Evaluating the need for acid treatment prior to δ13C and δ15N analysis of freshwater fish scales: effects of varying scale mineral content, lake productivity and CO2 concentration

Abstract

In order to evaluate the need for using scale acidification to remove carbonates prior to stable isotope analysis, we compared acidified and non-acidified scales of six freshwater fish species (perch, roach, rudd, pike, tench and bream) with contrasting mineral content in their scales. Fish samples were taken from six lakes with variable trophic conditions, ranging from oligotrophic to hypertrophic, and differing in CO2 concentrations. The scale mineral content of the six species studied ranged between 31.8 and 61.3% dry weight (DW) in tench and perch, respectively. The elemental composition was characterised by high amounts of phosphorus, varying from 4.5 to 9.1% DW. The mineral fraction was dominated by apatite (range 24.4–49.2% DW), carbonates constituted a very small proportion of the total carbon content (average ± SD: 5.5 ± 1.7%). The average effect of acidification was very small for all species (average ± SD: 0.181 ± 0.122 and −0.208 ± 0.243 for carbon and nitrogen, respectively), albeit significant for five out of the six species (excepting tench that had the lowest mineral content). Linear regression slopes between acidified and untreated scales did not differ significantly from one for almost all the species and isotopes. The effects of acidification on the two isotopes were correlated with the relative carbonate content as well as with the CO2 concentration for carbon and total phosphorus for nitrogen. We conclude that the need for scale acidification depends on the different species and on the system studied, although in most cases the acidification effect will be biologically irrelevant. However, dual analysis of acidified and untreated scales may provide useful information on differences in stable isotope composition of dissolved inorganic carbon and on phytoplankton carbon fractionation generated by varying levels of CO2 availability.