Effects of Bythotrephes on the trophic position of native macroinvertebrates

We measured the delta N-15 and delta C-13 stable isotopes of pelagic invertebrate predators in six central Ontario lakes, three invaded by Bythotrephes and three not invaded, to determine the effects of Bythotrephes invasion on diet overlap and trophic position of the predators. Seasonally averaged, lake-specific isotope signatures of bulk zooplankton were used as the isotopic baseline for determining nitrogen-based trophic position or carbon enrichment of the predators. Trophic positions of native Chaoborus, Leptodora, and Mysis in invaded lakes were not elevated compared with noninvaded lakes, suggesting that these predators do not feed directly on Bythotrephes. Nitrogen signatures of Mysis and Chaoborus were similar as were those of Leptodora and Bythotrephes, suggesting that invasion impacts will be greatest on Leptodora as has been observed. The trophic position of large Mysis was generally the highest and most variable of all predators, probably because it feeds on copepods that are more enriched in the heavier nitrogen isotope than bulk zooplankton and because Bythotrephes tends to decrease the ratio of cladocerans to copepods in invaded zooplankton communities.