Stable N and C isotopes in the organic matrix of fish otoliths: validation of a new approach for studying spatial and temporal changes in the trophic structure of aquatic ecosystems

Stable isotope studies of long-term ecosystem change are often hampered by lack of archived tissue samples. Here, we provide a reliable method for extracting the organic matrix from fish otoliths, demonstrate differences in isotope values between the soluble and insoluble organic fractions, and provide the trophic enrichment factors (epsilon). The analyses were performed on otoliths from wild-caught adult Atlantic cod (Gadus morhua) and otoliths from a 6-month diet-switch experiment with juvenile cod. Acid hydrolysis of otolith powder followed by ultrafiltration enabled us to purify and separate the soluble and insoluble organic material. There was a significant 1.72 parts per thousand and 0.15 parts per thousand difference in delta N-15 and delta C-13, respectively, between the soluble and insoluble organic fraction of the otolith. This emphasizes the need for separation of the two fractions before analysis. The diet-specific trophic enrichment (epsilon(otolith-diet)) in the soluble fraction varied between -0.20 parts per thousand and 0.31 parts per thousand for nitrogen and between 0.09 parts per thousand and 0.23 parts per thousand for carbon. In conclusion, the organic matrix of otoliths provides an attractive archive of organic material for ecological stable isotope analysis that has the potential to provide important insights into historic changes in the trophic structure of aquatic ecosystems.