21 
rae fe 33:5 34:0 34:5 35:0 35-5 
/ 
25°W 20° 15° 
65° a = 
Fig. 4. Temperature-salinity relationships at S-3 (66°32'N, 18°50'W) in May/June for different 
years and periods. Solid curves: warm years; broken curves: cold years. Based on 
Thordardottir (1980). 
Climatic fluctuations affect life in the sea both directly and indirectly. In particular, it 
seems that the changes of recent decades in the density stratification of the surface layers have 
had marked biological implications. This has been confirmed by studies of primary production 
over the last 25 years in North Icelandic waters (Thordardottir, 1980). For the period late May 
- early June very marked year to year differences were found, normally in such a way that the 
productivity was low in cold years, but high in warm years. This, however, had less to do 
directly with temperature than with the density stratification of the surface layers (Fig. 4). 
There is in general a fair degree of correlation between primary production, vertical mixing and 
nutrient concentrations in the surface layers (Fig. 5). Thus it appears that low primary produc- 
tion occurring in the cold years can be attributed to marked stratification, preventing the 
renewal of nutrients in the surface layers, whereby continued growth of phytoplankton was 
greatly reduced. There are, however, certain discrepancies which need to be examined. Thus 
small nutrient concentrations do not necessarily imply very strong stratification. During periods 
