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

The contribution of autotrophic picoplankton (APP) to phytoplanktonicprimary production, investigated during the phytoplankton growingseason (March–September) in a macrophyte-dominated backwatersystem near Vienna, showed that APP mainly consisted of rod-shapedand coccoid cyanobacteria. Two stations were examined, exhibitingsimilar seasonal patterns in the development of picocyanobacteria,although the two sites differed in picocyanobacterial cell numbersand biomass by a factor of 1.5. Cell numbers determined by epifluorescencemicroscopy varied between 0.29 x 10⁴ and 34.5 x 10⁴ cells ml^–1at Station 1, and between 0.23 x 10⁴ and 19.1 x 10⁴ cells ml^–1at Station 2. At both sites, the mean cell volume of picocyanobacteriawas 0.5 µm³. Carbon fixation in the planktonic communityof the Kühwörter Wasser was dominated primarily bylarger phytoplankton, although the picoplankton community sometimessupplied up to 74% (mean: 35%) of total primary production.Distinct differences in chlorophyll a concentrations and primaryproduction between the two sites refer to a greater competitionbetween phytoplankton and macrophytes at Station 2. Communityrespiration deviated greatly in time and in level at the twostations, showing a higher dynamic in community metabolism atStation 1. At this site, community respiration losses rangedbetween 12 and 100% of gross production. Hence, community metabolismcomprised net autotrophic, balanced, and net heterotrophic situationsover the investigation period, whereas at Station 2, only netautotrophic situations could be determined.