Abundance of major phylogenetic groups in river biofilms during one annual cycle as determined by whole-cell hybridization of biofilm samples with rRNA-targeted fluorescent probes. Alpha, α-Proteobacteria; Beta, β-Proteobacteria; Gamma, γ-Proteobacteria; CF, CF cluster; GPHGC, gram-positive bacteria with high G+C contents. (A) Elbe River. (B) Spittelwasser River.