ze 
of intense productivity nutrients may simply be maintained at a low level, even though they are 
to a moderate degree diffusing upwards through a weak pycnocline. The effect of grazing will 
also complicate the simplified relationships. Thus the primary production was on the average 
considerably smaller during the warm years of the period 1958-1964 than during those years 
which were warm in the period 1970-1980. It has been postulated (Thordardottir, 1977) that this 
was probably due to the effect of zooplankton grazing, since zooplankton densities were much 
greater in the former period than in the latter, when e.g. Calanus finmarchicus which constitu- 
tes the main food of herring practically disappeared from the area. Changes in the composition 
of the phytoplankton could possibly account for the low densities of zooplankton in recent years 
(Astthorsson et al., 1983), but further studies on these relationships are under way. Finally, 
the changes in food conditions as the result of decreased primary production were probably the 
main cause of the collapse of the summer herring fisheries (Jakobsson, 1980). 
It is concluded that climatic changes may not only lead to marked changes in the physical 
and chemical environment of arctic regions, but also have great biological and economical con- 
sequences. 
NOz-N 
(umol titre’) 
( 46,/a2)"!-10°2 
PRIMARY PRODUCTION VERT: MIXING 
(ygC Itre!' hour"! ) 
58-64 65,67 7071 72 73 74 75 76 77 78 79 80 
68,69 
} 
YEARS 
Fig. 5. Comparison between mean primary production, index of vertical mixing (the reciprocal 
of vertical stability), concentration of nitrate and concentration of phosphate, in May/June for 
the section off Siglunes (mean for 7 stations). White columns: warm years; black columns: 
cold years. Based on Thordardottir (1980). 
