Improving the qualities of the trophic magnification factors (TMFs): A case study based on scaled Δ15N trophic position framework and separate baseline species

Scientific understanding of trophic magnification factors (TMFs) is conducive to formulating environmental manage-ment measures. Trophic position (TP) of species is the key parameter in TMFs assessment. Nitrogen stable isotopes (delta 15N) 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 Delta 15N between two adjacent trophic posi-tions; 2) determination of baseline species. Different from the widely used constant Delta 15N (3.4 parts per thousand) between two adja-cent trophic positions, which is called additive Delta 15N framework, Delta 15N gradually decreases as trophic position increases under scaled Delta 15N 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 delta 15N and total mercury (THg) of each species. On the one hand, we compared the TP of species under the additive Delta 15N framework and scaled Delta 15N 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 Delta 15N framework, the trophic position of the same species is higher, while TMFs is lower compared with the additive Delta 15N framework; even if in the two interconnected lakes, dis-tributed 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 dif-ference 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 Delta 15N framework because it has improved the accuracy of trophic position assessment.

Automated summary

Scientific understanding of trophic magnification factors (TMFs) is important for environmental management. Trophic position (TP) is a key parameter in TMFs assessment, and nitrogen stable isotopes (δ15N) can be used to estimate TP. However, there are limitations in determining δ15N differences between trophic positions and baseline species, which introduce uncertainty into TP and TMFs assessment. This study compared the TP and TMFs under additive and scaled δ15N frameworks in two interconnected freshwater ecosystems. The results showed that the scaled δ15N framework led to higher TP and lower TMFs compared to the additive δ15N framework. The study highlights the importance of using the scaled δ15N framework for accurate TMFs assessment.