Consequences of large temporal variability of zooplankton δ¹⁵ N for modeling fish trophic position and variation

Abstract

We use a temporal integration model (TIM) to determine how estimates of trophic variation, using ␦ 15 N, depend on consumer growth dynamics and temporal isotopic variation (␦ 15 N) of food sources. Consumers are rarely in isotopic equilibrium with their food sources, so instantaneous comparisons between the ␦ 15 N 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 ␦ 15 N, then examine the consequences of this variability for understanding the isotopic composition of planktivorous fish communities. We use time series of ␦ 15 N for Daphnia, calanoid copepods, and particulate organic matter (Ͼ200 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 (⌬ ␦N ) can affect estimates of trophic position and intrapopulation isotopic variability. In general, we found that using a seasonal average of Daphnia ␦ 15 N to estimate the trophic position of planktivorous fish is nearly equivalent to using a TIM. However, temporal variation in the ␦ 15 N 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 ␦ 15 N.

Trophic Niche Width, Shift, and Specialization of Dascyllus aruanus in Toliara Lagoon, Madagascar

“…Our results confirm the statement that body size of fish is often correlated with d 15 N (Matthews and Mazumder, 2005). This relationship may be interpreted as an ontogenetic shift to higher trophic positions (Reñ ones et al, 2002), shift in habitat use (Genner et al, 2003), or temporal variability of food sources (Matthews and Mazumder, 2005). In our study, the stomach content analyses reveal that variation in d 15 N corresponded to ontogenetic diet changes in the type (benthic vs. planktonic) and the size of prey.…”

Using δ13C stable isotopes to quantify individual-level diet variation

“…We propose three possible explanations for this conflict. First, if prey isotopic signatures vary temporally or spatially, individuals feeding on the same prey taxa, but collected in different times or places will show variation in signatures, so that one will observe isotopic variance that is not actually related to diet variation (Matthews and Mazumder 2004;Matthews and Mazumder 2005). In our study, we tried to mitigate this problem by testing for seasonal and spatial effects on the consumers' isotopic ratios, but we acknowledge we cannot rule out those effects entirely.…”

“…If there are more food sources than we can discriminate with isotopes (Phillips and Gregg 2003), isotope variation may underestimate diet variation among individuals (Matthews and Mazumder 2004). On the other hand, if food sources show isotopic variation in space and/or time and consumers were sampled in different places or times, one will observe isotopic variation that is not necessarily related to diet variation (Dalerum and Angerbjörn 2005;Matthews and Mazumder 2005). Moreover, for a given level of diet variation, populations using more isotopically variable prey will themselves show higher isotope variances (Matthews and Mazumder 2004).…”

Spatiotemporal dynamics of C and N isotopic signature of zooplankton: a seasonal study on a man-made lake in the Mediterranean region

“…This variability is related to external loadings, phytoplankton species composition and primary productivity, as well as sources and concentrations of dissolved inorganic C and N (Grey and Jones, 2001;Caroni et al, 2012). Zooplankton are crucial in transferring matter and energy in pelagic food webs (Matthews and Mazumder, 2003). Vulnerability to fish predation (de Bernardi et al, 1987;Sprules and Bowerman, 1988;Einsle, 1996;Mauchline, 1998) and differential exploitation and availability of food resources can lead to seasonal variations in the dominance of different taxa and developmental stages (Smyntek et al, 2012).…”