Vertical distribution of polycystine radiolarians in the central North Pacific

Abstract

Stratified opening/closing net tows in the upper 2000 m of the central North Pacific reveal the major features of the vertical zonation of polycystine radiolarians. Depth profiles of total radiolarian densities show high values at the surface, sharp decrease to a minimum at about 50 m, a maximum exceeding surface levels at about 100 m, and a decrease then to very low values at about 300 m which persist through the rest of the sampled water column. Individual species profiles indicate that the following zones can be identified: (1) A surface zone characterized by a sharp subsurface decrease from high densities at the surface. Densities are an order of magnitude lower by depths of 50–100 m. The rate of decrease resembles the temperature decrease in the thermocline. (2) An upper subsurface maximum zone characterized by species which are absent or very scarce at the surface and reach a pronounced maximum at about 50–100 m. Hydrographic features within the zone are the shallow salinity minimum and the oxygen maximum. A regional chlorophyll maximum occurs at about the same depth as the radiolarian maximum. (3) A lower subsurface maximum zone distinguished from the upper one by occurrence of the maximum about 100 m deeper. Species in this zone are generally scarce at the depth of the upper maximum while species in the upper maximum are scarce at the depth of this maximum. This zone is below the shallow salinity minimum and within the depth of the subjacent salinity maximum which characterizes the habitat of other (tripylean) radiolarians in this area. (4) A deep zone characterized by species which are never abundant but which are consistently absent from the upper few hundred meters. The North Pacific Intermediate Water, previously identified as a tripylean habitat, lies in the depth range of these species, though they may also extend deeper. Despite their lower densities, these species are conspicuous in deep-sea surface sediments.