Spatial variation and transport of abundant copepod taxa in the southern Gulf of St. Lawrence in autumn

Abstract The spatial distribution of zooplankton is influenced by complex interactions among population dynamics, vertical positioning and advection. We used empirical observations and particle tracking simulations to investigate variation in horizontal distribution of Calanus spp. and three neritic copepod taxa (Acartia spp., Pseudocalanus sp. and Temora longicornis) in the southern Gulf of St. Lawrence (sGSL) in October 2018. These copepods are potential prey for fish and the endangered North Atlantic right whale, Eubalaena glacialis. We quantified horizontal variation in abundance from a plankton survey, and obtained vertical distributions from a subset of locations and a process study conducted over ~1 day. Horizontal distributions of Calanus finmarchicus and Pseudocalanus sp. were widespread, whereas those of Calanus hyperboreus and C. glacialis were centred in the interior of the sGSL and those of Acartia spp. and T. longicornis were centred on the western side. Horizontal distributions of C. hyperboreus–C. glacialis and C. finmarchicus–Pseudocalanus sp. pairs were highly correlated. Contrasting distributions among C. finmarchicus and its congeners reflect interspecific differences in life history strategy and its interaction with ocean circulation. In autumn, a shift to increased current strength and flow-through circulation may lead to enhanced losses of Calanus spp. in the absence of upstream replenishment.