Feeding level and diet quality influence trophic shift of C and N isotopes in Nile tilapia (Oreochromis niloticus (L.))

Many scientists use naturally occurring stable isotopes to reconstruct the diets of animals. However, isotopic ratios may be affected not only by the composition of the diet but also by the amount of food consumed. Thus, an experiment using tilapia (Oreochromis niloticus) was carried out to test the influence of feeding level on delta13C and delta15N of fish given a semi-synthetic wheat gluten/wheat starch based diet. In addition, the effect of diet quality was tested by comparing tilapia given this feed with tilapia fed a fish meal/wheat meal based diet. Forty-four tilapia were reared individually. After a prefeeding phase, fish were randomly assigned to five groups, four on the semi-synthetic diet at different feeding levels and one group on the fish meal/wheat meal based diet fed at the equivalent of the highest level of the semi-synthetic diet. The experiment lasted eight weeks. Proximate composition, gross energy content and delta13C and delta15N values were determined in feed and fish, for delta13C separately in the lipids and the lipid-free matter. Delta13C in the lipids and the lipid-free matter and delta15N of tilapia fed the semi-synthetic diet decreased significantly with increasing feeding rate. The absolute values of the trophic shift in fish fed the semi-synthetic wheat based diet were significantly higher than in fish fed the fish meal/wheat meal based diet. The different delta13C and delta15N values in tilapia fed the same diet at different feeding levels and the influence of feed quality on the trophic shift add to the uncertainty involved in the use of stable isotopes in ecological research.     

Tracing the pathways of Neotropical migratory shorebirds using stable isotopes: A pilot study

We evaluated the potential use of stable isotopes to establish linkages between the wintering grounds and the breeding grounds of the Pectoral Sandpiper (Calidris melanotos), the White-rumped Sandpiper (Calidris fuscicollis), the Baird's Sandpiper (Calidris bairdii), and other Neotropical migratory shorebird species (e.g., Tringa spp.). These species molt their flight feathers on the wintering grounds and hence their flight feathers carry chemical signatures that are characteristic of their winter habitat. The objective of our pilot study was to assess the feasibility of identifying the winter origin of individual birds by: (1) collecting shorebird flight feathers from several widely separated Argentine sites and analyzing these for a suite of stable isotopes; and (2) analyzing the deuterium and ¹⁸O isotope data that were available from precipitation measurement stations in Argentina. Isotopic ratios (δ¹³C, δ¹⁵N and δ³⁴S) of flight feathers were significantly different among three widely separated sites in Argentina during January 2001. In terms of relative importance in separating the sites, δ³⁴S was most important, followed by δ¹⁵N, and then δ¹³C. In the complete discriminant analysis, the classification function correctly predicted group membership in 85% of the cases (jackknifed classification matrix). In a stepwise analysis δ¹³C was dropped from the solution, and site membership was correctly predicted in 92% of cases (jackknifed matrix). Analysis of precipitation data showed that both δD and δ¹⁸O were significantly related to both latitude and longitude on a countrywide scale (p < 0.001). Other variables, month, altitude, explained little additional variation in these isotope ratios. Several issues were identified that will likely constrain the degree of accuracy one can expect in predicting the geographic origin of birds from Argentina. There was unexplained variation in isotope ratios within and among the different wing feathers from individual birds. Such variation may indicate that birds are not faithful to a local site during their winter stay in Argentina. There was significant interannual variation in the δD and δ¹⁸O of precipitation. Hence, specific locations may not have a constant signature for some isotopes. Moreover, the fractionation that occurs in wetlands due to evaporation significantly skews local δD and δ¹⁸O values, which may undermine the strong large-scale gradients seen in the precipitation data. We are continuing the research with universities in Argentina with a focus on expanding the breadth of feather collection and attempting to resolve the identified issues.     

