Long-term variation in phytoplankton productivity during spring in Icelandic waters

Annual variations in primary productivity (uptake of C-14) measurements in Icelandic waters during spring since 1958 are analysed for four geographically defined regions, which correspond to major hydrographical features. The overall means by region range from 4.3 to 9.2 mg C m(-3) h(-1). Annual variation in the shelf region north-east of Iceland reflects the major changes observed in environmental conditions, especially during the 1970s. The seasonal development of phytoplankton productivity depends on surface salinity conditions and its effects on the stability of the water column. Given favourable conditions, the spring bloom may start to develop in late March/early April and usually has its peak in May. In the Arctic Water north-east of Iceland, there is a single, well-defined peak, whereas a sequence of peaks is frequently observed in the Atlantic Water of the south-western shelf. The differences between the regions show the importance of the physical factors affecting phytoplankton dynamics. (C) 1998 International Council for the Exploration of the Sea.