Using otolith organic matter to detect diet shifts in Bardiella chrysoura, during a period of environmental changes

Abstract

Accurate knowledge on fish trophic ecology and its modifications is crucial for understanding the impact of global change on ecosystems. In this context, we investigated the value of the δ 13 C and δ 15 N of otolith soluble organic matter (SOM) for identifying temporal diet shifts in American silver perch Bairdiella chrysoura over a 30-yr period characterized by strong changes in its population size and habitats within the Terminos Lagoon (Mexico). We first compared the otolith SOM isotopic signatures from present-day adults to those of muscle and the main local prey. Our results suggest that otolith SOM can be confidently extracted and analyzed for both present and past otoliths of this species. The mean otolith SOM signatures obtained (−15.92 ± 1.35 ‰ for δ 13 C and 9.38 ± 0.93 ‰ for δ 15 N) were consistent with those of the diet as 85% of the individual signatures were included within the prey isotopic niche area. Moreover, this study supports a trophic enrichment factor between diet and otolith (TEF diet-otolith ) close to 0 for δ 15 N, while for δ 13 C, the TEF otolith-muscle of + 0.02 ‰ warrants further investigation. Then, we compared past and contemporary otolith SOM signatures to investigate temporal diet shifts in B. chrysoura. This showed that δ 13 C and δ 15 N differed significantly between the past and present period even if the temporal shift remained relatively small (respectively +1.17 ‰ and -0.55 ‰). The present study substantiates the use of otolith SOM δ 13 C and δ 15 N as a proxy of fish present and past trophic position, opening the possibility for major progress in studies of temporal changes in food web ecology.

Assimilation of nitrogen and carbon isotopes from fish diets to otoliths as measured by nanoscale secondary ion mass spectrometry

“…Drinking water containing high concentrations (20-35%) of the heavy isotope deuterium can cause harmful effects or even death in animals, since they cannot metabolize the deuterium due to the high relative mass between 2 H and 1 H ( 2 H/ 1 H = 2). 23,24 However, nitrogen and carbon stable isotopes have displayed no negative effects in animal or clinical use in light of the low relative mass between the heavy and light isotopes ( 15 N/ 14 N = 1.07, 13 C/ 12 C = 1.08). 25,26 The tilapia that were fed the 15 N-spiked algae did not show any abnormal behaviors, such as hyperirritability or convulsions, which have been observed in mice, rats and dogs that drank deuterium water.…”

“…Algae were cultured again in 15-liter plastic tanks for approximately 1 week, then spiked simultaneously with 15 N-labeled ammonium chloride (98 atom%) and 13 C-labeled sodium bicarbonate (98 atom%, Sigma-Aldrich) every day for 8 days using 1 ppm of both chemicals in each tank. The algae were centrifuged, oven-dried and fed to the tilapia for 14 days.…”

“…In addition, the chemical composition, such as stable isotope ratios, of otoliths also provides valuable information on fish ecology . The carbonate‐bound nitrogen isotopic compositions of organic materials in fish otoliths may record information on the food sources, diet changes and trophic levels of fishes . However, a fundamental understanding of otolith nitrogen stable isotopes remains limited and the isotopic assimilation of δ 15 N compositions from diets to otoliths has rarely been investigated.…”

“…Transect analysis with a 12 μm interval between two raster areas covered the newly deposited growth increments when fish ate isotopically spiked algae and partial areas before the feeding experiments. The carbon and nitrogen isotopic ratios were normalized to conventional delta notation (δ13 C and δ 15 N values, respectively) using the innermost area of each transect as a standard. The uncertainty for each raster was estimated by propagation of the standard error.…”

Fractionation of otolith nitrogen stable isotopes measured by peroxodisulfate oxidation‐bacterial conversion and isotope ratio mass spectrometry

“…However, the same mass was insufficient to simultaneously measure the δ 15 N values of the insoluble organic matrix . Sirot et al followed the purification method of Gronkjer et al to measure the δ 15 N oto values of American silver perch ( Bairdiella chrysoura ) using otoliths with masses of 68–155 mg.…”