Composition and distribution of mesozooplankton in the Nansen Basin,Arctic Ocean,during summer

Information about the large-scale zooplankton distribution in the Nansen Basin, that part of the Arctic Ocean most directly influenced by the inflow of Atlantic Water, is still scarce. During the Polarstern expedition ARK IV/3 in July/August 1987 zooplankton from the upper 500 m was sampled along two transects across the western Nansen Basin. Spatial variations in faunal composition, abundance, and biomass are described. Special emphasis is given to the distribution patterns and possible drifting routes of species advected into the Arctic Ocean through Fram Strait. Forty-five species and eight taxa not determined to species level were identified. Copepods clearly dominate in terms of species number, abundance, and biomass. Ostracods and chaetognaths comprise the most important groups of the non-copepod zooplankton. Maximum species numbers were recorded over the abyssal plain around 83°N. Total abundance and biomass decreased by more than 80% towards the northern part of the basin. Cluster analysis revealed two major faunal assemblages, broadly overlapping over the central Nansen Basin, and three principal types of spatial distribution. The well-defined zonation observed in the large-scale zooplankton distribution closely resembles the major hydrographic and topographic features. The opposite movement of the Polar Surface Water and the Atlantic Water in the southern Nansen Basin might cause pronounced deviations between the drifting routes of species with different water mass preferences advected through Fram Strait.