H. KALCHEVA1, D. TERZIYSKI2, R. KALCHEV1, K. DOCHIN2 and A. IVANOVA2
1 Institute of Zoology, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria
2 Institute of Fisheries and Aquaculture – Plovdiv, BG-4003 Plovdiv, Bulgaria
KALCHEVA, H., D. TERZIYSKI, R. KALCHEV, K. DOCHIN and A. IVANOVA, 2010. Control of zooplankton and nutrients on bacterioplankton in fish ponds with carp larvae. Bulg. J. Agric. Sci., 16: 284-297
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 0.1026 μm3), 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 0.9 μm and zooplankton biomass, while in 2008 cladocerans directly reduced 2 μm-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.