Seasonal variations in population dynamics of Calanus finmarchicus in relation to environmental conditions in the southwestern Norwegian Sea

A seasonal study of the life cycle and reproductive biology of the copepod Calanus finmarchicus – the dominant zooplankton species and a key link between the lower and higher trophic levels within the southwestern Norwegian Sea - is presented. C. finmarchicus reproduces, feeds and accumulates lipids during spring and summer within surface waters, which enables it to survive overwintering at depth until the following spring. An inflow of warm and saline Atlantic Water (AW) enters from the southwest and meets the cold and less saline subarctic waters (SAW) flowing from the north and west. This confluence establishes the dynamic Iceland Faroe Front. The study includes seven cruises, conducted from July 2013 to July 2014. Ascent occurred earlier and over a prolonged period in AW compared to the SAW. The peak egg production per capita occurred in April (43 eggs female−1 d−1) and May (13 eggs female−1 d−1) in SAW and AW, respectively. The higher productivity in SAW can be explained by larger female size, increased lipid content and better feeding conditions. The increased egg production rates in May in AW were not reflected in an increased abundance of young copepodite stages in June, potentially due to a decoupling between early ascent and the late onset of the bloom. Descent, largely comprised by the pre-adult stages, to overwintering depths was already initiated in June in both regions. This coincides with increased lipid content, being significantly higher at depth compared to the surface in late summer. A smaller portion of individuals remained within the upper layers to spawn a second generation in August in both regions, increasing the abundance and biomass. The reproductive phenology and stage development of C. finmarchicus likely influence feeding and competition between herring and mackerel and the duration of their stay in Faroese waters. •C. finmarchicus ascend earlier in warm Atlantic Water than in cold Subarctic Water.•Females show increased egg production capacity within the subarctic region.•Increased female size and better feeding conditions are seen in the subarctic region.•Initiation of descent was observed as early as in June in both regions.•Descended individuals had higher lipid content than individuals at the surface.