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Influence of velocity and chlorophyll standing stock on periphyton 13C and 15N in the Ste. Marguerite River system, Quebec

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In a previous laboratory experiment, water velocity had a strong negative effect on periphyton 13C and 15N, 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. 13C signatures were more variable (–17.7‰ to –31.5‰) than 15N signatures (–0.4‰ to 5.7‰). Velocity and chlorophyll were significant predictors of periphyton 13C 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 13C. However, none of the variability in 15N could be explained, possibly a result of severe boundary-layer N depletion. Similarly, the velocity and chlorophyll predictors could not explain the among-reach signature differences for either isotope. 13C signatures tended to be 13C-depleted in the tributary streams and 13C-enriched in the downstream reaches.

Dans une expérience antérieure en laboratoire, la vitesse du courant avait un fort effet négatif sur le 13C et le 15N du périphyton, ce qui reflète probablement l’effet du courant sur les échanges dans la couche limite. Afin de déterminer si de tels patrons s’observent in situ à diverses échelles spatiales, nous avons examiné plusieurs sections présentant chacune une gamme étendue de courants et de biomasses de chlorophylle. Les échantillons de périphyton ont une composition uniforme et comprennent principalement un très petit nombre d’espèces de diatomées vivantes et mortes. Les signatures de 13C sont plus variables (–17,7 ‰ à –31,5 ‰) que celles de 15N (–0,4 ‰ à 5,7 ‰). La vitesse du courant et la chlorophylle sont des variables explicatives significatives du 13C du périphyton dans les sections, particulièrement aux échelles intermédiaires (10–500 m) où des alternances persistantes entre les zones de courants rapides et lents (rapides–coulées–fosses) causent des fluctuations inverses du 13C du périphyton. Toutefois, rien de la variabilité de 15N ne peut être expliqué, probablement à cause d’un important épuisement de N dans la couche limite. De même, les variables explicatives, la vitesse du courant et la chlorophylle, ne réussissent pas à élucider les différences dans les signatures de ni l’un ni l’autre des deux isotopes. Les signatures de 13C ont tendance à être réduites en 13C dans les tributaires et enrichies en 13C dans les sections d’aval.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 1, 2007

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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