Life cycle and distribution of Calanus finmarchicus in deep basins on the Nova Scotia shelf and seasonal changes in Calanus spp.

Abstract

The deep basins on the Nova Scotia shelf contain high concentrations of Calanus finmarchicus, C. glacialis, and C. hyperboreus at depths below 200 m. From May to late fall these were as high as 20 000 m-3. The dominant species by numbers were C. finmarchicus; however, its biomass was equalled or exceeded by C hyperboreus. The life cycle of C. finmarchicus in the region of 2 of the largest basins showed that breeding started late in winter with a peak in April. High concentrations of Calanus suggested that the basins had higher levels of C. finmarchicus production than the adjacent shelf with mean depths less than 100 m. All 3 species of Calanus started to accumulate below 200 m in May as Stages CIV and CV. C. glacialis and C. hyperboreus did not reproduce in significant numbers on the NW half of the shelf but did accumulate in the basins as they were advected from the NE shelf region. Resting stages of Calanus resided at depths below 200 m in water at between 8.5 and 11.degree. C for an estimated 7 to 8 mo. The fate of these copepods is uncertain, but evidence suggested that a large proportion were preyed on by euphausiids in the deep regions of the basin. The deep basins on the shelf make it possible for C. finmarchicus populations to dominate the Nova Scotia shelf zooplankton community for most of the year; without the basins the shelf zooplankton would probably resemble that of the Grand Banks which has no deep basins, and is dominated by microzooplankton.