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

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 15 N: 14 N (␦ 15 N) relative to their food resources owing to the preferential excretion of 14 N. Abiotically, increases in ␦ 15 N may also reflect different sources of biologically available nitrogen, including nitrogen resulting from denitrification of inorganic fertilizer. We studied variation in ␦ 15 N among freshwater turtle populations to assess spatial variation in ␦ 15 N 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 ␦ 15 N was strongly correlated with Lemna ␦ 15 N and was elevated in sites influenced by agricultural activity. Furthermore, trophic position of turtles was not associated with ␦ 15 N but was consistent with expected values for primary consumers in freshwater systems, indicating that differences in tissue ␦ 15 N 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.Résumé : L'abondance naturelle des isotopes stables d'éléments dans les tissus animaux est influencée par des facteurs tant biotiques qu'abiotiques. Sur le plan biotique, les animaux s'alimentant à des niveaux trophiques élevés présentent des rapports 15 N: 14 N (␦ 15 N) enrichis par rapport à leurs ressources alimentaires en raison de l'excrétion préférentielle du 14 N. Sur le plan abiotique, des augmentations du ␦ 15 N pourraient également refléter différentes sources d'azote biodisponible, dont de l'azote produit par la dénitrification d'engrais inorganiques. Nous avons étudié les variations du ␦ 15 N dans des populations de tortues d'eau douce afin d'évaluer la variabilité spatiale de ␦ 15 N et de déterminer si cette variabilité peut être attribuée à des différences en ce qui concerne les sources d'azote ou l'enrichissement trophique. Nous avons examiné les rapports d'isotopes stables d'azote et de carbone dans des lentilles d'eau (genre Lemna L.) et des tortues peintes (Chrysemys picta (Schneider, 1783)) dans des écosystèmes aquatiques qui devraient présenter des incidences différentes de l'activité agricole et de la dénitrification d'engrais inorganiques. Pour l'ensemble des sites étudiés, les ␦ 15 N de C. picta étaient fortement corrélés aux ␦ 15 N de Lemna et étaient élevés dans les sites influencés par l'activité agricole. En outre, la position trophique des tortues n'était pas associée au ␦ 15 N, mais concordait avec les valeurs attendues pour des consommateurs primaires dans des systèmes dulcicoles, ce qu...

Nuclear introgression without mitochondrial introgression in two turtle species exhibiting sex‐specific trophic differentiation

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“…Direct links between elevated nitrogen loads and δ 15 N in food webs are shown in streams and rivers draining urban (Hicks et al., 2017; Turner et al., 2015) and agricultural landscapes (Vandermyde & Whitledge, 2008; Winemiller et al., 2011). Moreover, positive associations between agricultural land use and nitrogen levels in freshwater aquatic trophic consumers have been documented for fish (Harrington et al., 1998), turtles (Hofmeister et al., 2013), and waterfowl (Hebert & Wassenaar, 2001). Thus, differences observed in δ 13 C and δ 15 N for mussels, snails, and fish suggest a regime shift in basal resources.…”

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