Seasonal cycle of zooplankton southwest of Iceland

Abstract

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‰] and the Atlantic water (S > 35.0‰ southwest of Iceland. The sampling was carried out at approximately monthly intervals from February to November 1991. The mean temperature of the upper 50 m of the water column varied greatly during the year. Lowest temperatures were recorded in March–April (5–7δC) and the highest in July–August (11–12δC). 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 a. 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 (˜33% of the total zooplankton), followed by Temora Iongicornis (˜27%), Evadne nordmanni (˜17%), Podon leuckarti (˜8%) and Pseudocalanus spp. (˜6%). On the other hand, in the Atlantic water the plankton were completely dominated by C.finmarchicus (˜92%), while euphausiid eggs and larvae ranked second (˜3%) and third (˜2%), respectively. The annual mean dry weight biomass was about one and a half times greater in the Coastal water (˜6 g m−2 than in the Atlantic water (˜4 g m−2 Further, the annual mean abundance was approximately four times higher in the Coastal water (˜120 000 individuals m−2 compared to the Atlantic water (˜32 000 individuals m−2 This is presumably reflecting the higher production of the zooplankton community in Coastal water compared to Atlantic water.