Seasonal variations in biomass, abundance and composition of zooplankton in the subarctic waters north of Iceland

The seasonal variations in biomass, abundance and species composition of zooplankton in relation to hydrography and chlorophyll a were studied in the subarctic waters north of Iceland. The sampling was carried out at approximately monthly intervals from February 1993 to February 1994 at eight stations arranged along a transect extending from 66°16′N–18°50′W to 68°00′N–18°50′W. The mean temperature at 50 m depth showed a clear seasonal pattern, with lowest water temperatures in February (∼1.1°C) and the highest in July (∼5.4°C). The spring growth of the phytoplankton began in late March and culminated during mid-April (∼7.0 mg Chl a m⁻³). Both the biomass and the abundance of total zooplankton were low during the winter and peaked once during the summer in late May (∼4 g m⁻² and ∼38,000 individuals m⁻²). A total of 42 species and taxonomic groups were identified in the samples. Eight taxa contributed ∼90% of the total zooplankton number. Of these Calanus finmarchicus was by far the most abundant species (∼60% of the total zooplankton). Less important groups were ophiuroid larvae (∼9%), Pseudocalanus spp. (∼8%), Metridia longa (∼4%), C. hyperboreus (∼3%), Acartia longiremis (∼2%), chaetognaths (∼2%) and euphausiid larvae (∼2%). The dominant copepods showed two main patterns in seasonal abundance: C. finmarchicus, C. hyperboreus and C. glacialis had one annual peak in numbers in late May, while Pseudocalanus spp., M. longa and A. longiremis showed two maxima during the summer (July) and autumn (October/November). Ophiuroid larvae and chaetognaths (mainly Sagitta elegans) peaked during the middle of July, while the number of euphausiid eggs and larvae was greatest from May to July. The succession in population structure of C. finmarchicus indicated its main spawning to be in April and May, coincident with the phytoplankton spring bloom. A minor spawning was also observed sometime between August and October. However, the offspring from this second spawning contributed only insignificantly to the overwintering stock of C. finmarchicus. Received: 12 September 1997 / Accepted: 1 March 1998