Seasonal variations in fish delta13C and delta15N in two West African reservoirs, Sélingué and Manantali (Mali): modifications of trophic links in relation to water level

Most regions in the tropics undergo high seasonal precipitation that produces cyclic patterns of riverine discharge, resulting in periods characterized by low and high water levels. Many chemical and bio-logical factors are affected by this hydrologic seasonality, and it therefore appeared to be very likely that aquatic food webs would also differ during the low and high water periods. Available carbon sources for fish are thought to be less varied during low water periods, but flooding during high water periods could bring fish into contact with a greater abundance and diversity of food sources such as terrestrial plants or the biofilms that grow on submerged terrestrial plants. At low water levels, higher fish densities may lead to more piscivory and less omnivory when compared with the high water periods. Therefore, trophic links within the fish communities may then be modified by water level changes in tropical reservoirs. To address this prediction, we performed stable isotope analyses of the most common species in Sélingué and Manantali, two large reservoirs in Mali (West Africa). Allochthonous and littoral carbon sources were shown to support fish production to a significant extent, even during low water periods. However, the allochthonous or littoral carbon contributions that sustained the top-predators production were indeed greater during the high water periods as expected. The expected higher omnivory in the high water period might have shortened the food chain when compared with the low water period. Some carnivorous fish species were shown to feed at lower trophic levels during high water periods in both reservoirs, but this was not a general pattern. Flooding did not, therefore, necessarily result in a shorter food chain when water levels were high.     

Seasonal variation in natural abundance of δ13C and 15N in Salicornia brachiata Roxb. populations from a coastal area of India

High and fluctuating salinity is characteristic for coastal salt marshes, which strongly affect the physiology of halophytes consequently resulting in changes in stable isotope distribution. The natural abundance of stable isotopes (δ¹³C and δ¹⁵N) of the halophyte plant Salicornia brachiata and physico-chemical characteristics of soils were analysed in order to investigate the relationship of stable isotope distribution in different populations in a growing period in the coastal area of Gujarat, India. Aboveground and belowground biomass of S. brachiata was collected from six different populations at five times (September 2014, November 2014, January 2015, March 2015 and May 2015). The δ¹³C values in aboveground (?30.8 to ?23.6?‰, average: ?26.6?±?0.4?‰) and belowground biomass (?30.0 to ?23.1?‰, average: ?26.3?±?0.4?‰) were similar. The δ¹³C values were positively correlated with soil salinity and Na concentration, and negatively correlated with soil mineral nitrogen. The δ¹⁵N values of aboveground (6.7–16.1?‰, average: 9.6?±?0.4?‰) were comparatively higher than belowground biomass (5.4–13.2?‰, average: 7.8?±?0.3?‰). The δ¹⁵N values were negatively correlated with soil available P. We conclude that the variation in δ¹³C values of S. brachiata was possibly caused by soil salinity (associated Na content) and N limitation which demonstrates the potential of δ¹³C as an indicator of stress in plants.     

Field-deployable measurements of free-living individuals to determine energy balance: fuel substrate usage through δ13C in breath CO2 and diet through hair δ13C and δ15N values

Carbon isotopes of breath CO₂ vary depending on diet and fuel substrate used. This study examined if exercise-induced δ¹³C-CO₂ changes in substrate utilization were distinguishable from baseline δ¹³C-CO₂ variations in a population with uncontrolled diet, and compared hair isotope values and food logs to develop an isotope model of diet. Study participants included nine women with diverse Body Mass Index (BMI), age, ancestry, exercise history, and diet. Breath samples were collected prior to and up to 12?h after a 5- or 10?K walk/run. Indirect calorimetry was measured with a smartphone-enabled mobile colorimetric device, and a field-deployable isotope analyzer measured breath δ¹³C-CO₂ values. Diet was assessed by food logs and δ¹³C, δ¹⁵N of hair samples. Post-exercise δ¹³C-CO₂ values increased by 0.54?±?1.09‰ (1 sd, n?=?9), implying enhanced carbohydrate burning, while early morning δ¹³C-CO₂ values were lower than daily averages (p?=?0.0043), indicating lipid burning during overnight fasting. Although diurnal δ¹³C-CO₂ variation (1.90?±?0.77‰) and participant baseline range (3.06‰) exceeded exercise-induced variation, temporal patterns distinguished exercise from dietary isotope effects. Hair δ¹³C and δ¹⁵N values were consistent with a new dietary isotope model. Notwithstanding the small number of participants, this study introduces a novel combination of techniques to directly monitor energy balance in free-living individuals.     

The relative isotopic abundance (δ13C, δ15N) during composting of agricultural wastes in relation to compost quality and feedstock

Variations in the relative isotopic abundance of C and N (δ¹³C and δ¹⁵N) were measured during the composting of different agricultural wastes using bench-scale bioreactors. Different mixtures of agricultural wastes (horse bedding manure?+?legume residues; dairy manure?+?jatropha mill cake; dairy manure?+?sugarcane residues; dairy manure alone) were used for aerobic–thermophilic composting. No significant differences were found between the δ¹³C values of the feedstock and the final compost, except for dairy manure?+?sugarcane residues (from initial ratio of ?13.6?±?0.2?‰ to final ratio of ?14.4?±?0.2?‰). δ¹⁵N values increased significantly in composts of horse bedding manure?+?legumes residues (from initial ratio of +5.9?±?0.1?‰ to final ratio of +8.2?±?0.5?‰) and dairy manure?+?jatropha mill cake (from initial ratio of +9.5?±?0.2?‰ to final ratio of +12.8?±?0.7?‰) and was related to the total N loss (mass balance). δ¹³C can be used to differentiate composts from different feedstock (e.g. C₃ or C₄ sources). The quantitative relationship between N loss and δ¹⁵N variation should be determined.     

Effect of different ratios of wheat to corn flour in the diet on the development and isotopic composition (δ13C, δ15N) of the red flour beetle Tribolium castaneum

The back-calculation of the diet is a common application of stable isotopes in animal ecology. The method is based on a predictable relation between the isotopic signature of the diet and the animal’s tissues. Frequently, the assumption of a constant difference in isotopic signatures (trophic shift) is made. Carbon isotopic ratios of C₃ and C₄ plants differ by approximately 10 ‰, making wheat (C₃-plant) and corn (C₄-plant) ideal materials for isotopic studies in nutritional ecology and especially for testing the back-calculation method. In this experiment, red flour beetles, Tribolium castaneum, were reared on wheat flour, corn flour and three different mixtures thereof, either in pure flour or with the addition of yeast inoculum or yeast grains. Development of T. castaneum on these experimental diets was monitored, and isotopic signatures of carbon and nitrogen in emerging adults were analysed. The values of trophic shift of C and N isotopes for wheat and corn flour were different, and the values for the mixtures did not correspond to those expected from a linear mixing model. The latter can be taken as an indication that the tiny larvae of T. castaneum may be capable of differentiating between particles of wheat and corn flour, making this animal model unsuitable for testing the back-calculation method.