Seasonal variability in planktonic foraminiferal fluxes in the subtropical North Pacific: Implications for the effects of global warming

Planktonic foraminifera are valuable paleoceanographic indicators because their shell assemblages reflect ocean surface conditions. However, studies examining relationship with environmental factors in the subtropical Northwest Pacific remain limited. Herein, we investigated seasonal changes in foraminiferal fluxes and assemblages using bi-weekly sediment-trap samples collected from October 2019 to August 2020 in the North Pacific Subtropical Gyre (NPSG). Fluxes and assemblages showed significant seasonal variation, and cluster analysis grouped species into three clusters. Group A (symbiont-bearing warm-water species) was regulated primarily by SST (sea surface temperature) (~28.5 °C), likely due to photosymbiosis. Group B (deep-dwelling species) appeared to be driven by annual reproduction cycles and food availability. Group C ( Globigerinella siphonifera ) showed no clear seasonality. We also report, for the first time, three morphotypes of Orbulina universa (Type I–III) with distinct seasonal variations in the North Pacific. Notably, Type II exhibited a flux pattern similar to Group A, suggesting a preference for warm, stratified waters. By integrating 18 sediment-trap records from the North Pacific, we found that assemblage composition was primarily determined by SST, whereas the magnitude of total foraminiferal fluxes (TFFs) was closely associated with primary productivity and organic carbon (OC) flux. OC and TFFs were particularly low in the NPSG, reflecting its strongly stratified oligotrophic conditions. Although no major assemblage shift was detected over the past two decades, long-term SST records revealed significant warming, particularly in winter, suggesting enhanced stratification. These results imply that future warming may expand subtropical waters, potentially reducing foraminiferal fluxes and affecting the marine carbon cycle.