Detecting trophic-level variation in consumer assemblages

1. Stable isotopes of nitrogen are useful for quantifying the trophic structure of food webs, but only if the variation in trophic enrichment (Delta(N)), which is the difference in delta(15)N between a consumer and its food, is small relative to the value of Delta(N) itself. 2. We examined the sources of variation in zooplankton Delta(N) by measuring the trophic enrichment (Delta(N)) of seven species of freshwater cladocerans, and by testing for an effect of age and temperature on the Delta(N) of Daphnia pulicaria. 3. We found that Delta(N) was similar among Cladocera and was not correlated with body size. Overall, the Delta(N) for D. pulicaria was 1.4 parts per thousand (SE = 0.69, n = 57), as was expected for the detritus diet that we used in our experiments. We found no effect of temperature (15-25 degrees C) on Delta(N), but found that Delta(N) of D. pulicaria increased with increasing age (10-30 days). 4. We developed a new method to test for trophic-level variation in a group of consumers that explicitly accounts for the uncertainty in Delta(N). Using this approach, we confirmed that natural assemblages of zooplankton feed at several trophic levels in lake food webs.