Interannual variation in diapausing copepods and associated water masses in a continental shelf basin, and implications for copepod buoyancy

Abstract

Oceanographic surveys were conducted in Roseway Basin, western Scotian Shelf, during late-summer from 2007 through 2009 to measure the magnitude of interannual variation in the spatial distribution of diapausing copepods Calanus finmarchicus and C. hyperboreus and associated water mass characteristics. Calanus spp. abundance, energy density and hydrography were measured at depths >50m along transects using a Towed Underwater Biological Sampling System equipped with an Optical Plankton Counter (OPC) and a conductivity-temperature-depth (CTD) sensor, as well as at fixed stations using a Biological Net and Environmental Sampling System equipped with nets, OPC and CTD. Water mass density and in some cases salinity explained variation in the deep copepod layer across both time and space, whereas temperature did not. Water mass density, copepod energy density and thickness of the copepod layer were statistically lower during 2008 than 2007 or 2009. The copepod layer was absent from the western Basin margin during 2008 where low density continental water resided that year, whereas during 2007 and 2009 higher density continental slope water and copepods were each present along the western margin. Our results suggest that water mass density is an important characteristic defining the spatial and interannual ecology of the deep copepod layer in Roseway Basin. The 26 σt isopycnal may be a lower density limit to diapausing Calanus spp. habitat on continental shelves with shallow bathymetry, that helps the animals maintain neutral buoyancy during diapause.