Seasonal variation of planktonic foraminifera assemblage in response to seasonal shift of inter-tropical convergence zone in the northeastern equatorial pacific

Abstract

A time-series sediment trap was operated at a water depth of 4950 m from July 2003 to May 2004 at KOMO station (10o30'N, 131o20'W) in the northeastern equatorial Pacific, with the aim of understanding the temporal variation of planktonic foraminifera assemblages in response to the seasonal shift of Inter-Tropical Convergence Zone (ITCZ). A total of 22130 planktonic foraminifera specimens belonging to 30 species and 11 genera were identified, which shows a distinct seasonal variation with high values (125~288 specimens m-2 day-1) in the winter to spring (December-May) and low values (16 ~ 23 specimens m-2 day-1) in the fall (September-November). In addition, seasonal ecological differences of foraminifera assemblages are distinctly recognizable: omnivorous foraminifera occurred predominantly. during the summer season, whereas herbivorous ones were dominant during the winter season. Such seasonal variations correspond to the seasonal shift of the ITCZ. Enhanced occurrence of herbivorous species during the winter-spring season seems a result of surface water mixing generated by the southward shift of the ITCZ. The increase in omnivorous species during the summer season may be due to the northward movement of the ITCZ caused by weakened wind speed, resulting in the intensification of water column stratification and nutrient-poor environment. A significant reduction of planktonic foraminifera specimens during the fall is attributed to heavy precipitation and reduction in light intensity. © 2014, Korea Ocean Research and Development Institute. All rights reserved.