Bulk Net Zooplankton Samples Mask Species-Specific Iron Isotope Signatures

Stable iron isotope ratios (δ 56 Fe) provide new insights into marine iron cycling, while isotopic compositions of plankton, the key vectors of iron transfer, remain poorly constrained. Here, we report for the first time species-specific δ 56 Fe measurements in wild zooplankton. Bulk samples of zooplankton (138–1280 μg Fe g –1 ) exhibited δ 56 Fe values ranging from −0.20‰ to 0.25‰, whereas crustacean mesozooplankton exhibited distinct signatures: Neocalanus spp. (−0.76‰ and −0.37‰), Euphausia pacifica (−1.37‰ and −0.82‰), Orchomene spp. (−0.18‰), and Cyphocaris spp. (−2.24‰), all with lower Fe contents (11–93 μg g –1 ). These results suggest that bulk samples retain residual detrital or adsorbed Fe, while specific taxa reflect dietary Fe sources closer to those of fish tissues. The results provide the first species-specific δ 56 Fe data for wild marine zooplankton and demonstrate that isotopic fractionation during assimilation produces the critical shift linking seawater Fe to lighter signatures observed in higher consumers. This study establishes a foundation for integrating zooplankton δ 56 Fe into future assessments of Fe cycling and trophic transfer in marine ecosystems.