Variability of <i>Calanus</i> spp. spatial aggregations in the subarctic shelf and oceanic region using <i>in-situ</i> data and the coupled biophysical ocean model SINMOD

Abstract Calanus finmarchicus is the most studied zooplankton species in the North Atlantic due to its significant trophic role in pelagic ecosystems, especially for the production of many fish species of commercial interest. Our study investigates the spatial dimensions and temporal variability of C. finmarchicus patches in the Lofoten-Vesterålen Basin along a coast–ocean transect, as well as the drivers of their formation, using abundance data from six field campaigns (2015–2023) and simulated high-resolution data from a coupled biophysical ocean model (SINMOD). Recently published results from a meta-analysis of the vertical distribution of C. finmarchicus in the Atlantic Subarctic Province were used in model simulations to improve the representation of the vertical distribution of this species in the Norwegian Sea. Our findings reveal variability between years in patch size, as well as differences in aggregation sizes between coastal patches (1–42 km length, 5–130 m thickness) and offshore patches (1–38 km length, 21–118 m thickness). Model simulations at 1.3 km resolution and observations from the Laser Optical Plankton Counter both reveal the presence of patches, with noticeable differences between shelf and offshore regions. These patches exhibit significant variability over time and space, reflecting the dynamic nature of the system, where mesoscale physical processes seem to play a significant role in shaping patch size and influencing their formation. This research highlights the importance of high-resolution biophysical modelling and in-situ data for accurately modelling zooplankton patchiness.