Bacterial biomass, activity, and production within the hyporheic zone of a north-temperate stream

Bacterial biomass, activity, and production were examined within the hyporheic zone of a sandy-bed, north-temperate stream over three seasons. Date (November, February, and May), zone (upwelling, downwelling), and depth (10 cm, 50 cm into the bed) were significant main effects on bacterial variables. Biomass and production were significantly higher in May than during November and February, and generally higher at the 10 cm depth than at the 50 cm depth. Activity was higher in the downwelling zone in November followed by a minor peak in May. Production in the downwelling zone in May was 10-fold that of the upwelling zone. Response by bacteria to DOC enrichments (glucose, humic acid) was inconsistent and statistically insignificant. However, some general trends emerged which were consistent with observations for unenriched sediments. Bacterial activity increased with DOC enrichment only in the downwelling zone during November and May. Bacterial production in the downwelling zone increased with DOC enrichment only during November. Hyporheic bacterial patterns reflected generally higher metabolic activities (electron transport activity, thymidine uptake into DNA) within the downwelling zone, a region of greater surface-subsurface hydrologic exchange within the streambed and presumably higher DOC quality from surface-derived labile sources.