Temporal diet changes recorded by stable isotopes in Asiatic black bear (Ursus thibetanus) hair

Carbon and nitrogen stable isotope ratios were measured in hair samples of the Asiatic black bear (Ursus thibetanus) inhabiting the Northern Japanese Alps (NJA) (n?=?20) and the periphery of Nagano City (NC) (n?=?6), in Nagano Prefecture, Japan. The hair of NJA bears, which did not have access to anthropogenic foods, showed lower values of δ¹³C and δ¹⁵N than that of NC bears which had access to garbage and corn fields, especially during the summer. These results reflect somewhat differing diets between the NJA and NC bears. We attempted to assess the feeding history during the hair growth cycle using the growth section analysis method. Each hair sample had been cut into 3?mm lengths from root to tip, labeled, and analyzed along the hair growth. We measured the carbon and nitrogen stable isotope ratios of each 3?mm length of hair sample from one NC bear which had been killed while raiding a corn field. The sections showed wide ranges of isotope ratios, from ?23.2‰ to ?14.6‰ for δ¹³C, and from 0.3‰ to 4.6‰ for δ¹⁵N. It was shown that the diet of this bear shifted dramatically from principally C₃ plants to more C₄ plants and to foods of animal origin. An analysis of the whole hair reflects just the average feeding habit during hair growth, but the present method can trace its diet history. This method can contribute to obtain precise ecological information of wildlife.     

Nutrient cycling responses to fire frequency in the Kruger National Park (South Africa) as indicated by Stable Isotope analysis

Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N 2 fixation by legumes balances the N losses caused by fires. In this study, the N 2 fixation activity of woody legumes was estimated by analyzing foliar i 15 N and proportional basal area of N 2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N 2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N 2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N 2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil i 15 N and fire frequency found in this study indicates that the effects of fires on ecosystem i 15 N are unpredictable. Similar soil i 15 N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases i 15 N) and N 2 fixation other than that associated with woody legumes (which lowers i 15 N) on isotopic signatures.     

Isotope variations in white-tailed kites from various habitats in California: Possible limitations in assessing prey utilization and population dynamics

White-tailed kite (Elanus leucurus) populations in the 1930s were close to extirpation in the United States. But by the 1940s, an upward trend towards recovery was apparent and continued to their current stable population levels. These dramatic fluctuations in kite numbers may have been related to changes in rodent prey populations due to the conversion of native habitats to agriculture. To address this question, we evaluated the use of stable isotope analysis in determining if a shift in diet could be isotopically differentiated in current and historic kite populations. We first compared δ ¹³C, δ ¹⁵N, and δ ³⁴S values from present-day kite flight feathers and prey fur samples from four locations in California. The total ranges of isotope values for kite and their rodent prey were similar within each site. Carbon isotope values ranged from m 27.1 to m 22.2 in Arcata, m 26.1 to m 16.9 in Davis, m 27.0 to m 15.0 in Cosumnes, and m 28.2 to m 11.6 in Santa Barbara. Nitrogen isotope values ranged from 3.2 to 15.7 in Arcata, 2.8 to 12.7 in Davis, 4.0 to 15.7 in Cosumnes, and 1.7 to 20.0 in Santa Barbara. Sulfur isotope values ranged from m 7.8 to 12.4 in Arcata, m 1.1 to 9.2 in Davis, 0.7 to 10.9 in Cosumnes, and m 8.6 to 15.6 in Santa Barbara. Carbon, nitrogen, and sulfur isotope values at each site reflect typical trophic enrichments due to physiological processes. At each site, δ ¹³C and δ ¹⁵N values reflected the influence of a predominantly C₃ or a mixed C₃/C₄ plant community. Sulfur isotope values reflect the influence of predominant marine or terrestrial sulfur sources at each site. However, variability in isotope values may limit the usefulness of such analyses for addressing prey utilization and population dynamics.     

A Dual 13C and 15N Long Term Labelling Technique to Investigate Uptake and Translocation of C and N in Beech (Fagus sylvatica L.)

Abstract

A continuous dual ¹³CO₂ and ¹⁵NH₄ ¹⁵NO₃ labelling experimental set-up is presented that was used to investigate the C and N uptake and allocation within 3-year old beech (Fagus sylvatica L.) during one growing season. The C and N allocation pattern was determined after six, twelve and eighteen weeks of growth. The carbon uptake was distinctly different in the three phases examined: The first six weeks after budbreak were dedicated to leaf growth with a R/S (root to shoot) ratio of 0.14 for the new carbon. The second growth phase showed a balanced R/S ratio of C allocation and after week 13, the root compartment was the main carbon sink (R/S = 6.97).

Nitrogen allocation was more basipetal as compared to carbon. In the second growth phase, R/S of N_new was 5.57 but fell to 3.54 for the third growth phase probably due to formation of reserves in buds and stem.