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

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

Carbon and nitrogen isotope compositions of organic matter in coastal marine sediments (the Gulf of Trieste, N Adriatic Sea): indicators of sources and preservation

Abundant populations of frondose epilithic macroalgae from a variety of carbonate-rich tropical waters were significantly depleted in phosphorus relative to carbon and nitrogen when compared to macroalgae from temperate siliciclastic waters. Percent carbon (C) and percent nitrogen (N) dry weight contents were similar between tissues from the siliciclastic and carbonate environments (means of 22.6% vs. 20.1% and 1.0% vs. 1.2%, respectively), but phosphorus (P) levels were two-fold lower (0.15% vs. 0.07%) in the carbonate-rich systems. Accordingly, the molar C:N tissue ratios were comparable between macroalgae from the siliciclastic and carbonate sites (mean of 29.2 vs. 23.1), whereas large differences were observed for the C:P (mean of 430 vs. 976) and N:P ratios (mean of 14.9 vs. 43.4). In addition, alkaline phosphatase activity was low and often undetectable in the macroalgae from siliciclastic habitats (mean of 7.3 μM PO4
 3− released g dry wt−1 h−1) compared to seven-fold higher rates (52.5 μM PO4
 3− released g dry wt−1 h−1) observed in the macroalgae from carbonate systems. Seawater samples taken adjacent to benthic macroalgae from the carbonate-rich tropical waters contained relatively high levels of dissolved inorganic nitrogen with low concentrations of soluble reactive phosphorus (SRP), and showed elevated N:SRP ratios (mean=36) compared to siliciclastic environments (mean <3). These data support the precept that availability of N limits the productivity of macroalgae in temperate siliciclastic waters but, conversely, suggest that availability of P, rather than N, may be of paramount importance in limiting primary production of macroalgae in carbonate-rich tropical waters.

Nutrient availability to marine macroalgae in siliciclastic versus carbonate-rich coastal waters

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kutz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m−2 in some spots, corresponding to a thallus area index of nearly 17 m2Ulva·m−2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self-shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.

J. Faganeli

Papers

C:N:P ratios and stable carbon and hydrogen isotopes in the benthic marine algae, Ulva rigida C.Ag. and Fucusvirsoides J.Ag