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

Abstract

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 (ε). 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‰ and 0.15‰ difference in δ15N and δ13C, 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 (εotolith-diet) in the soluble fraction varied between −0.20‰ and 0.31‰ for nitrogen and between 0.09‰ and 0.23‰ 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.

Life history traits of the fish community in Lake Annecy: evidence from the stable isotope composition of otoliths

“…The hypothesis of a generic relationship provides a promising area of research. If this hypothesis is verified, it will allow the use of δ 13 C oto as a proxy element of the average trophic position of different species in a lake community as a complement to new methods developed for studies of the organic matrix of fish otoliths (Gronkjaer et al, 2013;McMahon et al, 2011McMahon et al, , 2013 (Dufour et al 1998;Dufour, 1999). Samples were collected in 1995Samples were collected in -1996.…”

Differential recovery of δ13C in multiple tissues of white sucker across age classes after the closure of a pulp mill

“…Although it has been advocated repeatedly (i.e., Lindenmayer and Likens 2010), few monitoring programs have consistent and continuous data from before development begins through operation and closure. To overcome these deficiencies, hard tissues have been suggested as useful archives of information obtained from stable isotope ratios (Grønkjaer et al 2013). This natural archiving can accommodate the construction of historical ranges of ␦ 13 C signatures in natural environments.…”

“…In fish, hard tissues with slow turnover, such as scales (Trueman et al 2012) and otoliths (Grønkjaer et al 2013), have been used as archives of stable isotopes, which can be measured in individual annuli several years later (Trueman et al 2012). Few studies, however, have used hard tissues to examine historical patterns linked to a specific anthropogenic activity (Farwell 2000), although some have examined large-scale phenomena (Trueman et al 2012).…”

Assimilation of nitrogen and carbon isotopes from fish diets to otoliths as measured by nanoscale secondary ion mass spectrometry

“…New approaches have recently been developed to measure the organic carbon and nitrogen isotopes of fish otoliths . The otolith mass required for the analysis of otolith δ 15 N composition has been greatly reduced from 100 to 2 mg .…”