Interactions between natural bacterial assemblages and dissolved organic carbon (DOC) were investigated in two complementary batch experiments. In the first, a positive relationship was found between the proportion of electron transport system (ETS) active bacteria and the diversity of DOC in microcosms enriched with an increasing number of organic substrates. In a second experiment, bacterial and nutrient dynamics were measured in microcosms with natural bacterial populations and organic matter from rivers and lakes of different trophic levels. The interactions between the bacterial assemblages and DOC from different sources was investigated using source systems (rivers or lakes) and blended (different proportions of river and lake water) batch cultures. In each experiment, the number of total and ETS-active bacteria, the fluorescein diacetate (FDA)-hydrolytic activity, and the total (DOC), biodegradable (BDOC) and refractory (RDOC) dissolved organic carbon were measured four times during 5 days. The results suggested that the temperature, more than trophic level, controlled planktonic bacterial production. Furthermore, bacterial activity was stimulated in micrososms where river and lake waters were mixed. For the oligotrophic microcosms, this observation can be explained by a greater diversity of the organic nutrients ("qualitative" stimulation of bacteria), whereas for the meso-eutrophic microcosms, the production of new pools of dissolved organic carbon (both biodegradable and total) could account for the observed "quantitative" stimulation of the bacteria. These experiments suggest that the mixing of bacteria and organic matter from two different systems can give rise to novel nutrient and bacterial dynamics that are likely similar to those that occur in river-lake ecotones.