Activity profiles of bacterioplankton in a eutrophic river

1. The significance of microbial diversity in processing dissolved organic matter (DOM) is largely unknown. We investigated the range of functional diversity in the bacterioplankton from a eutrophic river by profiling extracellular enzyme activities (EEA) and substrate‐induced respiration (SIR) patterns. 2. The EEA profiles consisted of assays for 21 hydrolases, measured using fluorogenic substrates arrayed on 96‐well microplates. Commercially available BiOLOG® GN and ECO plates, which contained 120 different substrates, were used for the SIR profiles. 3. The EEA data were more dynamic than the SIR. Five enzymes, leucine aminopeptidase, alkaline phosphatase, alanine aminopeptidase, arginine aminopeptidase and β‐glucosidase, showed consistently high activity; ten others were ubiquitous at lower activity levels; the remainder were detected intermittently. The SIR data showed less temporal variability. With one exception (citrate), the 20 substrates that generated the largest responses were all saccharides or their derivatives. 4. The EEA and SIR data did not generally correlate. Both methods were effective for ordinating bacterioplankton although, unlike the SIR, the EEA ordination followed a clear temporal trajectory. 5. Because the SIR profiles are based on a culture response, whilst the EEA profiles measure activity of the extant community, the latter appear to be more directly linked to the mechanics of DOM processing.