This article was downloaded by: [University North Carolina - Chapel Hill] On: 12 April 2013, At: 18:05 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Critical Reviews in Food Science and Nutrition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bfsn20
The use of stable isotopes for food web analysis a
a
E. Wada , H. Mizutani & M. Minagawa
a
a
Laboratory of Biogeochemistry and Sociogeochemistry, Mitsubishi Kasei Institute of Life Sciences, Machida, Tokyo, 194, Japan Version of record first published: 29 Sep 2009.
To cite this article: E. Wada , H. Mizutani & M. Minagawa (1991): The use of stable isotopes for food web analysis, Critical Reviews in Food Science and Nutrition, 30:4, 361-371 To link to this article: http://dx.doi.org/10.1080/10408399109527547
PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Critical Reviews in Food Science and Nutrition, 30(3):361—371 (1991)
The Use of Stable Isotopes for Food Web Analysis E. Wada, H. Mizutani, and M. Minagawa
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Mitsubishi Kasei Institute of Life Sciences Laboratory of Biogeochemistry and Sociogeochemistry, Machida, Tokyo 194 Japan
ABSTRACT: General aspects in isotope biogeochemistry was summarized with emphasis on δ 15 N and δ 13 C contents in plants and animals in natural ecosystems. In the estuary, the variation of isotope ratios were principally governed by the mixing of land-derived organic matter, marine phytoplankton, and seagrasses. A clear cut linear relationship between animal δ15N and its trophic level was obtained in the Antarctic food chain system. Several current efforts to use the stable isotopes for food web analysis were demonstrated for some terrestrial and marine systems as well as human food web. KEY WORDS: δ 1 5 N, δ 1 3 C, food web, isotope biogeochemistry.
I. INTRODUCTION Stable isotopes of light elements, such as nitrogen and carbon, undergo fractionation in chemical and biochemical reactions. Since isotope fractionation is regulated by the difference in thermodynamic properties among isotopic molecules, biogenic materials in nature have their own isotopic composition. This is the so-called dynamic stable isotope (SI) fingerprint. Therefore, a goal of isotope biogeochemistry is to decipher the SI code of biogenic substances by precise measurement of isotope ratios. In principle, a fluctuation of an element's isotope ratio provides information about its origin, pathways, and the reaction mechanisms concerning its formation. On a macroscopic level, the fluctuation can provide a key to solve biogeochemical phenomena involving food habits and behavior of an animal, and transport or organic matter in an ecosystem. Fundamental steps of isotope effects are classified into the following: (1) reaction from A to B, (2) partition of A between two phases, and
(3) branch reaction. These elemental isotope effects operate in a multicomponent system; bulk isotope discriminations result from environmental effects. Consequently, we can expect wellordered dynamics of the SI fingerprint, i.e., an isotopically ordered world, which can provide a new way of viewing material cycling in the ecosystem. In fact, stable isotope studies of food webs show regular features, irrespective of the high complexity of an ecosystem. In an ecosystem, nitrogen and carbon isotope discrimination takes place in certain sites, as indicated by the circled numbers in Figure 1. The number 1 means the occurrence of isotope fractionation in biotic cycles of nitrogen and carbon. Reactions 2 and 3 contribute to the isotope fractionation between ambient substrate and bulk primary producers. The 8 15 N and 813C values of substrate, the reversibility of substrate transport through membrane systems, the supply of reducing substances, and the inherent isotope fractionation of enzyme-substrate reactions determine the magnitude of apparent isotope fractionation between ambient substrate and
1040-8398/91/$.50 © 1991 by CRC Press, Inc.
361
hi/
Euphotic jyj layer
Substrate So
Si + E:
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Detritus Excreta
NH 3 \ f
f Mineralization
NH4
(Carnivores)
I FIGURE 1. Schematic model for nitrogen and carbon isotope fractionation in an ecosystem. Numerical values stand for major sites of isotopic variations of primary producers. (1) Isotopic compositions of substrate; (2,3) kinetic isotope effects in the uptake processes by plants; (4) branch point in metabolic pathways; (5) enrichment of 1SN in a feeding process; and (6) mineralization. (From Wada, E., Isotopenpraxis, 23, 320, 1987. With permission.)
