The investigation was conducted in the Kopački Rit Nature Park (eastern Croatia) which is established to protect the floodplain area of the Danube River. The samples were collected monthly in four sites (channels and lakes) with increasing distance from the river. The aim of investigation was to examine trophic structure of culturable bacterioplankton in relation to inundation-isolation cycles on sampling sites with unequal degree of connection with the Danube river. A change in the ratio of copiotrophs (r-strategists) to oligotrophs (K-strategists) was expected as the different organic carbon sources emerged. The composition of variables was revealed by Principal Component Analysis of abiotic water properties (temperature, water depth, transparency, pH, electrical conductivity, dissolved oxygen, nutrient concentrations) and the chlorophyll-a concentration. Scores of significant components were used in a Multiple Regression as independent variables and the relationship between the scores and abundance of colony forming units (CFUs) was examined. Regression model was significant only for copiotrophs, and their relative dominance was noticed in couple of samples on more isolated sampling sites. In all other samples oligotrophs were a dominant group, especially during the flood pulses or drainage after it. Their elevated abundance was equally distributed in all groups revealed by Principal Component Analysis, and corresponds to the fluvial action. Also concerning the η (eta) squared and R squared values from the two-way ANOVA (sites/samplings) oligotrophs are highly influenced by the sampling period i.e. inundation-isolation cycles. They are probably driven by the allochthonous organic matter provided by floods or have allochthonous origin itself. At the same time, copiotrophs are to some extent coupled with the established water properties, i. e. bottom-up controlled and their abundance is influenced by the localisation of sampling sites in the floodplain. These results describe the development of the conditions that promote habitat specific succession of the culturable bacterioplankton. Established differences had disappeared after additional inundation-isolation cycles.
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