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

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 N-15:N-14 (delta N-15) relative to their food resources owing to the preferential excretion of N-14. Abiotically, increases in delta N-15 may also reflect different sources of biologically available nitrogen, including nitrogen resulting from denitrification of inorganic fertilizer. We studied variation in delta N-15 among freshwater turtle populations to assess spatial variation in delta N-15 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 delta N-15 was strongly correlated with Lemna delta N-15 and was elevated in sites influenced by agricultural activity. Furthermore, trophic position of turtles was not associated with delta N-15 but was consistent with expected values for primary consumers in freshwater systems, indicating that differences in tissue delta N-15 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.