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)