Temperature and diet affect carbon and nitrogen isotopes of fish muscle: can amino acid nitrogen isotopes explain effects?

Abstract

Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) are widely used in food-web studies to determine trophic positioning and diet sources. However in order to accurately interpret stable isotope data the effects of environmental variability and dietary composition on isotopic discrimination factors and tissue turnover rates must be validated. We tested the effects of temperature and diet on tissue turnover rates and discrimination of carbon and nitrogen isotopes in an omnivorous fish, black bream (Acanthopagrus butcheri). Fish were raised at 16 °C or 23 °C and fed either a fish-meal or vegetable feed to determine turnover rates in fish muscle tissue up to 42 days after exposure to experimental treatments. Temperature and diet affected bulk tissue δ15N turnover and discrimination factors, with increased turnover and smaller discrimination factors at warmer temperatures. Fish reared on the vegetable feed showed greater bulk tissue δ15N changes and larger discrimination factors than those reared on a fish-meal feed. Temperature and diet affected bulk tissue δ13C values, however the direction of effects among treatments changed. Analyses of δ15N values of individual amino acids found few significant changes over time or treatment effects, as there was large variation at the individual fish level. However glutamic acid, aspartic acid and leucine changed most over the experiment and results mirrored those of treatment effects in bulk δ15N tissue values. The results demonstrate that trophic discrimination for δ15N and δ13C can be significantly different than those typically used in food-web analyses, and effects of diet composition and temperature can be significant. Precision of compound-specific isotope analyses (0.9‰) was larger than our effect size for bulk δ15N diet effects (0.7‰), therefore future experimental work in this area will need to establish a large effect size in order to detect significant differences. Our results also suggest that compound-specific amino acid δ15N may be useful for determining essential and non-essential amino acids for different animals.