Elevated levels of ∂15N in Riverine Turtles: Trophic enrichment or anthropogenic input?

Background/Question/Methods

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.

Results/Conclusions

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.