Dynamical Controls of the Eastward Transport of Overwintering Calanus finmarchicus From the Lofoten Basin to the Continental Slope

Diapausing populations of Calanus finmarchicus at depth in the Lofoten Basin (LB) return to the continental shelf and slope off the Lofoten-Vesteralen Islands during the phytoplankton spring bloom to feed and spawn, forming surface swarms with a great abundance. To study how overwintering populations of C. finmarchicus move with the deep currents and return to the shelf, Lagrangian transport characteristics of particles in deep water between 2008 and 2019 were analyzed using Global Ocean Reanalysis and Simulation re-analysis data and Lagrangian Coherent Structures (LCSs). Our analyses revealed that persistent eastward transport of diapausing C. finmarchicus between LB and continental slope occurred mainly between 600 and 1,100 m in the Arctic Intermediate Water. The consistency of the vertical distributions of C. finmarchicus abundance and salinity further suggests that physical factors control the horizontal distribution of the species. Hovmoller diagrams of kinetic energy indicate that there is an eastward advection of mean current at depth. The co-occurrence between the eastward transport of LCSs and the eastward advection of the mean current provides direct evidence that the life history of C. finmarchicus is subjected to physical control in the Norwegian Sea. Plain Language Summary Overwintering populations of the copepod Calanus finmarchicus in the Lofoten Basin (LB) are critical for maintaining its life cycle and food web in the northern Norwegian Sea. Horizontal advection and particle tracking simulation between 2008 and 2019 were performed using reanalysis data. Our analyses revealed that eastward transport between 600 and 1,100 m in the Arctic Intermediate Water below the Atlantic Water was responsible for moving C. finmarchicus from the LB to the continental slope. The consistency of the vertical distributions of C. finmarchicus and salinity further suggests that physical factors control the horizontal distribution of the species. The trend of decreasing abundance of C. finmarchicus from west to east in January results from the eastward transport of the species and its accumulation on the continental slope. Eastward transport from the basin to the slope is a key process in the life cycle of C. finmarchicus and demonstrates the importance of large-scale physical-biological interactions in Nordic Seas.