Factors Affecting Stable Carbon and Nitrogen Isotopic Signatures in Food Webs of the Ste. Marguerite River (Quebec, Canada)

The use of stable C and N isotopes in food web studies, and as environmental indicators, requires deeper understanding of the factors that affect the signatures of primary producers. A laboratory experiment by Trudeau and Rasmussen (2003) demonstrated the negative effect of water velocity on periphyton C and N signatures (dC and dN). Under a range of water velocities normally found in lotic systems (0.05-0.62 m s), periphyton had significantly more negative signatures in faster currents. The present field study was conducted in the Ste. Marguerite River system and compared periphyton signatures found in sites with different flow regimes (0-1.27 m s). Regression analyses on periphyton dC in the eight sections sampled yielded similar patterns to those found in the laboratory. The tendency of periphyton to have more negative signatures in high water velocity sites was statistically significant for dC, which ranged from -17.7‰ to -31.5‰. In addition, regression analyses showed that periphyton biomass (chlorophyll a concentration ranging from 0.3 to 31.0 mg m) had a positive effect on periphyton dC. Our field study however, failed to replicate the pattern for dN previously found in the laboratory, and showed no significant trends for this element. All of the periphyton communities collected were dominated by a few diatom species and varied in the thickness of the biofilm. Fractionation in favour of C by the periphyton communities likely leads to build-up of the heavier isotope in the boundary layer. At lower water velocities, as well as higher biomass (presumably greater C uptake rates), this build-up eventually results in periphyton having less negative dC. Failure of the dN to reflect this process may result either from N limitation, which would prevent isotopic fractionation altogether, or possibly from local variability in the N source signature from groundwater. This study