Stable isotope values of lotic invertebrates: Sources of variation, experimental design, and statistical interpretation

In a subset of a stream food web, whole-body isotope values of delta C-13 a nd delta N-15 were measured for eight populations of lotic invertebrates. O bserved isotopic differences among species corresponded broadly to their tr ophic status, as also revealed by gut content analysis, but with some excep tions. Species within a guild of grazer/scraper mayflies differed significa ntly in delta C-13; a predatory caddisfly (Rhyacophila dorsalis) and a coll ector/gatherer stonefly (Leuctra inermis) had statistically indistinguishab le values of delta C-13 and delta N-15. Th, variation associated with the m ean isotope value of each population was partitioned into the variation amo ng individuals and the variation that arises from analysis by isotope ratio mass spectrometry. For some taxa, within-population variance was lower tha n or equal to the variance attributable to the measurement error of the mas s spectrometer. The highest but conservative estimate of within-population variation was a mean coefficient of variation of 11% for delta N-15 in a pr edator R. dorsalis. The minimum detectable difference between two populatio ns was negatively associated with the number of replicate samples and the n umber of individual animals combined in each replicate. The optimum number of replicate samples, therefore, varies depending on the hypotheses of inte rest.