product or bulk primary producers. For instance, the nitrogen isotope fractionation during nitrate assimilation by a marine diataom is inversely related to its growth rate under light-limited growing conditions.2 In general, the 815N and 813C values of animals reflect the isotopic values of their diets, even though animals appear to incorporate dietary I5N preferentially over 14N.3>4 Isotopic analysis of an animal can thus be used to reconstruct its diet when food sources have different 815N values. Furthermore, theI5N enrichment at a single feeding process (number 5 in Figure 1) ranges from 1.3 to 5.3, averaging 3.4 ± \.\%o irrespective of biochemical differences, such as the existence of a urea cycle, the production of uric acid, or differences in habitat (Figure 2). 4 This fact suggests that the 8I5N of the animal's body can be a useful parameter to analyze its trophic level in an ecosystem and can provide a possible analytical method to classify components of the
362
animal kingdom from the point of view of biogeochemical nitrogen cycling. On the other hand, the 813C values of animal tissue are very close to those in their diet, and a small increase in 13C content (about \%o enrichment in animal body) sometimes occurs with an increasing trophic level.5 Consequently, the differences in 8I3C values between C3 and C4 plants, or between terrestrial plants and marine algae, have been used for the food habit analysis of an animal in an ecosystem. Current progress in isotope biogeochemistry can be summarized as follows. Plants exhibit a variety of isotope compositions, depending upon environmental conditions, and animal 815N and 813C values reflect its diet: "You are what you eat" Humans are a case in point. For a 50-kg human, there is in vivo 225 g of heavy isotopes of the light elements (Figure 3). Ingestion involves intake of about 3 g of 13C and 0.5 g of I5 N per day. Isotopic compositions are different
/
o-"
20 \sM) throughout the year, and diatoms are the most dominant group among phytoplankton. Krill, a herbivore, acts as a central dietary source for carnivorous zooplankton, seals, whales, and other animals. In general, species diversity is lower in the Antarctic Ocean than in temperate and tropical regions. Therefore, several representative animals can be easily assigned to trophic levels (TL) in a food web by their feeding habits and provide an excellent opportunity for the examination of the food chain effect. In fact, a clearcut linear relation emerges between the chemical parameter (animal 81SN) and the ecological one (TL), i.e., 815N (%o) = 3.3 x ( T L - 1) - O.2.8 As indicated in Figure 5,8 15 N values of an animal increases with increasing trophic level, and the 15 N abundances provide a new way to classify components of the animal kingdom for assessing fluxes of biogenic substances in an ecosystem. The enrichment factor of 3.3%e per TL also successfully describes a monthly change of food web structure involving phytoplankton, rotifers, copepods, chironomids, and pond smelt in a eutrophic lake in Japan.9
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
6 13 C°/oo FIGURE 4. Relation between 815N and 813C values of various biogenic substances from the Otsuchi estuarine system. T and M denote the land-derived organics and the marine organic matter, respectively. The solid, straight line T1M1 was obtained from a linear regression analysis of the bay sediments. (A) Plant remains in the upper reaches, (A') POM in the upper reaches, (B) mountain soil, (C) river sediments in the upper reaches, (D) benthic animals in the river, (E) POM in the bay, (F) netted plankton in the bay, (G) fish collected from the seagrass meadow, and (H) seagrasses at the inner bay. Numerical suffixes stand for the trophic level in each ecosystem. (From Wada, E., Isotopenpraxis, 23, 320, 1987. With permission.)
IV. SEABIRD ROOKERIES One of the unique characteristics of a seabird rookery is its large annual influx of nutrients. For instance, more nitrogen, phosphorus, and potassium are deposited into a black-tailed gull (Lams crassirostris) rookery than into cultivated fields under the most fertilizer-intensive agriculture.10 Due to this influx of nutrients, a luxuriant plant community often forms in seabird rookeries.11 Although a study on the ecological structure of such a community is of considerable interest,
rookeries are usually found in isolated locations, making frequent visits impractical. Since nitrogen and carbon isotopes provide information integrated over time, the study of various samples from these rookeries could elucidate nutrient fluxes in such an ecosystem. We examined two rookeries: Kabushima, a black-tailed gull rookery in Hachinohe, Japan, and Cape Bird, an Adelie penguin (Pygoscelis adeliae) rookery on Ross Island, Antarctica. This study unfolded major pathways of material flow and associated ecogeochemical processes. In the 365
Large fish
> 100 cm
Fish 8 - 2 0 cm 13 %o
Small fish Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
= 2cm 1O%O
1.007
NO5 FIGURE 5.
7 7oo
Distribution of 815N%o in the Antarctic ecosystem.
following, we present a summary of this study. Some of the details have been presented elsewhere.12'13 Figure 6 shows isotopic relationships between seabirds and their diet. The enrichment of both 15N and 13C is apparent from the upward direction of the arrows, starting from the estimated overall 815N and 813C of the diet and ending at those of seabirds. The extent of 15N enrichment is similar for gulls and penguins, while the 13C enrichment of gull feathers is higher than that of penguin bodies. This difference in 13C enrichment between avian feathers and other organs has been reported previously.14 It was found that, in the rookeries, inorganic
366
nitrogen entering the cycle through precipitation and in situ dinitrogen fixation was an unimportant process in nitrogen cycling. Instead, microbial degradation of organic nitrogen of avian origin and volatilization of the resultant ammonia were the most important processes. While most organic nitrogen from soil was of avian origin, organic carbon from soil was not always so. Most soil organic carbon was autochthonous in the gull rookery, where a rich ornithocoprophilous plant community forms, whereas it was dominantly allochthonous in the penguin rookery. Figure 7 contrasts the two different types of nitrogen and carbon flow in the rookeries. Even
A ^
12-
Gull feather
A Sardine
Larvae A
5F 11-i 10-
Est.avg. A
9-
A Squid A Anchovy
8-
AMysids
7 -23
-22
-21
-20
-19
-18
-17
-16
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
6 1 3 CPDB(%O)
.—*
15-
^
14-
"5
13-
^
12-
B
A Fishes
11-
Penguin body
10-
/
9-
/
8-
Est.avg. A
765-
Krill A
43-
-30
-29
-28
-27
-26
-25
-24
-23
6 1 3 CPDB(°/OO) FIGURE 6. 815N and 813C of diets and seabirds. (A) Gulls in Kabushima. (B) Penguins in Cape Bird. Each symbol represents the average of several different samples. The estimated average was obtained not only from foods shown, but also from those for which either 815N or 513C was unavailable. Larvae are those of the Japan soldier fly. More details are given in Reference 13, although additional data are included in this figure.
