The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food-web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food-web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.

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Applying stable isotopes to examine food‐web structure: an overview of analytical tools

We report measurements of seasonal variability in the C-N stable isotope ratios of plants collected across the habitat mosaic of San Francisco Bay, its marshes, and its tributary river system. Analyses of 868 plant samples were binned into 10 groups (e.g., terrestrial riparian, freshwater phytoplankton, salt marsh) to determine whether C-N isotopes can be used as biomarkers for tracing the origins of organic matter in this river‐marsh‐estuary complex. Variability of d 13 C and d 15 N was high (;5‐10‰) within each plant group, and we identified three modes of variability: (1) between species and their microhabitats, (2) over annual cycles of plant growth and senescence, and (3) between living and decomposing biomass. These modes of within-group variability obscure any sourcespecific isotopic signatures, confounding the application of C-N isotopes for identifying the origins of organic matter. A second confounding factor was large dissimilarity between the d 13 C-d 15 N of primary producers and the organicmatter pools in the seston and sediments. Both confounding factors impede the application of C-N isotopes to reveal the food supply to primary consumers in ecosystems supporting diverse autotrophs and where the isotopic composition of organic matter has been transformed and become distinct from that of its parent plant sources. Our results support the advice of others: variability of C-N stable isotopes within all organic-matter pools is high and must be considered in applications of these isotopes to trace trophic linkages from primary producers to primary consumers. Isotope-based approaches are perhaps most powerful when used to complement other tools, such as molecular biomarkers, bioassays, direct measures of production, and compilations of organic-matter budgets.

/pdf/stable-carbon-and-nitrogen-isotope-composition-of-aquatic-39b36tyw9n.pdf

Stable carbon and nitrogen isotope composition of aquatic and terrestrial plants of the San Francisco Bay estuarine system

Stable nitrogen isotope ratios of whole organisms and tissues are routinely used in studies of trophic relationships and nitrogen flow through ecosystems, yet changes underlying increases in W15N from food source to consumer are not completely understood. In this study, the W15N of 16 amino acids in marine planktonic consumers and their food sources were examined using gas chromatography/combustion/isotope ratio mass spec- trometry of their N-pivaloyl-i-propyl-amino acid ester derivatives. Moderate increases in bulk W'5N with trophic position reflect an averaging of large increases in the W15N of some amino acids, and little or no change in others. Amino acids showing consistently large increases (e.g., glutamic acid changes by -7%o between food and consumer) provide greater scope for defining trophic position than the smaller isotopic changes in bulk material. In contrast, amino acids like phenylalanine show no change in W15N with trophic position and therefore preserve information about nitrogen sources at the base of the food web. The ability to acquire information about both trophic level and nitrogen sources at the base of the food web from single samples of consumer tissues offers a powerful new tool for elucidating pathways of N transfer through food webs.

https://www.jstor.org/stable/pdfplus/3072049.pdf

Trophic relationships and the nitrogen isotopic composition of amino acids in plankton

The importance of phytoplankton, benthic microalgae, fresh and salt marsh emergent plants, and terrestrial organic matter to the estuarine food web of Plum Island Sound, Massachusetts, USA, was evaluated using the stable isotopes of carbon, nitrogen and sulfur. Three distinct regions of the estuary were examined: the oligohaline upper estuary with high freshwater inputs and fringing fresh marsh, the middle estuary with extensive salt marsh, and the lower estuary with a greater proportion of open bay and a direct connection to open ocean. In each region, sources of organic matter for pelaglc and benthic aquatlc consumers were compared with the range of organic matter sources available in the entire estuary. Consumers in all regions relied most heavily on locally produced organic matter. In the upper estuary, most consumers had 613C values of -29 to 2 1 ' L and 634S values of about 8?L, whlch indicated dependence on a mixture of fresh marsh emergent vegetation and phytoplankton from the oligohaline region. In the middle and lower estuary, consumers had 613C values of -23 to -15 ' k and 634S values of 5 to 154., resembling a mixture of Spal-tina spp., benthic m~croalgae and marine phytoplankton. Terrestrial organic matter was of minimal importance in the upper estuary and was not evident in the food webs of the middle and lower estuary. The isotopic values of pelagic consumers reflected a greater dependence on phytoplankton than those of benthic consumers, which were closer to fresh or salt marsh emergent vegetation and benthic microalgae. Several of the patterns revealed by examination of organic matter sources for the food web of the Plum Island Sound estuary have implications for our general understanding of estuarine food webs. First, while there is substantial spatial heterogeneity in organic matter sources within a single estuary, consumers tend to utilize sources of organic matter produced In the same region of the estuary in which they reside. Second, consumer dependence on terrestrially derived riverine organic matter is minimal, even in the relatively small region of the estuary where ~ t s availability is the highest Third, benthic and pe lag~c organisms rely on different mixes of organic matter sources.

https://www.int-res.com/articles/meps/147/m147p031.pdf

Evidence for spatial variability in estuarine food webs

/pdf/the-role-of-standing-dead-spartina-alterniflora-and-benthic-2l8o0q83du.pdf

The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs: Considerations based on multiple stable isotope analysis

The stable isotope compositions (C, N, and S) of hve, senescent, and standing dead Spartina alterniflora were compared in order to determine the effects of aerial decomposition on the isotopic signature of aboveground S. dlterniflora tissue entering the food chain. Aerial decomposition of aboveground S. alterniflora resulted in a 6 to 18% increase in P S , and a 2 to 3 % decrease in 615N values; 613c values were unchanged. We describe mechanisms whereby the activity of fungi and epiphytic microorganisms may contribute to the observed shlfts in and 6I5N, respectively. The 613c value of salt marsh benthic and eplphytic microalgae ranged from -13.0% in summer to -17.6%0 in early spring. Average 6'\" values of microalgae and standing dead S. altemiflora were -0 3 and l.?%, respectively, reflecting the activity of NZ-fixing microorganisms. The 615N values for omnivorous and herbivorous salt marsh macrofauna ranged between 7.5 and 2.2% and for predatory Fundulus spp. averaged 9.2%. Given a presumptive + 3 % trophic shift in N assimilation, these results suggest that N2-fixing microorganisms associdted with microalgal communities were an important source of N to salt marsh consumers. The 6% 6'\"N, and 6\"'s values of primary producers were compared to the values of the following consumers: Fundulus spp., Uca spp., Ilyanassa obsoleta, and Littoraria irrorata. 6I3C VS 6 1 5 ~ and 613C vs 63JS dual isotope plots demonstrated that microalgae and standing dead S. alternjflora are important food resources in the North Carolina (USA) marshes we sampled. In addition, a compilation of literature values suggests that this is true in salt marshes throughout the East and Gulf coasts of North America. Future isotope studies of marsh food webs should include detrital Spartina spp. material in analyses of trophic structure.

The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs: considerations based on multiple stable isotope analysis

S. Y. Newell

Papers

The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs: Considerations based on multiple stable isotope analysis

The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs: considerations based on multiple stable isotope analysis