Seasonal and vertical distribution of living planktonic foraminifera in the western tropical Pacific Ocean

Abstract

To investigate the seasonal and vertical distributions of planktonic foraminiferal species in the western tropical Pacific Ocean, we collected planktonic foraminifera seasonally and diurnally from eight water depths from 0 to 600 m using the Multiple Opening/Closing Net and Environmental Sensing System. We identified 23 living foraminiferal species (size fraction 212–1000 μm) and examined their abundances at the sites of oligotrophic areas. Seasonal changes were characterized by an increase in the diversity and abundance of planktonic foraminifera in summer. We estimated the habitat depth of planktonic foraminifera based on their vertical distribution. Planktonic foraminifera can be classified into species living in the surface mixed layer, uppermost thermocline, upper thermocline, and lower thermocline in the water column. The uppermost thermocline layer is characterized by the occurrence of subsurface chlorophyll maximum (water depth, 100–150 m). The boundary between the upper and lower thermocline layers (approximately 300 m) is at the depth at which the dissolved oxygen concentration begins to rapidly decrease. The species inhabiting the surface mixed layer were Globigerinoides sacculifer and Globigerinoides ruber white in spring. In summer, G. sacculifer dwelled in the surface mixed layer during night time, whereas they existed in the uppermost thermocline during daytime. Many species were found in the uppermost thermocline layer in summer, including G. ruber white, Globigerinoides conglobatus, Globigerinella siphonifera, Pulleniatina obliquiloculata, Globoquadrina conglomerata, Orbulina universa, Neogloboquadrina dutertrei, Globigerinella calida, and Hastigerina pelagica. The species Globorotalia hirsuta dwelled in the upper thermocline in spring, whereas it occurred in the lower thermocline in summer. Globorotaloides hexagonus inhabited the lower thermocline in all seasons. Our results will provide useful information for reconstructing changes in water column structure using planktonic foraminiferal shells in marine sediments.