Quantity and composition of zooplankton at three stations in Isafjord-deep, northwest Iceland, were studied from February 1987 to February 1988 Sampling was made with 335 mum mesh Bongo-net at approximately monthly intervals along with hydrographic measurements and measurements of chlorophyll a concentrations. Spring warming of the water began in early May and maximum temperatures (8-10-degrees-C) were observed during July-September. The phytoplankton bloom started in early April, maximum chlorophyll a levels were observed in late April to the middle of May (c. 8 mg Chl a/m3), and a smaller 'autumn increase' was again observed in July to August (c. 3 mg Chl a/m3). The total zooplankton volume was highest in August (c. 32 ml/100 m3) while it was lowest in February 1987 (< 1 ml/m3). During the winter months euphausiids dominated in terms of volume (> 90 %) while during the summer months the combined volume of fish larvae and 'other zooplankton' (mainly copepods and cirripedes) was similar to that of the euphausiids. The total number of zooplankton stayed small through April. Increasing number through the following months led to a maximum in August (c. 90 000 individuals/100 m3), after which numbers decreased sharply to low winter levels. A total of 40 species and groups was recorded in Isafjord-deep, but only six (Acartia spp., Calanus finmarchicus, Pseudocalanus elongatus, Temora longicornis, Balanus balanoides and Verruca stroemia) usually constituted 80-90 % of the total zooplankton number, The dominant holoplanktonic copepods showed similar changes throughout the year, with a pronounced maximum in August. C. finmarchicus overwintered at copepodite stages IV and V and the overwintering generation spawned during late March and early April. Only a part of the spring generation became mature in June to produce the second generation during the year; the majority remained at younger copepodite stages during The summer and autumn and did not become mature until the following spring. The spring spawning of C. finmarchicus in March-April appeared to be closely linked to the phytoplankton spring bloom, while at that time the temperature in Isafjord-deep was near the annual low.
