The stable isotopes of nitrogen (␦ 15 N) and carbon (␦ 13 C) provide powerful tools for estimating the trophic positions of and carbon flow to consumers in food webs; however, the isotopic signature of a consumer alone is not generally sufficient to infer trophic position or carbon source without an appropriate isotopic baseline. In this paper, I develop and discuss methods for generating an isotopic baseline and evaluate the assumptions required to estimate the trophic position of consumers using stable isotopes in multiple ecosystem studies. I test the ability of two primary consumers, surface-grazing snails and filter-feeding mussels, to capture the spatial and temporal variation at the base of aquatic food webs. I find that snails reflect the isotopic signature of the base of the littoral food web, mussels reflect the isotopic signature of the pelagic food web, and together they provide a good isotopic baseline for estimating trophic position of secondary or higher trophic level consumers in lake ecosystems. Then, using data from 25 north temperate lakes, I evaluate how ␦ 15 N and ␦ 13 C of the base of aquatic food webs varies both among lakes and between the littoral and pelagic food webs within lakes. Using data from the literature, I show that the mean trophic fractionation of ␦ 15 N is 3.4‰ (1 SD ϭ 1‰) and of ␦ 13 C is 0.4‰ (1 SD ϭ 1.3‰), and that both, even though variable, are widely applicable. A sensitivity analysis reveals that estimates of trophic position are very sensitive to assumptions about the trophic fractionation of ␦ 15 N, moderately sensitive to different methods for generating an isotopic baseline, and not sensitive to assumptions about the trophic fractionation of ␦ 13 C when ␦ 13 C is used to estimate the proportion of nitrogen in a consumer derived from two sources. Finally, I compare my recommendations for generating an isotopic baseline to an alternative model proposed by M. J. Vander Zanden and J. B. Rasmussen. With an appropriate isotopic baseline and an appreciation of the underlying assumptions and model sensitivity, stable isotopes can help answer some of the most difficult questions in food web ecology.

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The transition to piscivory is a crucial ontogenetic niche shift for many primarily piscivorous fishes. An early transition to piscivory may increase growth, decrease mortality, and therefore enhance lifetime fitness. Although much is known about the extent and causes of variation in the timing of the shift to piscivory among species and among cohorts within a species, little is known about the extent and causes of variation in the timing of the switch to piscivory among individuals within a single cohort. Here, I link otolith age and growth analysis to direct diet and stable isotope analyses to examine variation in the timing of the transition to piscivory and its causes among individual members of the 1994 largemouth bass cohort in Paul Lake, Michigan. Stable isotope and direct diet analyses indicate that only a few members of the 1994 cohort were able to shift to and sustain piscivory in their first summer of life (early piscivores), while most cohort members would have to wait until their second summer of life to become piscivorous (late piscivores). Significant differences in growth rate between early and late piscivores emerged shortly after 18 June, the first date of possible piscivory by early piscivores, after which early piscivores began to grow at rates nearly twice that of late piscivores. Otolith and stable isotope analyses combined indicate that an early hatching date was necessary, but not sufficient, to explain variation in the timing of the transition to piscivory. All early piscivores were hatched early in the summer, but many early-hatched members of the 1994 cohort did not shift to piscivory in their first summer of life. A combination of at least 10 days of variation in hatching dates and higher-than-average growth rates was required for early piscivores to switch to and sustain piscivory in their first summer of life. Individuals that were able to make the early transition to piscivory most likely benefited from both increased survival and fecundity over much of their life, the combination of which would confer a substantial fitness advantage upon individuals able to make the early transition to piscivory.

We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor-quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcvstis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj). We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log-transformed) changes in gj. Log(gj) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in gj and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in gj among genotypes), a significant genotype x food-type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log-transformed data, a significant sediment-genotype-age x food-type interaction. The latter shows that phenotypic plasticity evolved over the period studied. Two constraints act on response to selection in the D. galeata-Lake Constance system. First, gj on a diet containing poor food is highly correlated with gj on a diet of good food, thus evolving resistance also meant evolving an increase in gj on both diets. Second, because genotypes with a high gj also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential gj after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized gj and led to animals of small adult body size even if their genotypes had the potential for high gj and large size. With realized gj reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.