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Distinguishing trophic variation from seasonal and size-based isotopic (15N) variation of zooplankton

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Abstract:

We measured the 15N of particulate organic matter (POM), Daphnia pulex (D), Holopedium gibberum (H), Leptodiaptomus tyrelli (LT), Epischura nevadensis (E), and Chaoborus trivittatus (C) over an annual cycle in Council Lake, a pristine, oligotrophic, fishless lake. Annual averages of the 15N of plankton (C, LT, E > D, H > POM) matched expected differences in trophic position, but seasonal patterns differed among species. During midsummer, the 15N of D, H, and LT increased above the 15N of E and C, despite little temporal or spatial variation in the 15N of POM (<41 µm). Larger size fractions of POM (41–200 µm) helped explain some of the temporal variability of zooplankton 15N. Body size of zooplankton was correlated with 15N for D, C, and E, which could indicate size-based, trophic-level variation; selective feeding; or physiological differences in 15N enrichment. We propose that detailed time series of 15N for multiple zooplankton species can help determine the seasonal variability of trophic structure in zooplankton assemblages.

Nous avons mesuré les 15N de la matière organique particulaire (POM), de Daphnia pulex (D), d'Holopedium gibberum (H), de Leptodiaptomus tyrelli (LT), d'Epischura nevadensis (E) et de Chaoborus trivittatus (C) durant un cycle annuel au lac Council, un lac oligotrophe non perturbé et sans poissons. Les valeurs moyennes annuelles de 15N du plancton (C, LT, E > D, H > POM) correspondent aux différences attendues, compte tenu des positions trophiques, mais les patrons saisonniers varient d'une espèce à l'autre. Durant la mi-été, les 15N de D, H et LT dépassent les 15N de E et C, malgré le peu de variation temporelle ou spatiale du 15N de POM (<41 µm). Les fractions de plus grande taille de POM (41–200 µm) aident à expliquer une partie de la variabilité temporelle du 15N du plancton. Il y a une corrélation entre la taille corporelle du zooplancton et le 15N chez D, C et E, ce qui pourrait indiquer une variation du niveau trophique, une alimentation sélective ou des différences physiologiques dans l'enrichissement en 15N en fonction de la taille. Nous croyons que des séries chronologiques détaillées de 15N pour plusieurs espèces du zooplancton pourraient aider à déterminer la variabilité saisonnière de la structure trophique des peuplements de zooplancton.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2007-01-01

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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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