Improving the qualities of the trophic magnification factors (TMFs): A case study based on scaled ΔN trophic position framework and separate baseline species.

Scientific understanding of trophic magnification factors (TMFs) is conducive to formulating environmental management measures. Trophic position (TP) of species is the key parameter in TMFs assessment. Nitrogen stable isotopes (δN) provide a powerful tool to estimate TP. However, some limitations could introduce considerable uncertainty into TP and TMFs assessment which mainly includes: 1) determination of ΔN between two adjacent trophic positions; 2) determination of baseline species. Different from the widely used constant ΔN (3.4 ‰) between two adjacent trophic positions, which is called additive ΔN framework, ΔN gradually decreases as trophic position increases under scaled ΔN framework, which has been confirmed by more and more laboratory studies and meta-analyses. In this study, we sampled in two similar littoral ecosystems separated by one natural dam, which is called Small Xingkai Lake and Xingkai Lake, analyzed the δN and total mercury (THg) of each species. On the one hand, we compared the TP of species under the additive ΔN framework and scaled ΔN framework with the White shrimp (Exopalaemon modestus) as baseline species in two lakes respectively. On the other hand, we explored the possible changes in TMFs based on TP. Our results show, under the scaled ΔN framework, the trophic position of the same species is higher, while TMFs is lower compared with the additive ΔN framework; even if in the two interconnected lakes, distributed the same baseline species, in the similar ecosystem, separate baselines should also be used. In this study, two frameworks of the food chain were compared in two interconnected freshwater ecosystems for the first time. The difference between TMFs of two lakes was obvious under scaled framework but not under additive framework. We also recommend that future TMFs assessments should be based on the scaled ΔN framework because it has improved the accuracy of trophic position assessment.