Microbial respiration within a floodplain aquifer of a large gravel-bed river

1. Aerobic respiration, productivity and the carbon turnover rate of microbial biofilms were determined at hyporheic and phreatic sites in the Kalispell Valley alluvial aquifer along a transect extending 3.9 km laterally from the main channel of the Flathead River, a sixth order river in Montana (U.S.A.). The effect of experimentally increasing bioavailable organic carbon (acetate) on the respiration rate of biofilms in this carbon-poor [dissolved organic carbon (DOC) < 2 mg L-1] aquifer was also measured. 2. Chambers containing natural substratum were placed in-situ and allowed to colonise for 20 weeks. After 4, 12 and 20 weeks, they were taken to the laboratory where oxygen flux was measured in a computer-controlled, flow-through respirometry system. 3. Respiration ranged from 0.01 to 0.33 mg O-2 dm(-3) h(-1) across sites, with means ranging from 0.10 to 0.17 mg O-2 dm(-3) h(-1). Productivity estimates ranged from 0.18 to 0.32 mg C dm(-3) day(-1) (mean 0.25, SE 0.03). The total organic carbon (TOC) of the microbial biofilms ranged from 18.2 to 29.7 mg C dm(-3). Turnover rate ranged from 3.2 to 5.6 year(-1) with a mean of 4.2 year(-1). 4. At the hyporheic site very close to the river, respiration did not significantly increase when samples were supplemented with labile carbon. Respiration increased with increasing DOC addition at hyporheic sites more distant from the river, suggesting a carbon-limitation gradient within the hyporheic zone. Microbes at the phreatic site did not respond to increasing DOC addition, suggesting that the phreatic biofilm is adapted to low carbon availability. 5. Comparing the volume of the alluvial aquifer (about 0.7 km(3)) to that of the river benthic sediments (to 0.25 m depth, which amounts to about 1.6 x 10(-4) km(3)) within the Flathead Valley, leads to the conclusion that interstitial microbial productivity is orders of magnitude greater than benthic productivity. Alluvial aquifers are often voluminous and microbial production is an enormous component of ecosystem production in rivers such as the Flathead.