Zooplankton Community Dynamics in the Norwegian Sea

Abstract

The copepod Calanus finmarchicus is an abundant species in the Northern Atlantic Ocean, including in the Norwegian Sea, where it accounts for a large part of the biomass of the zooplankton community. As a primary consumer, it is an important link between phytoplankton and higher trophic levels such as fish, sea birds or sea mammals. It also contributes substantially to biogeochemical cycles through its seasonal and diel vertical migrations. Illumination or water column characteristics play a major role in the vertical distribution of Calanus finmarchicus across the Norwegian Sea, yet precise mechanisms controlling its the vertical position are still poorly understood. Using zooplankton samples digitized using a waterproof scanner, ZooScan, I documented the spatial distribution of Calanus finmarchicus, compared two methods to evaluate its biomass, and studied its size distribution in response to environmental gradients within the study area. Here, I show that the affiliation of Calanus finmarchicus to coastal or oceanic waters is paramount to explain its horizontal distribution in late May-early June, and that its vertical distribution is primarily correlated to the amount of phytoplankton available. These results suggest that knowledge of the environmental conditions is paramount to understanding Calanus finmarchicus life-history, though a strong spatial variability in its abundance and biomass exists.