Respiration and lipid content of the Arctic copepod Calanus hyperboreus overwintering 1 m above the seafloor at 2,300 m water depth in the Fram Strait

In August 2000 high concentrations of the dominant herbivorous copepod Calanus hyperboreus were detected in the Arctic Fram Strait, west of Spitsbergen, I in above the seafloor at 2,290 in water depth. Individuals from that layer were sampled by a hyperbenthic net attached to the frame of an epi-benthic sledge. For comparison, the vertical distribution of C. hyperboreus in the water column was studied simultaneously by a multiple opening/closing net haul from 2,250 in depth to the surface. Maximum abundance was found close to the surface with 6.6 and 10.0 ind. m(-3) at 0-50 in and 50-100 in depth, respectively. However, the major fraction of the population (>40%) occurred between 1,000 and 1,500 in depth. In the deepest layer (2,000-2,250 in) abundance measured 2.2 ind. m(-3) and was twice as high as between 100 and 1,000 in depth. In comparison to individuals from surface waters, copepods from the hyper-benthic layer were torpid and did not react to mechanical stimuli. Stage CV copepodids and females from the deep sample contained 4-10% less lipid and showed significantly reduced respiration rates of 0.24 and 0.26 ml O-2 h(-1) g(-1) dry mass (DM) as compared to surface samples (0.49 and 0.43 ml O-2 h(-1) 9 1 DM). All these observations indicate that the hyperbenthic part of the population had already started a dormant overwintering phase at great depth. Based on the lipid deposits and energy demands, the potential maximum duration of the non-feeding dormant phase was estimated at 76-110 days for females and at 98-137 days for CV copepodids, depending on what indispensable minimum lipid content was assumed. In any case, the estimated times could not meet the necessary requirements for a starvation period of > 6 months until the next phytoplankton bloom in the following spring. The ecological implications of these results are discussed with respect to the life cycle and eco-physiological adaptations of C. hyperboreus to its high-Arctic habitat.