Flow history explains temporal and spatial variation of carbon fractionation in stream periphyton

We investigated factors that contribute to isotopic carbon fractionation in periphytic biofilms in a human-altered headwater stream with a flashy hydrograph. Water velocity had an important effect on periphyton d13C, explaining both temporal and spatial variation. We found that water velocity averaged over a certain period before sampling, rather than the instantaneous water velocity, explained a high percentage of both temporal and spatial variation of the periphyton d13C signature. The relationship between water velocity and periphyton d13C signature was particularly influenced by individual flow events during the recent flow history. A simple model based on a flow history of 3-4 weeks reliably estimated the d13C signature of periphyton from distinct reaches. The model clearly identified signature shifts caused by the deposition of activated sludge particles from a wastewater treatment plant onto the periphytic biofilms. We highlight the high spatial and temporal variability of periphyton d13C signatures (i.e., up to 3-6‰) in a heterogeneous flow environment with inputs from a wastewater treatment plant, and we explore its implications for food web analysis. Œ 2005, by the American Society of Limnology and Oceanography, Inc.