Fractionation of stable isotopes in the Arctic marine copepod Calanus glacialis: Effects on the isotopic composition of marine particulate organic matter

Fractionation of δ13C and δ15N between food, consumer, and faecal pellets was studied in the Arctic marine copepod Calanus glacialis Jaschnov, fed with isotopically distinct algal monocultures. Temporal variations in δ13C and δ15N of copepods that were fed ice algae and phytoplankton followed those of a control group consisting of starved animals. There were no significant trends in the δ13C and δ15N values of copepods that were starved for 42 days, suggesting that the isotopic composition of non-lipid body tissues is unaffected by the metabolic processes during prolonged periods of starvation. The stable isotopic composition of starved copepods therefore seems to reflect food consumed during the previous period of feeding and growth. Faecal pellets produced by feeding copepods were depleted in 13C and 15N by 6.3-11.2‰ and 0.7-9.1‰, respectively, relative to the food ingested. These results indicate that faecal pellet production is an important pathway for the trophic fractionation of δ13C, whereas other fractionation pathways, such as excretion of ammonia, may be relatively more important for δ15N. The strong depletion of 13C in faecal pellets compared to the food suggests that grazing by herbivorous copepods on primary production adds to the variability of δ13C in marine particulate organic matter.