The major components of the microbial loop (dissolved organic carbon, bacteria, nanoflagellates and ciliates) were examined in three Polish lakes (Masurian Lakeland, North-Eastern Poland): highly eutrophic (Lake Ryƒskie), mesotrophic (Lake Kuc) and humic, acid (Lake Smolak Du˝y). Microbial loop was distinctly differentiated among studied lakes. In the eutrophic lake, the microbial loop was characterized by a wide variety of biotic components, great taxonomic, trophic and size differentiation of ciliates (dominated by bacterivorous and predatory taxa). Probably, in this lake autochthonous dissolved organic carbon (DOC) prevailed. In the mesotrophic lake, the microbial loop comprised fewer components and the taxonomic and trophic variability of ciliates (dominated mainly by bacterivorous and algivorous taxa) was smaller. This lake contained almost the same amount of DOC, but part of it was probably less available and allochthonous origin. Much of the DOC in the humic lake being poorly available (humic substances prevailed) and hence there were lower densities of bacteria and still lower of nanoflagellates and ciliates (dominated by large-sized mixotrophic and small- -sized bacterivorous species). The only significant correlation between bacteria and ciliates suggests that the food chain in this lake is short and that the high concentrations of DOC of allochthonous origin are not readily assimilated. Additionally, low pH values restricted growth of the studied groups of organisms and decomposition of DOC by bacteria. The number of significant correlations differed among lakes and in different temporal scales. In the eutrophic lake the relationships were stronger in both seasonal and diel cycle. Practically no relationships were found in the humic lakes in both scales. Generally, diel correlations were stronger than those in the seasonal scale.
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