Consequences of large temporal variability of zooplankton δ15N for modeling fish trophic position and variation

We use a temporal integration model (TIM) to determine how estimates of trophic variation, using delta N-15, depend on consumer growth dynamics and temporal isotopic variation (delta N-15) of food sources. Consumers are rarely in isotopic equilibrium with their food sources, so instantaneous comparisons between the delta N-15 of a consumer and its diet provide little information about trophic variation, even if the trophic positions of the diet are known. In this paper, we focus on the trophic link between zooplankton and planktivorous fish. We first review the extent of temporal variability of zooplankton delta N-15, then examine the consequences of this variability for understanding the isotopic composition of planktivorous fish communities. We use time series of delta N-15 for Daphnia, calanoid copepods, and particulate organic matter (> 200 mu m) to generate theoretical diets for a model juvenile sockeye over a typical growing season. We use a TIM to predict the isotopic trajectory of individual juveniles feeding on these diets and explore how variance in growth rate and isotopic enrichment (Delta(delta N)) can affect estimates of trophic position and intrapopulation isotopic variability. In general, we found that using a seasonal average of Daphnia delta N-15 to estimate the trophic position of planktivorous fish is nearly equivalent to using a TIM. However, temporal variation in the delta N-15 of food sources, coupled with individual differences in the growth rate of consumers, can contribute to intrapopulation isotopic variation of consumers and lead to correlations between consumer size and delta N-15.