ECOSYSTEM ECOLOGY - ORIGINAL RESEARCH PAPER Variability and directionality of temporal changes in d 13 C and d 15 N of aquatic invertebrate primary consumers Ryan J. Woodland • Pierre Magnan • He ´le `ne Gle ´met • Marco A. Rodrı ´guez • Gilbert Cabana Received: 18 May 2011 / Accepted: 17 October 2011 Ó Springer-Verlag 2011 Abstract Seasonal oscillations in the carbon (d 13 C) and nitrogen (d 15 N) isotope signatures of aquatic algae can cause seasonal enrichment–depletion cycles in the isotopic composition of planktonic invertebrates (e.g., copepods). Yet, there is growing evidence that seasonal enrichment– depletion cycles also occur in the isotope signatures of larger invertebrate consumers, taxa used to define reference points in isotope-based trophic models (e.g., trophic base- lines). To evaluate the general assumption of temporal stability in non-zooplankton aquatic invertebrates, d 13 C and d 15 N time series data from the literature were analyzed for seasonality and the influence of biotic (feeding group) and abiotic (trophic state, climate regime) factors on isotope temporal patterns. The amplitude of d 13 C and d 15 N enrichment–depletion cycles was negatively related to body size, although all size-classes of invertebrates displayed a winter-to-summer enrichment in d 13 C and depletion in d 15 N. Among feeding groups, periphytic grazers were more variable and displayed larger temporal changes in d 13 C than detritivores. For nitrogen, temporal variability and magni- tude of directional change of d 15 N was most strongly related to ecosystem trophic state (eutrophic [ mesotrophic, oli- gotrophic). This study provides evidence of seasonality in the isotopic composition of aquatic invertebrates across very broad geographical and ecological gradients as well as identifying factors that are likely to modulate the strength and variability of seasonality. These results emphasize the need for researchers to recognize the likelihood of temporal changes in non-zooplankton aquatic invertebrate consumers at time scales relevant to seasonal studies and, if present, to account for temporal dynamics in isotope trophic models. Keywords Stable isotope Á Temporal effects Á Seasonality Á Trophic state Á Isotopic baselines Introduction Stable isotopes have rapidly become a powerful and flexible ecological method with broad applications from the micro- to the global scale. In the majority of applications (e.g., trophic ecology, Fry 1988; tracking of consumer movement, Hobson 1999), a critical analytical step involves defining and accurately measuring the isotopic signature, typically carbon (d 13 C %) and nitrogen (d 15 N %), of one or more basal food web components. Indeed, variance in the esti- mates of end-members directly affects the confidence intervals around the estimates of diet contribution obtained from mixing models for target consumers (Phillips and Gregg 2001). Establishing the representative isotopic sig- nature(s) of basal food web components serves three pri- mary functions, depending on the intended goals of the study. First, the isotope signatures of food sources can be used to estimate the contribution of specific prey or general Communicated by Robert Hall. Electronic supplementary material The online version of this article (doi:10.1007/s00442-011-2178-7) contains supplementary material, which is available to authorized users. R. J. Woodland Á P. Magnan Á H. Gle ´met Á M. A. Rodrı ´guez Á G. Cabana Centre de recherche sur les interactions bassins versants, e ´cosyste `mes aquatiques (RIVE), Universite ´ du Que ´bec a ` Trois-Rivie `res, C.P. 500, Trois-Rivieres, Quebec G9A 5H7, Canada R. J. Woodland (&) Water Studies Centre, Monash University, Clayton, Victoria 3800, Australia e-mail: ryan.woodland@monash.edu.au 123 Oecologia DOI 10.1007/s00442-011-2178-7