Introduction to stable isotopes in Ecological Studies

1
Introduction to stable isotopes in Ecological
Studies

• What are stable isotopes?
• How are they quantified?
• Patterns of natural abundance
• Fractionation
• Mixing models

• Uses in Ecological studies
• Source(s) of diet
• Sources of nutrients/pollutants
• Migration/movement
• Water-use efficiency studies
• Nitrification/denitrification

2

• Natural abundance of stable isotopes

Hobson 1999 Oecologia
3
Colorado Stable Isotope Lab, NAU, URL
http//jan.ucc.nau.edu/bah/cpsil.html
4

• del notation
• X (Rsample / Rstandard) 1 x 1000
• Where X is 13C, 15N, 34S and R is ratio heavy /
  light isotope, 13C/12C, 15N/14N, 34S/32S
• d 13C 13C/12C sample 1 x 1000
• 13C/12C standard
• Materials with ratios gt0.011 have d 13C gt1,
  otherwise lt1
• Standards vary by isotope
• C, Pee Dee Belemnite (PDB) , 13C/12C
  0.0112372
• N, atmospheric 15N/14N 0.0036765

5
Fractionation

• D dconsumer - dsource
• change in ratio of stable isotope as molecules
  move through environment
• Example terrestrial C3 plants, d13C -28,
  atmosphere -7.4, D -20.4
• Physical and chemical processes that determine
  representation of each isotope in a product is
  proportional to mass
• Mechanisms
• Equilibrium (heavy isotopes end up in bonds that
  are strongest, discriminate against light
  isotopes)
• Kinetic

CO2 (air) ? CO2 (leaf) ? Carboxylation
(Ps) Carboxylation fast d13C -7 D -4
d13C -11 slow d13C -7 D -29 d13C
-35 In practice, D -21, and plants have about
d13C -28 For terrestrial consumers,
fractionation of C is small 0 1
6
Fractionation, continued

• N fractionation in consumers D 3.4
• (Minagawa and Wada 1984, DeNiro and Epstein
  1978, Post 2002)
• Excretion
• Production of metabolites with light isotopes in
  deamination/transamination (heavy isotopes become
  concentrated)
• Assimilation fractionation
• Preferential use of heavy isotopes during
  protein biosynthesis
• Trophic position model (Cabana and Rasmussen
  1994)

2 if baseline is primary consumers (herbivores)
7
Hobson 1999 Oecologia
8
Mixing models

• If isotope values of sources can be determined
  (end members), partition contribution of each
  source to diet by solving simple linear
  equations
• dt fA dA fB dB fA fB 1 (2 sources, 1
  isotope)
• For more than 2 sources, need more isotopes
  (n-1)
• Built-in assumptions, equal assimilation
  (function of ), issues of fractionation, error
  propagation (Phillips 2001), tissue turnover

9
Waasermaan and Hobson 1998 PNAS
10
Vander Zanden et al 1999 Nature
11
Finlay et al 2002 Ecology
12
Finlay et al 2002 Ecology
13
Bastow et al 2002 Oecologia
14
Sa
Pa
BMA
POM
BMA
Sa
POM
Pa
Weinstein et al 2000 Trans. Am. Fish Soc.
15
McClelland and Valiela 1998 Limnol and
Oceanography
16
Peterson and Fry 1987 Ann. Rev. Ecol. Syst