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1. The impact of flash flooding on microbial distribution and biogeochemistry was investigated in the parafluvial zone (the part of the active channel lateral to the surface stream) of Sycamore Creek, a Sonoran Desert stream in central Arizona.

2. It was hypothesized that subsurface bacteria were dependent on the import of algal-derived organic matter from the surface stream, and it was therefore predicted that microbial numbers and rates of microbially mediated processes would be highest at locations of surface to subsurface hydrologic exchange and at times when algal biomass was high.

3. Prior to a flash flood on 19 July 1994, chlorophyll a was high (approximate to 400 mg m(-2)) in the surface stream and microbial numbers were highest at the stream-parafluvial interface and declined along parafluvial flowpaths, supporting the hypothesized algal-bacterial linkage. Immediately following the flash flood, chlorophyll a was low (approximate to 7mgm(-2)), and microbial numbers were reduced at the stream-parafluvial interface.

4. Counter to expectations, parafluvial functioning (in terms of nitrate production and dissolved oxygen decline along flowpaths) re-established immediately after the flood receded. Therefore, material other than algal exudates supported parafluvial metabolism immediately postflood, and terrestrially derived dissolved organic matter is the likely source.

5. Algae in the surface stream recovered quickly following flooding, but recovery of parafluvial bacteria lagged somewhat behind. These results highlight the importance of surface-subsurface interaction to stream ecosystem functioning and show that the nature of these interactions changes substantially in successional time.