Seasonal development of mixed layer depths, nutrients, chlorophyll and Calanus finmarchicus in the Norwegian Sea – A basin-scale habitat comparison

Seasonal patterns for mixed layer depths, nutrients, chlorophyll, and Calanus finmarchicus in different water masses between 62 and 70°N of the Norwegian Sea were compared using spatiotemporally aggregated basin-scale data. Norwegian Coastal Water was stratified throughout the year due to a low-salinity upper layer. The winter mixed layer depth was typically about 50–60 m, and the spring phytoplankton bloom peaked in late April. In Atlantic and Arctic Waters the winter mixed layer depths were much greater, typically about 175–250 m. Due to the requirement for thermal stratification, the phytoplankton build-ups there were slower and the peaks were delayed until late May. Seasonal development of mixed layer depths, nutrient consumption and chlorophyll was similar for the Atlantic and Arctic areas. Young Calanus copepodites of the first new generation in Coastal Water peaked in early May, preceding the peak in Atlantic Water by about 2 weeks, and that in Arctic Water by about 6 weeks. While the young G1 cohorts in Coastal and Atlantic waters coincided rather well in time with the phytoplankton blooms, the timing of the cohort in Arctic Water was delayed compared to the phytoplankton. Two or more Calanus generations in Coastal Water, and two generations in Atlantic Water were observed. Only one generation was found in Arctic Water, where scarce autumn data precludes evaluation of a possible second generation.

► We have analysed basin-scale environmental and plankton data from the Norwegian Sea.
► Seasonal cycles for environmental variables and Calanus finmarchicus are presented.
► Chl. a peaked earlier in spring in Coastal Water than in Atlantic and Arctic Waters.
► Calanus G1 development was delayed in oceanic regions compared to Coastal Water.
► The Calanus G1 peak seemed to lag behind the spring chlorophyll peak in Arctic Water.