Changes in phytoplankton biomass during a period of significant warming in the south-western Norwegian Sea

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Chlorophyll and hydrographic conditions were m onitored in the southern Norwegian Sea in spring in the period 1991-2008. The area includes two differe nt water m asses: warm and saline Atlantic water in the south, separated from colder and le ss saline Arctic water in the north by the IcelandFaroe Front (IFF). Our results show that the inter-annual variations in the chl a concentration (Fav) were sim ilar in both water m asses (regression analysis p < 0.01) but with consistently higher concentrations in the Arctic water m ass north of the IFF. Furtherm ore the inter-annual variations in density change through the upper 50 m (Ddif) were very similar and highly correlated (p << 0.01) in both water m asses, and a regression analyses confir med a statistically significant relationship (p < 0.01) between the Fav and Ddif for the Arctic water mass, but not for the Atlantic. Thus stratification is the main controlling factor for the phytoplankton biomass in May, especially in the Arctic water mass north of the IFF. Our results also indi cate that stratification is m ainly governed by temperature; this is conf irmed by a tight relati onship between the tem perature gradient in the pycnocline and the chl a concentration above the pycnocline (F apd), especially in the Arctic water mass to the north of IFF.