Stable carbon isotope fractionation in the marine copepod Temora longicornis : Unexpectedly low δ13C value of faecal pellets

Abstract

C fractionation effects in an experimental food chain were determined by performing a series of mesocosm experiments with the copepod Temora longicornis and using flagellates as food. The bulk copepods were enriched in 13C by 1.2 to 2.3 ‰ and the CO2 respired was enriched by 0.8 ‰ , compared to the isotopic composition of the food. Faecal pellets, however, were strongly depleted in 13C by 4.3 to 11.3 ‰ , in isotopic mass balance with the 13C enrichment of the copepod body and of the respired carbon dioxide. Compound-specific carbon isotope analyses indicated that solvent extractable sterols and alkenones were isotopically light in carbon in both the copepod body and in the faecal matter. These lipids reflected the isotopic nature of the consumed food, indicating that they are not fractionated in the copepod. The residual fraction of the faecal pellets (after extraction) showed the largest isotopic change, being depleted in 13C by about 16 ‰ compared to the same fraction in the diet. Curie point pyrolysis analyses indicated that proteins were the major constituents of this residual material from both the copepods and their faecal pellets. When food sources of different isotopic composition were alternately supplied to the copepods, the faecal pellet residue showed a relatively slow turnover rate compared to the alkenones, which were completely egested by the copepods. A similarly slow turnover rate was observed in the residual material from copepods, suggesting that it must be proteins that are being fractionated by the copepod T. longicornis. The low d13C value of faecal pellets adds another variable to the stable carbon isotopic signature of particulate organic carbon in the pelagic environment.