Elevated levels of δ¹⁵N in riverine Painted Turtles (Chrysemys picta): trophic enrichment or anthropogenic input?

ISSN： 1480-3283

Source： Canadian Journal of Zoology, Vol.91, Iss.12, 2013-01, pp. : 899-905

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Abstract

The natural abundance of stable isotopes of elements in animal tissue is influenced by both biotic and abiotic factors. Biotically, animals feeding at higher trophic levels are enriched in the ratio of ¹⁵N:¹⁴N (δ¹⁵N) relative to their food resources owing to the preferential excretion of ¹⁴N. Abiotically, increases in δ¹⁵N may also reflect different sources of biologically available nitrogen, including nitrogen resulting from denitrification of inorganic fertilizer. We studied variation in δ¹⁵N among freshwater turtle populations to assess spatial variation in δ¹⁵N and to determine whether this variation can be attributed to differences in nitrogen source or trophic enrichment. We examined nitrogen and carbon stable isotope ratios in duckweed (genus Lemna L.) and in Painted Turtles (Chrysemys picta (Schneider, 1783)) in aquatic ecosystems expected to be differentially affected by agricultural activity and denitrification of inorganic fertilizer. Across sites, C. picta δ¹⁵N was strongly correlated with Lemna δ¹⁵N and was elevated in sites influenced by agricultural activity. Furthermore, trophic position of turtles was not associated with δ¹⁵N but was consistent with expected values for primary consumers in freshwater systems, indicating that differences in tissue δ¹⁵N could be attributed to differences in initial sources of nitrogen in each ecosystem. Our results suggest that care must be taken when attributing differences in isotopic values of animal populations to trophic factors.