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Microbial Assemblages and Production in Alluvial Aquifers of the Flathead River, Montana, USA

Microbial communities of alluvial floodplain aquifers of 5th- and 6th-order reaches of the Flathead River, Montana, were quantified by sampling wells drilled along transects (500-5100 m) perpendicular to the channels. Cell densities decreased progressively with distance from the river at both reaches. Only 2-3% of the total microbial densities measured in the river water column (3.1 × 105 cells/mL) occurred in wells most distant from the river (9.0 × 103 cells/mL). The rapid decline in bacterial densities could be a result of the filtering effect of the bed sediments. Multiple regression analysis indicated that site distance from the river channel was the only variable examined that explained a significant amount of the variance in bacterial densities in the transects in the down-welling region. Bioavailability of dissolved organic carbon (DOC) also may have been important in the distribution of bacteria, but no spatial correlation was observed using DOC. However, continuously low bacterial densities in wells far from the river suggested that DOC in phreatic waters was primarily refractory. Fungal biovolume exceeded bacterial biovolume in a few wells and hyphae were common in many well samples (up to 89/mL), suggesting strong connectivity with the surface environment. Low fungal densities in the river and elevated densities observed in wells located down-gradient in the flow path suggested that vertical immigration through infiltration and percolation of water from surface soils, which were typically high in fungi, may have been an important process influencing aquifer communities and productivity. Protozoa were observed at all sites with densities ranging from 0 to 64/mL in the river and 0 to 213/mL in wells. The discovery of 24 genera of algae and the presence of chlorophyll-containing cells 4 km from the river provided strong evidence that riverine microbiota were entrained great distances in these highly transmissive aquifers. Incubation of small rocks in wells revealed an extensive epilithic microbial community compared to sparse populations in interstitial waters; mean density of epilithic bacteria was 6.3× 106 cells/cm2, whereas fungi and protozoa averaged 134/cm2 and 350/cm2, respectively. Epilithic bacterial production, estimated by the rate of 3H thymidine incorporation into DNA, ranged from 0.4 to 6.9 μg C m-2 h-1. Lack of higher epilithic biovolume and metabolism in wells near the river suggested that the very pristine waters of the Flathead River did not stimulate production. However, sporadic increases in DOC during spring runoff, storm events, subsurface wetting of the unsaturated zone, and vertical percolation of rain and snow melt may be used very efficiently by the microbial community. Additional study linking hydrogeomorphic processes in these alluvial aquifers to temporal and spatial distribution of carbon and nutrients in a source-sink context is needed to fully elucidate controls on the microbial communities in these very oligotrophic groundwater systems.