though the isotope ratios were the same from primary producers through food chains to the birds and their droppings, the difference in the ecogeochemical processes within these rookeries
would have had resulted in totally different isotope ratios for the constituents of the rookeries. These patterns in the gull and the penguin rookeries seem to represent two extremes of material
367
A.Kabushima:a gull world with plants
Atmospheric CO2
(-7. j Soil 10(-27) Uric a c i d
•
0 ( 0 )
Ammonium • 21 Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Org. matters #
7 (-27)
B.Cape Bird:
a penguin world without plants CO 2 (0) NH3 -14
Uric acid Soil 2 4 ( 0 ) Ammonium
0 (0) 42 20 (0)
l^ipCyanobacteria FIGURE 7. Major pathways of nitrogen and carbon around the seabird rookery at Kabushima (A) and at Cape Bird (B). The thick arrows indicate the flow of carbon, and the thin arrows indicate the flow of nitrogen. Major pathways estimated by the isotope ratios are shown. Numbers show nitrogen and carbon isotope ratios for each constituent of the rookery. The ratios are normalized for the convenience of comparison between the two rookeries, assuming the overall 815N and S13C values for avian diet to be 0%o. Those without parentheses are 61SN values, and those in parentheses are 813C values.
368
flow in seabird rookeries. Although absolute values of the isotope ratios change, relative values for constituents of most seabird rookeries would be somewhere within those given in Figure 7. Since seabirds often nest in places far from human habitation, the access to such a rookery is very limited. An isotopic study of a rookery might prove particularly useful in obtaining ecogeochemical relationships in these remote ecosystems.
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
V. HUMAN FOOD WEB The SI food web analysis is also applicable to humans, including not only ancient people who depended on natural foods, but also contemporary humans eating commercial foods. Although a number of isotope measurements of human samples have been performed for archaeological purposes, the relation of isotope compositions between tissue and diet has not been confirmed in these works. By measuring 13C and 15N contents of human hair from residents of Chicago, Illinois, with reference to those of commercial foods, a relationship between tissue and diet has been shown.15 Hair and other human tissues have a significant offset in both 813C and 815N (Minagawa, unpublished data). The 813C of hair is about 3.5%c heavier than that of bone collagen, which is often used in anthropological works. On the other hand, the 815N of hair seems to be close to that of bone collagen. Offsets among other tissues have not been revealed for humans. Hair samples are nevertheless useful in studying the contemporary human diet if a substance remains in the dietary process longer than a couple of months. Human feeding habits are generally too complicated to trace by two isotope tracers. However, the isotope distribution of carbon and nitrogen favorably distinguishes C3 plants, C4 plants, most marine fish, meat, and dairy products (Figure 8). Since most vegetables are cultivated in the presence of chemical fertilizers ultimately made from atmospheric nitrogen, the 815N of C3 plants tends to overlap that of leguminous plants, which can fix atmospheric nitrogen biologically. Most marine fish that are used as food by humans are usually carnivores, so that their 15N contents are
relatively high. The 813C and 8I5N of commercial foods in Japan appears to be approaching those of American foods, partly because some food resources, such as wheat, potato, and soybeans, are imported from the U.S. In addition, assorted feed for livestock cultured in Japan has been prepared by using imported materials, including corn (a C4 plant) from the U.S. The 813C and 815N of human hair is different in populations of different countries (Figure 9). 16 In general, vegetable foods decrease the 15N and 13 C content of diet simultaneously, whereas fish foods raises the 15N content. These effects are major reasons for the hair isotope characteristics of Indian, Chinese, Korean, and Japanese populations. On the other hand, the different usage of C3 and C4 plants makes a large difference in the I3C content of the diet. In particular, the 8I3C of human hair differentiates Europeans from Americans, because the ultimate source of organic matter for the former depends mainly on C3 plants, whereas the diet of the latter depends more heavily on corn. Because the replacement of nitrogen and carbon in human tissues occurs continuously with the feeding process, the change of food composition results in the change of the isotope composition of hair. In fact, the isotope patterns of Japanese who lived in America and Europe for more than 1 year were similar to those of the native population (Figure 9). The 813C and 815N of the average diet of a country can be estimated by calculating the weighted mean: the delta value of food is multiplied by the uptake proportion of protein of each food group. The direct measurement of 813C and 815N of the mean food, mixed according to consumption rates, is another approach to obtain mean isotope ratios of a national mean diet. However, such mixed food did not show any correlation in an isotope study.16 The estimation of isotope ratios of mean food by combining delta values of individual food groups shows a good correlation to the corresponding isotope ratios of hair from Japan, Korea, China, India, the Netherlands, and the U.S. 17 The isotopic offset between diet and hair is l.5%c and 4.3%o for S13C and 815N, respectively. Consequently, it should be emphasized that the isotope measurement of human tissues possibly indicates the characteristics of food
369
20
15
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
10
Human'C? •" •
2 2
0 Vegetable -35
-30
(C 3 )
-25
-20 613C V-
-15
-10
-5
FIGURE 8. The distribution of carbon and nitrogen isotopes in a human food web. Open circles: Japanese foods. (From Minagawa, M. et al., Chikyuu-Kagaku, 2,79,1986. With permission.) Closed circles: American foods. (Adapted from Schoeller, D. A. et al., Ecol Food Nutrit, 18, 159, 1986.)
