SUMMARY. Sources of carbon utilized by invertebrates and fish in four New Zealand streams were investigated using stable carbon isotope analysis. Complementary analyses were made to determine σ 13 C alteration (fractionation) by fish reared on a known food and to examine the distribution of 13 C/ 12 C ratios in different tissues. σ 13 C values of hatchery‐reared rainbow trout (white muscle tissue) were 13 C‐enriched compared with their food (trout pellets) by an average of 1.7% 0 . σ 13 C values of different organs and tissues never differed by more than 1.8% o in any fish species; white muscle always had an intermediate value. σ 13 C values of invertebrates ranged from‐28.4 to –20.4% o – At the more open sites invertebrates were 13 C‐depleted, reflecting some use of algal carbon. However, invertebrates from the most heavily shaded stream site were unusually enriched in 13 C. An undetected carbon source of terrestrial origin, perhaps 13 C‐enriched dissolved organic carbon in groundwater, may have been contributing indirectly to invertebrate biomass carbon. σ 13 C values for fish were not closely linked to aquatic invertebrate ratios; instead, at all sites their carbon was isotopically similar to terrestrial organic carbon. The reasons for this apparent anomaly are unclear.
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Rounick et al. (1985) studied this question.