Control of zooplankton and nutrients on bacterioplankton in fish ponds with carp larvae

Bacterioplankton dynamics was investigated in fish ponds with carp polyculture in two-year experiment - 2007 2008, summer months. A total of seven ponds, separated in two variants of fish stocking densities with larvae of Cyprinus carpio and Aristichthys nobilis, in ratio 3:1 - Var1 and 1:3 - Var2 were studied. The total number and biomasses of bacteria ranged widely - mean cell volumes from 0.0301 to 0.1026microm3, being twice higher in 2008 than in 2007. Most of bacteria were free-living, while those attached to detritus varied from 2 to 37%. The influence of abiotic factors, phytoplankton and zooplankton on the dynamics of bacterioplankton was revealed by different statistical analyses. Many abiotic factors - more strongly transparency and pH influenced bacteria negatively. The greatest negative impact on number - Spearman rank correlation RS=–0.64, morphology and size groups of bacteria in spatial and temporal aspect - redundancy analysis, RDA was caused by NO3-N, while PO4-P influence was positive - RS=0.35, in 2008 only. Phytoplankton and bacterioplankton - mostly presented by their cell volumes were positively related. Negative correlations - RDA were found in 2007 between free bacteria and rotifers and between cells larger than 0.9microm and zooplankton biomass, while in 2008 cladocerans directly reduced 2microm-large cells and indirectly stimulated smaller and largest cells. High numbers of bacteria with various sizes, found in presence of copepods mainly in 2008 in Var1, showed their indirect positive relationships. In 2008 the two stocking variants showed significant differences - ANOVA in morphological and size groups. Trophic cascades by carp larvae on lower levels - zooplankton and probably protozoa resulted in high abundances of small cells - mainly cocci in Var1, but of larger free cells and of attached to detritus bacteria and detritus particles in Var2. Our results support the findings that zooplankton grazing pressure - top-down control negatively affects number and cell sizes, while nutrients and phytoplankton (bottom-up control) and some abiotic factors determine morphology and biomass of bacterioplankton.