consumption patterns, if the isotopic composition is determined for most food consumed.
VI. SUMMARY Stable isotope ratios can provide insight into the intricacies of biogeochemical processes in ecosystems. The fluctuation of an element's isotope ratio can provide information regarding the element's origin and subsequent reaction pathways. Even though these elemental isotope ef-
370
fects operate in complex, multicomponent systems, persistent and characteristic patterns emerge that reflect underlying ecological processes. This article demonstrates the use of stable isotope ratio patterns of nitrogen and carbon to study food webs of a variety of ecosystems, such as estuaries, the Antarctic Ocean, seabird rookeries, and humans. Examples of the fluctuations of these elemental isotope ratios are reviewed in light of the underlying food habits of the ecosystem's inhabitants. The results demonstrate the overall usefulness of the stable isotope approach to food web analysis.
13
I
1
i
>
1
12
i
•
B
11
•
J
•3
s-8
H U
K
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
•2
"o C
•
tb I
8 •
Analytical error 1
-21
-20
-19
i
i
-18
-17
6"CPDO
i
-16
i
-15
~J~ i
-14
-13
of hair %o
FIGURE 9. Carbon and nitrogen isotope ratios of human hair. Boxes show the range of standard deviation of 813C and 815N of each population. (From Minagawa, H. et al., Chikyuu-Kagaku, 20, 79, 1986. With permission.) Initials indicate nations: (B) Brazil, (C) China, (K) Korea, (H) the Netherlands, (I) India, (J) Japan, and (U) U.S. The following data are also presented: (1, 2) for Japanese living in U.S., (3) for a Japanese living in Sweden, and (4) for a Japanese living in the Edo era (130 years ago).
REFERENCES 1. Wada, E., Isotopenpraxis, 23, 320, 1987. 2. Wada, E. and Hattori, A., Geomicrobiology J., 1, 85, 1978. 3. DeNiro, M. J. and Epstein, S., Geochim. Cosmochim. Acta, 42, 495, 1978. 4. Minagawa, M. and Wada, E., Geochim. Cosmochim. Acta, 48, 1135, 1984. 5. Fry, B. and Sherr, E. B., Contribut. Mar. Sci., 27, 13, 1984. 6. Minagawa, M., Winter, D. A., and Kaplan, I. R., Anal. Chem., 56, 1859, 1984. 7. Wada, E., Minagawa, M., Mizutani, H., Tsuji, T., Imaizuni, R., and Karasawa, K., Estuarine, Coastal Shelf Sci., 25, 321, 1987. 8. Wada, E., Terazaki, M., Kabaya, Y., and Nemoto, T., Deep-Sea Res., 34, 829, 1987. 9. Yoshioka, T., Wada, E., and Sayo, Y., Verh. Int. Verein. Limnol., 23, 573, 1988.
10. Mizutani, H., Mar. Sci. Monthly, 16, 226, 1984. 11. Ishizuka, K., Narita, K., and Mizutani, H., Ecology, A Comprehensive Series on Modern Biological Sciences, Vol. 12a, Numata, M., Ed., Nakayama Publishing, Tokyo, 1985, 116. 12. Mizutani, H., Hasegawa, H., and Wada, E., Biogeochemistry, 2, 221, 1986. 13. Mizutani, H. and Wada, E., Ecology, 69, 340,1988. 14. Mizutani, H., Annual Report, Mitsubishi Kasei Institute of Life Sciences, 1985, 92, 1986. 15. Schoeller, D. A., Minagawa, M., Slater, R., and Kaplan, I. R., Ecol. Food Nutrit., 18, 159, 1986. 16. The International Atomic Energy Agency (IAEA) distributes a mixed human diet prepared in Finland for the intercalibration of heavy metal contents of food, and δ13C and δ15N of lipid-free fraction of this IAEA mixed diet gave - 2 4 . 8 and +6.4%o, respectively. 17. Minagawa, M., Karasawa, K., and Kabaya, Y., Chikyuu-kagaku, 20, 79, 1986 (in Japanese with English abstract).
