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

Abstract

The contribution of autotrophic picoplankton (APP) to phytoplanktonic primary produc- tion, investigated during the phytoplankton growing season (March-September) in a macrophyte- dominated backwater system near Vienna, showed that APP mainly consisted of rod-shaped and coccoid cyanobacteria. Two stations 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 epifluorescence microscopy varied between 0.29 3 10 4 and 34.5 3 10 4 cells ml -1 at Station 1, and between 0.23 3 10 4 and 19.1 3 10 4 cells ml -1 at Station 2. At both sites, the mean cell volume of picocyanobacteria was 0.5 µm 3 . Carbon fixation in the planktonic community of the Kuhworter Wasser was dominated primarily by larger phytoplankton, although the picoplankton community sometimes supplied up to 74% (mean: 35%) of total primary production. Distinct differences in chlorophyll a concentrations and primary produc- tion 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 losses ranged between 12 and 100% of gross production. Hence, community metabolism comprised net autotrophic, balanced, and net heterotrophic situations over the investigation period, whereas at Station 2, only net autotrophic situations could be determined.