DISTRIBUTION OF POLYCYSTINE RADIOLARIA AND THEIR RELATION TO THE PHYSICAL-ENVIRONMENT DURING THE 1992 EL-NINO AND FOLLOWING COLD EVENT

Radiolarian density and composition were determined for two transects from 12 degrees N to 12 degrees S along 140 degrees W to evaluate the relationship between species distribution and physical conditions in t he equatorial Pacific. Highest standing stocks were measured during th e U.S. JGOFS Equatorial Pacific Survey II cruise (August 1992) at 2 de grees N in association with a convergent, tropical instability wave fr ont. 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 mea sured during Survey I (February/March), when El Nino conditions prevai led, were lower and less patchy than during Survey II. Radiolarian com positional data are evaluated using Q-mode factor analysis. The result is a six factor model that explains 82% of the plankton data set. El Nino conditions are dominated by an assemblage of species found in hig h abundance in the surface sediments of the western Pacific. This asse mblage is related to reduced flow in the South Equatorial Current and a concomitant increase in the influence of warm surface waters from th e western Pacific. The cold period following the El Nino can be charac terized by the interaction of two other assemblages related to subtrop ical and equatorial water masses, respectively. Both have distribution s similar to the patterns of meridional advection and have highest abu ndances in the convergent front encountered at 2 degrees N. The subtro pical assemblage is associated with warm, southward-flowing water to t he north, while the equatorial assemblage is associated with cool, nor thward-flowing water to the south of the front. These results indicate radiolarian distribution in the equatorial Pacific is primarily contr olled by ocean circulation with sea-surface temperature of secondary i mportance. Copyright (C) 1996 Elsevier Science Ltd.