ascent begins in April, and descent in July. For the. C. hyperboreus population an active downward trans- port of 8.1 g mA2 dry weight during 8 months of over-.
Marine Biology (1997) 128: 607±618
Ó Springer-Verlag 1997
H.-J. Hirche
Life cycle of the copepod Calanus hyperboreus in the Greenland Sea
Received: 25 January 1997 / Accepted: 11 February 1997
Abstract The seasonal ontogenetic migration of the Arctic copepod Calanus hyperboreus was described from surface-to-bottom hauls in the central Greenland Sea Gyre (GSG) and in the Westspitsbergen Current (WSC). All stages except females spent the winter below 500 m in the GSG and below 1000 m in the WSC. Seasonal ascent begins in April, and descent in July. For the C. hyperboreus population an active downward transport of 8.1 g m)2 dry weight during 8 months of overwintering was estimated, similar to ¯ux rates of particulate matter in sediment traps. Seasonal distribution of biomass was determined from weight measurements of single stages. Annual means varied from 4.0 to 9.2 g m)2 in two dierent years in the GSG and were 1.1 in 1 year in the WSC. The life cycle in the Greenland Sea was reconstructed from ®eld data on stage composition, vertical distribution, reproduction, and moult cycle phase from tooth development of CV. Laboratory experiments were conducted on the moulting of CIV and CV in fall. A 3-year (males) and 3- to 4-year (females) life cycle is proposed for the GSG and 2 to 3 years for the WSC. However, the small number of young larvae and the incomplete spring ascent by older copepodites observed in the WSC cast doubt on the reproductive success in the WSC. A suite of physiological strategies and adaptations performed by the developmental stages support survival of this species in harsh environments.
Introduction Due to its abundance and size the planktonic copepod Calanus hyperboreus Krùyer is a key species in subarctic and arctic plankton communities. Its main areas of distribution are the Greenland Sea (Hirche 1991) and Communicated by O. Kinne, Oldendorf/Luhe H.-J. Hirche Alfred Wegener Institute for Polar and Marine Research, Columbusstraûe, D-27568 Bremerhaven, Germany
Ban Bay together with the Canadian Archipelago (Conover 1988; Conover and Siferd 1993), from where it spreads south into the Norwegian Sea and Gulf of Maine (Conover 1962, 1988) and north into the Arctic Ocean (Hirche and Mumm 1992; Mumm 1993). Yearround information on its stage distribution is sparse from open ocean locations. Estimates of generation time vary between 1 and at least 4 years depending on location and sample availability (for a review see Smith and Schnack-Schiel 1990). In the Arctic Ocean according to Harding (1966), Dawson (1978) and Pavshtiks (1983) C. hyperboreus has at least a 3-year life cycle. Conover (1988) and Conover and Siferd (1993) suggested a 3- to 4-year life cycle for the Canadian Arctic. In contrast, shorter generation times of 1 to 2 years were reported by Smith (1990) for Fram Strait, by Ussing (1938) for Scoresby Sound (East Greenland), by Grainger (1959) for the Canadian Arctic, by Matthews et al. (1978) for fjords in southwestern and by Wiborg (1954) in northwestern Norway, and by Sùmme (1934) and éstvedt (1955) for the Norwegian Sea. Calanus hyperboreus undergoes a pronounced seasonal ontogenetic migration and spends the winter in diapause (Head and Conover 1983; Head and Harris 1985; Hirche 1989). Timing of the ascent is of enormous importance for the development of the spring phytoplankton bloom: ascent of a large biomass before or concurrent with the onset of phytoplankton development should strongly control the spring bloom. During their seasonal migrations copepods translocate a large biomass accumulated in the euphotic zone during the productive season between the surface layers and resting depths in spring and autumn. Longhurst and Harrison (1989) suggested that in high-latitude plankton communities with a large fraction of herbivorous copepods the removal of organic matter by seasonal ontogenetic migration would be substantial. The loss of biomass at overwintering depths is due to metabolic processes and predation. The aim of the present study was to describe the seasonal ontogenetic migration of Calanus hyperboreus,
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to analyse its life cycle and to assess the biomass of its population in the two main hydrographic domains of the Greenland Sea, the cold Arctic water of the central Greenland Sea Gyre (GSG), and the warmer Atlantic water of the Westspitsbergen Current (WSC). From stage composition observed in late fall (Hirche 1991) a multi-year generation time was expected with most of the older stages always present. This makes understanding of the dynamics of development during the seasonal cycle from stage distributions alone very dicult. Therefore additional experiments on the moulting of older copepodite stages were conducted and the molt cycle phase of copepodite stage V (CV) was evaluated from tooth development according to Miller and Nielsen (1988). Since teeth are the part of the copepod exoskeleton requiring the most time to form, their state of development gives an early indication for forthcoming ecdysis and allows one to distinguish between active and diapausing specimens.
Materials and methods Description of study area In the Greenland Sea the warm, salty Atlantic water of the WSC is separated from the cold, less saline waters of the GSG by the Arctic Front, a permanent front extending from Jan Mayen to Spitsbergen. In the GSG during most of the year temperatures are below )1 °C over the whole water column. With the formation of the Isodden (Wadhams et al. 1993), a layer of new ice covered the GSG
in the winter of 1988/1989 which had melted by the end of April (Ramseier personal communication). In early summer a shallow lens of low salinity water at the surface warmed up and reached temperatures as great as 0.3 °C (Table 1). In 1993 the highest temperatures were 5 °C at the surface, but
