Experimental determination of the sources of otolith carbon and associated isotopic fractionation

Abstract

Otolith stable carbon isotope ratios provide a unique and widely applicable environmental record. Unfortunately, uncertainty regarding the proportion of otolith carbon that derives from metabolized food versus dissolved inorganic carbon (DIC) in the water currently limits utilization of this marker. We manipulated the δ 13 C of food and ambient DIC in a factorial design with juvenile rainbow trout (Oncorhynchus mykiss). At the activity levels and total metabolic rates characteristic of fish in this study, 17% (±3% standard error, SE) of otolith C was metabolically derived, while >80% was derived from DIC in ambient water. We also estimated isotopic enrichment factors associated with physiological carbon transformations by measuring δ 13 C of blood and endolymph (which closely tracked otolith δ 13 C). There was substantial depletion in 13 C of blood relative to C sources (ε blood–sources = –16.9‰ ± 1.1‰ SE), but substantial enrichment in 13 C in otolith relative to blood (ε oto–blood = 13.3‰ ± 1.3‰ SE). Net isotopic enrichment between sources and the otolith was therefore slightly negative. Most of the isotopic enrichment between the blood and the otolith was associated with the movement of C from blood to endolymph, while enrichment associated with the precipitation of otolith aragonite from the endolymph was small.