Influence of velocity and chlorophyll standing stock on periphyton δ13C and δ15N in the Ste. Marguerite River system, Quebec

In a previous laboratory experiment, water velocity had a strong negative effect on periphyton delta C-13 and delta N-15, probably reflecting the effect of flow on boundary-layer exchange. To determine whether similar patterns could be detected in situ at different spatial scales, we examined several reaches, each with a wide range in water velocity and chlorophyll standing stock. Periphyton samples were uniform in composition, composed predominantly of living and dead diatoms of very few species. delta C-13 signatures were more variable (-17.7%o to -31.5%o) than delta N-15 signatures (-0.4% to 5.7%o). Velocity and chlorophyll were significant predictors of periphyton delta C-13 within reaches, mainly at medium scales (10-500 m) where persistent alternations between fast and slow current (riffles-runs-pools) produced inverse fluctuations of periphyton delta C-13. However, none of the variability in delta N-15 could be explained, possibly a result of severe boundary-layer N depletion. Similarly, the velocity and chlorophyll predictors could not explain the among-depleted in the tributary streams and reach signature differences for either isotope. delta C-13 signatures tended to be C-13-depleted in the streams and C-13-enriched in the downstream reaches.