222
9.1 Introduction
Biogeochemical markers such as stable isotopes and trace elements have been used
to investigate aspects of trophic and spatial ecology of marine mammals. Their use
has been increasing notably over the last decade. Together with traditional
approaches these markers can enhance considerably the knowledge about the trophic ecology and patterns of habitat usage of the animals (Graham et al. 2010;
Hobson et al. 2010) and hence, their conservation.
Stable isotope analysis (SIA) has been largely applied to study the ecology of
marine megafauna, including foraging, diet, habitat use, movements, ecological
partition within and between species and physiology (see reviews by Hobson 1999;
Kelly 2000; Newsome et al. 2010). Isotope fractionation, also called trophic discrimination or trophic enrichment, is considered the difference in the isotopic composition between an animal’s tissue and its diet (Martínez del Rio et al. 2009). It
results from different factors, including multiple ecological, behavioral, and physiological processes of different ecosystem components, as light, temperature, and
precipitation (Ben-David and Flaherty 2012). For example, the food preference
(herbivore, omnivore or carnivore), the tissue being analyzed and the diet macronutrient abundance of stable isotopes of different elements (commonly carbon, nitrogen, oxygen, hydrogen and sulfur) have effects over the fractionation (Barnes et al.
2009). Stable isotopes of carbon (
13
C/
12
C) and nitrogen (
15
N/
14
N) are the main components of all biological material, thus they are the elements most used in SIA. For
the application of SIA, it is assumed that the stable isotope composition of an organism reflects that of its assimilated diet (DeNiro and Epstein 1978, 1981; Peterson
and Fry 1987), with some predictable difference found between the sources and the
consumers (Peterson and Fry, 1987). These ratios are usually presented in the conventional delta notation (δ), which means they are in parts per-mil (‰) of the ratios
found in international measurement standards (e.g. Vienna Pee Dee Belemnite for
carbon and atmospheric N 2 for nitrogen) (Ben-David and Flaherty 2012).
Isotopic composition of carbon and nitrogen depends on the biogeochemical
characteristics of the ecosystem, to the element fractionation, the composition and
assimilation efficiency, and the animal thermoregulation system (ectothermic or
endothermic) (Caut et al. 2009; Martínez del Rio et al. 2009; Robbins et al. 2010;
Borrell et al. 2012; Martínez del Rio and Carleton 2012).
Nitrogen isotopic values of consumer tissues are generally
15
N-enriched relative
to its diet (DeNiro and Epstein 1981; Peterson and Fry 1987). This increment is
tissue-dependent (e.g. Lesage et al. 2001; Caut et al. 2009), but a general enrichment of 3–4‰ is widely considered in trophic studies (Post 2002). Due to this significant increase along the food web, the nitrogen isotope composition of a consumer
is commonly used to estimate its trophic position (DeNiro and Epstein 1981;
Minagawa and Wada 1984; Post 2002).
In the case of carbon nitrogen isotopes, although some enrichment occurs along
food webs, it presents a lower modification in its composition and thus this element
is mainly used to determine the basal sources of food webs (Kelly 2000).
E. Seyboth et al.
9.1 Introduction
Biogeochemical markers such as stable isotopes and trace elements have been used
to investigate aspects of trophic and spatial ecology of marine mammals. Their use
has been increasing notably over the last decade. Together with traditional
approaches these markers can enhance considerably the knowledge about the trophic ecology and patterns of habitat usage of the animals (Graham et al. 2010;
Hobson et al. 2010) and hence, their conservation.
Stable isotope analysis (SIA) has been largely applied to study the ecology of
marine megafauna, including foraging, diet, habitat use, movements, ecological
partition within and between species and physiology (see reviews by Hobson 1999;
Kelly 2000; Newsome et al. 2010). Isotope fractionation, also called trophic discrimination or trophic enrichment, is considered the difference in the isotopic composition between an animal’s tissue and its diet (Martínez del Rio et al. 2009). It
results from different factors, including multiple ecological, behavioral, and physiological processes of different ecosystem components, as light, temperature, and
precipitation (Ben-David and Flaherty 2012). For example, the food preference
(herbivore, omnivore or carnivore), the tissue being analyzed and the diet macronutrient abundance of stable isotopes of different elements (commonly carbon, nitrogen, oxygen, hydrogen and sulfur) have effects over the fractionation (Barnes et al.
2009). Stable isotopes of carbon (
13
C/
12
C) and nitrogen (
15
N/
14
N) are the main components of all biological material, thus they are the elements most used in SIA. For
the application of SIA, it is assumed that the stable isotope composition of an organism reflects that of its assimilated diet (DeNiro and Epstein 1978, 1981; Peterson
and Fry 1987), with some predictable difference found between the sources and the
consumers (Peterson and Fry, 1987). These ratios are usually presented in the conventional delta notation (δ), which means they are in parts per-mil (‰) of the ratios
found in international measurement standards (e.g. Vienna Pee Dee Belemnite for
carbon and atmospheric N 2 for nitrogen) (Ben-David and Flaherty 2012).
Isotopic composition of carbon and nitrogen depends on the biogeochemical
characteristics of the ecosystem, to the element fractionation, the composition and
assimilation efficiency, and the animal thermoregulation system (ectothermic or
endothermic) (Caut et al. 2009; Martínez del Rio et al. 2009; Robbins et al. 2010;
Borrell et al. 2012; Martínez del Rio and Carleton 2012).
Nitrogen isotopic values of consumer tissues are generally
15
N-enriched relative
to its diet (DeNiro and Epstein 1981; Peterson and Fry 1987). This increment is
tissue-dependent (e.g. Lesage et al. 2001; Caut et al. 2009), but a general enrichment of 3–4‰ is widely considered in trophic studies (Post 2002). Due to this significant increase along the food web, the nitrogen isotope composition of a consumer
is commonly used to estimate its trophic position (DeNiro and Epstein 1981;
Minagawa and Wada 1984; Post 2002).
In the case of carbon nitrogen isotopes, although some enrichment occurs along
food webs, it presents a lower modification in its composition and thus this element
is mainly used to determine the basal sources of food webs (Kelly 2000).
E. Seyboth et al.