Seasonal to interannual changes in planktonic foraminiferal assemblages in the northwestern North Pacific: Sediment trap results encompassing a warm period related to El Niño
Abstract Planktonic foraminifera provide a record of the upper ocean environment through their species assemblage and individual tests. To investigate the relationship between foraminifera and oceanographic conditions and the impact of El Nino on foraminifera, we analyzed foraminiferal fluxes and relative abundances by using sediment trap samples collected biweekly at three sites in the northwestern North Pacific: Site 40N (39°60′N, 165°00′E), Site KNOT (43°58′N, 155°03′E), and Site 50N (50°01′N, 165°02′E) from 1998–2001, a period that included an El Nino event. Based on foraminiferal production and assemblage composition, we divided the sampling duration into several periods during which certain characteristic oceanographic properties were observed. These sampling periods were classified into five types (I–V) based upon four factors: 1) the predominant foraminiferal group, 2) total foraminiferal fluxes (TFFs), 3) organic matter (OM) fluxes, and 4) hydrographic conditions, which included sea-surface temperature (SST) and thermal structure. Our results suggest that our observed seasonal changes in foraminifera were closely related to water-mass properties in addition to SST. If species compositions were the same, then water-mass properties were the most important factors affecting the seasonal variation of foraminiferal abundance in the northwestern North Pacific. Although one of the major controlling factors for foraminiferal fluxes is food availability, the controlling factors for each type (Types I–V) are different because of specific oceanographic situations, such as phytoplankton blooms, which result in an excess food supply for foraminifera. At Site KNOT, high surface temperatures and weak winds related to an El Nino period in 1998 would have caused a low nutrient supply and water-column stratification, and resulted in the relatively low fluxes of total foraminifera, Neogloboquadrina pachyderma , and Globigerina bulloides and high fluxes of Neogloboquadrina dutertrei . The impact of El Nino on foraminifera was variable depending on the strength of the El Nino event and site location. However, there were some instances of common annual foraminiferal patterns; the patterns observed at Sites 40N and KNOT in 1998 were similar to those observed off Chile in 1997–1998 and in 1991–1992, respectively.