Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Hydrobiologia, Jun 2017

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T 0.5) or the change in mass (G 0.5). Ecological studies increasingly analyse tissues collected non-destructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T 0.5 and G 0.5 of the nitrogen stable isotope (δ15N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. δ15N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the δ15N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T 0.5 was 84 (muscle) to 145 (scale) days; G 0.5 was 1.39 × body mass (muscle) to 2.0 × body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.

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Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish Georgina M. A. Busst . J. Robert Britton 0 0 G. M. A. Busst J. R. Britton (&) Department of Life and Environmental Sciences, Faculty of Science and Technology, Bournemouth University , Poole BH12 5BB , UK Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T0.5) or the change in mass (G0.5). Ecological studies increasingly analyse tissues collected nondestructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T0.5 and G0.5 of the nitrogen stable isotope (d15N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. d15N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the d15N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T0.5 was 84 (muscle) to 145 (scale) days; G0.5 was 1.39 9 body Handling editor: Michael Power mass (muscle) to 2.0 9 body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies. Isotopic equilibrium; Fin tissue; Scale; Stable isotope Introduction Knowledge on the stable isotope turnover rates of tissues of a consumer species is fundamental for the correct interpretation of their isotopic ecology (Boecklen et al., 2011) . Stable isotope turnover rates tend to be expressed as half-lives, i.e. the time for stable isotope values to reach 50% equilibrium with a new diet (Vander Zanden et al., 2015) . Estimates of isotopic turnover rates are important for understanding how, for example, ontogenetic dietary shifts affect stable isotope data (e.g. Buchheister & Latour, 2010; Hertz et al., 2015) , and for incorporating into the design of manipulative field studies and mesocosm experiments where the duration of the study could be confounded if they are not of sufficient length for isotopic equilibrium to be reached (Jackson et al., 2013; Tran et al., 2015) . Consumer tissues tend to be considered at equilibrium with their diet after 4–5 halflives (Hobson & Clark, 1992) . Increasingly, the sampling of tissues for stable isotope analysis utilises the non-invasive or non-destructive sampling of tissues, such as using fin tissue and scales rather than dorsal muscle for fishes (Busst et al., 2015; Busst & Britton, 2016; Vasˇek et al., 2017) . Knowledge on the turnover rates of these tissues is often missing, leading to problems in their application to ecological studies (Busst & Britton, 2016). Moreover, the determination of the turnover rates of wild consumers can be problematic, as they tend to assimilate a range of prey items that vary temporally and spatially in isotopic content (Perga & Gerdeaux, 2005) . Consumer species are also unlikely to feed on the same proportions of prey items on a daily basis and consumer isotopic turnover rates are also influenced by a number of other factors, including temperature fluctuations and life-history events (Bearhop et al., 2002; Bosley et al., 2002; Witting et al., 2004) . Turnover rates also differ between fish species and between different tissues of individual species (e.g. McIntyre & Flecker, 2006; Church et al., 2009; Carleton & Del Rio, 2010) . Rather than relying on data collected in the wild, an alternative approach is the use of experimental diet-switch studies completed in controlled conditions (Heady & Moore, 2013; Xia et al., 2013a, b; Busst & Britton, 2016) . In these studies, diet tends to be fixed so that the food items have relatively consistent stable isotope values that should provide more reliable turnover estimates in the tissues (Logan et al., 2006). These approaches should also provide enhanced understandings of the mechanisms involved in isotopic replacement (Buchheister & Latour, 2010; Heady & Moore, 2013) . The data generated also favour the testing of different models to determine the best-fitting model that provides the best estimate of the turnover rate, and enable the relative contributions of growth and metabolism to turnover to be estimated (Fry & Arnold, 1982; Hobson & Clark, 1992; Hesslein et al., 1993) . Estimating the relative contributions of growth and metabolism to isotopic turnover rates is important, as both play important roles in the isotopic replacement of tissues following a diet-switch (Xia et al., 2013a, b) . Growth rate represe (...truncated)


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Georgina M. A. Busst, J. Robert Britton. Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish, Hydrobiologia, 2017, pp. 1-12, DOI: 10.1007/s10750-017-3276-2