Diel changes in the carbon and nitrogen content of the copepod Metridia lucens

Abstract

Diel changes in the near-surface (0-50 m) abundance, prosome length, and carbon and nitrogen content of the copepod Metridia lucens were measured in Deep Cove, Doubtful Sound, New Zealand (45°27'S, 167°9'E) between 3 and 6 September 1996. Metridia lucens showed maximal abundance at night, suggesting a pattern of normal diel vertical migration (DVM). The change in abundance suggested that the descent of the population occurred ~1 h prior to dawn and the ascent ~1 h after dusk. However, a proportion of the population remained near the surface during the day. Although there was no diel pattern in the prosome length of M.lucens collected near the surface, there was a marked diel cycle in the measured carbon and nitrogen contents, with maximal values being measured towards the end of the night prior to the downward migration. We suggest that this diel cycle in the measured elemental content was caused by DVM occurring more strongly in those animals which had a better body condition, i.e. a higher elemental content per unit length.