Diel changes in vertical distribution and feeding activity of copepods in ice-covered Resolute Passage, Canadian Arctic, in spring 1992

Abstract

Abstract To examine the role of copepods in the marine ecosystem under sea ice, diel changes in their vertical distribution and feeding activity were observed in May 1992 in Resolute Passage, Canadian Arctic. Copepods were numerically abundant, making up 98% of the zooplankton assemblage. Pseudocalanus was the dominant copepod, accounting for 92% of the copepods in number. All stages, particularly copepodid stages IV and V of Pseudocalanus, showed upward migration at night, concentrating just below the ice between 2000 and 2300 h and descending into the deeper water around midnight. Gut pigment content of Pseudocalanus just below the ice showed diel changes, with higher values in the day than at night. Gut evacuation rates were independent of the initial gut pigment contents. The ingestion rate of the Pseudocalanus population was high at 2000 h in the top 0–1 m layer under the ice, reaching more than 20 μg pig m−3 h−1. In the layers deeper than 3 m, the ingestion rates of the population was relatively high around 1800–8000 h and 0600 h, with a maximum of nearly 0.5 μg pig.m−3 h−1. These higher ingestion rates did not extend throughout the night in the whole water column. Cumulative daily ingestion rates for the population in the top 0–3 m and the whole water column were calculated to be 0.30 and 2.57 mg pig.m−2 d−1, respectively. Similarly, consumption of the daily ice algal production was estimated to be 4.2% in the upper water column and 36.8% for the total water column. Scanning electron microscopic observations of in situ gut contents revealed that ice algal diatoms were not the nutritive sources for Pseudocalanus. We suggest that naked autotrophs could be an important food source for copepods under the ice in Resolute Passage in the spring.