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 management measures. Trophic position (TP) of species is the key parameter in TMFs assessment. Nitrogen stable isotopes (delta N-15) 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 N-15 between two adjacent trophic positions; 2) determination of baseline species. Different from the widely used constant Delta N-15 (3.4%) between two adjacent trophic positions, which is called additive Delta N-15 framework, Delta N-15 gradually decreases as trophic position increases under scaled Delta N-15 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 N-15 and totalmercury (THg) of each species. On the one hand, we compared the TP of species under the additive Delta N-15 framework and scaled Delta N-15 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 N-15 framework, the trophic position of the same species is higher, while TMFs is lower compared with the additive Delta N-15 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 Delta N-15 framework because it has improved the accuracy of trophic position assessment.

Automated summary

Understanding trophic magnification factors (TMFs) is important for environmental management. Trophic position (TP) is a key parameter in TMFs assessment and can be estimated using nitrogen stable isotopes (δN-15). However, there are limitations in determining the ΔN-15 between trophic positions and selecting baseline species, which introduce uncertainty into TP and TMFs assessment. This study compared two frameworks (additive ΔN-15 and scaled ΔN-15) of the food chain in two interconnected freshwater ecosystems for the first time and found that TMFs differed significantly between the two lakes under the scaled framework but not under the additive framework. Based on these findings, we recommend using the scaled ΔN-15 framework for future TMFs assessments to improve the accuracy of TP assessment.