Within-lake horizontal heterogeneity of bacterioplankton community composition (BCC) was investigated in the large and shallow subtropical Taihu Lake (2338 km(2), maximum depth < 3 m). Samples were collected at 17 sites along a trophic gradient ranging from mesotrophic to hypertrophic areas in August and September 2004. These sites cover two alternative stable states of shallow lakes, which are basically characterized by the dominance or the lack of submerged macrophytes. In the case of Taihu Lake, the macrophyte-dominated state is characterized by clear water and immobilized sediment, and the state largely lacking macrophytes is characterized by the dominance of phytoplankton, frequent wind-driven re-suspension of sediments, and a high turbidity. Three different methods, i.e. denaturing gradient gel electrophoresis (DGGE), reverse line blot hybridization (RLB) with probes targeting 17 freshwater bacterial groups, and 16S rRNA gene cloning and sequencing, were used for analysis of BCC. The BCC varied strongly between the two alternative ecological states, but less pronounced between phytoplankton-dominated sites even spanning chlorophyll a gradients from 16.5 (mesotrophic) to 229.8 microg l(-1) (hypertrophic). The 16S rRNA gene library representing the turbid water state contained many sequences closely related to sequences previously obtained from soil or freshwater sediment samples. Furthermore, sequences representing two new lineages of freshwater Actinobacteria were obtained from the investigated samples. Comparative statistical analyses of BCC along the investigated ecological gradients revealed that the dominance of submersed macrophytes was the most influential factor on BCC, responsible for a major part of the observed within-habitat heterogeneity of BCC in Taihu Lake.