371
Critical Reviews in Food Science and Nutrition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bfsn20
The use of stable isotopes for food web analysis a
a
E. Wada , H. Mizutani & M. Minagawa
a
a
Laboratory of Biogeochemistry and Sociogeochemistry, Mitsubishi Kasei Institute of Life Sciences, Machida, Tokyo, 194, Japan Version of record first published: 29 Sep 2009.
To cite this article: E. Wada , H. Mizutani & M. Minagawa (1991): The use of stable isotopes for food web analysis, Critical Reviews in Food Science and Nutrition, 30:4, 361-371 To link to this article: http://dx.doi.org/10.1080/10408399109527547
PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Critical Reviews in Food Science and Nutrition, 30(3):361—371 (1991)
The Use of Stable Isotopes for Food Web Analysis E. Wada, H. Mizutani, and M. Minagawa
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Mitsubishi Kasei Institute of Life Sciences Laboratory of Biogeochemistry and Sociogeochemistry, Machida, Tokyo 194 Japan
ABSTRACT: General aspects in isotope biogeochemistry was summarized with emphasis on δ 15 N and δ 13 C contents in plants and animals in natural ecosystems. In the estuary, the variation of isotope ratios were principally governed by the mixing of land-derived organic matter, marine phytoplankton, and seagrasses. A clear cut linear relationship between animal δ15N and its trophic level was obtained in the Antarctic food chain system. Several current efforts to use the stable isotopes for food web analysis were demonstrated for some terrestrial and marine systems as well as human food web. KEY WORDS: δ 1 5 N, δ 1 3 C, food web, isotope biogeochemistry.
I. INTRODUCTION Stable isotopes of light elements, such as nitrogen and carbon, undergo fractionation in chemical and biochemical reactions. Since isotope fractionation is regulated by the difference in thermodynamic properties among isotopic molecules, biogenic materials in nature have their own isotopic composition. This is the so-called dynamic stable isotope (SI) fingerprint. Therefore, a goal of isotope biogeochemistry is to decipher the SI code of biogenic substances by precise measurement of isotope ratios. In principle, a fluctuation of an element's isotope ratio provides information about its origin, pathways, and the reaction mechanisms concerning its formation. On a macroscopic level, the fluctuation can provide a key to solve biogeochemical phenomena involving food habits and behavior of an animal, and transport or organic matter in an ecosystem. Fundamental steps of isotope effects are classified into the following: (1) reaction from A to B, (2) partition of A between two phases, and
(3) branch reaction. These elemental isotope effects operate in a multicomponent system; bulk isotope discriminations result from environmental effects. Consequently, we can expect wellordered dynamics of the SI fingerprint, i.e., an isotopically ordered world, which can provide a new way of viewing material cycling in the ecosystem. In fact, stable isotope studies of food webs show regular features, irrespective of the high complexity of an ecosystem. In an ecosystem, nitrogen and carbon isotope discrimination takes place in certain sites, as indicated by the circled numbers in Figure 1. The number 1 means the occurrence of isotope fractionation in biotic cycles of nitrogen and carbon. Reactions 2 and 3 contribute to the isotope fractionation between ambient substrate and bulk primary producers. The 8 15 N and 813C values of substrate, the reversibility of substrate transport through membrane systems, the supply of reducing substances, and the inherent isotope fractionation of enzyme-substrate reactions determine the magnitude of apparent isotope fractionation between ambient substrate and
1040-8398/91/$.50 © 1991 by CRC Press, Inc.
361
hi/
Euphotic jyj layer
Substrate So
Si + E:
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Detritus Excreta
NH 3 \ f
f Mineralization
NH4
(Carnivores)
I FIGURE 1. Schematic model for nitrogen and carbon isotope fractionation in an ecosystem. Numerical values stand for major sites of isotopic variations of primary producers. (1) Isotopic compositions of substrate; (2,3) kinetic isotope effects in the uptake processes by plants; (4) branch point in metabolic pathways; (5) enrichment of 1SN in a feeding process; and (6) mineralization. (From Wada, E., Isotopenpraxis, 23, 320, 1987. With permission.)
