Relation of radiolarian distributions to subsurface hydrography in the North Pacific

Based on opening-closing net plankton collections, the vertical and horizontal distributions of some triplyean radiolarian species have been plotted for the eastern North Pacific. In a mid-ocean profile of the upper 400 m, there emerge: (1) a zone including subarctic near-surface waters and extending southward at greater depths, (2) a narrow zone to the south concentrated above the first zone, and (3) a zone yet farther south ranging from the surface to a lower boundary that shoals northward. These zones correspond generally to the following respective hydrographic features: (1) the Subarctic Water Mass and the Intermediate Water, centered on a salinity minimum, (2) an unnamed mid-latitude zone characterized by a salinity maximum above the salinity minimum and (3) the warm, high salinity, Central Water Mass. Toward the North American continent, distributions of species from the two more northern (and deeper) zones overlap as the respective water masses become mixed in the California Current. The southernmost (and shallowest) zone retains hydrographic and biologic continuity as a surface layer at the outer edge of the California Current. Semi-closed circulation systems in the hydrographic zones provide physical and biological continuity within the zones with minor interchange among them.

The horizontal distributions of these radiolarians in the upper 150 m are comparable to those of other kinds of plankton in the same area, and they are generally consistent, hydrographically, with previously reported distributions of the same species in other oceans.