Large-scale oceanic distribution and population structure of Calanus finmarchicus in relation to physical food and predators

The investigation was carried out from 62 degreesN to 73 degreesN and from WE to I PW in the Norwegian Sea during 19 June-12 July 1997. Regional diffe rences in the phase of the seasonal development of the plankton community w ere evident, most pronounced across the Arctic front. In the Coastal and ea stern Atlantic domains, post-bloom conditions prevailed, characterised by l ow chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg prod uction rates of Calanus finmarchicus were low (< 10 eggs female I day(-1)), older copepodite stages dominated, and the seasonal descent to deeper wate rs had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates w ere higher (a maximum of 29 eggs female(-1) day(-1)), but the dominance of stages' CI-CIII indicated that considerable spawning had already occurred p rior to the spring bloom. The seasonal descent had barely started. Both inv ertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing northwestwards. No tig ht relationship between total abundance of invertebrate or fish predators a nd that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that the se invertebrate predators develop parallel to the development of the new co hort of C. finmarchicus. In early summer, C. finmarchicus had reached overw intering stages, and had started to accumulate in deeper waters in areas wi th the highest abundance of horizontally migratory planktivorous fish.