product or bulk primary producers. For instance, the nitrogen isotope fractionation during nitrate assimilation by a marine diataom is inversely related to its growth rate under light-limited growing conditions.2 In general, the 815N and 813C values of animals reflect the isotopic values of their diets, even though animals appear to incorporate dietary I5N preferentially over 14N.3>4 Isotopic analysis of an animal can thus be used to reconstruct its diet when food sources have different 815N values. Furthermore, theI5N enrichment at a single feeding process (number 5 in Figure 1) ranges from 1.3 to 5.3, averaging 3.4 ± \.\%o irrespective of biochemical differences, such as the existence of a urea cycle, the production of uric acid, or differences in habitat (Figure 2). 4 This fact suggests that the 8I5N of the animal's body can be a useful parameter to analyze its trophic level in an ecosystem and can provide a possible analytical method to classify components of the
362
animal kingdom from the point of view of biogeochemical nitrogen cycling. On the other hand, the 813C values of animal tissue are very close to those in their diet, and a small increase in 13C content (about \%o enrichment in animal body) sometimes occurs with an increasing trophic level.5 Consequently, the differences in 8I3C values between C3 and C4 plants, or between terrestrial plants and marine algae, have been used for the food habit analysis of an animal in an ecosystem. Current progress in isotope biogeochemistry can be summarized as follows. Plants exhibit a variety of isotope compositions, depending upon environmental conditions, and animal 815N and 813C values reflect its diet: "You are what you eat" Humans are a case in point. For a 50-kg human, there is in vivo 225 g of heavy isotopes of the light elements (Figure 3). Ingestion involves intake of about 3 g of 13C and 0.5 g of I5 N per day. Isotopic compositions are different
/
o-"
20 \sM) throughout the year, and diatoms are the most dominant group among phytoplankton. Krill, a herbivore, acts as a central dietary source for carnivorous zooplankton, seals, whales, and other animals. In general, species diversity is lower in the Antarctic Ocean than in temperate and tropical regions. Therefore, several representative animals can be easily assigned to trophic levels (TL) in a food web by their feeding habits and provide an excellent opportunity for the examination of the food chain effect. In fact, a clearcut linear relation emerges between the chemical parameter (animal 81SN) and the ecological one (TL), i.e., 815N (%o) = 3.3 x ( T L - 1) - O.2.8 As indicated in Figure 5,8 15 N values of an animal increases with increasing trophic level, and the 15 N abundances provide a new way to classify components of the animal kingdom for assessing fluxes of biogenic substances in an ecosystem. The enrichment factor of 3.3%e per TL also successfully describes a monthly change of food web structure involving phytoplankton, rotifers, copepods, chironomids, and pond smelt in a eutrophic lake in Japan.9
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
6 13 C°/oo FIGURE 4. Relation between 815N and 813C values of various biogenic substances from the Otsuchi estuarine system. T and M denote the land-derived organics and the marine organic matter, respectively. The solid, straight line T1M1 was obtained from a linear regression analysis of the bay sediments. (A) Plant remains in the upper reaches, (A') POM in the upper reaches, (B) mountain soil, (C) river sediments in the upper reaches, (D) benthic animals in the river, (E) POM in the bay, (F) netted plankton in the bay, (G) fish collected from the seagrass meadow, and (H) seagrasses at the inner bay. Numerical suffixes stand for the trophic level in each ecosystem. (From Wada, E., Isotopenpraxis, 23, 320, 1987. With permission.)
IV. SEABIRD ROOKERIES One of the unique characteristics of a seabird rookery is its large annual influx of nutrients. For instance, more nitrogen, phosphorus, and potassium are deposited into a black-tailed gull (Lams crassirostris) rookery than into cultivated fields under the most fertilizer-intensive agriculture.10 Due to this influx of nutrients, a luxuriant plant community often forms in seabird rookeries.11 Although a study on the ecological structure of such a community is of considerable interest,
rookeries are usually found in isolated locations, making frequent visits impractical. Since nitrogen and carbon isotopes provide information integrated over time, the study of various samples from these rookeries could elucidate nutrient fluxes in such an ecosystem. We examined two rookeries: Kabushima, a black-tailed gull rookery in Hachinohe, Japan, and Cape Bird, an Adelie penguin (Pygoscelis adeliae) rookery on Ross Island, Antarctica. This study unfolded major pathways of material flow and associated ecogeochemical processes. In the 365
Large fish
> 100 cm
Fish 8 - 2 0 cm 13 %o
Small fish Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
= 2cm 1O%O
1.007
NO5 FIGURE 5.
7 7oo
Distribution of 815N%o in the Antarctic ecosystem.
following, we present a summary of this study. Some of the details have been presented elsewhere.12'13 Figure 6 shows isotopic relationships between seabirds and their diet. The enrichment of both 15N and 13C is apparent from the upward direction of the arrows, starting from the estimated overall 815N and 813C of the diet and ending at those of seabirds. The extent of 15N enrichment is similar for gulls and penguins, while the 13C enrichment of gull feathers is higher than that of penguin bodies. This difference in 13C enrichment between avian feathers and other organs has been reported previously.14 It was found that, in the rookeries, inorganic
366
nitrogen entering the cycle through precipitation and in situ dinitrogen fixation was an unimportant process in nitrogen cycling. Instead, microbial degradation of organic nitrogen of avian origin and volatilization of the resultant ammonia were the most important processes. While most organic nitrogen from soil was of avian origin, organic carbon from soil was not always so. Most soil organic carbon was autochthonous in the gull rookery, where a rich ornithocoprophilous plant community forms, whereas it was dominantly allochthonous in the penguin rookery. Figure 7 contrasts the two different types of nitrogen and carbon flow in the rookeries. Even
A ^
12-
Gull feather
A Sardine
Larvae A
5F 11-i 10-
Est.avg. A
9-
A Squid A Anchovy
8-
AMysids
7 -23
-22
-21
-20
-19
-18
-17
-16
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
6 1 3 CPDB(%O)
.—*
15-
^
14-
"5
13-
^
12-
B
A Fishes
11-
Penguin body
10-
/
9-
/
8-
Est.avg. A
765-
Krill A
43-
-30
-29
-28
-27
-26
-25
-24
-23
6 1 3 CPDB(°/OO) FIGURE 6. 815N and 813C of diets and seabirds. (A) Gulls in Kabushima. (B) Penguins in Cape Bird. Each symbol represents the average of several different samples. The estimated average was obtained not only from foods shown, but also from those for which either 815N or 513C was unavailable. Larvae are those of the Japan soldier fly. More details are given in Reference 13, although additional data are included in this figure.
