Seasonal changes in planktonic bacterial assemblages of two Ohio streams

1. Monthly water samples were collected from two Ohio streams (Bixon and Mahoning) for 14 months to assess the extent of seasonal changes in the bacterial assemblage and in a population within the assemblage. At the assemblage level, abundances of total bacteria and colony forming units (CFUs) were measured. Populations of Burkholderia (Pseudomonas) cepacia were quantified by colony hybridization using a species‐specific rDNA probe.2. Total bacterial numbers were highest in early winter and were lowest during summer and when the streams were covered with ice. Changes in the number of CFUs were more variable than changes in total number, but did not exhibit a clear seasonal pattern.3. B. cepacia was not detected during summer, but up to 8000 ml–1 were present in November. Seasonal changes in assemblage‐level measurements were dissimilar to population‐level changes in B. cepacia abundance.4. The temporal changes observed in this study suggest that populations of stream bacteria, such as B. cepacia, exhibit seasonal blooms that are undetected by assemblage level measurements.