Microbial Production in the Hyporheic Zone of a Coastal Floodplain River

Microbes living on saturated sediments influence stream ecosystem processes by altering the amount and chemical composition of materials delivered from the watershed to the river. Although many factors control rates of microbial processes, available dissolved organic matter (DOM) is often limiting. This limitation is of key interest in the hyporheic (subsurface) zone where microorganisms are dependent upon allochthonous sources of DOM for respiration and production. In floodplain rivers, long hyporheic flowpaths (hundreds of meters) occur beneath productive riparian terraces. At this scale, advecting DOM is rapidly utilized at the head of the flowpath leaving a large proportion of the hyporheic microbial community potentially DOM limited. An alternative source of labile DOM however, is the infiltration of DOM from overlying riparian soils. I investigated how variation in DOM and microbial activity was related to differences among the successional stages of overlying vegetation and positions along flowpaths in the hyporheic zone of a floodplain terrace on the Queets River, WA. Samples for dissolved organic carbon (DOC) and microbial production were collected seasonally from 30 wells during 2000-2001. Dissolved organic carbon ranged from 0.5-2.0 mg/L over the year and was higher in wells overlain by mixed old-growth conifer than young alder. There was insufficient DOC in advecting surface water to support hyporheic respiration in the terrace, suggesting that riparian soils were a potential DOM source for microbial metabolism. Microbial growth experiments demonstrated that hyporheic microorganisms were capable of metabolizing riparian soil leachates. Microbial production was higher at the head of the flowpaths than the end of the flowpaths; however it did not decrease in a fashion predicted from other studies. Although microbial production was higher in wells overlain by older trees, production was not statistically related to the overlying patch structure. The inconsistent patterns of microbial production along flowpaths and among successional patches demonstrates that neither advecting surface water nor riparian soil inputs completely determines rates of microbial activity in the Queets River floodplain hyporheic zone. Instead, subsurface production reflects a mixing of DOM sources that results in a heterogeneous pattern of microbial activities.