Distribution of polycystine radiolaria and their relation to the physical environment during the 1992 El Niٌo and following cold event

Radiolarian density and composition were determined for two transects from 12°N to 12°S along 140°W to evaluate the relationship between species distribution and physical conditions in the equatorial Pacific. Highest standing stocks were measured during the U.S. JGOFS Equatorial Pacific Survey II cruise (August 1992) at 2°N in association with a convergent, tropical instability wave front. Radiolaria accumulate at the surface of the front and subduct to at least 100 m depth and over 100 km to the north. Standing stocks measured during Survey I (February/March), when El Niٌo conditions prevailed, were lower and less patchy than during Survey II. Radiolarian compositional data are evaluated using Q-mode factor analysis. The result is a six factor model that explains 82% of the plankton data set. El Niٌo conditions are dominated by an assemblage of species found in high abundance in the surface sediments of the western Pacific. This assemblage is related to reduced flow in the South Equatorial Current and a concomitant increase in the influence of warm surface waters from the western Pacific. The cold period following the El Niٌo can be characterized by the interaction of two other assemblages related to subtropical and equatorial water masses, respectively. Both have distributions similar to the patterns of meridional advection and have highest abundances in the convergent front encountered at 2°N. The subtropical assemblage is associated with warm, southward-flowing water to the north, while the equatorial assemblage is associated with cool, northward-flowing water to the south of the front. These results indicate radiolarian distribution in the equatorial Pacific is primarily controlled by ocean circulation with sea-surface temperature of secondary importance.