though the isotope ratios were the same from primary producers through food chains to the birds and their droppings, the difference in the ecogeochemical processes within these rookeries
would have had resulted in totally different isotope ratios for the constituents of the rookeries. These patterns in the gull and the penguin rookeries seem to represent two extremes of material
367
A.Kabushima:a gull world with plants
Atmospheric CO2
(-7. j Soil 10(-27) Uric a c i d
•
0 ( 0 )
Ammonium • 21 Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
Org. matters #
7 (-27)
B.Cape Bird:
a penguin world without plants CO 2 (0) NH3 -14
Uric acid Soil 2 4 ( 0 ) Ammonium
0 (0) 42 20 (0)
l^ipCyanobacteria FIGURE 7. Major pathways of nitrogen and carbon around the seabird rookery at Kabushima (A) and at Cape Bird (B). The thick arrows indicate the flow of carbon, and the thin arrows indicate the flow of nitrogen. Major pathways estimated by the isotope ratios are shown. Numbers show nitrogen and carbon isotope ratios for each constituent of the rookery. The ratios are normalized for the convenience of comparison between the two rookeries, assuming the overall 815N and S13C values for avian diet to be 0%o. Those without parentheses are 61SN values, and those in parentheses are 813C values.
368
flow in seabird rookeries. Although absolute values of the isotope ratios change, relative values for constituents of most seabird rookeries would be somewhere within those given in Figure 7. Since seabirds often nest in places far from human habitation, the access to such a rookery is very limited. An isotopic study of a rookery might prove particularly useful in obtaining ecogeochemical relationships in these remote ecosystems.
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
V. HUMAN FOOD WEB The SI food web analysis is also applicable to humans, including not only ancient people who depended on natural foods, but also contemporary humans eating commercial foods. Although a number of isotope measurements of human samples have been performed for archaeological purposes, the relation of isotope compositions between tissue and diet has not been confirmed in these works. By measuring 13C and 15N contents of human hair from residents of Chicago, Illinois, with reference to those of commercial foods, a relationship between tissue and diet has been shown.15 Hair and other human tissues have a significant offset in both 813C and 815N (Minagawa, unpublished data). The 813C of hair is about 3.5%c heavier than that of bone collagen, which is often used in anthropological works. On the other hand, the 815N of hair seems to be close to that of bone collagen. Offsets among other tissues have not been revealed for humans. Hair samples are nevertheless useful in studying the contemporary human diet if a substance remains in the dietary process longer than a couple of months. Human feeding habits are generally too complicated to trace by two isotope tracers. However, the isotope distribution of carbon and nitrogen favorably distinguishes C3 plants, C4 plants, most marine fish, meat, and dairy products (Figure 8). Since most vegetables are cultivated in the presence of chemical fertilizers ultimately made from atmospheric nitrogen, the 815N of C3 plants tends to overlap that of leguminous plants, which can fix atmospheric nitrogen biologically. Most marine fish that are used as food by humans are usually carnivores, so that their 15N contents are
relatively high. The 813C and 8I5N of commercial foods in Japan appears to be approaching those of American foods, partly because some food resources, such as wheat, potato, and soybeans, are imported from the U.S. In addition, assorted feed for livestock cultured in Japan has been prepared by using imported materials, including corn (a C4 plant) from the U.S. The 813C and 815N of human hair is different in populations of different countries (Figure 9). 16 In general, vegetable foods decrease the 15N and 13 C content of diet simultaneously, whereas fish foods raises the 15N content. These effects are major reasons for the hair isotope characteristics of Indian, Chinese, Korean, and Japanese populations. On the other hand, the different usage of C3 and C4 plants makes a large difference in the I3C content of the diet. In particular, the 8I3C of human hair differentiates Europeans from Americans, because the ultimate source of organic matter for the former depends mainly on C3 plants, whereas the diet of the latter depends more heavily on corn. Because the replacement of nitrogen and carbon in human tissues occurs continuously with the feeding process, the change of food composition results in the change of the isotope composition of hair. In fact, the isotope patterns of Japanese who lived in America and Europe for more than 1 year were similar to those of the native population (Figure 9). The 813C and 815N of the average diet of a country can be estimated by calculating the weighted mean: the delta value of food is multiplied by the uptake proportion of protein of each food group. The direct measurement of 813C and 815N of the mean food, mixed according to consumption rates, is another approach to obtain mean isotope ratios of a national mean diet. However, such mixed food did not show any correlation in an isotope study.16 The estimation of isotope ratios of mean food by combining delta values of individual food groups shows a good correlation to the corresponding isotope ratios of hair from Japan, Korea, China, India, the Netherlands, and the U.S. 17 The isotopic offset between diet and hair is l.5%c and 4.3%o for S13C and 815N, respectively. Consequently, it should be emphasized that the isotope measurement of human tissues possibly indicates the characteristics of food
369
20
15
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
10
Human'C? •" •
2 2
0 Vegetable -35
-30
(C 3 )
-25
-20 613C V-
-15
-10
-5
FIGURE 8. The distribution of carbon and nitrogen isotopes in a human food web. Open circles: Japanese foods. (From Minagawa, M. et al., Chikyuu-Kagaku, 2,79,1986. With permission.) Closed circles: American foods. (Adapted from Schoeller, D. A. et al., Ecol Food Nutrit, 18, 159, 1986.)
