Elevated levels of ∂15N in Riverine Turtles: Trophic enrichment or anthropogenic input?
The natural abundance of stable isotopes of nitrogen in animal tissue is influenced by both biotic and abiotic factors. For instance, animals feeding at higher trophic levels are enriched in the ratio of 15N:14N (∂15N) relative to their food resources due to the preferential excretion of 14N. In addition, variation in ∂15N may reflect different sources of biologically available nitrogen. In ecosystems experiencing high agricultural activity, denitrification of inorganic fertilizer can cause 15N enrichment of available nitrogen that is transmitted throughout the food web, and this effect is compounded in rivers, where denitrification rates are elevated relative to ponds and rivers. We found that ∂15N content in painted turtles in the Mississippi river is higher than any other freshwater turtle reported. We subsequently studied variation in ∂15N among freshwater turtle populations to assess spatial variation in ∂15N and to determine whether this variation can be attributable to differences in nitrogen source or trophic enrichment. We examined nitrogen stable isotope ratios in duckweed (Lemna sp.) and in painted turtles (Chrysemys picta) in: 1) two sites along the Mississippi River in a water catchment characterized by heavy agricultural use, 2) two pond systems in agricultural regions, and 3) an isolated lake in a pristine area.
Across sites, C. picta ∂15N was strongly correlated with Lemna ∂15N. ∂15N of agricultural pond turtles was lower than ∂15N of riverine turtles, and ∂15N of turtles in the pristine site was lower than turtle ∂15N in all but one other site. Furthermore, trophic position of turtles was not associated with 15N, suggesting that differences in tissue ∂15N 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.