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Terrestrial carbon contribution to lake food webs: could the classical stable isotope approach be misleading?

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Abstract:

Carbon stable isotope analyses have been widely used to estimate terrestrial carbon contribution to lake secondary production. In such approaches, phytoplankton is tacitly assumed as a single, isotopically homogenous source. Such assumption might be valid if (i) zooplankton do not feed selectively on specific algal taxa within bulk phytoplankton, or (ii) although zooplankton do feed selectively, the variability in the 13C values amongst the different algal taxa is small compared with the variability between the 13C values of bulk phytoplankton and terrestrial end-members. In a summer field study of six coastal lakes of British Columbia, Canada, we tested these assumptions using gut pigments and stable isotope measures on zooplankton and particulate organic matter. Results revealed that filter-feeding cladocerans positively selected cryptophytes, shown to be substantially 13C-enriched compared with bulk phytoplankton and even with the terrestrial end-member. Comparing a classical two-source mixing model and a model that accounted for algal isotopic heterogeneity, we showed that the use of a classical two-source model can result in an overestimation of terrestrial contribution to zooplankton secondary production.

Les analyses en isotopes stables du carbone ont été largement utilisées pour estimer la contribution du carbone d’origine terrestre à la production secondaire des lacs. Dans ces approches isotopiques, le phytoplancton est tacitement considéré comme une source unique et isotopiquement homogène. Cette supposition peut être valide sous l’hypothèse selons laquelle (i) le zooplancton ne se nourrit pas sélectivement de certains taxons algaux au sein du phytoplancton ou (ii)même si le zooplancton se nourrit de façon sélective, la variabilité des 13C entre les différentes taxons algaux n’excède pas la variabilité des 13C entre la source algale globale et la source terrestre. Nous avons testé ces hypothèses au cours d’une étude de terrain menée sur six lacs côtiers de Colombie-Britannique (Canada) en été en analysant les pigments du tube digestif et la composition isotopique du zooplancton et de la matière organique particulaire. Les résultats montrent que les cladocères filtreurs sélectionnent les cryptophytes, dont la composition isotopique est particulièrement enrichie en 13C par rapport au phytoplancton total et même à la matière organique d’origine terrestre. En comparant les estimations issues d’un modèle de mélange classique à deux sources à celles d’un modèle prenant en compte l’hétérogénéité isotopique des algues, nous montrons que le modèle classique à deux sources conduit à une surestimation de la contribution du carbone terrestre à la production secondaire zooplanctonique.

Document Type: Research Article

Publication date: December 1, 2008

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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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