SEASONAL CYCLE OF ZOOPLANKTON SOUTHWEST OF ICELAND

Seasonal variations in biomass, abundance and species composition of zooplankton in relation to environmental parameters and chlorophyll a were studied in both the Coastal water [salinity (S) < 35.0 parts per thousand] and the Atlantic water (S > 35.0 parts per thousand) southwest of Iceland. The sampling was carried out at approximately monthly intervals from February to November 1991. The mean temperature of the upper 50m of the water column varied greatly during the year. Lowest temperatures were recorded in March-April (5-7 degrees C) and the highest in July-August (11-12 degrees C). The spring bloom of the phytoplankton probably occurred earlier in the Coastal water (early May) than in the Atlantic water (mid-to late May) and in neither water mass was there an autumn peak in the standing stock of chlorophyll alpha. The seasonal variations in both biomass and abundance of total zooplankton followed more or less those of the most abundant zooplankters, and were characterized by a seasonal low during the winter and two maxima during the summer (May-June) and early autumn (July-September). In the Coastal water, Calanus finmarchicus was the most abundant species (similar to 33% of the total zooplankton), followed by Temora longicornis (similar to 27%), Evadne nordmanni (similar to 17%), Podon leuckarti (similar to 8%) and Pseudocalanus spp. (similar to 6%). On the other hand, in the Atlantic water the plankton were completely dominated by C. fumarchicus (similar to 92%), while euphausiid eggs and larvae ranked second (similar to 3%) and third (similar to 2%), respectively. The annual mean dry weight biomass was about one and a half times greater in the Coastal water (similar to 6 g m(-2)) than in the Atlantic water (similar to 4 g m(-2)). Further, the annual mean abundance was approximately four times higher in the Coastal water (similar to 120 000 individuals m(-2)) compared to the Atlantic water (similar to 32 000 individuals m(-2)). This is presumably reflecting the higher production of the zooplankton community in Coastal water compared to Atlantic water.