5. Cell numbers determined by ep ifluorescence microscopy varied between 0.29 x 10(4) and 34.5 x 10(4) cells ml(-1) at Station 1, and between 0.23 x 10(4) and 19.1 x 10(4) cells ml(-1 ) at Station 2. At both sites, the mean cell volume of picocyanobacteria wa s 0.5 mu m(3). Carbon fixation in the planktonic community of the Kuhworter Wasser was dominated primarily by larger phytoplankton, although the picop lankton community sometimes supplied up to 74% (mean: 35%) of total primary production. Distinct differences in chlorophyll a concentrations and prima ry production between the two sites refer to a greater competition between phytoplankton and macrophytes at Station 2. Community respiration deviated greatly in time and in level at the two stations, showing a higher dynamic in community metabolism at Station 1. At this site, community respiration l osses ranged between 12 and 100% of gross production. Hence, community meta bolism comprised net autotrophic, balanced, and net heterotrophic situation s over the investigation period, whereas at Station 2, only net autotrophic situations could be determined.