Spatial heterogeneity and niche differentiation in oceanic zooplankton

Abstract

The annual mean abundance (1958 to 1986) and geographical distributions of four of the major biomass species of copepod in the northern North Atlantic Ocean are shown together with their seasonal vertical distributions (1971 to 1974) from Ocean Weather Station I. The arcticboreal species Euchaeta norvegica and Calanus finmarchicus have sympatric distributions with their maximum numerical abundance in the cold water current system between Labrador and Greenland. The distribution of the temperate species Pleuromamma robusta and Metridia lucens are allopatric with respect to the two previous species but have sympatric distributions centering around the north-eastern oceanic region. The four species dominate the copepod biomass of the oceanic epiplankton and minimise interspecific competition by a) seasonal displacement of their main reproductive periods, b) occupying different trophic levels, c) having diverse reproductive strategies, d) residing in different positions in the water column and e) having different migratory behaviour throughout the year. The vertical distributions and migratory behaviour of these four copepods suggest a highly structured community with individual species occupying distinct niches with minimum overlap between competitors; even though the vertical distributions of the species are constantly changing through diel, ontogenetic and seasonal migrations.