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

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 d 13 C values amongst the different algal taxa is small compared with the variability between the d 13 C 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 13 C-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. Résumé :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 d 13 C entre les différentes taxons algaux n'excède pas la variabilité des d 13 C 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 13 C 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.

Do planktonic rotifers rely on terrestrial organic matter as a food source in reservoir ecosystems?

“…Proposed explanation of obtained results remains in accordance with other studies aimed to explain what happens with terrestrial organic matter in aquatic ecosystems . It is worth of note that after higher river inflow the δ 13 C signature in rotifer‐dominated fraction of zooplankton in the Dobczyce Reservoir tended towards the value characteristic of terrestrial C 3 plants in northern latitudes , which points to the terrestrial origin of incorporated carbon . The coinciding high development of rotifers might be explained by their use of allochthonous fine POM or bacteria developing on these particles .…”

“…Recent studies to assess the sources and transformation of organic matter using stable isotopes have been used to examine the paths of carbon in lake food webs . Indeed studies on lacustrine zooplankton and benthic organisms have shown their importance in transmission of terrestrial organic matter into aquatic ecosystems , and a strong correlation in the stable isotope signature in the zooplankton and the stable isotope signature of POM .…”

Using Stable Isotope Mixing in a Great Lakes Coastal Tributary to Determine Food Web Linkages in Young Fishes

“…Also, we assumed that all phytoplankton taxa were isotopically similar. If certain taxa were δ 13 Cenriched and preferentially consumed by invertebrates, then we may overestimate the role of terrestrial-derived POM in the food web (Perga et al 2008).…”

Assessing the reliability of fatty acid–specific stable isotope analysis for trophic studies

“…Phytoplankton stable isotope signature has rarely been measured directly (Grey, Jones, & Sleep 2000) as phytoplankton cells are difficult to isolate from other particles (bacteria, heterotrophic protists and detritus of different origins) that compose the bulk particulate organic matter. In addition, in SIA studies, phytoplankton is usually tacitly considered as a single, isotopically homogenous dietary source but the isotopic heterogeneity within the phytoplankton community was recently shown to be a potential source of overestimation of terrestrial C contribution to lake food webs (Perga, Kainz, & Mazumder 2008).…”