consumption patterns, if the isotopic composition is determined for most food consumed.
VI. SUMMARY Stable isotope ratios can provide insight into the intricacies of biogeochemical processes in ecosystems. The fluctuation of an element's isotope ratio can provide information regarding the element's origin and subsequent reaction pathways. Even though these elemental isotope ef-
370
fects operate in complex, multicomponent systems, persistent and characteristic patterns emerge that reflect underlying ecological processes. This article demonstrates the use of stable isotope ratio patterns of nitrogen and carbon to study food webs of a variety of ecosystems, such as estuaries, the Antarctic Ocean, seabird rookeries, and humans. Examples of the fluctuations of these elemental isotope ratios are reviewed in light of the underlying food habits of the ecosystem's inhabitants. The results demonstrate the overall usefulness of the stable isotope approach to food web analysis.
13
I
1
i
>
1
12
i
•
B
11
•
J
•3
s-8
H U
K
Downloaded by [University North Carolina - Chapel Hill] at 18:05 12 April 2013
•2
"o C
•
tb I
8 •
Analytical error 1
-21
-20
-19
i
i
-18
-17
6"CPDO
i
-16
i
-15
~J~ i
-14
-13
of hair %o
FIGURE 9. Carbon and nitrogen isotope ratios of human hair. Boxes show the range of standard deviation of 813C and 815N of each population. (From Minagawa, H. et al., Chikyuu-Kagaku, 20, 79, 1986. With permission.) Initials indicate nations: (B) Brazil, (C) China, (K) Korea, (H) the Netherlands, (I) India, (J) Japan, and (U) U.S. The following data are also presented: (1, 2) for Japanese living in U.S., (3) for a Japanese living in Sweden, and (4) for a Japanese living in the Edo era (130 years ago).
REFERENCES 1. Wada, E., Isotopenpraxis, 23, 320, 1987. 2. Wada, E. and Hattori, A., Geomicrobiology J., 1, 85, 1978. 3. DeNiro, M. J. and Epstein, S., Geochim. Cosmochim. Acta, 42, 495, 1978. 4. Minagawa, M. and Wada, E., Geochim. Cosmochim. Acta, 48, 1135, 1984. 5. Fry, B. and Sherr, E. B., Contribut. Mar. Sci., 27, 13, 1984. 6. Minagawa, M., Winter, D. A., and Kaplan, I. R., Anal. Chem., 56, 1859, 1984. 7. Wada, E., Minagawa, M., Mizutani, H., Tsuji, T., Imaizuni, R., and Karasawa, K., Estuarine, Coastal Shelf Sci., 25, 321, 1987. 8. Wada, E., Terazaki, M., Kabaya, Y., and Nemoto, T., Deep-Sea Res., 34, 829, 1987. 9. Yoshioka, T., Wada, E., and Sayo, Y., Verh. Int. Verein. Limnol., 23, 573, 1988.
10. Mizutani, H., Mar. Sci. Monthly, 16, 226, 1984. 11. Ishizuka, K., Narita, K., and Mizutani, H., Ecology, A Comprehensive Series on Modern Biological Sciences, Vol. 12a, Numata, M., Ed., Nakayama Publishing, Tokyo, 1985, 116. 12. Mizutani, H., Hasegawa, H., and Wada, E., Biogeochemistry, 2, 221, 1986. 13. Mizutani, H. and Wada, E., Ecology, 69, 340,1988. 14. Mizutani, H., Annual Report, Mitsubishi Kasei Institute of Life Sciences, 1985, 92, 1986. 15. Schoeller, D. A., Minagawa, M., Slater, R., and Kaplan, I. R., Ecol. Food Nutrit., 18, 159, 1986. 16. The International Atomic Energy Agency (IAEA) distributes a mixed human diet prepared in Finland for the intercalibration of heavy metal contents of food, and δ13C and δ15N of lipid-free fraction of this IAEA mixed diet gave - 2 4 . 8 and +6.4%o, respectively. 17. Minagawa, M., Karasawa, K., and Kabaya, Y., Chikyuu-kagaku, 20, 79, 1986 (in Japanese with English abstract).
371
