Aggregation of the Arctic copepod Calanus hyperboreus over the ocean floor of the Greenland Sea

According to combined observations from vertical plankton tows, dredging with epibenthic nets 1 m above the ocean floor, video recordings and acoustic data from a scanning sonar obtained during descent and during deployment on the ocean floor, the calanoid copepod Calanus hyperboreus was aggregated in high concentrations near the ocean floor of the Greenland Sea between 2300 and 2500 m during late July and August. Concentrations were highest very close to the ocean floor and decreased rapidly further upward. These nearly mono-specific aggregations were apparently drifting in cloud-like formations with a horizontal extension of ca. 270 m with the near-bottom currents. Maximum abundances observed were up to 2 orders of magnitude higher than in the water column. The biomass in the bottom 20 m layer was around 18% of the biomass in the rest of the water column. Stage composition, reduced metabolic rates and insensibility to mechanical stimuli indicate that these C. hyperboreus were representing the resting population. The fact that high concentrations were observed during deployments lasting >1 d and in 3 years suggests that aggregation near the ocean floor is a regular, rather than an extraordinary, pattern in the life history of C. hyperboreus in the Greenland Sea, but there is need for comparison with other seas and eventually other Calanus species.